CHARLESTON, W.Va. (AP) — One of just two major makers of emergency air packs for U.S. coal mines has stopped selling the devices because they often don't automatically start and the industry fears it will lead to a shortage, officials said Wednesday.

Underground mines must increase stockpiles as their operations grow. Government rules require the nation's 50,000 underground miners to wear an emergency air pack on their belt and operators to cache extra air ones in work areas, on vehicles used to bring miners underground and along escapeways.

With one company not planning to sell them for the foreseeable future, the industry worries there will be a shortage, said National Mining Association lobbyist Bruce Watzman.

The new danger comes as the industry deals with its worst loss of life in 40 years — the deaths of 29 miners in an explosion at a West Virginia underground mine last month.

Monroeville, Pa.-based CSE Corp. controls about half the U.S. market. Small oxygen cylinders used to start its air packs automatically have long been known to fail. The devices, known as self-contained self-rescuers, are designed to generate enough oxygen from chemicals for a miner to breathe for about an hour in toxic conditions due to fire or explosion.

The federal Mine Safety and Health Administration won't allow CSE to resume shipments until the problem is fixed, spokeswoman Amy Louviere said.

"If they try to go back into production before they solve the problem, we will issue a formal letter of stop sale," Louviere said.

MSHA will soon give options to mine operators about how to comply with regulations without CSE's devices, she said.

CSE said its air packs remain safe and haven't been pulled from mines because workers are trained to start the device manually. However, MSHA is advising miners to try a spare before attempting a manual start.

Starting it manually is dangerous because miners have to breathe in carbon monoxide, smoke and other toxins to blow air into the packs, said Randy Harris, engineering consultant to the West Virginia's mine safety director.

The problem has been noted before in studies by the National Institute for Occupational Safety and Health, which found a 16 percent failure rate in its most recent report. Harris found a much higher rate in tests he conducted for West Virginia and Kentucky regulators.

Of the 50 that West Virginia and Kentucky jointly tested, two-thirds of them failed, which Harris called "a highly unacceptable rate."

West Virginia probably will have to cite mines that can't get enough air packs, Harris said.

That's another frustration for the coal industry. Although coal generates half the nation's electricity, it's not a big enough industry to attract large manufacturers. CSE, for instance, is a family-run business and makes its air packs essentially by hand.

"Once again we're in a situation where we're an industry that unfortunately has too few technology providers because of the size of the marketplace," said Watzman.