The second lumber mill explosion and fire in B.C. in four months is raising questions about the kind of wood that mills in the province are cutting and the amount of explosive dust that could be in the air inside the buildings.

The Lakeland Mills sawmill in Prince George blew up in what witnesses described as "a ball of flame" Monday night, killing one man and injuring 24 more workers, nine of them with serious or critical injuries.

The blast was similar to one that killed two people and destroyed a mill in Burns Lake in January. The conclusions from investigations into the incidents are months away, but one expert suspects that dust particles suspended in the air inside the mills could be the culprit. And others point to the kind of wood the mills are working on.

"I was ... fearful that there would be another occurrence of what happened in Burns Lake," Industrial hygienist Neil McManus told CBC News.

"There's a lack of appreciation, it appears, among the general public about the potential for organic materials, when they're finely powdered, to be able to explode."

The provincial government has ordered the immediate inspection of dust levels in all B.C. sawmills.

'Extraordinary' incidents

"These explosions — two of them in such short order — are extraordinary," said B.C. Labour Minister Margaret MacDiarmid. "There are people who've worked in this industry for over 30 years and have never seen an accident of this type."

Both of the destroyed mills were cutting wood from trees killed by pine beetles, which is much dryer than timber from live trees and its dust particles can be more easily ignited. The probability of dust explosions in B.C. sawmills wasn't much of an issue as recently as 10 years ago, but since then, the harvest of pine beetle timber has increased significantly.

[IMAGEGALLERY galleryid=2262 size=small]

About 70 per cent of the lumber that Lakeland Mills in Prince George was cutting was from trees killed by pine beetle infestation.

"You can see increases in the harvest level anywhere from 60 per cent to 100 per cent more," said Harry Nelson, an assistant forestry professor at the University of B.C. "I'd say, on average, mills have been harvesting and cutting around 70- to 80-per-cent pine over the last five or six years."

Mills built decades ago were designed to handle wetter, green timber.

"Every mill in that area needs to be looked at through this lens," said independent B.C. MLA Bob Simpson. "Do we have a build-up of combustible dust like we haven't experienced before? If so, let's deal with it."