Even 'Safe' Levels of Air Pollution Tied to Higher Mortality

Diana Phillips

December 27, 2017

Exposure to air pollution at levels well below current quality standards is linked to an increased risk for death, according to new data published in the December 26 issue of JAMA.

Numerous groups, including elderly adults, women, nonwhites, and Medicaid-eligible individuals, are particularly vulnerable to the ill-effects of daily exposure to increased levels of fine particulate matter (PM2.5) and warm-season ozone, Qian Di, MS, from the Department of Environmental Health, Harvard T. H. Chan School of Public Health in Boston, Massachusetts, and colleagues write.

The findings suggest the National Ambient Air Quality Standards (NAAQS), which were established by the US Environmental Protection Agency under the Clean Air Act, should be reevaluated, the authors write.

To estimate the association between all-cause mortality and short-term exposure to daily ambient PM2.5 and warm season (April 1 - September 30) ozone concentrations, the researchers conducted a case-crossover study using claims data for all deaths among US Medicare participants between 2000 and 2012, comparing daily air pollution exposure on the date of death (case day) with exposure on control days. For each participant, control days were the same day of the week and in the same month as case days to control for potential confounding effects and seasonal patterns.

Using published and validated air pollution prediction models, the researchers predicted the daily 24-hour PM2.5, 8-hour maximum ozone, and daily air and dew point temperatures, assigned by Zip code. They used conditional regression models to estimate the relative risk for all-cause mortality associated with short-term exposures to the pollutants, restricting their analyses to days with daily air pollution concentrations below 25 μg/m3 for PM2.5 and 60 ppb for ozone, which are well below the current daily NAAQS of 35 μg/m3 for daily PM2.5 and 70 ppb for 8-hour maximum ozone.

The analysis included more than 22 million case days and more than 76 million control days. Nearly 94% of all case and control days had PM2.5 levels below 25 μg/m3, and approximately 95% of deaths occurred during these days. Ozone levels were below 60 ppb for more than 91% of the days, during which more than 93% of deaths occurred, the authors report.

However, daily increases in pollutant exposures were associated with increased risk for all-cause mortality. "[E]ach 10-μg/m3 daily increase in fine particulate matter and 10–parts-per-billion daily increase in warm-season ozone exposures were associated with a statistically significant increase of 1.42 and 0.66 deaths per 1 million persons at risk per day, respectively," they write. "The risk of mortality remained statistically significant when restricting the analysis to days with PM2.5 and ozone levels much lower than the current daily NAAQS."

In subgroup analyses, the effect sizes were larger for certain populations. The relative risk increase (RRI) and absolute risk difference of all-cause mortality associated with PM2.5 exposure for Medicaid-eligible individuals, respectively, were 1.49% (95% confidence interval [CI], 1.29% - 1.70%) and 3.59 (95% CI, 3.11 - 4.08) per 1 million persons at risk per day. Similar for adults older than 70 years, the RRI was 1.38% (95% CI, 1.23% - 1.54%), and the absolute risk difference was 5.35 (95% CI, 4.75 - 5.95) per 1 million persons at risk per day; for women, the RRI was 1.20% (95% CI, 1.07% - 1.33%), and the absolute risk difference was 1.56 (95% CI, 1.39 - 1.72) per 1 million persons at risk per day. Further, the effect estimates for the association between PM2.5 exposure and mortality increased with age, and they were higher for black adults than for white adults, the authors note.

Similar patterns were observed between ozone exposure and all-cause mortality, but there was less contrast between groups, the authors explain. Population density levels did not significantly influence the short-term associations between PM2.5 or ozone exposure and mortality.

The study findings are consistent with effects reported in the literature, the authors write. "This study further demonstrates that in more recent years, during which air pollution concentrations have fallen, statistically significant associations between mortality and exposures to PM2.5 and ozone persisted."

In an accompanying editorial, Junfeng Zhang, PhD, from the Nicholas School of the Environment and Duke Global Health Institute in Durham, North Carolina, notes that the findings "add unique evidence, applicable to both rural residents and more vulnerable groups, to raise public awareness concerning health risks associated with low-level PM2.5 and ozone pollution."

Dr Zhang agrees with the study authors that the current NAAQS for these pollutants should be revisited.

The findings may also have implications for monitoring exposure to air pollutants, Dr Zhang writes: "The study showed that when PM2.5 or ozone concentration was higher on a particular day, more deaths occurred 2 days later. Predictions of pollutant concentrations for the next few days, such as weather forecasting, can be made readily available to the public," he states, noting that such information "could allow individuals at increased risk to reduce or mitigate their exposure."

The identification of vulnerable populations can further guide the development of personalized protection strategies, "such as staying indoors on heavy pollution days and during exacerbations of underlying respiratory conditions, and wearing personal protective equipment," Dr Zhang writes. He stresses, however, that such individual-level protections "are only a complement to the ultimate solution of emissions control."

Multiple study authors reported receiving grants from federal agencies including the National Institutes of Health, Environmental Protection Agency, Health Effects Institute, National Cancer Institute, and US Department of Justice. One coauthor is an expert consultant for the US Department of Justice about health impacts of Clean Air Act violations. Dr Zhang reports receiving research funding from Underwriters Laboratories Inc, serving as a member of the Oxides of Nitrogen Primary NAAQS Review Panel for the Environmental Protection Agency, and receiving honorarium from the RB Company in London, England.

JAMA. 2017;318:2431-2432, 2446-2456. Article abstract, Editoral extract

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