For years, federal testing has shown the most commonly used air pack in U.S. coal mines can make breathing more difficult than other models, with some medical experts now warning they could be impossible to use in a crisis.

Still, the federal Mine Safety and Health Administration continues to insist that air packs manufactured by CSE Corp. of Monroeville, Pa., work properly. MSHA suggests poor training is to blame for persistent reports the devices fail when miners need them most.

Long-term studies of CSE air packs by MSHA and the National Institute for Occupational Safety and Health suggest that while they do create oxygen, they don't necessarily help miners breathe.

Fearing other fatal mining accidents like this year's disasters in West Virginia and Kentucky, some regulators are now treating CSE air packs as if they contain only half the one-hour supply of oxygen the manufacturer promises.

"There's a problem with what's out there," said James Dean, West Virginia's acting mine safety chief.

MSHA maintains that tests on devices recovered from West Virginia's Sago Mine and the Darby No. 1 Mine in Kentucky show the CSE air packs were in working order.

Belt-wearable CSE air packs are the smallest and lightest available, making them popular with miners already loaded with gear. Officials estimate as many as six in 10 U.S. coal miners carry one underground every day.

Yet miners who've used CSE air packs to escape accidents have complained they barely worked and, in some cases, failed altogether.

Sago survivor Randal McCloy Jr. has said four of the packs assigned to his crew failed after the Jan. 2 explosion trapped them underground. Eleven of his co-workers died of carbon monoxide poisoning.

Roger Perry, a Sago miner who escaped the blast, said breathing with a CSE air pack was like being smothered.

"I was taking it on and off because I couldn't breathe enough," Perry told The Associated Press. "It's like there's no air there."

His account corroborates what government scientists have uncovered: As many as 13 years ago, NIOSH found CSE air packs require users to work harder to breathe than similar devices by rival manufacturers. And those studies show CSE air packs generate high amounts of carbon dioxide, which makes breathing even more difficult.

But Les Boord, director of the NIOSH National Personal Protective Technology Laboratory, says the air packs meet performance standards specified in federal regulation.

Because tests conducted for the long-term field study differ from tests for certification, the study results don't necessarily show a problem with the devices, Boord said.

"It's kind of comparing two different things," he said.

CSE product manager Don Pannell says his company's air packs work as designed when users properly inspect them and mine operators properly train workers how to use them.

However, Pannell said CSE is working on better training equipment that will let miners experience the breathing resistance and heat generated in the air packs before they are in an emergency.

Right now, he said, "They don't really know what to expect."

Medical experts at Harvard, Duke and the University of Kentucky say the combination of high breathing pressure and high levels of carbon dioxide could lead a panicked miner to conclude an air pack isn't working.

MSHA says the air packs at Sago and Darby worked as designed because they generated oxygen.

"There was oxygen capacity left in all those units," agrees Pannell, of CSE. "Any of those units could have helped to get people out of the mines."

But there is a distinction between producing oxygen and facilitating breathing.

A 2002 report on the MSHA-NIOSH study, the latest available, shows nearly 30 percent of CSE air packs, especially those that had been carried in the field, stopped removing carbon dioxide in less than an hour.

The study also found about 20 percent of CSE air packs forced the user to inhale at least 6 percent carbon dioxide — triple the amount allowed by federal regulation.

Carbon dioxide levels should be 4 percent or lower, the study said, because irregular heartbeat and other problems can occur at higher levels. At somewhere around 10 percent, the user likely would pass out.

CSE, however, maintains that its air packs function properly if miners and mine operators follow its inspection guidelines.

Four percent level might be uncomfortable, "but it wouldn't be intolerable," said Robert Banzett, an associate professor at the Harvard School of Public Health. "If they can't breathe more, it's going to make them feel horribly in need of air."

CSE's training video instructs users to slow their pace when breathing gets difficult. But medical experts say the sensation of suffocating would drive most miners to remove the packs, regardless of the threats they face.

"There is no kind of perception that's more primal," said Michael Reid, chairman of the physiology department at the University of Kentucky's College of Medicine. "It's a protective response. It's Darwinian."

Perry, for example, said he felt better without his air pack at Sago, even though that meant breathing carbon monoxide and methane.