High levels of arsenic trioxide have been found in 31 cleanup workers at Giant Mine in Yellowknife but it's not clear how employees were exposed to the highly toxic substance. 

Aboriginal Affairs and Northern Development is overseeing the massive cleanup at the former gold mine. There are 237,000 tonnes of arsenic trioxide dust stored in underground chambers at the site. 

Dave Grundy Workers Compensation Yellowknife

Dave Grundy, of the N.W.T Workers' Safety and Compensation Commission, says the arsenic trioxide has to be fully contained before clean-up work resumes at the former gold mine in the spring. (CBC)

"Arsenic doesn't make people sick right away, so we're really concerned about the long-term effects that it has on our northern workers," says Dave Grundy, president of the Workers Safety Compensation Commission of Northwest Territories and Nunavut. "There's a disconnect somewhere where these people are getting exposure and they're not supposed to."

The remediation project is shut down for the winter but when in operation, people working at the site have to undergo regular and rigorous health checks, including weekly urine samples. It's part of a medical monitoring program. 

Grundy says doctors determined "elevated levels" mean people must have more than 35 micrograms of arsenic per litre of urine. 

The contractor, by law, has to report those results to the Workers Safety Compensation Commission (WSCC). 

Aboriginal Affairs says workers who tested for elevated levels of arsenic immediately stopped working in high-risk areas but continued to work on site in other, low-risk areas. 

'We're really concerned about the long-term effects that it has on our northern workers.'- Dave Grundy

"We need to look at why did they test over? What activities they were doing, take them out of high-risk work so we can keep them safe," said Jane Amphlett, manager of operations with Aboriginal Affairs.

The federal cleanup team says so far, none of the affected workers have submitted a claim to Workers Compensation. 

"Later if a worker has an issue and they thought it was because of an arsenic issue, then they can submit a claim to WSCC," said Amphlett. "But right now it's just a record. So WSCC has that on file, so they have that information."

Grundy says reports started coming in about three months ago and so far they've received 28 about exposure to arsenic. He said before work starts up again in the spring, the arsenic has to be fully contained to prevent further exposure. He says Workers Compensation is working with contractors to ensure proper procedures are in place.

"We have to be assured they're doing everything they can do to stop any of this exposure," he said. "There's hot spots in the mine... we have to ensure people are wearing the clothing they're supposed to wear."