The concentration-response between long-term PM_2.5 exposure and mortality; A meta-regression approach

Long-term exposure to ambient fine particulate matter (≤ 2.5 μg/m³ in aerodynamic diameter; PM_2.5) is significantly associated with increased risk of premature mortality. Our goal was to provide an updated meta-analysis of all-cause and cause-specific mortality associated with exposure to PM_2.5 and to better estimate the risk of death as a function of air pollution levels.

We systematically searched all published cohort studies examining the association between long term exposure to PM_2.5 and mortality. We applied multivariate linear random effects meta-analysis with random effects for cohort, and study within cohort. Meta-regression techniques were used to test whether study population or analytic characteristics modify the PM_{2.5 -}mortality association and to estimate the shape of the concentration-response curve.

A total of 53 studies that provided 135 estimates of the quantitative association between the risk of mortality and exposure to PM_2.5 were included in the meta-analysis. There were 39 studies from North America, 8 from Europe, and 6 from Asia. Since 2015, 17 studies of long-term air pollution exposure have been published, covering, wider geographic areas with a wider range of mean exposures (e.g. <12 or > 20 μg/m³). A penalized spline showed the slope decreased at higher concentrations but appeared to level off. We found that the inverse transform of average PM_2.5 well approximated that spline and provided a parametric estimate that fit better than a linear or logarithmic term for average PM_2.5. In addition, we found that studies using space time exposure models or fixed monitors at Zip-code scale (as compared to land use regression method), or additionally controlling for area level socio-economic status, or with mean exposure less than 10 μg/m³ were associated with higher mortality effect estimates.

This meta-analysis provides strong evidence for the adverse effect of PM_2.5 on mortality, that studies with poorer exposure have lower effect size estimates, that more control for SES increases effect size estimates, and that significant effects are seen below 10 µg/m³. The concentration -response function produced here can be further applied in the global health risk assessment of air particulate matter.