NASHVILLE, Tenn. (AP) - New safety rules will not be approved any time soon even though they could prevent accidents like the ones last year at a Tennessee metal powders plant, where fireballs fueled by iron dust contributed to five deaths.

The Occupational Health and Safety Administration is developing rules that would require many industries to better control combustible dust hazards. The rules were recently moved to a long-term agenda, despite pleas from the Chemical Safety Board to put them on the fast track. The Chemical Safety Board investigated accidents Hoeganaes Corp., a plant near Nashville, where five people died a year ago.

"Hoeganaes should have made a believer out of everybody. It's appalling what went on there," said Bill Kauffman, a retired professor of aeronautical engineering with the University of Michigan.

Kauffman helped develop rules in the 1980s that have led to a steep decline in deaths from grain dust explosions and was an expert on a panel last May that discussed crafting the new regulations. He believes some officials at OSHA are trying to make them too complicated.

"They seem to be splitting hairs - 'This dust. That dust.' - Why don't they just say 'Anything that burns?'" he said.

OSHA spokeswoman Diana Petterson did not offer much explanation. She said the agency continues to develop the rules, and preventing worker injuries and deaths remains a priority. She would not discuss the matter further.

Combustible dust is the technical term for any dust that will catch fire, and most will if it is ground finely enough - including dust from chemicals, plastics, metals and foods.

The Chemical Safety Board has been studying the hazards of combustible dust since a series of deadly fires and explosions in 2003.

"We really don't know why OSHA is doing this," said Rafael Moure-Eraso, chairman of the Chemical Safety Board. "We do know that workers keep dying."

A 2006 study by the board found at least 281 dust explosions and fires in the U.S. between 1980 and 2005. They killed 119 workers and injured 718. According to more recent figures, there's been no change in the frequency of deaths and injuries from dust accidents, despite more inspections and an OSHA education program.

One of the worst dust explosions in recent memory occurred in 2008 at the Imperial Sugar Co. refinery in Port Wentworth, Ga., where 14 workers died dozens were injured.

The board recommended that OSHA develop regulations for controlling dust hazards as part of its 2006 report, more than a year before Imperial Sugar. OSHA did not begin the process until 2009 and the agency has offered no date for when it might be completed.

Two weeks ago, OSHA published its twice-yearly regulatory agenda. A note about combustible dust regulations reads "next action undetermined."

Chris Sheburne, whose husband Wiley was killed after a flash fire at Hoeganae s' Gallatin facility, said she was disappointed.

"What if something else happens? Then what?" she said.

Jim Dale, with the Metal Powders Industries Federation, said his organization has been looking on its own at how to best address dust hazards.

"It's a tough question with many members saying, 'We've never had a problem,'" he said. "They don't want to spend millions to deal with a risk that is overstated. It could put a lot of companies out of business."

He said industry employers will do what it takes to make sure their workers are safe.

"We're not taking a big sigh of relief here because this has slipped to the back burner," he said.