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Air in the U.S. has gotten cleaner for decades, adding years to people's lives and preventing millions of asthma attacks, but nearly half of Americans still live with unhealthy air pollution, a new report finds. 

The report comes as the Trump administration is considering rolling back some key air quality regulations. 

Air quality across the country has improved dramatically since regulations like the Clean Air Act were put in place in the 1970s to govern sources of pollution like coal-fired power plants and emissions from diesel trucks. Despite that progress, the air is still unhealthy and polluted in many parts of the country. In 2023, nearly half of the country's inhabitants — 156 million people — lived in places heavy in smog or soot pollution that harms their lungs, hearts, and brains, according to the newest edition of the American Lung Association's 

"Both these types of pollution cause people to die. They shorten life expectancy and drive increases in asthma rates," says Mary Rice, a pulmonologist at Harvard University.

Pollution levels vary widely across the country, the report finds, with the worst soot pollution, averaged over the whole year, centered on California cities like Fresno and Bakersfield. Ozone pollution is highest in the Los Angeles region. Phoenix, Arizona, and Dallas, Texas, also rank in the top 10 most smog-heavy cities. Nationwide, people of color are about twice as likely to live somewhere with high soot and ozone pollution as white Americans.

The report, now in its 26th year, comes as the Trump administration pushes federal agencies to reconsider many of the regulations that have resulted in cleaner air in recent decades.  Environmental Protection Agency administrator Lee Zeldin has announced his intention to re-evaluate 

, which were last tightened in 2024, claiming the tighter rules restrict 

s on emissions reduction requirements to dozens of coal-fired power plants, which produce some of the most health-harming particle pollution.

Doctors and health experts have expressed alarm at the proposed changes.

"Any rollback of environmental regulations has the potential to have really widespread public health impacts," says Neelu Tummula, an ear, nose, and throat physician and a spokesperson for this year's report. 

"The U.S. can protect the environment and grow the economy at the same time," EPA spokesperson Molly Vaseliou wrote in an email, adding that "the Trump Administration is taking steps in the right direction to ensure EPA adheres to the agency's core mission of protecting human health and the environment and Powering the Great American Comeback."

In October of 1948, a thick, choking smog settled over the small Pennsylvania town of Donora.

The town was home to two steel plants. Pollution from those plants, like 

, lingered near ground level for five days. The pollution morphed into a thick, acidic mix that sickened nearly half the town's residents.

In the months and years after the smog lifted, scientists figured out that breathing in pollutants from the smog worsened people's respiratory problems and contributed to 20 deaths. Air pollution, they concluded, was far more dangerous than previously thought.

The incident, along with smog that regularly blanketed cities like Los Angeles, pushed the federal government to develop the 

, which was first implemented in 1963 and then updated significantly in 1970 and several times since. The law focused on lowering levels of several pollutants known to harm human health: particulate matter (also known as particle pollution), ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and lead.

In the years since, study after study has found profound health improvements linked to better air quality. 

"The Clean Air Act is one of the greatest success stories in our country," says Rice. 

The law requires that the EPA revisit the science every five years, reassessing what health researchers have learned about the risks of breathing in poor air and adjusting the regulations accordingly. 

"So the air quality standards have kept pace with the science over time, and levels of fine particulate matter have declined over the last 50-plus years," Rice says. "As a result, life expectancy is longer, and asthma rates have gone down."

One landmark study informing the regulations spanned 20 years. The 

, led by researchers from Harvard University, followed about 8,000 people who lived in six different U.S. cities—some heavily polluted, like Steubenville, Ohio, and others where air pollution was mild, like Portage, Wisconsin.

The scientists measured people's lung physiology at the start of the study and checked in with the participants every year.

"We were not just going into a community, but going into people's homes," says Douglas Dockery, one of the leaders of the study and an emeritus professor at Harvard. "It was real shoe-leather epidemiology." They kept track of those who died and of what causes over 20 years.

The results, which they published in 1993, were unambiguous. 

"People in dirtier communities died sooner than people in clean places," says C. Arden Pope, a researcher at Brigham Young University and another author of the study — about two years earlier than a similar person in the cleaner cities.

The magnitude of the impact was so large that "we almost didn't believe it," Pope says. In fact, a study published around the same time had investigated the impact on life expectancy from a much more familiar killer: cancer.

"And here we were, saying that the effects of air pollution in the six communities we studied was about that big," Pope says.

To check the results, Pope led a follow-up study that analyzed health outcomes for more than 500,000 people across the country, in more than 150 cities. 

 came to the same conclusion: air pollution shortened people's lives significantly.

What Pope found particularly striking was that pollution levels in all six of the original cities studied didn't exceed the pollution limits that existed at the time — yet even at those allegedly "safe" levels, people's lives were being prematurely shortened. The outcomes showed that the rules weren't yet tight enough to protect people's health, Pope says.

The new research kicked off an effort to strengthen pollution rules. After the two studies were published, the American Lung Association pushed the EPA to tighten the 

Now scientists also know how air pollution harms the body

Scientists have also explored how and why different types of air pollution harm health.

Some particles, for example, are so small they can be inhaled deep into people's lungs, where they irritate the organ's delicate lining and trigger respiratory problems, from asthma to COPD.

But the problems don't stop in the lungs. The tiniest particles can pass through the lungs' barriers and slip into the bloodstream — "and from there, anywhere," says Tummula. "They contribute to a lot of inflammation within the bloodstream—but can also disrupt plaque within the arteries," she says. 

Exposure to long-term pollution accelerates the 

 in the arteries around the heart, increases the chances of 

, and increases the likelihood of strokes.

Recent research has also found that air pollution directly impacts the brain. Higher exposure to particle pollution is linked to "problems with cognition and brain function," says Wilson. Exposure to high pollution can disrupt children's learning and hasten the onset of 

Pollution dropped for decades — but progress is slowing

Since the Clean Air Act first passed, air pollution has dropped by nearly 80 percent across the country, according to this year's State of the Air report. 

Cleaner air keeps 2.4 million asthma attacks from happening every year, says Tummula. 

"These are each a person we're talking about," she says — 2.4 million people every year who can go to school or a full day of work without being interrupted by a health problem.

The improvements progressed steadily for years, with soot pollution levels dropping about 40 percent since 2000, when the SoTA report began its annual roundup. 

But recently, some of those improvements have plateaued. 

One major factor, the report says, are climate change-intensified wildfires, which have started burning bigger areas and more intensely. The smoke they produce now impacts parts of the county that historically had little experience with it, like the Midwest and northeastern U.S. Fine particle pollution from wildfire smoke has now eroded about a quarter of air quality improvements since 2000, according to a study 

Climate change is complicating the fight against air pollution in other ways, too, says Rice. For example, ozone, the primary component in smog, forms more readily in hot, sunny weather, and climate change is bringing more hot days to the U.S.

A more immediate concern, Rice says, may come from changes in the federal government as the Trump administration makes efforts to cut staffing and spending. Scientists and technicians at the EPA monitor the air and study the health effects of different pollutants. Their work contributes to how regulations on pollution are updated. EPA leaders have proposed eliminating the agency office that performs that research. That would be a "terrible mistake," Rice says.

"We are best equipped to protect our health when we have all the information, like weather and air quality," she says. Without it, she worries more people's health will suffer.

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Alejandra Borunda

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Despite improvements in air quality in past decades, 156 million Americans still breathe in too much soot or ozone, says the annual State of the Air report from the American Lung Association.

Air pollution still plagues nearly half of Americans. That does a number on our health

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