Long-term exposure to even the lowest levels of air pollution, well below national standards, is associated with increased mortality, a study of more than 60 million people shows. Racial minorities and those with low incomes are particularly vulnerable.
"The enormous sample size in this study, which includes the entire Medicare cohort, allowed for unprecedented accuracy in the estimation of risks among racial minorities and disadvantaged subgroups," write Qian Di, MS, from the Department of Environmental Health, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, and colleagues.
They report their findings online today in the New England Journal of Medicine.
"The findings of Di et al stress the need for tighter regulation of air-pollutant levels, including the imposition of stricter limits on levels of [fine particulate matter (PM2.5)]," Rebecca E. Berger, MD, NEJM Group Editorial Fellow, and colleagues write in an accompanying editorial. "Despite compelling data, the Trump administration is moving headlong in the opposite direction."
Other large studies have found an association between low levels of fine particles, those with a mass median aerodynamic diameter smaller than 2.5 μm, and mortality. "However, most of these studies have included populations whose socioeconomic status is higher than the national average and who reside in well-monitored urban areas," the researchers explain. "Consequently, these studies provide limited information on the health effects of long-term exposure to low levels of air pollution in smaller cities and rural areas or among minorities or persons with low socioeconomic status."
The current study included all Medicare beneficiaries from 2000 through 2012, with a median follow-up of 7 years and a total of more than 460 million person-years. The researchers estimated and validated ambient levels of ozone and PM2.5 on the basis of prediction models published previously. Using a survival analysis, they estimated the risk for all-cause mortality associated with long-term exposure to PM2.5 and ozone concentrations below the current National Ambient Air Quality Standard of 12 μg per cubic meter and 50 parts per billion, respectively.
Each increase of 10 μg per cubic meter in annual exposure to PM2.5 was independently associated with a 7.3% (95% confidence interval [CI], 7.1% - 7.5%) increase in all-cause mortality. Each increase of 10 parts per billion in warm-season ozone exposure was independently associated with a 1.1% (95% CI, 1.0% - 1.2%) increase.
When the researchers restricted their analysis to person-years with PM2.5 and ozone exposure below the National Ambient Air Quality Standard, those same increases were still associated with significantly higher risk for all-cause mortality (13.6% [95% CI, 13.1% - 14.1%] and 1.0% [95% CI, 0.9% - 1.1%], respectively).
The relationship between PM2.5, ozone, and all-cause mortality "was almost linear, with no signal of threshold down to 5 μg per cubic meter and 30 [parts per billion], respectively," they write.
Most at Risk
The researchers also conducted subgroup analyses to identify populations at higher and lower risk for pollution-associated all-cause mortality.
For PM2.5, the risk for death was higher among men, blacks, and people with Medicaid eligibility compared with the rest of the population. The effect estimate was three times as high among black persons as the overall population.
For ozone, the risk for death was higher among white, Medicaid-eligible individuals.
"With air pollution declining, it is critical to estimate the health effects of low levels of air pollution — below the current [National Ambient Air Quality Standard] — to determine whether these levels are adequate to minimize the risk of death," the authors conclude. "Since the Clean Air Act requires the [Environmental Protection Agency] to set air-quality standards that protect sensitive populations, it is also important to focus more effort on estimating effect sizes in potentially sensitive populations in order to inform regulatory policy going forward."
"Do We Really Want to Breathe Air That Kills Us?"
Dr Berger and colleagues note in their editorial that different types of transportation and coal-fired generation of electricity are responsible for much of the air pollution in the United States.
"In March, Trump signed an executive order that lifted a moratorium on new leases for coal mined on public and tribal lands and began a process to dismantle guidelines intended to reduce emissions from coal-fired electricity plants. Earlier this month, he announced his intention to withdraw the United States from the Paris climate agreement," they explain.
"In addition, [Environmental Protection Agency] Administrator Scott Pruitt has not ruled out the possibility of revoking a waiver included in the 1970 Clean Air Act that allows California to set limits on automotive tailpipe emissions that are more stringent than national standards; 15 states have adopted California's standards. Revoking this waiver could have the effect of exposing more than 100 million Americans to higher levels of automobile emissions," the editorialists add.
"Trump's proposed budget includes crippling cuts to the EPA, including cuts in funding for both federal and state enforcement of regulations. The increased air pollution that would result from loosening current restrictions would have devastating effects on public health," Dr Berger and colleagues explain.
Writing that we must "redouble our commitment to clean air," the editorialists ask, "Do we really want to breathe air that kills us?"
The authors have disclosed no relevant financial relationships. Dr Berger reports that she is an NEJM Group Editorial Fellow at the New England Journal of Medicine. One editorialist reports being employed by the New England Journal of Medicine as editor-in-chief. One editorialist reports being an NEJM Group Editorial Fellow at the New England Journal of Medicine. One editorialist reports being employed by the New England Journal of Medicine as deputy editor.
N Engl J Med. Published online June 28, 2017.
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