Systematic review and meta-analysis of epidemiological time-series studies on outdoor air pollution and health in Asia

Asia is experiencing rapid increases in industrialization, urbanization, and motor vehicle transport with correspondingly high levels of pollution. We undertook a systematic review and meta-analysis of the Asian time-series literature to assess the evidence from health effects of short-term exposure to outdoor air pollution. Eighty-two studies provided estimates suitable for quantitative meta-analysis. Summary estimates for daily mortality and hospital admissions were calculated for particulate matter with aerodynamic diameters less than 10 and 2.5???m (PM₁₀ and PM_2.5), nitrogen dioxide, sulfur dioxide, ozone, and carbon monoxide. For 10???g/m³ increases in PM₁₀, daily mean numbers of deaths increased by 0.27% (95% confidence interval: 0.12%, 0.42%), 0.86% (0.34%, 1.39%), and 0.36% (0.09%, 0.62%) for all cause, respiratory, and cardiovascular deaths, respectively. Associations in the 65+ age group tended to be larger than those for all ages combined. All other pollutants were positively associated with both mortality and hospital admissions. We found no evidence to suggest that the size and direction of PM₁₀ estimates were modified by either the average level of particles in the city or the year of publication (a proxy for more up-to-date statistical methods and the most recent pollutant concentrations and sources). There were insufficient reports for PM_2.5 to enable a quantitative meta-analysis. Our findings were generally consistent with the range of effects found in other parts of the world and suggest that global guidelines and health impacts based largely on evidence derived from the USA and Europe are likely to be reasonably reliable.