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Planning your Literature Review

While planning your review, in addition to finding and analyzing the reviews in dissertations, you might ask yourself questions such as the following:

What is my central question or issue that the literature can help define?

What is already known about the topic?

Is the scope of the literature being reviewed wide or narrow enough?

Is there a conflict or debate in the literature?

What connections can be made between the texts being reviewed?

What sort of literature should be reviewed? Historical? Theoretical? Methodological? Quantitative? Qualitative?

What criteria should be used to evaluate the literature being reviewed?

How will reviewing the literature justify the topic I plan to investigate?

From: Writing the successful thesis and dissertation: entering the conversation , by Irene L. Clark

source: Kent State University's Literature Reviews Libguide

Organizing the Review

Categorizing the Literature

When categorizing the writings in the review, the researcher might consider

• the methodology employed;
• the quality of the findings or conclusions;
• the document’s major strengths and weaknesses;
• any other pivotal information.

He/She might consider such questions as:

• what beliefs are expressed?
• Is there an ideological stance?
• What is being described? Is it comprehensive or narrow?
• Are the results generalizable?

Remember that you are relating other studies to your study. How do the studies in your lit. review relate to your thesis? How are the other studies related to each other?

From http://libguides.redlands.edu/content.php?pid=32380&sid=239161

Literature Review -Created by North Carolina State University Libraries

Watch this video for more information about writing a literature review.

• Writing a Literature Review

What is a Literature Review?

A literature review is a comprehensive and up-to-date overview of the principal research about the topic being studied.

The aim of a literature review is to show "that the writer has studied existing work in the field with insight" (Haywood and Wragg, 1982). It is not enough merely to show what others in your field have discovered. You need to view the work of others with insight to review critically. An effective review analyses and synthesizes material, and it should meet the following requirements: (Caulley, 1992)

• Compare and contrast different authors' views on an issue
• Group authors who draw similar conclusions,
• Criticise aspects of methodology,
• Note areas in which authors are in disagreement,
• Highlight exemplary studies,
• Identify patterns or trends in the literature
• Highlight gaps in and omissions in previous research or questions left unanswered
• Show how your study relates to previous studies,
• Show how your study relates to the literature in general,
• Conclude by summarising what the literature says.

A literature review has a number of purposes. It enables you to:

• Set the background on what has been researched on a topic.
• Show why a topic is significant to a subject area.
• Discover relationships between ideas.
• Identify major themes & concepts.
• Identify critical gaps & points of disagreement.
• Help the researcher turn a network of articles into a coherent view of the literature.

Source: University of Melbourne's Literature Review Libguide

Literature Review Samples

• Otterbein's Institutional Repository You can browse by collection and then department and student scholarship. Look up samples of literature reviews in theses and dissertations.
• OhioLink's ETD Browse by institution and look up samples of literature review in the students' theses and dissertations
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What is a literature review?

Analysis and critical evaluation of research previously done on your topic.

Literature review helps you:

• find a gap in the research so you can contribute something original
• justify your project and set the context for your topic

You will cite articles, standards, books, etc. on your topic that helped you develop your project (ideas, processes, models, useful quotations).

Some examples of literature review in the PDF below. Go through each example and pick one that is the best out of the four.

Writing your literature review

Take notes as you read!

Use a Synthesis Matrix, local version below adapted from the Synthesis Matrix technique developed by North Carolina State University).

• Synthesis Matrix - Local Version

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What is a Literature Review?

Definition and purpose.

• Further Information

Not to be confused with a book review, a literature review surveys scholarly articles, books and other sources (e.g. dissertations, conference proceedings) relevant to a particular issue, area of research, or theory, providing a description, summary, and critical evaluation of each work. The purpose is to offer an overview of significant literature published on a topic.

Similar to primary research, development of the literature review requires four stages:

• Problem formulation —which topic or field is being examined and what are its component issues?
• Literature search —finding materials relevant to the subject being explored
• Data evaluation —determining which literature makes a significant contribution to the understanding of the topic
• Analysis and interpretation —discussing the findings and conclusions of pertinent literature

Literature reviews should comprise the following elements:

• An overview of the subject, issue or theory under consideration, along with the objectives of the literature review
• Division of works under review into categories (e.g. those in support of a particular position, those against, and those offering alternative theses entirely)
• Explanation of how each work is similar to and how it varies from the others
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research

In assessing each piece, consideration should be given to:

• Provenance —What are the author's credentials? Are the author's arguments supported by evidence (e.g. primary historical material, case studies, narratives, statistics, recent scientific findings)?
• Objectivity —Is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness —Which of the author's theses are most/least convincing?
• Value —Are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

A literature review may constitute an essential chapter of a thesis or dissertation, or may be a self-contained review of writings on a subject. In either case, its purpose is to:

• Place each work in the context of its contribution to the understanding of the subject under review
• Describe the relationship of each work to the others under consideration
• Identify new ways to interpret, and shed light on any gaps in, previous research
• Resolve conflicts amongst seemingly contradictory previous studies
• Identify areas of prior scholarship to prevent duplication of effort
• Point the way forward for further research
• Place one's original work (in the case of theses or dissertations) in the context of existing literature

The literature review itself, however, does not present new primary scholarship.

An example of a literature review is:

Allen, R.C. (1996). Socioeconomic Conditions and Property Crime: A Comprehensive Review and Test of the Professional Literature. The American Journal of Economics and Sociology, 55 (3), 293.

Tips for finding literature reviews in your field:

• ask your professor or advisor for recommendations for good examples of literature reviews in your discipline. You can also check out the literature review sections of peer review journal articles, theses, or dissertations.
• Search ProQuest Dissertations and Theses for dissertations and these to look at their literature review and also to look at their bibliography.
• certain discipline specific databases like PsycINFO have ways to limit your search to just review or systematic review articles. 

