The Alberta Energy and Utilities Board is investigating a firethat broke out Monday at a Shell Canada plant outside Edmonton.
Shell said a small fire at its Scotford upgrader near Fort Saskatchewan released hydrogen and hydrogen sulphide into the air.
Crews took about 45 minutes to extinguishthe fire in the upgrader's hydro conversion unit, which refines bitumen from the tarsands into lighter crude oil.
Shell spokesman Randy Provencal told CBC News the company's air monitoring showed no traces of toxic hydrogen sulphide or sulphur dioxide, and thatthere's no reason for anyone to be concerned.
"Emergency crews are still on scene and we are conducting monitoring of ambient air quality," Provencal said Monday evening.
"We've notified the regulatory agencies and some of them are currently on site."
No homes have been evacuated and there have been no injuries reported.
There was a similar problem in the same unit in September 2006 when a cloud of sour gas caught fire forcing a plant evacuation and the company to warn residents nearby to stay indoors.
One week later, hydrogen sulphide and sulphur dioxide were released from another unit of the plant, prompting another warning.
Also on Monday, one of Shell's sour gas pipelines in southern Alberta began to leak, forcing the evacuation of about 10 homesnear Pincher Creek.