Exposure to outdoor air pollution and its human-related health outcomes: an evidence gap map

Abstract

Objectives: Outdoor air pollution is a serious environmental problem worldwide. Current systematic reviews (SRs) and meta-analyses (MAs) mostly focused on some specific health outcomes or some specific air pollution.

Design: This evidence gap map (EGM) is to identify existing gaps from SRs and MAs and report them in broad topic areas.

Data sources: PubMed, Cochrane, Scopus and Web of Science were searched from their inception until June 2018. Citations and reference lists were traced.

Eligibility criteria: SRs and MAs that investigated the impact of outdoor air pollution on human health outcomes were collected. This study excluded original articles and qualitative review articles.

Data extraction and synthesis: Characteristics of the included SRs and MAs were extracted and summarised. Extracted data included authors, publication year, location of the corresponding author(s), publication journal discipline, study design, study duration, sample size, study region, target population, types of air pollution and health outcomes.

Results: Asia and North America published 93% of SRs and MAs included in this EGM. 31% of the SRs and MAs (27/86) included primary studies conducted in 5-10 countries. Their publication trends have increased during the last 10 years. A total of 2864 primary studies was included. The median number of included primary studies was 20 (range, 7-167). Cohort studies, case cross-over studies and time-series studies were the top three most used study designs. The mostly researched population was the group of all ages (46/86, 53%). Cardiovascular diseases, respiratory diseases and health service records were mostly reported. A lack of definite diagnostic criteria, unclear reporting of air pollution exposure and time period of primary studies were the main research gaps.

Conclusions: This EGM provided a visual overview of health outcomes affected by outdoor air pollution exposure. Future research should focus on chronic diseases, cancer and mental disorders.

Keywords: evidence gap map; health outcomes; outdoor air pollution.

Figure 1

Systematic literature search process for eligible SRs and MAs. SR, systematic review; MA, meta-analysis.

Figure 2

Regionwise and countrywise distributions of eligible SRs and MAs. The data of regions and countries are based on the information of corresponding authors. SRs, systematic reviews; MAs, meta analyses.

Figure 3

Numbers of inclusive eligible SRs and MAs in six research fields between 2004 and 2018. The number of eligible SRs and MAs saw an increasing trend from 2004 to 2018. Most of the included studies were published in the last 6 years. SRs, systematic reviews; MAs, meta analyses.

Figure 4

EGM for health outcomes affected by outdoor air pollution. The colours of the bubbles represent the confidence of included SRs and MAs; red colour represents low confidence and yellow colour represents medium confidence. The size of the bubbles indicates the relative number of included SRs and MAs, and a larger bubble represents a larger study sample size in each grid. EGM, evidence gap map; SRs, systematic reviews; MAs, meta-analyses.