Further information on the literature review may be found in:

Adapted from " How to Write a Literature Review ", courtesy of the UC-Santa Cruz Library, retrieved 30th October 2008 from the Web.

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Environmental Science

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Useful resources on literature or systematic reviews

What is a literature review.

• Geographical information systems
• Government and other sources of environmental information, including datasets
• Statistical resources
• Griffith school of environment - systematic quantitative literature review Developed by the school of environment at Griffith University for environmental science students, this resource shows how to systematically search the literature using online databases; save and manage the information and quantify the current status of the literature.
• Systematic searching for environmental evidence using multiple tools and sources Environmental evidence - the official journal of the collaboration for environmental evidence, 2017, 6(23). https://doi.org/10.1186/s13750-017-0099-6
• Guidelines for systematic reviews in environmental management Collaboration for environmental evidence. 2013. Guidelines for systematic review and evidence synthesis in environmental management. Version 4.2. http://environmentalevidence.org/wp-content/uploads/2014/06/Review-guidelinesversion-4.2-finalPRINT.pdf
• GEOS6001 qualitative research and literature reviews Slides prepared by Tiffany Morrison for GEOS6001 research philosophy, design & implementation 11/03/2014
• Literature reviews guide How to search for, store, organise, evaluate and critique information for your literature review. Includes techniques, books, articles and more to help you do your literature review.
• Systematic reviews guide

A literature review is a critical assessment of the literature pertaining to a particular topic or subject.  It is a 'systematic, explicit, and reproducible method for identifying, evaluating and synthesising the existing body of completed and recorded work produced by researchers, scholars, and practitioners'  Fink, (2005).

What's its purpose?

• justify your research
• provide context for your research
• identify new ways, to interpret and highlight gaps in previous research
• ensure that the research has not been done before
• signpost a way forward for further research
• show where the research fits into the existing literature
• highlight flaws in previous research
• UQ Student Support - writing a literature review A guide to the process of planning and writing a literature review.
• Annotated bibliography A useful guide on how to write entries for your annotated bibliography, from UQ Student Support
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Environmental Science

Literature reviews.

"A literature review discusses published information in a particular subject area, and sometimes information in a particular subject area within a certain time period.

A literature review can be just a simple summary of the sources, but it usually has an organizational pattern and combines both summary and synthesis. A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information. It might give a new interpretation of old material or combine new with old interpretations. Or it might trace the intellectual progression of the field, including major debates. And depending on the situation, the literature review may evaluate the sources and advise the reader on the most pertinent or relevant." Source:  http://writingcenter.unc.edu/handouts/literature-reviews/ This link opens in a new window

The library has several books and ebooks in our collection which can help answer your questions about writing literature reviews. To find books and ebooks on writing a literature review, please search the  Multi-Search  or the  Online Library Catalog .

Further Help

This information is intended to be a guideline, not expert advice. Please be sure to speak to your professor about the appropriate way to write a literature review in your class assignments and projects.

Campus Students

To access academic support, visit your Brightspace course and select “Tutoring and Mentoring” from the Academic Support pulldown menu.

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To access help with citation and more, visit Academic Support via modules in Brightspace:

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A systematic literature review of environmental concerns in smart-cities

I K Raharjana 1

Published under licence by IOP Publishing Ltd IOP Conference Series: Earth and Environmental Science , Volume 245 , The International Conference Research Collaboration of Environmental Science 12 March 2018, Universitas Airlangga, Surabaya, Indonesia Citation I K Raharjana 2019 IOP Conf. Ser.: Earth Environ. Sci. 245 012031 DOI 10.1088/1755-1315/245/1/012031

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1 Information Systems, Faculty of Science and Technology, Universitas Airlangga, Kampus C UNAIR Mulyorejo, Surabaya, Indonesia

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The concept of smart-city is to use information and communication technology to efficiently manage assets and resources in order to support sustainable urban environment. Examples of smart-city implementation are the use of ICTs in government systems, energy management, buildings, infrastructure, health services, population, and others. The purpose of this study is to identify and analyze existing study of environmental concerns in smart-cities. Search terms with relevant keywords were used to identify appropriate studies. Filtering and scoring techniques were used to obtain relevant studies. 345 papers were obtained from abstract search results, 248 were successfully downloaded. 14 manuscripts pass abstract filtering and quality assessment, it included into analysis phases. Environmental concern is an integral part of smart city management. Some smart-city frameworks have identified environmental factors that must be considered. The majority of studies focus on areas of resource, governance, and urban infrastructure. In general, the application of smart-cities that focus on resource and urban infrastructure will have a positive impact on the environment. This can be maximized by smart-cities governance that supports environmental policy.

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More about the Literature Review...

Did you know the library has an entire guide to help you write a literature review?  Click the link below to learn more!

• Literature Review: Conducting & Writing by Britt McGowan Last Updated Mar 22, 2024 117816 views this year

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The UWF Writing Lab can help you with grammar, proofreading, and answer questions about your paper.  For a full list of their services, look at the Lab Hours and Resources menue.

What Is A Literature Review?

A Literature Review Is Not:

• just a summary of sources
• a grouping of broad, unrelated sources
• a compilation of  everything  that has been written on a particular topic
• literature criticism (think English) or a book review

So, what is it then?

A literature review is an integrated analysis-- not just a summary-- of scholarly writings that are related directly to your research question.  That is, it represents the literature that provides background information on your topic and shows a correspondence between those writings and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

Steps for Conducting a Lit Review

1. Choose your topic, define your question

• Your literature review should be guided by a central research question.  Remember, it is not a collection of loosely related studies in a field but instead represents background and research developments related to a specific research question, interpreted and analyzed by you.

