CHARLESTON, W.Va. -- United Mine Workers officials turned out Thursday to support the Obama administration's rule to require mine operators to do more to control the buildup of explosive coal dust in underground mines.

UMW safety officers and rank-and-file miners were among about two-dozen people who attended the last of four U.S. Mine Safety and Health Administration public hearings on an emergency rule governing "rock-dusting" of underground tunnels.

In September, MSHA chief Joe Main issued the rule to require coal companies to apply more crushed stone to the walls, floors and other surfaces underground to control potential dust ignitions. The rule has already taken effect on an emergency basis, and now MSHA is taking public input on a final version.

"MSHA has the full support of the United Mine Workers," said Linda Raisovich-Parsons, deputy director of the union's safety department. "Sufficient evidence exists to warrant an emergency temporary standard."

Experts from the National Institute for Occupational Safety and Health urged a toughening of federal "rock dusting" standards in reports published in 2006 and 2009, but MSHA officials did not act until another NIOSH report was published this May, a month after 29 miners died in an explosion at Massey Energy's Upper Big Branch Mine in Raleigh County.

Coal dust is highly explosive, and can turn what might be minor ignitions of methane gas in underground mines into massive blasts that take many more lives. Federal and state investigators and independent experts believe that's what happened on April 5, when a huge explosion ripped through the UBB Mine, causing the worst U.S. coal-mining disaster in 40 years.

Mine safety experts have known for decades how to prevent coal dust explosions: Apply large amounts of "rock dust," usually powdered limestone, to wall and floor surfaces underground. Even if there is an explosion, the rock dust mixes with coal dust and helps prevent it from fueling a larger blast.

Under the 1969 federal Coal Mine Health and Safety Act, coal companies must apply enough rock dust so that "incombustible content" of mine dust in clean-air intake tunnels makes up at least 65 percent of all dust measured. In "return air" sources, rock dusting must be adequate to make the incombustible content 80 percent of all dust measured.

That 41-year-old law, though, is based on coal-dust surveys of U.S. mines conducted in the 1920s. More recent NIOSH studies, conducted after a series of disasters in 2006, found that more modern and highly mechanized mining practices produce significantly finer coal dust that requires more rock dust to control.

Under MSHA's changes, the incombustible content of dust in all areas of underground mines would have to be at least 80 percent. Higher incombustible percentages would be required where methane is present.

MSHA estimated that the change would cost the nation's 415 underground coal mines a total of $22 million a year for additional rock dust and for employee time applying that rock dust. The agency noted that six coal dust explosions killed 46 miners over the past 26 years, an average of two miners per year.

"MSHA acknowledges that the requirements in this ETS probably would not have prevented all of the deaths from the six explosions, and estimates that the ETS would have prevented approximately one to one and a half deaths per year," MSHA said in a Federal Register notice.

At Thursday's public hearing, several Massey employees and a lobbyist for the West Virginia Coal Association complained that MSHA should be more focused on allowing mine operators to use scrubbers mounted to mining machines to control dust and allowing extended cuts of coal that involve less moving of machines underground.

"That's a major concern we have here in West Virginia," said Chris Hamilton, the coal association's vice president. "That's a much greater concern here throughout West Virginia and Appalachia than putting an additional 5 or 10 or 15 percent more rock dust."

MSHA officials have said Massey's scrubbers weren't meeting dust standards, and that the company is merely upset about the agency blocking extended cuts, which allow more coal to be more quickly mined, but also can create roof-control problems.

On Thursday, MSHA also announced a second phase of its "Rules to Live By" enforcement initiative to target the types of violations agency officials believe can lead to explosions and fires.

"Too many miners have lost their lives in catastrophic accidents over the past 10 years," Main said. "That is simply unacceptable."

Reach Ken Ward Jr. at kw...@wvgazette.com or 304-348-1702.