Investigators looking into two mill explosions in B.C. have found common elements that may have set off the fiery explosions that levelled the buildings and killed four men.

The ignition source of both fires appears to be at the conveyor level in the basement, where electrical or mechanical equipment was in operation at the time, said Jeff Dolan, WorkSafe BC's director of investigations.

But Dolan said the main factor in both cases is the area was contained and that was a key ingredient in setting off the explosion.

To start a fire you need fuel, ignition and oxygen, but an explosion needs fuel dispersion and containment, Dolan explained.

"It's the containment that causes the fire to become explosive because it is contained or confined within an area, which it then exceeds," he told reporters in a conference call on Monday.

Investigators know the containment area for the explosion at the Babine Forests Products mill in Burns Lake in January was about 15 metres by 15 metres, but they aren't sure yet of the area that contained last month's fire at the Lakeland mill in Prince George.

"They may be coincidental, but they certainly can't be ignored," Dolan said of the findings.

Robert Luggie, 45, and Carl Charlie, 42 were killed in the Burns Lake mill blast.

Alan Little, 43 and Glenn Roche, 46, died in the Prince George explosion while many others received serious injuries in both explosions.

Roberta Ellis, vice president of corporate services with WorkSafe BC, said the agency has told other mill employers to pay extra attention to similar locations inside their sawmills.

Dolan didn't want be specific about the warning or the size of the area that employers may need to be concerned about.

"The spaces are enclosed by hard surfaces -- four walls a ceiling and a floor -- they're not airtight. It could be a large piece of equipment that creates the containment," he said.

He said it took investigators about 10 weeks to determine that a contained area may have led to the Burns Lake explosion because temperatures were down to -41C at the time and the water fire crews poured onto the mill froze the investigation scene.

WorkSafe BC has already issued a directive to every sawmill employer in the province to clean up dust in their operations.

Dolan said investigators are looking at dust, natural gas and propane as possible fuel sources for the Babine mill explosion, although it's too early in the Lakeland investigation to provide such a list.

Betty Pirs, vice-president of prevention services at WorkSafe BC, said some of the mills are attempting to clean up their sites using compressed air.

The practice, called blowdown, is used to get dust off beams and other areas that can't be easily reached by other means.

"That's highly dangerous," she said. "As you can imagine, it's extremely dangerous, because it creates the very fuel dispersion that Jeff's been talking about."

WorkSafe has issued a bulletin telling employers to use alternative means to clean up their mills.