The Environmental Protection Agency on Wednesday tightened rules that limit toxic air pollution from factories, refineries and other industrial facilities, reversing one of former President Donald Trump’s signature environmental protections.

In an update posted on its website, the agency quietly signaled that it had finalized changes to its “Once In, Always In” rule, which requires facilities classified as “major” sources of toxic air pollution to always maintain strict pollution controls, even if they are later reclassified.

The EPA considers a facility a “major source” if it emits 10 tons or more of one hazardous air pollutant per year or 25 tons or more of two or more such pollutants, including arsenic and benzene. Inhaling these pollutants poses a range of health risks.

Short-term exposure to high levels of benzene, a sweet-smelling chemical found in gasoline and other petroleum products, can cause headaches, dizziness, and loss of consciousness. Long-term exposure is associated with a higher risk of cancer, especially leukemia and other cancers that affect the blood and bone marrow.

The Once In, Always In policy was first formulated in a 1995 memo by a senior EPA air official under President Bill Clinton. But Trump officials unofficially ended the policy in 2018 and formally rescinded it in 2020.

Industry groups have chafed at the rule, calling it too burdensome. In public comments, the National Association of Manufacturers wrote that “while compliance with existing federal regulations can already be costly and confusing, waves of new regulations could make that burden unsustainable.”

Wednesday’s change would not fully restore the Clinton-era policy because it would allow some plants to move away from being “major” sources of pollution if their emissions fall. But most plants would still have to meet strict standards for toxic pollutants, including benzene and mercury.

“Manufacturers appreciate the compromise EPA has attempted to reach by not completely restoring this flawed policy, but this rule represents the type of regulatory assault and overreach that will only hinder the growth and investment in manufacturing that the country needs right now,” Chris Phalen, vice president of national policy at the National Association of Manufacturers, said in a statement Wednesday.

Environmental and public health advocates have strongly supported the “Once In, Always In” policy, saying it has reduced pollution in poor and minority communities that are often near industrial facilities. They praised the EPA’s actions Wednesday while calling on the agency to go even further.

“While today’s rule is not a complete repeal of the 2020 rule, it is an important step forward,” said Harold Wimmer, president and CEO of the American Lung Association, in a statement. “Families and communities living near major sources of pollution, like everyone else in this country, deserve to breathe clean air that won’t make them sick.”

Trump’s rollback has allowed about 4,000 companies to increase their toxic emissions, according to environmental law firm Earthjustice.

“Communities living near mega-polluters have long been forced to endure a toxic cocktail of deadly chemicals known to cause cancer and other serious health problems,” Patrice Simms, vice president of healthy communities at Earthjustice, said in a statement. “No corporation should ever be allowed to shirk responsibility and pollute with impunity.”