Importance
The Environmental Protection Agency (EPA) is required to re-examine its National Ambient Air Quality Standards (NAAQS) every 5 years, but evidence of mortality risk is lacking at air pollution levels below the current daily NAAQS, in unmonitored areas and for sensitive subgroups.
Objective
To estimate the association between short-term exposures to ambient PM2.5 and ozone and at levels below the current daily NAAQS and mortality in the continental US.
Design, Setting, and Participants
Case-crossover design and conditional logistic regression to estimate the association between short-term exposures to PM2.5 and ozone (mean of daily exposure on the same day of death and one day prior) and mortality in 2-pollutant models. The study included the entire Medicare population from January 1, 2000 to December 31, 2012 residing in 39,182 zip codes.
Exposures
Daily PM2.5 and ozone levels in a 1 km × 1 km grid were estimated using published and validated air pollution prediction models based on land use, chemical transport modeling, and satellite remote sensing data. From these gridded exposures, daily exposures were calculated for every zip code in the US. Warm-season ozone was defined as ozone levels for the months April to September of each year.
Main Outcome and Measure
All-cause mortality in the entire Medicare population from 2000 to 2012.
Results
During the study period, there were 22,433,862 million case days and 76,143,209 control days. Of all case and control days, 93.6% had PM2.5 levels below 25 μg/m3, during which 95% of deaths occurred (21,353,817 of 22,433,862), and 91.1% of days had ozone levels below 60 ppb, during which 93.4% of deaths occurred (20,955,387 of 22,433,862). The baseline daily mortality rate was 137.33 and 129.44 (per 1 million persons at risk per day) for the entire year and for the warm season, respectively. Each short-term increase of 10 μg/m3 in PM2.5 (adjusted by ozone) and 10 ppb (parts-per-billion, 10−9) in warm-season ozone (adjusted by PM2.5) were statistically significantly associated with a relative increase of 1.05% (95% confidence interval [CI]: 0.95%, 1.15%) and 0.51% (95% CI: 0.41%, 0.61%) in daily mortality rate, respectively. Absolute risk differences in daily mortality rate were 1.42 (95% CI: 1.29, 1.56) and 0.66 (95% CI: 0.53, 0.78) per 1 million persons at risk per day. There was no evidence of a threshold in the exposure-response relationship.
Conclusions and Relevance
In the US Medicare population from 2000-2012, 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.