(The Hill) – Blistering heat waves and more frequent wildfires are reversing a generation of U.S. clean air gains, a new study has found.

The peer-reviewed research by the climate analytics firm First Street Foundation projected that by midcentury, the increased levels of microscopic soot particles and ozone molecules entering Americans’ lungs will be back to the levels they were at in 2004 — before a decades-long federal campaign to clean up the air.

Climate change is driving the U.S. from a pattern where the average bad air days are “unhealthy for some to ones which are unhealthy to all,” coauthor Jeremy Porter told The Hill.

Porter said that federal regulations drove consistent improvements in air quality from 1963 until about 2016 — when the negative impacts of climate change surpassed the positive pressure from clear air enforcement.

“We’re seeing the biggest uptick in the most hazardous [air] days,” Porter said, though he noted that every category of unhealthy air “was sliding up” in frequency.

“We’re wiping out two decades in air quality gains,” he added. 

Those changes have already had subtle but far-reaching effects, according to the study. 

For example, falling air quality has driven up the number of days when children in the U.S. West can’t safely play outside nearly fivefold since 2000.

And about 14 million American households (about 10 percent) can expect to experience at least a week of Environmental Protection Agency (EPA)-designated “unhealthy” air quality each year. 

In some hot spots, those numbers are even worse: About 6 million of those households — located in hot spots across the West Coast, Midwest and Northeast — can expect two weeks a year of bad air. 

While these declines in air quality will happen across the country, they will be particularly pronounced on the West Coast, where ozone rising from baking asphalt combines with toxic particulates from wildfires and the burning of fossil fuels, First Street researchers found.

Over the next thirty years — the length of the average mortgage — this region will see a noticeable uptick in bad air days, according to the study. Los Angeles, for example, currently experiences 47 days annually when the air is — at minimum — unsafe for children and those with chronic illness; by 2054, First Street data projects that Angelenos will face an additional week each year when it’s unhealthy for those groups be outside.

In California, “sensitive” groups make up most of the population: About 28 million people are elderly, young or suffer from heart disease or diabetes — making up more than 70 percent of the population.

And California isn’t alone. First Street researchers found that by 2054, most American cities — led by New York, Chicago, Los Angeles, Houston, Philadelphia and Jacksonville — will see dramatic growth in the number of households in areas with at least a week and a half of bad air per year.

These changes are already showing up, driven by two very different contaminants, each with ties to climate change: PM2.5 and ozone.

PM2.5 is the official shorthand for particulate matter less than 2.5 microns across, or smaller than about one-thirtieth the width of a human hair.

These floating particles are small enough to enter the bloodstream, interfering with a wide array of physical systems. But they mostly are products of combustion, either of fossil fuels, agricultural waste, vehicle exhaust — or the floating smoke of forest fires.

As the incidence of enormous and destructive forest fires has increased throughout the 21st century, so have PM2.5 levels, First Street researchers wrote last year in the journal Fire.

That study found that a “mass-release” of PM2.5 from a wildfire was enough to predict the number of unhealthy air days in nearby municipalities “without significant computational burden.” 

Nationwide, the increase of PM2.5 levels from forest fires and the rising ozone levels from increased heat now expose over 83 million people — about a quarter of the population — to “unhealthy” air quality, according to First Street.

Of those, about 10 million face “very unhealthy” — and 1.5 million face “hazardous” — air quality, characterized by the kind of haze that reddened eyes and bloodied noses across the Northeast and upper Midwest as wildfires burned out of control across Canada in the summer of 2023.

In some places, that risk is particularly concentrated. Most counties on the West Coast are expected to experience three weeks per year of poor air quality days, according to the First Street study.

It found that some hot spots, like the San Francisco metro area, California’s Central Valley and Southern California can expect 3 months of air too unhealthy for sensitive groups — children, the elderly, or those with diabetes or heart disease — to go outside.

This pollution is already costly in terms of lives. According to a 2021 study in Nature, PM2.5 kills about 47,000 Americans per year. By some estimates, better air quality have saved a quarter million American lives since regulations went into place — improvements that are now eroding.

To make the public health situation worse, while rising heat drives up PM2.5 levels by causing the number of forest fires to spike, it also drives up the impact of those heightened levels on the people breathing it in. Since both heat and PM2.5 burden the circulatory system, they combine to drive up heart attack deaths higher than either would alone, a 2020 study found.

The impacts of PM2.5 are the major cause in declining air quality, but not the only one. According to First Street research published in Atmospheric Environment: X, rising heat — largely driven by the continued burning of fossil fuels — will push up ground-level amounts of ozone, particularly in the heart of cities.

In contrast to its role in the atmosphere, where it blocks cancer-causing ultraviolet radiation, ground-level ozone has a more insidious effect. Ozone is formed when heat and sunlight force a reaction between two pollutants that are characteristic of agricultural and fossil fuel pollution: volatile organic compounds and nitrous oxide.

Once inhaled, ozone bypasses the body’s first line of defenses against pollution in the nose and mouth and reacts with the cells lining of the lungs — damaging them and causing them to leak food-dissolving enzymes into the airways, according to an EPA fact sheet. Ozone also causes “a train of events leading to lung inflammation.” 

“The statistical signals are clear. We are seeing rapid increases in air pollutants after decades of legislation to reduce pollution,” Matthew Eby, CEO of First Street said in a statement. 

After decades of improvement, Eby added, “the concern moving forward is that climate is much harder to regulate than industry.”