Background The western Canadian province of Alberta attracts skilled workers from across Canada to work in the oilfields. We investigated whether information from Workers Compensation Board (WCB) claims would provide unbiased estimates on the rate of injury in such migrant workers. Work injuries in Alberta are compensated by the Alberta WCB regardless of province of residence.

Methods The Alberta WCB provided claims data with home province, sex, age, industry and time lost from work. Denominator data came from Statistics Canada, linking census and taxation information. We also recruited a cohort of workers in Fort McMurray, the hub city for oil and gas, and followed them for 4 months to record work injuries.

Results From Statistics Canada, we had 1,720,716 people working in Alberta in 2012 whose home was Alberta and 10403 whose home was Newfoundland. The overall rate of injury (with no correction possible for days employed) was lower in the migrant workers, after adjustment for age, sex and industry. Within claims, the pattern of time loss differed importantly: those from Newfoundland had a marked deficit in claims with time loss 1-28 days (OR=0.18: 95%CI 0.12-0.27). WCB reporting among the 151 cohort members was lowest among those from out of province or recently settled: overall only 38% of loss time injuries were reported. Those in precarious employment were more likely to self-medicate or quit their job to avoid being labelled with a history of injury.

Conclusion Injury risk in inter-provincial workers could not be estimated using only WCB data.