In Reply Dr Lipfert criticizes our study for not including indoor-generated particles in our exposure. Indoor particles are a potential confounder, not an omitted part of outdoor particles. Differences in indoor exposure across persons cannot confound the results of our case-crossover study; the exposure contrast was within person, between case and control days. To confound, daily variation in particles from cooking, smoking, etc, must be correlated with daily variation in outdoor PM2.5 of the same person. The Medicare Beneficiary Survey showed that 86% of beneficiaries were nonsmokers. It seems implausible that smokers consistently smoked more or that people fried more food on higher pollution days.
Schwartz JD. Air Pollution and Mortality in the Medicare Population—Reply. JAMA. 2018;319(20):2135–2136. doi:10.1001/jama.2018.3943
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