2. Decide on the scope of your review

• How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

3. Select the databases you will use to conduct your searches

4. Conduct your searches and find the literature. Keep track of your searches!

• Review the abstracts and conclusions carefully. This will save you time.
• Write down the keywords you used and where you found them
• Use RefWorks to keep track of your citations.

5. Review the literature! This is the most time consuming part.

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions. Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited?; if so, how has it been analyzed?
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Peer-reviewed

Research Article

The effects of gases from food waste on human health: A systematic review

Roles Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliations Department of Schulich School of Medicine and Dentistry, Western University, London, ON, Canada, Department of Health Sciences, Western University, London, ON, Canada

Roles Investigation, Methodology, Writing – review & editing

Affiliation Department of Geography and Environment, Western University, London, ON, Canada

Roles Supervision, Validation, Writing – review & editing

• Paulina Rudziak, 
• Evans Batung, 
• Isaac Luginaah

• Published: March 27, 2024
• https://doi.org/10.1371/journal.pone.0300801
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Food waste is a routine and increasingly growing global concern that has drawn significant attention from policymakers, climate change activists and health practitioners. Amid the plurality of discourses on food waste-health linkages, however, the health risks from food waste induced emissions have remained under explored. This lack of evidence is partly because of the lack of complete understanding of the effects of food waste emissions from household food waste on human health either directly through physiological mechanisms or indirectly through environmental exposure effects. Thus, this systematic review contributes to the literature by synthesizing available evidence to highlight gaps and offers a comprehensive baseline inventory of food waste emissions and their associated impacts on human health to support public health decision-making. Four database searches: Web of Science, OVID(Medline), EMBASE, and Scopus, were searched from inception to 3 May 2023. Pairs of reviewers screened 2189 potentially eligible studies that addressed food waste emissions from consumers and how the emissions related to human health. Following PRISMA guidelines, 26 articles were eligible for data extraction for the systematic review. Findings indicate that emissions from food waste, such as hydrogen sulphide, ammonia, and volatile organic carbons, can affect human endocrine, respiratory, nervous, and olfactory systems. The severity of the human health effects depends on the gaseous concentration, but range from mild lung irritation to cancer and death. This study recommends emission capture technologies, food diversion programs, and biogas technologies to reduce food waste emissions.

Citation: Rudziak P, Batung E, Luginaah I (2024) The effects of gases from food waste on human health: A systematic review. PLoS ONE 19(3): e0300801. https://doi.org/10.1371/journal.pone.0300801

Editor: Helen Onyeaka, University of Birmingham, UNITED KINGDOM

Received: August 21, 2023; Accepted: March 5, 2024; Published: March 27, 2024

Copyright: © 2024 Rudziak et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: Funding for this study was provided by Western University’s Undergraduate Student Research Internship. The funder provided support in the form of a bursary for the author [PR], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: Funding for this study was provided by Western University’s Undergraduate Student Research Internship in the form of a bursary. The funding does not interfere with the full and objective presentation, peer review, editorial decision-making, or publication of research submitted to PLOS ONE.

Introduction

Food waste is a common global issue–the latest Food and Agricultural Organization (FAO) report on food waste approximates 1.3 billion tonnes of food in the world is lost or wasted each year [ 1 ]. Food waste emits harmful gases, such as CO 2 , H 2 S, CH 4 , N 2 O, and PM 2.5 , that are detrimental to human health [ 2 – 4 ]. Emissions from food waste can negatively impact human health directly and indirectly. These impacts may include an increasing number of respiratory issues, and mild and severe headaches [ 2 , 5 ]. The human health impacts have led to many global emission-related food waste policies and goals. For example, the FAO plans to halve the per capita global food waste at the retail and consumer levels in response to Sustainable Development Goal 12− Responsible Consumption and Production by 2030 [ 1 ]. In this study, composting, a form of food waste, is used to measure and characterize food waste emissions [ 2 , 6 ].

Level of analysis

For this systematic review, food is defined as edible products that contribute to human nourishment [ 7 ]. The definition of food waste will be “discarded food” from households, restaurants, and food catering services. Kitchen food waste mainly consists of fruit, vegetables, meat and bones, bread, fish bones, pasta, shellfish, rice, eggshells, coffee grounds, and dairy products [ 8 ]. Residential food waste can quickly decompose and produce odours because of its nature, high organic content, chemical mixtures, and sensitivity to room temperature [ 6 ].

Although food waste occurs in multiple stages of the food chain, focusing on the consumer level is important because of the relationship between consumption and food waste reinforced by the rapid industrialization, urbanization and economic development that have increased food waste levels [ 9 ]. Food waste estimates show that anthropogenic influence accounts for nearly one pound of wasted food per person per day [ 10 , 11 ]. Wasted food is equivalent to over 30 million acres of cropland each year, mainly accounting for grains, oilseeds, vegetables, fruit, and dairy [ 10 ]. Fruits and vegetables are among the most wasted products [ 11 ]. Restaurants are guilty of serving portions too large for people to gauge higher price and profit margins. As a result, lots of food is wasted if the consumer does not like the idea of eating leftovers the next day [ 12 ]. Moreover, hotels, convention centres, resorts, and banquet halls that host large gatherings throw away a lot of viable food [ 13 ].

Metaphorically, if food waste were a country, it would be the third-largest greenhouse gas emitting country [ 1 ]. Thus, research on food waste emissions from households and consumer outlets needs to be the prime area of focus for their direct and indirect impacts on human health. Furthermore, most studies addressing food waste emissions are in contexts that vary in climatic conditions; thus, differences between food waste emissions will be assessed to account for different climatic conditions of composting areas. Warmer months with low aeration provide optimal conditions for food waste to emit gases at high concentrations [ 9 ]. Hence, food waste gas concentrations will be discussed in relation to odour thresholds (maximum tolerable gaseous concentration without odour annoyance) and olfactometry thresholds (the ability to change olfactory cell physiology) [ 9 , 14 ].

