WASHINGTON - The U.S. Mine Safety and Health Administration proposed Friday to toughen its 27-year-old asbestos exposure limit - a threshold so weak that it impeded the discovery of a health disaster from a Montana vermiculite mine.

MSHA's lapse also has had fallout for Sacramento and scores of other cities, where the asbestos-tainted Montana ore was shipped for decades for use in making attic insulation and other products. As in the mining town of Libby, Mont., workers at the plants continued to be exposed to the dust for years, while patterns of asbestos-related illnesses went unnoticed.

The new rule is designed to prevent a recurrence. David Dye, an MSHA deputy chief, said it "would help improve the health of American miners."

Since 1994, MSHA has allowed miners to be exposed to 20 times more of the potentially deadly or disabling fibers than is permitted for nonmining workers. Public health experts have yet to identify a safe exposure level to asbestos.

Agency officials began efforts to toughen the rule in response to the Libby catastrophe, in which routine MSHA air samplings failed from 1978-92 to detect asbestos.

After a Seattle Post-Intelligencer report in late 1999, federal health officials found that asbestos-related illnesses had killed and sickened hundreds of miners and Libby residents.

In early 2000, an audit by the Labor Department's inspector general's office found that MSHA had directed its testing laboratory to initially analyze dust samples with microscopes too weak to specifically identify asbestos. Only if the tests produced general counts of fibers that exceeded the agency's threshold of 2 fibers per cubic centimeter of air were the labs permitted to get a closer look with more expensive, high-powered microscopes. The weaker microscopes were unable to see the smallest fibers, reducing the counts.

As a result of the audit, MSHA increased its use of electron microscopes and is proposing to toughen its exposure threshold to 0.1 fiber per cubic centimeter, the same as the U.S. Occupational Safety and Health Administration's limit.

Once the stronger microscopes were deployed, MSHA turned up elevated asbestos counts in a Minnesota taconite mine.

At hearings during the rulemaking process, representatives of the mining, stone, sand and gravel industries made similar arguments.

Experts for the W.R. Grace & Co., which owned the Montana vermiculite mine from 1963 until it closed in 1990, also argued that most particles in the air in Montana were cleavage fragments, smaller particles created by breakage during the crushing or grinding process.

Dr. Aubrey Miller, a Denver-based, senior medical officer and toxicologist for the Environmental Protection Agency, expressed skepticism.

"Some of W.R. Grace's experts feel that Libby fibers are nearly all cleavage fragments," said Miller, who helped unravel the Libby disease toll. "If this is true, then clearly cleavage fragments are toxic."

Still, an early draft of MSHA's proposed rule, a copy of which was obtained by the Minneapolis Star Tribune, stated that cleavage fragments may be "erroneously" counted as asbestos fibers and that the proposed rule "would minimize these errors." The formal proposal, with input from an inter-agency task force, dropped that language and said MSHA is sticking to its current definition of asbestos. MSHA spokesman Dirk Fillpot said any change would require consultation with other agencies, scientific evaluation and "unnecessary delays."

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• The Bee's Greg Gordon can be reached at (202) 383-0005 or ggordon@mcclatchydc.com.

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