Title, Date of Publication & Journal

‘Air pollution (particulate matter) exposure and associations with depression, anxiety, bipolar, psychosis and suicide risk: a systematic review and meta-analysis’, Environmental Health Perspectives

Study’s main claims – and are they supported by the data

The paper suggests that that people experiencing higher levels of air pollution on average tend to have a higher risk of depression, anxiety and suicide. It doesn’t prove that the level of pollution causes these mental health problems

Strengths/Limitations

Strengths

• This study is a systematic review, meaning that the authors have searched for relevant research papers and combined their results. The combination of the different studies means that the results are based on a large number of participants and are reasonably precise. The methods used appear appropriate and are well described.
• The results from the individual studies are reasonably similar.
• The reviewers have assessed the quality of the individual studies.
• The authors include a discussion on the route by which any relationship between pollution and mental health problems might work – i.e. biological changes in the body that might happen as a result of air pollution particles.
• The authors are appropriately cautious with their conclusions and suggest further research rather than claiming a causal relationship.
• The authors have made an attempt to control for confounding factors by selecting adjusted results from the individual studies.

Limitations

• The research question is a very complex one. There are many different ways of measuring exposure to air pollution. Nobody knows how long it might take for any effects of exposure to air pollution to manifest themselves as mental health problems. Methods to diagnose different mental health problems vary in different settings and mental health diagnoses vary over time. The authors have had to make many decisions about which studies to include, which data to extract, how to group studies and how to analyse them. The authors themselves have shown that making slightly different decisions about how to do the review can lead to different conclusions (through sensitivity analyses).
• All of the studies included are observational in nature and may involve retrospective data collection or data collected for a different purpose. These types of study are known to have a high risk of bias; e.g. due to missing data, inconsistency in recording, selecting favourable results. The review authors used a very general tool to assess quality; there are more established tools designed to specifically detect the problems inherent in these types of study.
• Although the authors have considered confounding factors carefully, there are likely to be additional factors that weren’t measured by the individual studies.

Glossary

Bias: under or over estimating the true result

Retrospective: collecting data on something that happened in the past

Observational study: the researcher simply observes the participants without any experimentation

Confounding variable: something related to both exposure (in this case air pollution) and outcome (in this case mental health problem).

Any specific expertise relevant to studied paper (beyond statistical)?

My main expertise is in the area of systematic reviews and meta-analysis. My knowledge of epidemiological studies is more limited.

Before The Headlines is a service provided to the SMC by volunteer statisticians: members of the Royal Statistical Society (RSS), Statisticians in the Pharmaceutical Industry (PSI) and experienced statisticians in academia and research.  A list of contributors, including affiliations, is available at http://www.sciencemediacentre.org/working-with-us/for-journalists/headlines-for-journalists/