The Trump administration declined to impose stricter limits on smog-forming pollutants Wednesday, saying national ozone standards set in 2015 are sufficient. Public-health advocates had pressed for lower levels of ozone, arguing the hazardous gas disproportionately affects the most vulnerable Americans.

Formed when chemicals from power plants, cars and industrial operations are exposed to heat and sunlight, ozone is linked to an array of illnesses including childhood asthma and lung disease.

The Clean Air Act requires the Environmental Protection Agency to set air pollution standards every five years to a level that protects public health. Under the Obama administration in 2015, the EPA lowered the standard from 75 parts per billion to 70 parts per billion, averaged over an eight-hour period.

EPA Administrator Andrew Wheeler said in a call with reporters Wednesday the decision to keep the current ozone standards followed a careful review of the best available science regarding its health effects, noting that pollution levels have sharply declined since the creation of the agency in 1970.

“Air quality has made progress in this country for decades and the last several years have seen spectacular improvements benefiting the health of millions of Americans,” he said.

Members of the EPA’s Clean Air Scientific Advisory Committee, all of whom have been appointed under President Trump, endorsed the idea of keeping the National Ambient Air Quality Standards for ozone at the current level.

But many public-health experts argued it should be lowered to 60 parts per billion, given the growing scientific evidence that points to the heavier pollution burden carried by low-income and communities of color and ozone’s impact on at-risk groups. An average level of 70 parts per billion suggests that some communities breathe air that is much more heavily polluted, they say.

The problem is made worse by a warming planet, said Paul Billings, senior vice president for advocacy at the American Lung Association. Higher temperatures make ozone more likely to form.

“We’ve seen rollback after rollback under this administration. I can’t point to a single rule that’s reduced air pollution,” Billings said in a phone interview. “We know climate change makes things worse because it creates more hot, stagnant conditions that lead to more ozone pollution.”

Ozone is a persistent problem that affects more than 1 in 3 Americans, Billings said. Using EPA air quality data for the period between 2016 and 2018, the ALA found that 137 million people live in counties that received failing ozone grades in its State of the Air Report.

While ozone high in the atmosphere helps protect against the sun’s hazardous ultraviolet rays, it is a lung-damaging pollutant when it is closer to the ground.

Earlier this month, the EPA rejected more stringent standards for another air pollutant, fine particulate matter, despite an agency finding that reducing it significantly could save between 9,050 and 34,600 American lives each year.

The pollution levels set by the EPA have economic implications. A community that is deemed out of compliance with national air quality standards may not be able to proceed with industrial operations or transportation projects. Several trade associations had urged the administration to retain the current ozone requirements and hailed the EPA’s move.

“This decision, based on sound science, advances important goals while supporting sustainable domestic growth,” said Rachel Jones, vice president of energy and resources policy at the National Association of Manufacturers, in a statement. “We have long supported smart policies that protect the environment and improve public health, and the policy announced today is the right approach.”

The Trump administration said it is only the second time since the 1970 passage of the Clean Air Act that the agency has completed a review of an air pollutant within the five-year time frame required by Congress.

“The process had gotten out of control,” Wheeler said.

But North Carolina State University environmental engineering professor H. Christopher Frey, who chaired the Clean Air Scientific Advisory Committee from 2012 to 2015, said in an email that Trump political appointees rushed the process and overhauled the outside advisory board in a way that led to a truncated, distorted outcome.

“EPA did not receive credible review of the scientific basis of the ozone standard from its cherry-picked panel,” Frey said. “The review process was essentially a charade.”

The scientific advisory committee “ lacked the expertise, and the interest, to thoroughly deliberate and advise EPA regarding the likely or anticipated effects of ozone on severely asthmatic children, and regarding environmental justice aspects of disparity in health risk,” he said.

Bernard Goldstein, a professor emeritus at the University of Pittsburgh who is a former assistant administrator for research and development at the EPA, faulted Wheeler with failing to take covid-19, the respiratory disease caused by the coronavirus, into account when setting standards for either ozone or fine particles.

“There were so many ways he could have done it,” Goldstein said. “Instead, what he did was to ignore it.”

Wheeler said the agency’s career scientists told him this summer that it would take time to determine if poor air quality makes covid-19 worse.

President-elect Joe Biden campaigned on combating both climate change and air pollution, particularly for so-called “fence-line” communities near industrial sites. While the incoming administration could reassess the newly finalized ozone standards, that, too, would take time.

“The Biden EPA needs to restore scientific integrity, but particularly to all of its national ambient air quality standards,” Billings said. “It’s not a simple flip-the-switch go in the opposite direction. They have to follow the science and follow the statute, which this process has not done.”

Biden announced he will nominate Michael S. Regan, North Carolina’s top environmental official, to be EPA administrator. Regan, who would be the first Black man to run the agency if confirmed, had struggled to breathe during “high ozone days” while growing up with asthma in eastern North Carolina.

“I’ve always been curious about the connections between our environment and our health, how the world around us contributes to, or detracts from, our enjoyment of life,” Regan said Saturday after being picked.

“We will be driven by our convictions that every person in our great country has the right to clean air, clear water and a healthier life, no matter how much money they have in their pockets,” he added.