The Environmental Protection Agency yesterday tightened air quality standards for the most dangerous particulate matter, setting limits for the first time on the tiniest particles of soot, dust, dirt and smoke.

In its first revision of standards for particulate matter since 1971, the agency extended controls to particles that have largely escaped regulation because of an enforcement system that encouraged the trapping of large but less harmful pollutants.

The new standards will restrict industrial emissions of the small particles considered most threatening to health because they can reach the deepest part of the lung and contribute to respiratory problems.

"They will provide health protection with an adequate margin of safety against particulates of most concern," J. Craig Potter, the EPA's assistant administrator for air and radiation, said of the regulations at a news briefing.

The announcement brought criticism both by environmentalists, who said the regulations did not go far enough, and industry, which said they went too far.

The burden of meeting stricter standards is expected to be heaviest for large generators of particulate matter, such as electric utilities and iron and steel plants. The estimated cost to industry of new pollution controls is $1.9 billion.

States will have no more than five years to bring their jurisdictions into compliance. According to the EPA, 70 areas in 19 states, concentrated in the West, are likely to have difficulty in achieving the standard.

The new regulations amount to a new way of measuring particulate matter, a broad class of solid substances of varying size or nature. State and local officials monitor the air for particulates to assure compliance with federal ambient air standards. Industry is required to restrict emissions so as not to exceed the standards.

Under the old regulations, the total of all particulate matter was limited daily to 260 micrograms per cubic meter of air. Since the larger particles weighed the most, industry captured them while releasing the more dangerous small ones.

The rules unveiled yesterday are aimed at particles no larger than 10 micrometers -- a tenth of the thickness of a human hair. They will be limited to 150 micrograms per day. New monitoring devices will screen out all larger particles, so that more of the small ones will have to be captured by industry to comply.

Although some of the larger particles will be trapped in the devices, they are no longer legally restricted from entering the atmosphere.

David Doniger, senior attorney for the National Resources Defense Council, said the agency should have set much lower daily limits. He said the new limit will achieve "marginal" health benefits where small particles are common. But the standard is "substantially weaker," he said, in areas where large-particle pollution is a problem.

Doniger predicted "backsliding" by factories until they implement new pollution controls because they are no longer legally bound to comply with old standards.

Potter said industry will proceed on a dual track during the compliance period, continuing old pollution controls while preparing new ones.

The American Iron and Steel Institute in a statement said the EPA action was of "unnecessary strigency" and augurs "draconian" results for the industry. Steel companies, which have spent more than $3.5 billion to control particulate emissions, face the possibility of another $500 million in costs, it said.