Comment on "Effect of exposure to ambient PM2.5 pollution on the risk of respiratory tract diseases: a meta-analysis of cohort studies"

Abstract

Dear Editor, We reviewed the paper by Liu et al[1], as part of an umbrella review on PM2.5 and the incidence of respiratory outcomes. We determined that the authors did not account for the PM2.5 increments based upon which primary studies reported their results. For instance, Young et al[2] reported an odds ratio (OR) of 1.20 (95% confidence interval [CI]: 0.99, 1.46) per 3.6 μg/m3 PM2.5 for incident asthma. Likewise, Bennett et al[3] reported an OR of 1.08 (95% CI: 0.79, 1.48) per 1 μg/mm3 PM2.5 for incident wheezing. Liu et al[1] simply pooled these and other results "as is", ignoring the differing PM2.5 increments, meaning that their pooled estimates are of limited value