Association of Short-term Exposure to Air Pollution With Mortality in Older Adults

Abstract

Importance: The US Environmental Protection Agency is required to reexamine 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 fine particulate matter (PM2.5) and ozone, and at levels below the current daily NAAQS, and mortality in the continental United States.

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 1 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 United States. Warm-season ozone was defined as ozone levels for the months April to September of each year.

Main outcomes and measures: 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 95.2% of deaths occurred (21 353 817 of 22 433 862), and 91.1% of days had ozone levels below 60 parts per billion, during which 93.4% of deaths occurred (20 955 387 of 22 433 862). The baseline daily mortality rates were 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 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% 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 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.

Figure 1. Daily Air Pollution Concentrations in the Continental United States, 2000-2012

Daily mean of PM_2.5 (left panel) and 8-hour maximum ozone (right panel) concentrations were calculated and plotted by state. The time-series plots at the bottom indicate the national daily mean values across all locations. Red dashed lines indicate the daily NAAQS for PM_2.5 (35 μg/m³) and ozone (70 ppb). Boxplots show the distribution of daily PM_2.5 and ozone levels for each state. The line across the box, upper hinge, and lower hinge represent the median value, 75^th percentile (Q3), and 25^th percentile (Q1), respectively. The upper whisker is located at the smaller of the maximal value and Q3+1.5*interquartile (IQR); the lower whisker is located at the larger of the minimal value and Q1 – 1.5*IQR. Any values that lie beyond upper and lower whiskers are outliers.

Figure 2. Absolute Risk Difference and Relative Risk of Daily Mortality Associated with 10 μg/m3 Increase in PM2.5 and 10 ppb Increase in Ozone

As for the main analysis, subgroup analyses used a 2-pollutant analysis (with both PM_2.5 and ozone), based on the mean of daily exposure on the same day of death and one day prior (lag 01 day) as the exposure metric for PM_2.5 and ozone, and controlled for natural splines of air and dew point temperatures (each with 3 degrees of freedom). Vertical lines indicate effects for the entire study population. Subgroup analyses were conducted for each subgroup (e.g., male or female, White or non-White, Medicare-eligible or Medicare-ineligible, age groups, quartiles of population density). For the main analysis and each subgroup, we ran conditional logistic regressions to obtain RR, and calculated ARD based on baseline mortality rates (See Section 2, supplementary material). For ozone, analyses were restricted to the warm season (April to September). Numbers in the figure represent point estimates, 95% confidence intervals, and p-values for effect modifications. “Ref” indicates reference group when assessing effect modification; asterisks indicate a statistically significant effect estimate (at 5% level) compared with the reference group.

Figure 3. Estimated Exposure-response Curves for Short-term Exposures to PM2.5 and Ozone

A 2-pollutant analysis with separate penalized splines on PM_2.5 and ozone was conducted to assess the percentage increase in daily mortality at various pollution levels. Dashed lines indicate 95% confidence intervals. The mean of daily exposure on the same day of death and one day prior (lag 01 day) was used as metrics of exposure to PM_2.5 and ozone. Analysis for ozone was restricted to the warm season (April to September).