05/02/2012 01:51 EDT | Updated 07/02/2012 05:12 EDT

U.S. lab examining dust from B.C. mill but blast cause still unknown

VANCOUVER - An American laboratory is examining dust samples from a northern British Columbia mill explosion that killed two workers, but the province's workplace safety agency is cautioning that sawdust from dry wood remains only one of several possible causes.

The Jan. 20 explosion at the Babine Forest Products mill in Burns Lake was the first of two deadly mill explosions this year. The second incident last week in Prince George killed another two workers.

Officials at WorkSafeBC, the province's workers' compensation board, hope a U.S. lab can determine whether dust from pine-beetle infested wood was a factor in the Burns Lake explosion.

The agency said that if that was the case, new guidelines may be needed to limit dust in the province's lumber mills.

"If this (dust) is identified as being critical, that would give us the information to look at what has happened here and what we should do differently," WorkSafeBC vice-president Roberta Ellis told reporters Wednesday during an update on the Burns Lake investigation.

There has been speculation that both disasters may have been linked to airborne sawdust, which can be highly explosive if left uncontrolled, but Ellis stressed investigators are still far from determining the cause of either fatal blast.

Investigators are working to determine what fuelled the explosion, and what ignited that fuel.

They've narrowed down potential fuel sources to dust, natural gas or propane, and are trying to find out whether a hot surface, friction from motors or saw blades, or electricity sparked the blast. They're also looking into whether production levels, the type of wood processed at the mill, the facility's ventilation system or the extremely cold weather increased the risk of an explosion.

The province has workplace safety rules that require mill operators to control dust levels, but they mostly relate to keeping the air safe for workers to breathe.

Ellis said WorkSafeBC has not looked into whether pine-beetle wood, which is dead long before it is cut and produces a dust that is drier and finer than green wood, increases the risk of a catastrophic explosion.

"We've had discussions with the industry over the years over milling techniques — has the production process changed as a result of beetle-kill wood," Ellis said.

"I'm not aware of anyone in B.C. sending out samples to labs for testing."

Still, the agency was concerned enough about dust that it issued a sweeping directive to the province's 341 mills last week, ordering them to immediately conduct inspections to assess the risk of dust and take steps to bring it under control.

The agency also conducted 70 inspections following the Burns Lake explosion, resulting in 90 orders for violations of occupational health and safety regulations.

WorkSafeBC hasn't revealed the nature of those violations, but Ellis said the inspection results would be released soon.

The investigation into the Burns Lake explosion is expected to take another three months, and it's not clear when the Prince George investigation will be complete.

Ellis said it took more than two months after the Burns Lake explosion before investigators could even access the site because of extreme winter weather and concerns about the structural integrity of the mill.

They completed their scene examination in mid-April and are now analyzing evidence that includes natural gas piping and propane tanks, Ellis said.

The Burns Lake explosion killed 45-year-old Robert Luggi and 42-year-old Carl Charlie, and injured 19 others.

Last week's explosion in Prince George killed 43-year-old Alan Little and 46-year-old Glenn Francis Roche. Two dozen workers were injured.