7/9/2026: This story has been updated to include the most recent EPA PM 2.5 numbers for Allegheny County and other areas of Pennsylvania.
A federal appeals court ruling late last month upheld a key Biden administration air quality standard for fine particulate matter, or soot. The new rule, finalized in 2024, means that Pittsburgh and other parts of Pennsylvania could soon be out of compliance with Clean Air Act standards.
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The Clean Air Act requires the EPA to review its ambient air quality standards every 5 years, but it also allows the U.S. Environmental Protection Agency to update them more frequently.
The first Trump administration conducted its own review in 2020, but did not make changes. The next review would have been due in 2025, but in the last year of the Biden administration, the EPA reviewed new research and tightened the standard.
The EPA said at the time that new studies showed lower levels of air pollution were needed to protect public health, and lowered the limit for soot pollution in the U.S. from 12 to 9 micrograms per cubic meter of air.
Industry groups and states governed by Republicans sued to block the standard from going into effect.
Eventually, the administrations changed, and the Trump EPA joined the lawsuit. But on June 26, the U.S. Court of Appeals for the District of Columbia Circuit ruled in the Biden EPA’s favor.
The plaintiffs and the Trump EPA argued that the Biden administration hadn’t conducted a full evaluation of the standard and should have considered costs.
But the appeals court ruled that the Biden administration followed the law of the Clean Air Act in arriving at its decision, and that cost should not be a factor in determining how much soot is safe in the air. So the rule stands.
What is the PM 2.5 standard, and why is it important?
PM 2.5 are tiny particles (particulate matter smaller than 2.5 microns, which are one thousandth of a millimeter) that are mainly caused by combustion, particularly fossil fuels. Typical sources of particulate matter include industrial facilities, such as coal-fired power plants and steel mills, and vehicle exhaust.
The reason these particles are so dangerous is their size: They’re small enough (1/30th the width of a human hair) to get deep into the lungs and are associated with heart attacks, decreased lung function, and asthma. They are also associated with neurological problems.
“We know PM2.5 pollution has major health consequences, cardiovascular impacts, respiratory impacts… neurological conditions like Alzheimer’s disease and dementia,” said Sage Lincoln, an associate attorney at Earthjustice, one of the environmental groups that sued to keep the standards.
EPA estimated when it came out with the new rules in 2024 that the lower standard would save 4,500 lives a year and essentially save $77 in health costs for every $1 companies and consumers have to pay to comply with the new rule.
How does this affect places in Pennsylvania?
Several cities in Pennsylvania would be affected by a stricter PM 2.5 standard, including Pittsburgh. Allegheny County had been out of compliance with the former standard of 12 micrograms per cubic meter for years. Just a few years ago, it finally got into compliance by lowering PM 2.5 levels, and this April was determined to be in attainment with the standards. But its air is still not clean enough to meet the 9 micrograms per cubic meter standard.
A state-run air monitor in Liberty, Allegheny County, near the U.S. Steel Clairton coke works, had the highest PM 2.5 levels in the state, averaging 11.6 micrograms per cubic meter, according to a DEP report to the EPA in March 2025 recommending Allegheny County and three other areas in the state, Philadelphia, Lancaster County, Harrisburg-Carlisle-York, for nonattainment of EPA’s updated standard.
Subsequent data published by the EPA show that PM 2.5 levels in Allegheny County are somewhat lower. The Liberty monitor reported an average of 10.1 micrograms per cubic meter, which represents a three-year average from 2023 to 2025. At those levels, the monitor has lower air pollution levels than Philadelphia, with a reading of 10.6.
Lancaster and Harrisburg-York areas also reported lower levels of PM 2.5, both with monitors averaging 9.0 micrograms per cubic meter from 2023 to 2025.
If the standards go into effect, states will have six years to meet the new standards.
New EPA rule could rein in air pollution in Western Pennsylvania
Large factories with lots of emissions, like U.S. Steel’s operations in the Mon Valley near Pittsburgh, would likely have to come up with plans to lower their pollution.
Matthew Mehalik, executive director of the Breathe Project, says that could mean requiring stricter pollution controls at plants or taller smokestacks to increase dispersion of soot.
“It might require updating equipment that is old and outdated and leaks,” said Mehalik. “None of this stuff happens overnight. The whole thing is set up to ratchet down pollution over the course of five-year to 10-year periods of time.”
When will the new standard go into effect?
The EPA has thus far not enforced the 2024 standard pending this litigation. There is a lawsuit in the Northern District of California to force the EPA to do just that. In the meantime, the EPA could appeal the decision to the Supreme Court or propose a new, less stringent standard.
What is the Trump administration’s rationale for a less stringen soot standard?
The Trump administration has stressed the costs of regulations on industry. Trump has made several executive orders to that effect. The agency says the air in the U.S. has improved over the past 20 years, and that tightening the standard would make it harder to do things like permit large industrial projects, and implementing it would cost around $470 million a year.
“Under President Trump, we will ensure air quality standards for particulate matter are protective of human health and the environment while we unleash the Golden Age of American prosperity,” EPA Administrator Lee Zeldin said in a statement.
Reached for comment on the ruling, an EPA spokesperson said: “EPA is reviewing our options for appeal.”