The current solutions to food waste emissions

Alternatives to food waste emissions have been explored, such as generating biofuels to power vehicles, heat homes, and generate electricity [ 15 ]. In this manner, food waste emissions are reused rather than released into the atmosphere which can have deleterious effects due to imbalances. Biofilters for composting sites have been experimented with to understand which filtering method best limits food waste emissions [ 14 , 16 ]. In addition, food diversion systems have also been explored to ship untouched food from restaurants and catering events to local food banks and provide nourishment to people in need of food [ 13 ]. Despite these innovations, the net effect of the unprocessed food waste is negative as emissions continue to harm human health. It is therefore unsurprising that alternative methods to releasing food waste emissions into ambient air are being intensively explored [ 16 ].

Relevance of this study

Previous systemic reviews addressing food waste have focussed on production and agricultural food waste, and loss prevention and mitigation techniques, but less so on human health outcomes and consumer food waste emissions [ 17 – 20 ]. This systematic review is unique in that it combines the studies reporting on various foods that emit toxic emissions from a consumer level and studies focusing more on the human consequences of these gaseous emissions. Such a review illustrates a clear link of food waste emissions and the impact on human health.

Hypothesis and objectives

For this review, we hypothesize that food waste emissions will pose respiratory, nasal, and social health issues based on the current food waste evidence in the literature. Accordingly, the objective of this study is to review analyses of how gas emissions from food waste can impact human health both directly and indirectly. The objective will be accomplished by summarizing findings from literature and creating an inventory of food waste gases and their associated health effects. In this systematic review, direct human health impacts relate to physiological effects on the human body. Indirect human health effects relate to secondary outcomes, such as fresh-water acidification, which can lead to human health effects.

We began the design by searching the international database of Prospectively Registered Systematic Reviews (PROSPERO) in “Health” and “Social care” for similar or identical reviews prior to study commencement, and none were found [ 21 ]. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were then employed to systematically identify and assess approaches to ensure consistent methods and analyses for this review [ 22 ]. To inform this systematic review, the databases Web of Science, OVID(Medline), EMBASE, and Scopus were searched from inception to 3 May 2023. These databases were selected based on their relevance to environmental and human health research. No restrictions on language or publication date were set in the databases. Three strings were searched in each database. The strings included keywords ‘food waste’ AND ‘human health’ AND ‘gases,’ and synonyms for gases were also searched, including ‘emissions’ and ‘vapours.’ An additional search strategy was later added to review compost emissions more broadly. The key words ‘compost emissions’ AND ‘human health’ were searched in Web of Science, OVID(Medline), EMBASE, and Scopus ( S1 Fig ).

For the first search strategy, a pair of reviewers screened 1020 potentially eligible titles and abstracts in the systematic review software Rayyan, after removing 483 duplicates ( Fig 1 ) [ 23 ]. This software allows for uploads of search strategies from databases into a project folder, detects duplicates among the added titles and abstracts, and allows for highlights of keywords to help each reviewer with the inclusion of articles.

• PPT PowerPoint slide
• PNG larger image
• TIFF original image

https://doi.org/10.1371/journal.pone.0300801.g001

For the second and third search strategy, a pair of reviewers screened 668 potentially eligible titles and abstracts for the systematic review in the systematic review software Covidence, after removing 348 duplicates [ 24 ]. Covidence was used for the second search strategy because of the upgrades that were made available to facilitate the screening process with relative ease. Following training, pairs of reviewers independently screened all titles and abstracts, followed by full-text articles that were identified as potentially eligible. When necessary, a consensus was reached through a discussion.

The inclusion criteria for titles and abstracts consisted of primary research related to food waste emissions from the human consumer level, such as household food waste, restaurant food waste, and purposeful viable food discards from food retailers. Articles that report a mix of municipal solid waste, everyday garbage consisting of everything and anything, for food waste emissions data were not considered for the review [ 25 ]. This is because emissions from unsorted municipal solid waste cannot be generalized to the food waste within. The articles of focus are studies that report emissions from consumer food waste/ scraps and organic compost. Food waste from production, manufacturing, and commercial farming was also not considered because of differences in definitions of food waste across studies and the vast complexities involved. Title and abstracts also had to mention human health in context to be included for full-text screening.

A total of 70 (6.8%) articles were marked as discrepancies between reviewers after title and abstract screening and were resolved after discussion. To facilitate full-text screening, articles were restricted to the English language [ 26 ]. After title and abstract screening, 80 articles were eligible for full-text screening, and two articles were excluded because they did not have the full-text published. In total, 78 full-text articles from the search strategy itself were screened by a pair of reviewers. An excel sheet was formulated for full-text screening with identical inclusion criteria ( S2 and S3 Figs). Criteria for inclusion consisted of primary research articles that addressed food waste, waste emissions, human health, and articles published in the English language. The link to human health could be direct or indirect (measured or mentioned in context). Editorials, commentaries, and reviews were excluded to focus on evidence-based primary research. After resolving 13 (32.5%) discrepancies, 19 out of 78 articles were included for data analyses after full-text screening, and 59 out of 78 articles were excluded ( Fig 1 ). The excluded articles lacked addressing human health directly or indirectly and consisted of varying definitions of food waste that were too challenging too extract. The bibliography of included articles was searched for relevance to minimize the risk of not including relevant studies. Seven articles were included from searching citations of included articles, expanding the included full-text articles for data analysis to 26 articles ( S4 Fig ).

