As anticipated, the Environmental Protection Agency announced on Friday that it was proposing to update and tighten national air quality standards for fine-particle soot.

Somewhat surprisingly, however, the agency said that by the time these new standards were fully in force in 2020, all but six counties in the United States would be in compliance with them as a result of steps taken to abide by other tightened rules.

The E.P.A. also predicted that the costs of compliance would be relatively modest. Depending on the final standard adopted, it said, the costs of compliance could be as low as $2.9 million, with an anticipated $88 million a year in benefits, or range as high as $69 million, an investment that would yield $5.9 billion in benefits. The cost-benefit analysis shows that investing in pollution control yields returns, the agency said.