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

Dear Editor,

We reviewed the paper by Liuet al[1], as part of an umbrella review on PM2.5and the incidence of respiratory outcomes.We determined that the authors did not account for the PM2.5increments based upon which primary studies reported their results.For instance, Younget al[2]reported an odds ratio (OR) of 1.20 (95% confidence interval [CI]: 0.99, 1.46) per 3.6 μg/m3PM2.5for incident asthma.Likewise, Bennettet al[3]reported an OR of 1.08 (95% CI: 0.79, 1.48) per 1 μg/m3PM2.5for incident wheezing.Liuet al[1]simply pooled these and other results "as is", ignoring the differing PM2.5increments, meaning that their pooled estimates are of limited value.

Results from primary studies should be standardized to the same pollutant increment prior to pooling.In our opinion, this could have been detected at the time of peer review, since the authors did not report the PM2.5increment upon which their pooled estimates were based.While we did not examine their findings for other outcomes, we suspect that the same error applies.

We have identified this issue in other recent systematic reviews, including Pranataet al[4], Zhaoet al[5], Hanet al[6], and Zhanget al[7].

Yours Sincerely,

Ammanie Abdul-Fatah1,3, Jia Lu Gao2,3, David M.Stieb3,4,✉

1Department of Health Sciences,

Carleton University,

Ottawa, Ontario K1S 5B6,

Canada;

2Department of Population Medicine,

University of Guelph,

Guelph, Ontario N1G 2W1,

Canada;

3Environmental Health Science and Research Bureau,

Health Canada,

Ottawa, Ontario K1A 0K9,

Canada;

4School of Epidemiology and Public Health,

University of Ottawa,

Ottawa, Ontario K1G 5Z3,

Canada.

✉Tel/Fax:+1-604-666-3701/+1-604666-5741, E-mail:dave.stieb@hc-sc.gc.ca.

RECEIVE IMMEDIATE NOTIFICATION FOR EARLY RELEASE ARTICLES PUBLISHED ONLINE

To be notified by e-mail whenJournalearly release articles are published online, sign up at jbr-pub.org.cn.