To evaluate the quality of each publication selected for the systematic review, a modified version of the “McMaster University’s Critical Review Form” was used to thoroughly critique the studies ( S5 and S6 Figs) [ 27 ]. Reviewers individually used the modified form to critique articles in the following areas:

• purpose of the study,
• background information,
• study design,
• interventions,
• results, and
• conclusions of article.

The Joanna Briggs Institute critical appraisal form for case reports was used for the one included case report [ 28 ]. To facilitate the appraisal process, excel files were created using the checklists of the critical appraisal reports. Critical appraisals were completed in duplicate, and opinions on the credibility of articles were discussed and decided amongst the reviewers. To facilitate the write-up process, annotations of included full-text articles were created. Included in the annotations were methodologies, results, limitations, and recommendations. The annotations helped to determine common themes among included articles to help articulate the systematic review and helped determine the set-up of the results summary table. The summarization of ideas in articles is subjective to a certain degree but allows researchers to compare views and extract shared meanings [ 29 ].

Results and discussion

Studies analyzed research in various countries with different food waste emission regulations. Olfactory and odour thresholds differed among countries, likely because of the change in climate and governmental stipulations [ 4 , 9 , 14 ]. The 26 included studies were conducted mainly in Asia (38%), Europe (35%), and the United States of America (23%) ( Fig 2 ). Most of the articles were also published within the last decade (65%) ( Fig 3 ). The number of articles reporting mild to severe direct human health impacts is summarized in Table 1 . Because of the variability in composition, temperature, climate, and moisture of food waste across countries, the summary of food waste emissions in Table 2 is not generalizable for the entire world, but the impact on human health from such gases is universal.

The python-generated map includes circles of various sizes representing the number of studies included from various countries. China, Europe, and the United States of America were the main origins of studies conducted. Most studies were conducted in Asia.

https://doi.org/10.1371/journal.pone.0300801.g002

The year 2017 had the most frequent publication date, and the last decade showed the most publications in general.

https://doi.org/10.1371/journal.pone.0300801.g003

https://doi.org/10.1371/journal.pone.0300801.t001

https://doi.org/10.1371/journal.pone.0300801.t002

Classification of food waste emissions

Typical for composting conditions, food waste was mixed with a percentage of wood chips, usually 30% in included studies [ 39 ]. A typical food waste composting process includes sorting/crushing food mechanically, hydrothermal hydrolysis, then conversion into a liquid for anaerobic digestion or conversion into a solid for aerobic fermentation [ 31 ]. During all stages of the composting process food waste emissions can be released in various levels of toxicity into the surrounding environment [ 31 ]. Composting conditions across included studies were not variable enough to emit higher amounts of toxic gases, but higher concentrations of food waste gases were typically collected in winter months [ 9 , 31 ]. Most studies used a gas chromatography-mass spectrometer to identify the emissions from food waste [ 4 , 9 , 14 , 31 ]. Emissions from food waste during various stages of composting were compared to odour thresholds recommended by environmental protection agencies of the countries represented in the studies [ 14 ].

The most common harmful gases captured from food waste include some volatile organic carbons (VOCs) (such as terpenes and sulphur compounds); 1,2,4-trimethylbenzne; aromatic compounds and hydrogen sulphide because of their associated human health effects [ 9 , 14 , 16 ]. Food waste composting is influenced by the concentration of emissions, which is dependent on aeration, temperature, moisture, and storage [ 34 ]. Food waste decomposes more at higher temperatures, and odour intensity increases linearly with increasing storage time and increasing temperatures [ 6 ]. The measure of total organic carbon can be used to quantify the amount of organic matter present within compost [ 6 ]. Quantification of organic matter is important to determine the homogeneity of the compost, and if different factors contribute to high emission concentrations or high odour intensity levels.

Odour intensity can be measured by two evaluators smelling buckets of food waste at 1, 4, 7, 10, and 14 days apart, and at 20°C and 8°C temperature differences [ 6 ]. Generally, the more extended storage periods at 7, 10, and 14 days at 20°C had a scaled odour intensity described as overpowering, intolerable for any length of time, and acute exposure could change olfactory physiology. The odours in common with all the buckets were ammonia and isovaleric acid. Both have low odour thresholds and strong scents, possibly causing the strong odours from all the buckets evaluated at 20°C. At 8°C, the emissions were considered very distinguishable, irritable, and objectionable [ 6 ]. The most common harmful gases captured from food waste include some volatile organic carbons (VOCs) (such as terpenes and sulphur compounds); 1,2,4-trimethylbenzne; aromatic compounds; and hydrogen sulphide that tested above their designated odour thresholds [ 9 , 14 , 16 ]. Hundreds of VOCs emitted from compost, and they pose a hazard to human health [ 34 ]. The VOCs shown on Table 2 mainly include terpenes, butane, halogenated compounds, aromatic compounds, isobutane, pentane, butane, dimethyl sulphide, and dimethyl disulphide [ 16 , 31 , 34 , 40 ]. VOCs emit odours during conditions of low aeration and high moisture levels, increasing the volatility of compounds by increasing their vapour pressure and availability of microorganisms to degrade the food waste [ 34 ]. Given the various health effects described in Table 2 , VOC emissions variably break down because of the uneven distribution of methanogenic bacteria used to break down composted food and varying molecular weights of compounds [ 34 , 40 ]. The VOCs with lower molecular weights can quickly enter the gaseous phase and become volatile [ 41 ]. In the aqueous compost phase, VOCs can solubilize easily under high temperatures and are emitted at high rates into the surrounding atmosphere [ 34 , 40 ]. Concentrations of VOCs are higher and more concerning for human health in indoor settings than in outdoor settings [ 16 ]. Human health is compromised from long-term VOC emissions from food waste because of the carcinogenicity of VOCs, their ability to irritate olfactory cells, change their physiology, and compromise endocrine, respiratory, and nervous systems [ 31 ].

