Mount Marcy, the highest peak in New York State’s Adirondack Park, offers breathtaking views to hikers on a clear day. But despite being hundreds of miles from the nearest smokestack, summer air pollution levels here can sometimes be worse than in Times Square in New York City. “Often you’re going to feel lung burn in a place where you’re expecting the air to be clean and clear,” says John Sheehan, a spokesperson for the Adirondack Council, an environmental advocacy group. The haze in this bucolic setting is emblematic of a larger problem: even though New York has some of the U.S.’s strictest air-quality regulations, it suffers from elevated rates of air-pollution-related illnesses and associated premature mortality. Much of that problem is because of emissions blown in from other states.

Anywhere from 90,000 to 360,000 Americans die prematurely every year because of air pollution, various past studies have estimated. Most reduction efforts to date have focused on curtailing local sources of emissions. But according to a new paper published Wednesday in Nature, nearly half of premature deaths linked to poor air quality occur beyond the borders of the state where the pollutants or their chemical precursors were released. In the study, the researchers say their findings highlight the need for cooperation among states to address the problem. The Environmental Protection Agency’s Cross-State Air Pollution Rule (CSPAR) mandates such cooperation—but the agency recently denied a petition from New York to aid in dealing with out-of-state pollution sources.

The new study is the first to comprehensively assess how air pollutants potentially move from one state to others—potentially a large number—and the effects those emissions have in the latter, says co-author Steven Barrett, an atmospheric scientist and aerospace engineer at the Massachusetts Institute of Technology. The researchers did this work by modeling where pollution from electric power plants, industry, transportation and other sources traveled in the contiguous 48 states in 2005, 2011 and 2018 and by assessing emissions-related deaths in each of them.

The team found that about 40 percent of such deaths occur outside the state where the pollution was emitted. Nearly 70 percent of deaths related specifically to electric power generation—the sector with the highest cross-border impact on premature mortality—occurred in states other than the one where the involved plant was located. Wyoming and North Dakota were allegedly the highest net exporters of emissions-related mortality (though this was partly because of their relatively low populations). New York had both the highest net and absolute mortality from out-of-state pollution sources, because of a confluence of factors that included being downwind from polluter states and having a large population.

But the authors did find some reason for optimism: because of recent emissions reductions from the electric power sector—linked to the increasing retirement of coal-fired plants—the number of premature deaths associated with poor air quality decreased by about 13,000 nationwide over the 13-year period they assessed.

Julian Marshall, a civil and environmental engineer at the University of Washington, who was not involved in the study, says air pollution is “shockingly” bad for human health. Marshall co-authored a November 2019 paper in Environmental Science & Technology showing that in 36 U.S. states, most emissions-related health impacts stem from out-of-state coal-fired power plants rather than other fuel sources. He says an important aspect of the Nature study’s findings is that if pollution affecting states such as New York is largely from elsewhere, it makes it harder for single-state fixes to be effective. “This means we need a multipronged solution if we actually want to address air pollution,” he says. “There’s no getting around that.”

Such a strategy would require the federal government to step in as a mediator, says Matthew Shapiro, a political scientist who studies cross-border pollution at the Illinois Institute of Technology and was not involved in the new study. “Otherwise you have this race-to-the-bottom scenario, where local authorities are just going to pursue what is less costly for them and more environmentally hazardous for downwind states.”

Working through CSPAR, the EPA has typically been the arbiter of cross-state pollution disputes from electricity production—the only sector the rule covers. But when New York petitioned the EPA for help with about 350 emissions sources it had identified from upwind states, the agency declined to step in. It officially denied the petition in October 2019. New York State, New York City and New Jersey have challenged the denial in court, and the Environmental Defense Fund, Sierra Club, and Adirondack Council have joined the lawsuit on the plaintiffs’ side. Environmental Defense Fund attorney Graham McCahan says oral arguments before the District of Columbia Court of Appeals are likely to occur in May. “We hope to get a decision in time for New York to get some relief before this ozone season in the warmer months of the year,” he says. (Ozone, which is a pollutant at ground level, increases during the spring and summer months as heat and sunlight react with other air pollution chemicals.) Asked for a response, an EPA spokesperson said that the agency does not comment on pending litigation.

Meanwhile, as of December 2019, President Donald Trump’s administration had reversed 16 federal air pollution and emissions regulations and was on track to eliminate nine more, according to an analysis by the New York Times. Barrett says it is difficult to assess what impact these rollbacks will have on premature deaths from air pollution. “Changes that are made now will take many years to have an effect,” he says. If air pollution regulations are reinstated in the next year or two, he adds, the impact of Trump’s rollbacks would probably ultimately be negligible.

In the Adirondack Mountains—where the passage of the Clean Air Act of 1970 led to significantly improved air quality—Sheehan says the effects of the rollbacks are already emerging. “We are in a position where we’re starting to see measurable increases in [air pollutants] from previous years. And that’s starting to go in the wrong direction for the first time.”