Refineries, power plants and other polluting facilities would have to cut their soot emissions under a proposed rule the Biden administration announced Friday that aims to reduce the deadly air pollution inhaled by Americans, particularly communities of color.

The Environmental Protection Agency’s proposed rule comes after the Trump administration rejected tougher soot standards in 2020 and embraced industry groups’ argument that the existing regulations were good enough. At the time, EPA scientists, health experts and environmental advocatesurged the opposite, saying the agency needed to tighten the standards to protect public health.

In revisiting this debate, the EPA has found that the current allowable levels of fine soot are taking an unacceptable toll. The agency now plans to lower those limits beyond what the Obama administration set in 2012.

But the EPA isn’t saying precisely how tough it’s prepared to be.

Under its draft rule, the agency would reduce the limit on industrial fine soot particles from an annual average of 12 micrograms per cubic meter of air to between 9 and 10 micrograms. Yet on a phone call with reporters, EPA Administrator Michael Regan said the agency is still considering a range of options, including a lower a threshold of 8 micrograms or a higher limit of 11.

“There’s no predetermined outcome here,” Regan said, saying that agency officials wanted to put out a proposal while soliciting input from outside experts, some of whom have pushed for tougher limits than the agency is proposing. The EPA expects to settle on a new standard this summer. Whatever the outcome, Regan said the net result would advance public health.

“Fine particulate matter is both deadly and extremely costly,” Regan said, adding that strengthening the standard would deliver “transformative benefits” nationally.

“This proposal is anchored in the best available science and reflects input from the public and scientific experts,” he said.

The EPA also announced that it does not plan to strengthen the daily exposure limit, which currently allows soot pollution to reach 35 micrograms per cubic meter. In doing so, the agency departed from the advice of a scientific panel, the Clean Air Scientific Advisory Committee, the majority of whose members favored lowering the limit. EPA officials said that, in most parts of the country, a tougher annual standard would ensure that daily pollution peaks are also lowered.

At issue are fine particles measuring less than 2.5 micrometers in diameter, or one-thirtieth the width of a human hair. Released into the air from power plants, diesel trucks and factories, they are so tiny that they can enter the lungs and bloodstream, triggering asthma, heart attacks and strokes, and are among the most lethal of air pollutants.

Polluting industries have argued against tougher restrictions and are likely to challenge the new standard.

For their part, some advocates for tougher pollution rules were frustrated by the administration’s proposal.

“We’re so disappointed that EPA is ignoring the advice from the independent scientific experts who made very clear recommendations,” said Paul Billings, a vice president of the American Lung Association.

“We don’t want to leave thousands of potential lives saved off the table — we want to maximize the public health protection,” he said, adding that both the daily and annual soot exposure limits should be strengthened beyond what the EPA is proposing.