Seasonal fluctuations in food waste emissions

Dietary changes in the summer months create food waste mainly consisting of fruits and vegetables, compared to high fat and protein foods consumed in the winter months [ 9 ]. Fruits and vegetables contain large amounts of sugars and carbohydrates that are easily converted into oxygenated organic compounds such as acetaldehyde. In January, people consume more eggs and meats that contain sulphur proteins, so sulphur malodors are common [ 9 ].

The difference between the measurements during the two seasons was higher water content and higher temperature during summer months, making decomposition easier [ 9 , 34 ]. This difference is specific to China’s summer and winter months and follows Chinese odour regulations [ 9 ]. During September emission measurement collections, thresholds were exceeded for toluene (0.65173 mg m -3 ), dimethyl sulphide (0.00776 mg m -3 ), and acetaldehyde (0.00282 mg m -3 ) across multiple compost sites [ 9 ]. Ammonia emissions exceeded the threshold (1.0629 mg m -3 ) at one compost plant during September emission collections. During both September and January emission collections, thresholds were exceeded for methanethiol (0.00014 mg m -3 ), dimethyl disulphide (0.00862 mg m -3 ), and hydrogen sulphide (0.000581 mg m -3 ) across multiple compost sites. Hydrogen sulphide was the only gas that exceeded its threshold at every compost location in both September and January collections, suggesting its emission of a powerful odour [ 9 ]. A study completed in Taiwan compared food waste emissions during typically high temperature and humidity conditions of the compost plant to olfactory thresholds [ 14 ]. The emissions p-Cymene and ethylbenzene exceeded their olfactory thresholds (12 ug m -3 ) and (13 ug m -3 ), respectively, compromising the health of workers. Concentrations below the emission thresholds stated are considered safe for workers in the compost area [ 9 , 14 ].

The emissions stated all had low odour thresholds, which means that even the slightest concentration above threshold levels can irritate workers and residents nearby, measured using an odour activity value (OAV). Hydrogen sulphide, dimethyl sulphide, dimethyl disulphide, methanethiol and acetaldehyde had large OAV values, causing great and sustained annoyance among workers and residents nearby. Large OAV values can detriment workers’ and residences’ health, well-being, and quality of life [ 9 , 14 ]. Hydrogen sulphide is arguably the most lethal food waste emission, having the capability to cause cardiovascular, respiratory, neurological, and vision complications that can lead to impaired functioning and death [ 3 ]. Toluene has a low OAV value and poses minimal annoyance for workers and residents nearby [ 9 ].

Connection to human health

The evidence provided here shows a positive linkage between food waste emissions and both direct and indirect human health impacts. Food waste can disrupt olfactory cell functioning, atmospherically spread to nearby residents, and cause occupational safety concerns [ 4 ]. One of the main concerns of food waste emissions is the possible impairment of olfactory cells [ 4 , 9 , 14 ]. Some studies used the odour thresholds of chemical compounds that varied among studies, to determine the hazard potential for workers at the compost site and residents living nearby [ 9 , 14 , 16 ]. Odours can irritate olfactory cells, which are vulnerable to impairment from strong odours. This can in turn, decrease the safety and quality of life for individuals affected [ 14 , 41 , 42 ]. For example, individuals may not be able to tell when food is spoiled by the smell and may ingest harmful products [ 4 , 14 ]. In another example, the odours from Taiwanese food waste plants forced residents nearby to protest, causing a shutdown and an investigation of the plants [ 4 ]. Proper aeration and controlling temperatures of food decomposition are essential to lower concentrations of food waste emissions [ 16 , 31 ].

Table 2 includes information about respiratory health issues related to food waste emissions. Respiratory health issues range from least severe, lung irritation, to most severe, carcinogenicity. The respiratory health reports came from complaints of compost plant workers and residents living nearby them [ 4 , 9 , 16 ]. The highly volatile gaseous compounds, such as VOCs, had the most detrimental respiratory human health effects [ 16 ].

Moreover, the emissions of food waste, such as ammonia gas, have social health costs in addition to environmental health and human physiological health. For example, ammonia is a precursor for the formation of Particulate Matter 2.5, which are ultra-fine particles known to cause severe respiration complications by lodging deep into alveoli, obstructing their function and shape [ 43 ]. Particulate matter can travel long distances, affecting populations not only in the nearby composting location [ 2 ]. Arguably, a possible solution to human exposure to food waste emissions is to locate composting sites in very sparsely populated areas [ 2 ]. However, an environmental inequity would arise because rural inhabitants would be exposed to polluted air from food they mostly did not throw away.

Key components in food waste

In addition to gaseous chemical emissions, biological aerosols and endotoxins are emitted from food waste compost and pose potentially serious human health respiratory repercussions [ 44 ]. Such toxicity is especially harmful to humans because of the biological aspect that can create more severe human health complications [ 45 ].

Bioaerosols

Bioaerosols are aerosols with any biological origin; bacteriological and fungal aerosols are mainly reported in this review [ 44 ]. The highest concentration of bioaerosols is found at the boundary areas for site collection, which may be due to accumulation from being carried upwind [ 45 ]. Peaks in bioaerosol emissions occur at the 100 and 150m boundary downwind of the composting sample, possibly because of the buoyancy effect, causing some bioaerosols to rise above sampling height until cooled to sink back to a sampling height [ 45 ]. Inhalation of bioaerosols pose respiratory challenges for humans, such as inducing allergies, sensitivity, and infectious disease [ 46 ]. A standard for microbes in the air is not universally settled, but not exceeding 1000cfu/m 3 is recommended [ 39 ]. Size of bioaerosol particles matter since smaller particles can penetrate deeper into the respiratory system, making it more difficult to for lungs to recover [ 46 ]. Bioaerosols of particles >7.1μm reach the nasal cavity, 4.7–7.1μm reach the pharynx, 3.3–4.7μm reach the trachea and primary bronchi, 2.1–3.3μm reach the secondary bronchi, 1.1–2.1μm reach the terminal bronchi, and 0.65–1.1μm reach the alveoli [ 47 ]. Such deep penetrations can pose respiratory challenges as mentioned previously [ 46 ]. Bacteria emitted from a United Kingdom compost site were <0.6μm in diameter, threatening deep tissues of human lungs. Some larger particles were actinobacteria and firmicutes, with a diameter >3.3μm.

