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“It could be an enormously significant impact,” Mr. Revesz said.

The Obama administration had sought to reduce planet-warming greenhouse gas emissions under the Clean Power Plan by pushing utilities to switch away from coal and instead use natural gas or renewable energy to generate electricity. The Obama plan would also have what’s known as a co-benefit: levels of fine particulate matter would fall.

The Trump administration has moved to repeal the Obama-era plan and replace it with the Affordable Clean Energy rule, which would slightly improve the efficiency of coal plants. It would also allow older coal plants to remain in operation longer and result in an increase of particulate matter.

Particulate matter comes in various sizes. The greatest health risk comes from what is known as PM 2.5, the range of fine particles that are less than 2.5 microns in diameter. That is about one-thirtieth the width of a human hair.

The E.P.A. has set the safety threshold for PM 2.5 at a yearly average of 12 micrograms per cubic meter. While individual days vary, with some higher, an annual average at or below that level, known as the particulate matter standard, is considered safe. However, the agency still weighs health hazards that occur in the safe range when it analyzes new regulations.

Industry has long questioned that system. After all, fossil fuel advocates ask, why should the E.P.A. search for health dangers, and, ultimately, impose costs on industry, in situations where air is officially considered safe?

Mr. Wehrum, who worked as a lawyer and lobbyist for chemical manufacturers and fossil fuel businesses before moving to the E.P.A., echoed that position in two interviews. He noted that, in some regulations, the benefits of reduced particulate matter have been estimated to total in the range of $40 billion.