Two deadly explosions at British Columbia mills have cast a pall over an annual day of mourning for workers killed or injured on the job.

WorkSafeBC held its day of mourning ceremony Friday, just four days after two workers were killed when a sawmill in Prince George exploded -- three months after another explosion in Burns Lake killed two mill workers.

George Morfitt, chairman of WorkSafeBC's board of directors, said the explosions serve as a sad reminder that some workers are still at risk on the job.

"The tragic events in Prince George earlier this week and Burns Lake three months ago are powerful reminders of the reason we do this work."

The Lakeland mill in Prince George exploded on Monday, killing two workers and injuring many others. Alan Little, a 43-year-old shift supervisor, died Tuesday morning in Prince George, while Glenn Francis Roche, 46, died in an Edmonton hospital.

In mid-January, two workers were killed when a fireball slammed through the mill in Burns Lake. The victims were Robert Luggi, 45, and 42-year-old Carl Charlie.

Morfitt said that while those deaths were tragic, they are among more than 100 workplace fatalities that happen every year. Last year, WorkSafeBC recorded 142 workers killed on the job, including 42 who died in worksite accidents.

"These aren't numbers and statistics -- these are real human beings with families and friends who love them and who suffer greatly for their loss," said Morfitt .

"These workers must be remembered and mourned. We must learn from what happened to them, and determine how to prevent it from happening again so they did not die in vain."

The cause of the explosions in Prince George and Burns Lake remains a mystery, but excessive dust -- which can be highly explosive if left uncontrolled -- has emerged as a possibility.

Those concerns prompted the B.C. government to announce safety inspections of all the province's mills this week.

Labour Minister Margaret MacDiarmid told the ceremony that the day of mourning was a sombre occasion, and she promised the families of the victims would have answers about what happened.

"Today's day of mourning is particularly painful and poignant for us because of what's happened in British Columbia over the past three months," said MacDiarmid.

"The first (explosion) in Burns Lake was extraordinary, people couldn't remember something like that happening before, and yet just a few short days ago the unthinkable happened. There are so many questions, and we know that we must find out what happened. We must let the workers who were injured know, and those who lost loved ones. It's imperative."

WorkSafeBC has notified all mills they have fewer than two weeks to conduct a thorough inspection and implement an effective combustible dust control problem.

Follow-up inspections will be conducted by May 9 to ensure mills have done enough to comply with the order.