To the Editor In a case-crossover study, the authors noted that “In the US Medicare population from 2000 to 2012, short-term exposures to PM2.5 and warm-season ozone were significantly associated with increased risk of mortality. This risk occurred at levels below current national air quality standards, suggesting that these standards may need to be reevaluated.”1 Such studies of association fail to address the key causal question: How would public health effects change if exposure concentrations were reduced? Instead, they addressed an easier, noncausal question: What are the estimated ratios (or slope factors or regression coefficients) of health effects to past pollution levels in selected models and data sets? Answers to the second question are inadequate substitutes for answers to the first question for several reasons.
Cox LA. Air Pollution and Mortality in the Medicare Population. JAMA. 2018;319(20):2134–2135. doi:10.1001/jama.2018.3923
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