Scientists are just beginning to understand that there is no safe level of air pollution exposure. Breathing polluted air has lasting, insidious effects, including asthma, allergies, cognitive delays, developmental disorders, and premature births. All of these can contribute to shorter life spans, and in some cases can abruptly end a life. Countries, including the US, have air-quality standards to presumably prevent the worst of these health effects, but a new body of emerging research shows that people are dying prematurely from breathing the air even in places where air pollution levels were deemed “safe” by the US.
The latest of these studies was published Dec. 26 in the Journal of the American Medical Association (JAMA). It found that even one summer of Americans breathing air with pollution levels well below US national standards leads to a rise in premature deaths. The study looked at 22 million deaths of Medicare recipients (so, Americans over the age of 65) over a 13-year period from 2000 to 2012. It included air pollution data from the US Environmental Protection Agency, as well as other pollution data sets, processed through neural networks to predict air pollution concentrations “at each 1-km ×1-km grid in the continental United States, including locations with no monitoring sites.”
“We found that the mortality rate increases almost linearly as air pollution increases,” Francesca Dominici, professor of biostatistics at Harvard’s school of public health, and a senior author on the paper, said in a statement. Though the study focuses on the US, its basic conclusion applies broadly: the “safe” levels laid out by national health agencies everywhere are inherently far from safe. “Any level of air pollution, no matter how low, is harmful to human health,” Dominici said.
In the US, the air pollution impacted different demographics unevenly: The researchers found that low-income people (measured by eligibility for Medicaid) had a 225% higher mortality risk when exposed to air with with even slightly elevated PM2.5 levels than people who were wealthier and therefore not eligible for medicaid. Overall, women had a 25% higher mortality risk than men from the same pollution. The researchers speculate that “poverty, unhealthy lifestyle, poor access to health care, and other factors may make some subgroups more vulnerable to air pollution,” but the study didn’t specifically investigate the causes of these discrepancies. “The exact mechanism is worth exploring in future studies,” the researchers write.
In the US, exposure to very fine particulate matter known as PM2.5 is considered safe by the US Environmental Protection Agency’s national ambient air quality standards so long as a person breathes in an average of 12 micrograms per cubic meter of air (μg/m3) or less per day over the duration of a year. In the short term, the US considers it safe if PM2.5 levels don’t go over 35 μg/m3 within a day, so long as the yearly average comes out to 12 μg/m3 per day (in other words, a few days of higher PM2.5 here and there is acceptable, so long as there are only a few).
But in their study, the Harvard researchers found that for every 10 μg/m3 increase in daily PM2.5 exposure over the course of one summer, the daily mortality rate among adults 65 years and older increased by slightly more than 1%—even at levels within the acceptable 35 μg/m3 one-day limit. When it comes to ozone, the US considers exposure to 70 parts per billion over an eight-hour period to be the safe limit. The researchers found that every 1 ppb increase in daily ozone during the summer (again, at levels still below the 70-ppb limit) raised the daily mortality rate by 0.5%.
That might sound small, but extrapolated over the whole US population over the long term, those small increases add up For each 1 μg/m3 of PM2.5 added to the daily level over the course of one summer, 550 more people die prematurely every year. Likewise, 1 ppb more of daily ozone over the summer leads to 250 extra deaths per year.
This study on short-term air pollution followed work published by some of the same researchers in June showing that multiple years of exposure to air pollution—even at levels well below the “safe” level mandated by the US government—increased premature deaths in the country. That research concluded that if the level of PM2.5 could be lowered by just 1 ug/m3 nationwide, roughly 12,000 premature deaths would be avoided every year. And if ozone levels could be lowered by just 1 part per billion, another 1,900 premature deaths would be avoided annually.
The US EPA’s air quality standards are reviewed formally every five years and updated periodically; revisions for PM2.5 and ozone were last published in 2012 and 2015 respectively. The next time they’re reviewed, the Harvard researchers write, they hope officials will keep their research in mind. “This risk occurred at levels below current national air quality standards, suggesting that these standards may need to be reevaluated,” they write.