The findings, published in the Journal of the American Medical Association, reinforce the results of previous studies showing that fine inhalable particles (PM2.5) and ozone – especially “warm-season ozone” in April to September – are associated with increased mortality rates.
The new research shows that this exposure-related risk is particularly significant among elderly people who are poorer, female or African-American.
“This the most comprehensive study of short-term exposure to pollution and mortality to date,” said senior author Francesca Dominici, a professor of biostatistics at Harvard University. “We found that the mortality rate increases almost linearly as air pollution increases. Any level of air pollution, no matter how low, is harmful to human health.”
Continued exposure to PM2.5 is considered safe at a daily average of 12 micrograms per cubic meter of air (12ug/m3) or less over the course of a year, according to the U.S. Environmental Protection Agency’s National Ambient Air Quality Standards (NAAQS). The 24-hour threshold is 35 ug/m3. The 8-hour standard for warm-season ozone is 70 parts per billion, while there is no year-long guideline for exposure to the inorganic molecule.
To examine the effects of daily exposures to air pollution, the team used prediction models that offered precise estimates of PM2.5 and ozone for most of the United States. Next, they linked the air-pollution data with mortality data from the entire Medicare population within 39,182 ZIP codes – about 93 percent of all U.S. ZIP codes – from 2000 to 2012.
About 22 million Medicare recipients died during the analysis period.
For every 10 ug/m3 daily increase in PM2.5 and 10 parts per billion daily rise in warm-season ozone, the daily death rate increased by 1.05 and 0.51 percent, respectively, according to the report.
The public health impact of even small percentage increases in deaths among U.S. seniors is drastic. A rise of just 1 ug/m3 in daily PM2.5 exposure over the course of a summer would cause 550 extra deaths each year in the U.S. and 7,150 over the 13-year study period. An increase of merely 1 part per billion in daily ozone exposure over a single summer would lead to 250 additional deaths a year – equivalent to 3,250 over the entire study.
Short-term air pollution exposure was especially dangerous for a few subgroups. Medicaid-eligible recipients exposed to higher levels of PM2.5 had a mortality rate that was three times higher than those who did not qualify for the program. The mortality risk of women and people of color was about 25 percent higher than white people or men.
Lifestyle choices, limited access to health care, and poverty may contribute to such disparities, the team speculates.
An earlier study by some of the same researchers showed that long-term exposure to air pollution was connected to an elevated risk of premature deaths, even at proportions below national standards for extended exposures. Those findings are critical to an ongoing review of the EPA’s annual ambient air-quality standards.
Joel Schwartz, a professor of environmental epidemiology at Harvard, says the new findings suggest the EPA should also re-evaluate the daily standards.
“No matter where you live – in cities, in the suburbs, or in rural areas – as long as you breathe air pollution, you are at risk,” said lead author Qian Di, a doctoral student at Harvard.