"The present coal size study indicates that the coal dust in intake airways of U.S. mines is finer than that measured in the 1920s," the paper said.

In a follow-up paper, Marcia L. Harris and other NIOSH researchers recommended that the 65-percent standard for rock-dusting be increased to at least 72 percent.

"Modern underground coal mine conditions have changed since the early 20th century," that study said. "The coal dust deposited in mine intake airways is significantly finer. As such, more rock dust needs to be applied in order to prevent a propagating explosion in intake airways."

Still, over years, regulators have found that compliance even with the outdated coal-dust standards has been difficult.

Across the coalfields, there is a long history of mine explosions that were turned from small accidents into major disasters by the failure of mine operators to control coal dust.

The Upper Big Branch is the worst U.S. mining disaster since December 1970, when 38 miners died in an explosion at Finley Coal Co.'s Nos. 15 and 17 mines in Hyden, Ky.

The Hyden blast was ignited by the use of improper explosions underground. But federal investigators found that, "Excessive accumulations of coal dust, and inadequate applications of rock dust in parts of Nos. 15 and 16 mines permitted propagation of the explosion throughout the mines."

Since 1970s, coal dust has been linked to at least six major U.S. coal-mining explosions that killed 73 miners, according to a Charleston Gazette review of government records.

Last year -- like almost every other year -- the most frequently cited violation by MSHA inspectors was allowing the accumulation of combustible coal dust in underground mines. In 2009, coal operators nationwide were cited for violating coal-dust accumulation rules more than 9,200 times, accounting for 11 percent of all underground coal mine violations.

Inadequate rock-dusting is also among the top 10 most frequently cited violations at underground coal mines nationwide, with more than 1,200 violations nationwide in 2009, according to MSHA data.

In underground mines, dust is produced at the working face where coal is mined, at conveyor belts and coal transfer points, and by the normal movement of workers and machines. Coarse dust settles rapidly. But the fine coal particles remain airborne much longer, and can be blown relatively long distances in underground mines. This fine dust is known as float coal dust, and can be a major danger to underground miners.

After 13 miners died in a September 2001 explosion at the Jim Walter No. 5 Mine in Brookwood, Ala., the U.S. Government Accountability Office faulted MSHA for not having a clear program to regulate float coal dust. Several MSHA district managers told the GAO "the lack of a specific criteria for floating coal dust makes it difficult to determine what is an allowable level.

"As a result, mine inspectors must rely on their own experience and personal opinion to determine if the accumulation of floating coal dust is a safety hazard that constitutes a violation," the GAO said in a September 2003 report. "According to some inspectors we interviewed, this has led, in some cases, to inconsistencies in inspectors' interpretations of the procedures - some inspectors have cited violations for levels of floating coal dust that have not brought citations from other inspectors."

In response, MSHA said that float coal dust "must be evaluated on a case-by-case basis." There is no "shopping list or clear-cut formula to indicate when and to what degree that presence of coal dust poses a distinct hazard to the miners," MSHA officials told the GAO.

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

Researchers at the National Institute for Occupational Safety and Health published reports in 2006 and 2009 urging regulatory agencies to re-examine the standards, but no such action has been taken.

Joe Main, assistant labor secretary for the U.S. Mine Safety and Health Administration, said the NIOSH recommendations would get a closer look in the wake of last week's deadly explosion at Massey Energy's Upper Big Branch Mine in Raleigh County.

"I think you can pretty well expect that to be on the table," Main said in an interview Tuesday afternoon.

Gov. Joe Manchin said Tuesday he is considering emergency action to tighten West Virginia's dust standard until he can call the Legislature into special session to pass a new law.

"I'm going to do everything I can in this state," Manchin said. "I can't wait until the feds start moving."

Coal dust is highly explosive, as is methane gas, which is naturally liberated by geologic formations underground.

If methane builds to explosive levels and is ignited, coal dust can be tossed into the air and explode -- making underground blasts 10 times more powerful. When methane ignites in the presence of excessive dust, an explosion that might have caused minor damage or injured miners can easily shoot through mine tunnels, killing dozens of workers.

