Over the last few decades, air in the U.S. has undergone a remarkable transformation: pollution levels of health-damaging tiny particles have dropped by roughly 40% since 2000, primarily thanks to the country's decades-long effort to improve air quality through the Clean Air Act, a landmark environmental law.

Smoke from wildfires fueled by human-driven climate change, however, has erased roughly 25% of those air quality gains, according to a new study published Wednesday in Nature. "We've seen really remarkable improvements in air quality," says Marissa Childs, one of the authors of the study and a researcher at Harvard's Center for the Environment. "But wildfire smoke is undoing that progress in many states."

The effects are more pronounced in Western states, where smoke-laden days have become an annual fact of life. Schools keep kids inside during recess; emergency rooms know to prepare when wildfires break out nearby. The study found that since 2016, in states like California, Washington, and Oregon, wildfire smoke has added enough pollution to the air to wipe out nearly half of the total air quality gains made from 2000 onward.

The Midwest, South, and eastern states are not immune. "This is impacting way more places than we used to think and at a larger scale," says Childs. Even before this year's Canadian wildfires blanketed the Eastern seaboard in thick smoke, smoke plumes regularly tanked air quality far from the actual wildfires.

The Clean Air Act worked until now

The bipartisan Clean Air Act, signed into law in 1970, has had remarkable success cleaning up the nation's air. In its first few decades, levels of the six major pollutants it addressed dropped by more than 40%. Since 2000, the drop has continued nearly everywhere in the country.

One major target of the Clean Air Act is PM2.5 — tiny particles about 30 times smaller than the diameter of a human hair. Many different sources contribute to PM2.5 including dust, and soot from burning coal or gas. The super-small particles are also produced when anything burns such as forests, grasslands, and houses.

Closing or retiring coal and gas-fired power plants cut PM2.5 levels nearby. So did improving car and truck fuel efficiency and pollution-control technologies like catalytic converters — though pollution levels near major roadways still often exceed the EPA's daily standard. Nationally, PM2.5 levels dropped another 42% between 2000 and 2022.

"Overall, there was a big improvement — but it was not shared equitably," says Tarik Benmarhnia, an environmental epidemiologist at the University of California, San Diego. Communities of color remained exposed to higher pollution, even as total levels dropped. Black communities in particular breathe in much more heavy pollution from cars, heavy industry, and construction than any other groups. That's a pattern that holds nationwide and over decades, including into today's efforts to cut back fossil fuel pollution, which are at risk of continuing the disparities.

More smoke is not good for anyone's health

Overall, the country's air was getting cleaner. But Childs, who was living in California at the time watching wildfire seasons break record after record, could tell that wasn't the whole story.

Scientists were pinpointing how climate change exacerbated the burns. The answer, they found over and over, was a lot. Hotter, drier conditions sucked more moisture out of vegetation, priming it to burn explosively and extensively when a spark happened to catch.

Decades and even centuries of fire suppression — the long-held policy of the U.S. Forest Service and other agencies — also fed the wildfires. Many habitats across the Western U.S. evolved to experience frequent burns, which cleared away excess fuel, and Indigenous communities often used fire to keep those habitats open as well. Now forests are packed with many more trees.

The combination has led to wildfires that burn 10 times the acreage as 50 years ago. Massive, destructive burn years like 2020 are projected to become much more common as climate change marches forward, though aggressive forest management could blunt some of the worst outcomes, research shows. And wildfires are not just tied to the West. This year, wildfires burned from Canada's East to West coasts and deep into Louisiana.

Christopher Migliaccio, an immunologist at the University of Montana, studies the impact of wildfire smoke on human health. When he moved to Montana in 2000, wildfires weren't top-of-mind for most people. But within the past decade, "the concern has gotten huge," he says. "And it's gone global."

That's because the health impacts leak well outside the immediate realm of the fires. Smoke, and all its fine particles, can travel thousands of miles. "When you see a wildfire smoke plume, you see that pollution. Essentially, the smoke that you're seeing is PM2.5," says Colleen Reid, an environmental public health expert at the University of Colorado, Boulder.

It's not yet completely clear if wildfire smoke particles induce different health outcomes than PM2.5 from other sources, like roadways, though some research points that direction. But the tiny particles from fires and other pollution sources are so small they cross from lungs into the bloodstream, driving inflammation throughout the body. Even short-term exposure to wildfire smoke makes lung problems like asthma worse, as well as a panoply of other health issues, from heart attacks to neurological issues.

Migliaccio led a study that followed Montanans exposed to extremely high doses of smoke for 49 straight days in 2017. It found their lung function was depressed for at least two years afterward.

In 41 states, air quality had been getting better between 2000 and the 2010s. But as wildfires exploded, those improvements stopped or even reversed. Smoke was responsible for just intermittent "exceedances," when air pollution exceeds EPA's limits, in the early part of the record. By 2020-2022, wildfire smoke was the primary cause of bad air in four western states and a major contributor in 17 others.

Solutions are not straightforward

Wildfires are a natural and necessary ecological reality in many parts of the country. But research predicts the frequency and size of fires will grow precipitously in coming decades, increasing peoples' exposure to smoke.

The Clean Air Act effectively regulates point-source pollution, like soot from power plants. It is less effective at regulating risk from smoke, which drifts across state borders and affects people far from the wildfires themselves.

Dialing back the climate pressures that exacerbate wildfires is critical, says Childs. But so is creating forest and fire management policies that reduce exposure to very high concentrations of smoke. That could be, somewhat counterintuitively, increasing the number of prescribed fires, which can lessen the risk of catastrophic wildfires, though they also generate local smoke plumes.

In the meantime, people can take steps to protect themselves from inevitable smoke exposure, says Reid. Installing air filters in your home — and keeping them clean — can go a long way. Health experts recommend wearing N95 or KN95 masks if you have to go outdoors, and to avoid exercise in smoky air if possible.