Most of California has enjoyed a blissfully mild fire season so far this year, following heavy winter rains and snows that soaked the state and ended its three-year drought.

But as smoke blew in Tuesday and Wednesday from Wildfires burning hundreds of miles away near the Oregon border, Bay Area skies turned hazy, rekindling an unhealthy and growing trend.

A new study published Wednesday from researchers at Stanford University found that many of the improvements America has made in reducing Air Pollution since 2000 are now being diminished — particularly in Western states — as the frequency and ferocity of wildfires has increased.

“We have been remarkably successful at cleaning up air quality,” said Marshall Burke, an associate professor at Stanford’s Doerr School of Sustainability, and co-author of the study. “We have seen decades of improvements and public health benefits. But over the last 10 years, that progress has slowed down and started to reverse.”

Burke and his colleagues found that from 2000 to 2015, particle Pollution, called PM 2.5, declined 38% in the United States. But then the steady trend stopped. It increased 3% from 2016 to 2022.

In California and Nevada, the reversal was more dramatic. Particulate pollution fell 32% from 2000 to 2015, then jumped 14% from 2016 to 2022.

In those recent years, huge blazes burned in Big Sur, forests around South Lake Tahoe, towns in Wine Country, Paradise, the Santa Cruz Mountains and other areas, choking much of the state in thick smoke.

Similarly in the Pacific Northwest and the American Southwest, particulate pollution fell 19% from 2000 to 2015, then shot up 21% and 12%, respectively, from 2015 to 2022, largely due to wildfires, the researchers found.

Even the Midwest, the South the East Coast saw their improving air quality trends stall due to smoke from Western wildfires.

“Larger fires that have been burning in recent years are lofting the pollution way up into the atmosphere, which can transport these tiny particles thousands of miles away,” said Marissa Childs, a former Stanford doctoral student and co-author of the study who now is an environmental fellow at the Harvard University Center for the Environment.

Overall, wildfire smoke has either slowed or fully reversed air quality trends in 35 states since 2016, said the study, which was published in the journal Nature.

The trend is showing no signs of abating.

Smoke from more than 1,000 wildfires burning across Canada earlier this year drifted south, bringing unhealthy air to New York, Chicago, Minneapolis and other U.S. cities.

Wildfires have been on the increase for several reasons. A century of fire suppression has left forests unnaturally thick and unhealthy in some parts of the West, so that when they finally burn, the blazes are much larger than fires started centuries ago by lightning or native tribes.

Climate change also is causing more severe droughts and heat waves. And in many parts of the West, more people have moved to rural areas, increasing the risk of human-caused fires.

As a result, wildfire smoke is a becoming widening health problem.

America’s landmark air pollution law, the Clean Air Act, was signed by President Richard Nixon in 1970. It was designed to curb dangerous levels of smog from factories, power plants, trucks and cars. It has succeeded by setting standards that required cleaner-burning fuels, catalytic converters on trucks and cars, scrubbers and other clean-air equipment on smokestacks, and other changes.

“For over 50 years the Clean Air Act has delivered critical public health benefits,” said Will Barrett, national senior director for clean air advocacy at the American Lung Association. “Now, climate change is making the job of cleaning our air more difficult and threatening our incredible progress. We’re very concerned about it.”

The Biden administration is considering tightening the national health standards for particle pollution, something Barrett said is overdue.

But the Clean Air Act does not regulate wildfires, or even count their soot emissions toward state and local air pollution totals.

It considers them “exceptional events,” like volcanic eruptions and earthquakes, that are out of human control.

“You can’t put a scrubber on a burning forest,” said Michael Wara, director of the Climate and Energy Policy Program at Stanford’s Woods Institute for the Environment, and a co-author of Wednesday’s study. “We are going to need flexibility, new tools, and a willingness to experiment.”

Soot is among the most harmful types of air pollution.

It is made up of tiny particles is called PM 2.5 — for particulate matter 2.5 microns or smaller, so small that 30 of them can line up across the width of a human hair.

That microscopic soot drifts in the air, and can penetrate deep into people’s lungs. It can enter the bloodstream and increases the risk of asthma attacks, heart attacks, strokes and other serious health issues. Studies have shown an increase in hospital visits during smoky days, particularly by elderly residents.

Burke said that as climate change continues, smoky days are likely to be more common. The solution, he said, is more thinning of overgrown forests, more controlled burns, changes in federal air pollution rules to make it easier to conduct controlled burns that reduce the risk of massive fires, and other programs, like expanding initiatives to help low-income residents purchase air purifiers.

“The Clean Air Act was written for a different world,” he said. “It was written for a different logic. Wildfires are a different beast.”