Investigators digging into the cause of last week's explosion that killed 29 workers at Massey Energy's Upper Big Branch Mine in Raleigh County are focusing on their belief that coal-dust accumulations made the disaster far worse. The operation has been repeatedly cited not just for ventilation problems, but also for unsafe levels of coal dust.

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, experts say.

Under current federal regulations and West Virginia rules, coal companies must apply enough rock dust so that the "incombustible content" of mine dust in clean-air intake tunnels makes up at least 65 percent of all dust measured. In "return air" courses, rock dusting must be adequate to make the incombustible content 85 percent of all dust measured.

But those rules are based on dust surveys of U.S. mines conducted in the 1920s.

Four years ago, after a series of deadly mine disasters in West Virginia and Kentucky, NIOSH conducted the first comprehensive survey of coal dust in more than 80 years. The agency examined coal-dust particle size, which is important because smaller and finer dust particles require more rock-dusting to prevent explosions.

Michael J. Sapko and other NIOSH researchers examined coal-dust samples from 50 mines across the country.

"Underground coal mining technology has become highly mechanized, and this has resulted in increased coal production rates," Sapko wrote in an August 2006 paper. "Coal mining has become highly mechanized and this has resulted in increased coal production rates.

"The present coal size study indicates that the coal dust in intake airways of U.S. mines is finer than that measured in the 1920s," the paper said.

In a follow-up paper, Marcia L. Harris and other NIOSH researchers recommended that the 65-percent standard for rock-dusting be increased to at least 72 percent.

"Modern underground coal mine conditions have changed since the early 20th century," that study said. "The coal dust deposited in mine intake airways is significantly finer. As such, more rock dust needs to be applied in order to prevent a propagating explosion in intake airways."

Still, over years, regulators have found that compliance even with the outdated coal-dust standards has been difficult.

Across the coalfields, there is a long history of mine explosions that were turned from small accidents into major disasters by the failure of mine operators to control coal dust.

The Upper Big Branch is the worst U.S. mining disaster since December 1970, when 38 miners died in an explosion at Finley Coal Co.'s Nos. 15 and 17 mines in Hyden, Ky.

The Hyden blast was ignited by the use of improper explosions underground. But federal investigators found that, "Excessive accumulations of coal dust, and inadequate applications of rock dust in parts of Nos. 15 and 16 mines permitted propagation of the explosion throughout the mines."

Since 1970s, coal dust has been linked to at least six major U.S. coal-mining explosions that killed 73 miners, according to a Charleston Gazette review of government records.

Last year -- like almost every other year -- the most frequently cited violation by MSHA inspectors was allowing the accumulation of combustible coal dust in underground mines. In 2009, coal operators nationwide were cited for violating coal-dust accumulation rules more than 9,200 times, accounting for 11 percent of all underground coal mine violations.

Inadequate rock-dusting is also among the top 10 most frequently cited violations at underground coal mines nationwide, with more than 1,200 violations nationwide in 2009, according to MSHA data.

In underground mines, dust is produced at the working face where coal is mined, at conveyor belts and coal transfer points, and by the normal movement of workers and machines. Coarse dust settles rapidly. But the fine coal particles remain airborne much longer, and can be blown relatively long distances in underground mines. This fine dust is known as float coal dust, and can be a major danger to underground miners.

After 13 miners died in a September 2001 explosion at the Jim Walter No. 5 Mine in Brookwood, Ala., the U.S. Government Accountability Office faulted MSHA for not having a clear program to regulate float coal dust. Several MSHA district managers told the GAO "the lack of a specific criteria for floating coal dust makes it difficult to determine what is an allowable level.

"As a result, mine inspectors must rely on their own experience and personal opinion to determine if the accumulation of floating coal dust is a safety hazard that constitutes a violation," the GAO said in a September 2003 report. "According to some inspectors we interviewed, this has led, in some cases, to inconsistencies in inspectors' interpretations of the procedures - some inspectors have cited violations for levels of floating coal dust that have not brought citations from other inspectors."

In response, MSHA said that float coal dust "must be evaluated on a case-by-case basis." There is no "shopping list or clear-cut formula to indicate when and to what degree that presence of coal dust poses a distinct hazard to the miners," MSHA officials told the GAO.

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