A WorkSafeBC report written just weeks before an explosion and fire that killed two and levelled a sawmill in northern B.C. on Jan. 20 noted dust levels in the facility's basement were more than twice the acceptable level.

High dust levels in places like sawmills can pose respiratory difficulties for workers and can also be an explosion hazard.

Results from 10 samples taken at the Burns Lake mill in November found the unacceptably high dust levels in the basement cleanup area.

High concentrations of dust in the air were detected at the Burns Lake mill in November. (Jonathan Hayward/Canadian Press)

Several mill workers interviewed by CBC News talked about large amounts of dust inside the mill.

"All those saws in there make pine dust, and it's really dark in there, you can't see from one end of the mill to the other because of the dust," said John Wiebe, who was one of 19 workers injured in the blast and fire.

Another employee, Frankie Erickson Jr., said cold weather meant the mill was more tightly closed than usual.

"All the main windows that are supposed to be opened, were closed," Erickson said.

Dust, cold weather and poor ventilation are often a deadly combination, said Neil McManus, an industrial hygienist, chemist and expert on enclosed spaces.

McManus said the –20 C temperature at the time would have increased static electricity, which can spark an explosion:

"Cold temperature is one factor that might be important and the other factor is that they closed the windows, because containment of a dust cloud is one of the considerations for the cloud to be ignitable," said McManus.

'Wall of fire'

Wiebe said he saw the wall of fire coming towards him at the mill.

"You see one of those movies where the doors open up and boom, wall of fire, same way," he said.

McManus said that description and other eyewitness accounts of the roof and walls being blown off in a single large blast are also important.

"Dust explosions have that reputation," he said.

McManus said a key factor is that the wood being cut at the mill was from trees that had been killed by B.C.'s infestation of pine beetles. The wood is much dryer than lumber that is cut from green trees, so the dust would have been finer and more combustible.

He said a range of things could have sparked the blast, including a smaller natural gas or propane explosion, an electric spark or some form of open flame.

The WorksafeBC report on the November dust samples said the findings suggest the mill's water misters and ventilation systems were not adequately protecting workers.

With files from the CBC's Greg Rasmussen and The Canadian Press