In more than a half century since the Clean Air Act was enacted, there have been dramatic improvements in air quality in the United States, as regulations demanding less-polluting cars and factories helped lift cities from clouds of dirty smog.

But a big chunk of recent air quality progress has been rolled back for one reason — wildfire smoke — and it’s happening far beyond the smoldering forests of Western states.

Over the past two decades, air quality improvements have slowed or reversed in most of the country, eroding about a quarter of the recent gains, according to a new study in the journal Nature. Some states — Colorado, Montana, New Mexico, Washington and Wyoming — have rolled back 50 percent or more of their progress since 2000. In Oregon and Nevada, wildfire smoke has completely erased their gains.

“We had had so much success, and wildfires, just in five to six years, are really unraveling a lot of this progress,” said Marshall Burke, the paper’s lead author and a professor of Earth system science at Stanford University. “And that’s unfortunate.”

The study builds on previous work by Burke and his colleagues, who had created a detailed map of how much wildfire smoke there has been in the country over the past two decades. That effort relied on melding data from air quality sensors, satellite images of smoke plumes, and computer models that could estimate smoke levels in remote areas without sensors.

The new paper seeks to answer the question: What has all this wildfire smoke done to overall air pollution levels?

What they found was sobering.

Data from air quality sensors around the country had been showing steady improvement since 2000 in most states. But around 2016 — and earlier in some Western states — the trend broke. Since then, air quality progress has significantly slowed in 30 states. In 11 others, it began to reverse.

One of the main reasons is wildfire smoke.

The statistical analysis by Burke and his colleagues through 2022 found that wildfire smoke significantly influenced air quality trends in 35 of the contiguous states. States that are still improving would have done so faster without smoke, they found. In some Western states, air quality that otherwise would have kept improving is now getting worse because of that smoke.

“To me it was really a surprising number of states that were being affected,” said Marissa Childs, a co-author and a postdoctoral fellow at the Harvard University Center for the Environment. “We’re really starting to see that impact over the entire country.”

Wildfire smoke is not directly regulated by the Clean Air Act and is often harder to manage than other types of air pollution. Fires are irregular in duration and intensity. Smoke can travel huge distances, affecting places far from the source. The Biden administration is pushing to expand prescribed burns and tree-thinning projects — to keep fires smaller and less destructive — while encouraging protections against exposure to smoke.

“This isn’t a traditional source of air pollution where you can put a scrubber on a smoke stack or something like that,” said Jason Sacks, a research epidemiologist with the Environmental Protection Agency. “It’s going to be more of community and personal preparedness and trying to figure out what can I do to protect myself and reduce my exposures.”

This year has been particularly bad for smoke. Hundreds of wildfires burning across Canada poured smoke across the border into the Northeast and other parts of the country less accustomed to orange skies and choking air. There were days in June when New York City had some of the worst air quality of any major city in the world.

Childs said if their study had been redone with 2023 data, “the results we’d find would only be stronger.”

The consequences of more smoky days and deteriorating air quality could have profound impacts on Americans’ health. Across the country, researchers are trying to unravel what those effects may be.

Many studies have documented how smoky days can send patients into emergency rooms in greater numbers with respiratory, cardiovascular and other problems. Burke, Childs and other researchers published a second study this week in Proceedings of the National Academy of Sciences that found that visits to California emergency rooms between 2006 and 2017 for asthma, chronic obstructive pulmonary disease (COPD) and other respiratory symptoms rose more than 30 percent in the week after an extreme smoke day, relative to a day without smoke.

They also found a slight rise in all emergency visits after low or moderate smoke days but a decline in total visits on extreme smoke days, suggesting people may change their behavior under those conditions, possibly staying home and avoiding other potential injuries and accidents that might normally send them to a hospital.

In small towns and rural areas, wildfire smoke can quickly strain hospitals. The influx of respiratory patients can force others to miss out on care they might otherwise get, said Kyle Chapman, an associate professor at the Oregon Institute of Technology.

Chapman’s research found that a single day of unhealthy smoke — in the “red” level on the federal government’s color-coded scale — gave the hospitals he was studying in central Oregon a 41 percent chance of exceeding their capacity.

“What we’re finding is that there is a significant burden that is placed on hospital systems,” Chapman said.

The longer-term health consequences of wildfire smoke remain a murkier question.

Few studies have followed people over a multiyear period to document how wildfire smoke has affected their health. One study that did looked at residents from the small town of Seeley Lake, Mont., who lived through an extraordinary run of heavy smoke in 2017, as the Rice Ridge fire led to weeks of smoky days at “very unhealthy” and “hazardous” levels.

University of Montana toxicologist Christopher Migliaccio tracked residents who lived through that smoke and found they showed significant decreased lung function one and two years afterward, although some of these changes were subtle, he said.

“The lung function was decreased, clinically significant, more than what would be expected in the normal population,” he said. “But it’s not like suddenly they can’t walk out to the car because they’re going to lose their breath.”

He said more research is needed following people over a longer period, as well as looking into the potential cumulative effects from repeated exposure to heavy smoke.

“Especially like Montana, where almost every other year we’re getting smoke events — there’s got to be something there,” he said.

The work of Burke, Childs and others to quantify how much wildfire smoke there is separate from other sources of air pollution is a first step in trying to understand how harmful all this smoke may be.

Some research has already suggested that wildfire smoke can be worse than other sources of air pollution. In studying hospital admissions in Southern California, Tarik Benmarhnia, an environmental epidemiologist at the University of California at San Diego, and others found that smoke from wildfires can be up to 10 times more harmful on human health than other pollution particles.

“That’s also something that is not considered at all by regulations right now,” Benmarhnia said.

Wildfire season in California and the West is off to a slower start than normal. But with climate models predicting that warmer and drier conditions in the West will worsen, researchers say the onslaught of wildfire smoke isn’t going away anytime soon.

“This problem, if we don’t do something about it,” Childs said, “is only going to get worse.”