Even short-term exposure to air pollution at levels that are below current air quality standards appeared to increase the risk for death among older people, according to a newly published study involving 22 million Medicare recipient deaths.
Day-to-day changes in particulate matter smaller than 2.5 microns (PM2.5) and ozone ambient concentrations were significantly associated with higher risk for all-cause mortality at levels well below the current daily National Ambient Air Quality Standards (NAAQS), according to the findings published in the Journal of the American Medical Association.
Each 10-μg/m3 daily increase in fine particulate mater and each 10-ppb daily increase in warm-season ozone exposures were associated with statistically significant increases of 1.42 and 0.66 deaths per 1 million persons at risk per day, respectively.
Findings from the novel analysis – which included satellite-based measurements in addition to remote air monitors to assess pollution exposures – suggest that the current standards for air quality safety are not adequate to protect the public, said researcher Francesca Dominici PhD, of the Harvard T.H. Chan School of Public Health, Boston.
“Even very small changes in exposure to fine particulate matter and ozone increased mortality risk, and the relationship was linear,” Dominici told MedPage Today. “This shows that there is really no safe level of exposure and it tells us we need to lower levels as much as possible.”
Dominici said the inclusion of satellite data and other sources made it possible to measure pollution exposures in areas that are not monitored by the Environmental Protection Agency (EPA). Most EPA monitoring stations are in areas with higher population density and pollution levels.
The newly published analysis included 93% of the nation’s zip codes, she said.
“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,” Dominici and colleagues explained in their paper. “From these graded 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.”
During the study period, there were more than 22 million case days and 76 million 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, and 91.1% of days had ozone levels below 60 parts per billion, during which 93.4% of deaths occurred.
Among the main findings:
- 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 ppb 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 and no evidence was seen of a threshold in the exposure-response relationship.
Women, non-white individuals and the very elderly had higher mortality rate increases associated with PM2.5 than other groups.
The current NAAQS for daily PM2.5 is 35 μg/m3 and the current NAAQS for ozone is 70 ppb. When the researchers restricted their analysis to daily PM2.5 levels below 25 μg/m3 and ozone levels below 60 ppb, the associations with mortality remained.
The main study strengths cited by the researchers were its large size and the use of unique measures of daily pollution exposures.
“To our knowledge, this is the largest analysis of daily air pollution exposure and mortality to date, with approximately four times the number of deaths included in previous large studies. This study assessed daily exposures using air pollution prediction models that provide accurate estimates of daily levels of PM2.5 and ozone for most of the United States, including previously unmonitored areas,” they wrote.
The case-cross over design was a study limitation, which prevented analysis of long-term risks. The use of zip code data instead of residence information could also have led to measurement error.
In an editorial published with the study, Junfeng Zhang, PhD, of Peking University, Beijing, China, wrote that the findings make a strong case for lowering NAAQS standards. He cited estimates suggesting that pollution controls required by the Clean Air Act have prevented hundreds of thousands of premature deaths.
“It can be assumed that even greater health benefits could result from further emission reductions, which can be achieved through cleaner energy production (e.g., by renewable, nonpolluting sources such as wind and solar power) and hybrid vehicles and low-emission mass transportation),” Zhang wrote.
Dominici said current de-regulatory climate in Washington makes it unlikely that air quality standards will be lowered anytime soon.
“”Our government is not taking seriously enough the main sources of pollution, such as fossil fuel combustion, traffic and coal power plants,” she said. “The current administration is moving backward, not forward, in reducing pollution from these sources.”
Funding for this study was provided by the NIH, the EPA and the Health Effects Institute (HEI).
- Reviewed by F. Perry Wilson, MD, MSCE Assistant Professor, Section of Nephrology, Yale School of Medicine and Dorothy Caputo, MA, BSN, RN, Nurse Planner