The Environmental Protection Agency is toughening air quality standards for fine particulate matter, or soot, over the objections of some industry groups and lawmakers.

The agency unveiled final rules on Friday that ratchet down the annual exposure standard to 12 micrograms per cubic meter, a 20 percent reduction from the current standard of 15 micrograms.

EPA had faced a court-ordered deadline Friday to complete the rules, which will cover a dangerous microscopic pollution that is emitted by factories and power plants, diesel vehicles, ships and many other sources.

“EPA estimates that meeting the annual primary fine particle standard of 12.0 μg/m3 will provide health benefits worth an estimated $4 billion to $9.1 billion per year in 2020 — a return of $12 to $171 for every dollar invested in pollution reduction