Fungal aerosols are also considered bioaerosols and they range in size. Common fungal aerosols found around the world in compost are Penicillium , Aspergillus , Emericella , and Paecilomyes [ 39 , 48 ]. In some samples, such as a UK composting site, Capnodiales were found to have a diameter >3.3μm, and made up more than 25% of the fungal community in the compost. Eurotiales have a diameter between 4.7–1.1μm and made up more than 50% of the fungal community in the compost. Ascomycota have a diameter between 3.3–4.7um and make up a little more than 6% of the fungal community in the compost. Glomeromycota have a diameter between 1.1–0.65μm and make up 6% of the fungal community in the compost [ 47 ]. Table 3 classifies bioaerosols and gases by odour thresholds or diameter.

https://doi.org/10.1371/journal.pone.0300801.t003

Endotoxin emissions released from compost are potent and proinflammatory for human respiration; associated with ailments such as airway obstruction, pneumonitis, bronchitis and decreased lung function [ 50 ]. Endotoxins do not disperse into communities near compost sites as dispersal is maxed at 280m past the compost site. Thus, endotoxins mainly threaten the health of plant workers [ 51 ]. Endotoxin emission >50EU/m3 can impose damage to human respiration systems, very low emissions of endotoxins (~2.3EU/m3) were reported in the study, which cannot justify the respiratory complaints of the occupational workers [ 51 ]. Table 4 summarizes the range of colony forming units (CFUs) for fungal emissions and general endotoxin emissions from compost.

https://doi.org/10.1371/journal.pone.0300801.t004

Potential solutions to food waste emissions

Based on the findings, we propose potential solutions. Considering the health threats that food waste can pose to human health, we suggest several pathways in which this effect can be avoided. First, an eco-friendlier solution to uncompromised, ready-to-eat food is to partner with a food bank to provide ready meals and ingredients that would have otherwise been thrown away [ 13 ]. In about a year, a pilot study donated 24,703kg of recovered food, providing about 45,383 meals, which prevented the release of 82.8 MT CO 2 eE of CO 2 , 15.5 MT CO 2 E of CH 4 , and 8.5 MT CO 2 E of N 2 O [ 13 ]. Not only does redirecting food decrease landfill area use, but it also provides food to starving populations which can play a critical role in increasing the health and well-being of food insecure and malnourished groups.

Second, we also propose the use of organic food waste to create biofuel to power vehicles and provide electricity and heating [ 15 ]. Biofuel creation can still emit typical food waste gases, so a gas capture technology is necessary [ 2 ]. Biofuel for vehicle use could avoid climate change impacts caused by petrol fuel by capturing most food waste emissions and converting it into energy [ 15 ]. A study conducted in China measured the impact on human health from diverting food waste to create biofuel in Disability Adjusted Life Years (DALYs) [ 15 ]. The impact on human health from climate change and particulate matter was measured as 3.51x10 -6 DALYs and 2.6 x 10 −4 DALYs, respectively. Biogas used for electricity and heating had a human health damage of 10.65 x 10 −5 DALYs/ tonne food waste. Biogas for city gas had a human health damage of 26.46 x10 -5 DALYs/ tonne food waste. Biogas for vehicle use was calculated to have negative DALY values, considering the avoidance of petrol fuel [ 15 ].

A potential solution for food waste emissions from compost is to install a biofilter or bio-tricking filter to reduce gaseous food waste emissions, such as toxic hydrogen sulphide. A biofilter is a tower made from gravel, wood chips, food waste compost and coal bottom ash, whereas as a bio-trickling filter consists of a tower with polypropylene balls to filter material [ 2 , 14 , 16 ]. For example, a biofilter can capture 90% of ammonia emissions from composted food piles [ 14 ]. In general, less strong odorous gases, such as ammonia and VOCs, can be emitted at composting plants that use either biofilters or bio-trickling filters, because of deodorization [ 14 ].

Public policies

Reduction of compost gas emissions would be advantageous to the physical and social health of workers and close neighborhoods [ 52 ]. Policies for diverting food waste can include a food redistribution program to redistribute edible food to food banks, and education. Educating the public and commercial food retail can include topics on food waste, portion size, food purchasing, planning and preparation, and enacting more served plated food rather than buffet style [ 13 ]. A redistribution program can target restaurants, banquet halls, convention centres, and catering services to collect viable food to donate to local food banks. Educating the public on planning meals before grocery shopping can limit food waste. In addition, educating the public on how to properly store certain foods can help decrease wastefulness. Educating commercial food retailers on how to preserve prepared food, waste less food during preparation, and serve smaller portion sizes. Transitioning from buffet-style food businesses to plated foods can significantly decrease food waste and decrease food preparation costs for the businesses. In this manner, public sectors and individuals can be educated on limiting food waste.

Encouraging cities to enact “green bin initiatives” with capture technologies can help decrease malodor complaints and associated human health consequences for compost plant workers and people residing near the plant [ 2 ]. In addition, capturing emissions could also create biofuels to generate electricity for nearby residents [ 15 ]. In this manner, people would be recycling food waste into a clean, usable energy source.

Limitations and strengths

Notwithstanding our thorough search, only a few articles focused on consumer food waste, despite this sector growing in food waste gas emissions over the years as gross domestic product increases [ 13 ]. Furthermore, most of the articles in the search strategy that were excluded from this study focused on emissions during food production processes, rather than food waste processes. In addition, very few articles measured direct human health effects of food waste emissions, most focused on identification of gases and indirect human health links. This systematic review makes a crucial addition to the food waste literature because of the limited primary research and systematic reviews on consumer food waste and human health impacts, and calls upon researchers to explore more human impacts from food waste emissions.

A strength of this study is the duplication of abstract screening, full-text screening, and critical appraisals for all the search strategies that generated 1688 unique articles. PRISMA guidelines have also been thoroughly followed, increasing this article’s validity as a systematic review [ 22 ]. The critical appraisals of the included articles deemed each study to be conducted well, increasing the strength of the compiled data. Lastly, the compilation of data into Table 3 was a major accomplishment that strengthened the systematic review as a thorough synthesis of food waste emission impacts on human health.

Areas for future research

Future primary research needs to address human health outcomes directly through clinical trials, like how a study [ 6 ] used two individuals to rate odours from buckets of food waste. Direct observations of human health outcomes can strengthen the current literature and provide clear evidence for relevant stakeholders to enact policies. In addition, a more holistic approach to human health effects needs to be studied to address the interconnectedness of environmental health and human health. Moreover, a study understanding the difference in food waste thresholds across countries could help address the worldwide human health risk of varying waste emission concentrations to workers and the public. Understanding the differences in odour thresholds by means of guidelines or standardization on documenting gas emissions and limits could warrant a meta-analysis in the food waste literature. Lastly, studies on global distillation could help address the worldwide effect of food waste emissions from individual countries and help implement international regulations.

Conclusions

This study presents a compilation and categorization of evidence of food waste emissions and their impacts on human health. After analyzing 26 articles, this study found that food waste from consumers can emit gases, such as VOCs, ammonia, carbon dioxide, methane, and hydrogen sulphide, which can be a detriment to human health directly through physiological effects and indirectly through secondary environmental health effects. Hydrogen sulphide, aromatic compounds, and halogenated compounds posed the greatest risk to human health. These emissions complicated multiple organ systems and increased the chance of death and cancer. This study has the potential to inform decision-makers to enact food waste policies that can prevent the adverse human health effects of food waste emissions. Solutions to reduce food waste emissions have also been explored, such as food diversion programs, emission capture and filter technologies for compost, and creating biofuel. However, few policies have been implemented to reduce food waste emissions, including food redistribution and food waste education programs. Funding more primary research in the entire food waste sector is needed to raise awareness of associated health risks and help decision-makers formulate plans of action.

Supporting information

S1 fig. search strategy strings..

https://doi.org/10.1371/journal.pone.0300801.s001

S2 Fig. Full-text screening of identified articles 2021.

https://doi.org/10.1371/journal.pone.0300801.s002

S3 Fig. Full-text screening of identified articles 2022 and 2023.

https://doi.org/10.1371/journal.pone.0300801.s003

S4 Fig. Articles from references.

Articles included for data analysis from references of articles included from the search strategy (n = 4).

https://doi.org/10.1371/journal.pone.0300801.s004

S5 Fig. Critical appraisals of search strategy 2021.

https://doi.org/10.1371/journal.pone.0300801.s005

S6 Fig. Critical appraisals of search strategy 2022.

https://doi.org/10.1371/journal.pone.0300801.s006

S1 Graphical abstract. The effects of gases from food waste on human health: A systematic review.

https://doi.org/10.1371/journal.pone.0300801.s007

Acknowledgments

We are grateful for the editorial comments and guidance provided by Paulina Kowalewska, Dr. Beth Hundey, and Dr. Katrina Moser (Western University). Kevin McIntrye is acknowledged for providing biostatistical guidance on the study design.

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Evolution and performance analysis of thermal comfort indices for tropical and subtropical region: a comprehensive literature review

• Published: 20 May 2024

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• S. Patle   ORCID: orcid.org/0009-0003-9132-8801 1 &
• V. V. Ghuge 1  

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Thermal comfort is a significant aspect of human well-being and productivity, particularly in tropical and subtropical regions where high temperatures and humidity prevail yearly. However, research on indoor and outdoor thermal comfort in those regions is lacking when compared to other areas, and a significant gap exists in the comprehension related to the suitability and application of existing thermal comfort indices. This study intends to provide insights into existing human thermal comfort indices based on their applications in tropical and subtropical regions of Koppen climates for indoor and outdoor. PRISMA guidelines are used for conducting a systematic literature review using 86 articles. Of these, 38 were original thermal comfort indices articles and 48 were case studies (indoor and outdoor). This research discussed the most and least preferred indices as well as the approaches and parameters considered by the thermal comfort indices. The result shows that most efforts were made in temperate and continental climates to derive thermal comfort indices. For outdoor thermal comfort assessment, 72% of studies used Physiological Equivalent Temperature; 21% used Discomfort Index and Universal Thermal Climate Index. Whereas for indoors, > 90% of studies used the Predicted Mean Vote index and Operative Temperature. Overall, the used thermal comfort indices in reviewed studies underestimate the comfortable ranges observed in tropical and subtropical regions. The findings of this study can be utilized by researchers, urban designers, and planners to increase understanding related to human thermal comfort in tropical and subtropical climates.

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Patle, S., Ghuge, V.V. Evolution and performance analysis of thermal comfort indices for tropical and subtropical region: a comprehensive literature review. Int. J. Environ. Sci. Technol. (2024). https://doi.org/10.1007/s13762-024-05703-8

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A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE : Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

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Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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• Last Updated: May 21, 2024 11:14 AM
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