Brazilian mining company Vale have reported a nitrogen dioxide leak at its Copper Cliff smelter in Sudbury, Ontario and have confirmed the leak has been contained.
The company also reported that the toxic gas leak did not cause any injuries.
Vale spokesman Cory McPhee said the leak occurred at its acid plant when the company was starting a scheduled maintenance shutdown at the nickel smelter and the incident will not disturb the production.
"It looks at this point in time like it may be related to some of the shutdown work that was being done."
A level 3 emergency that was declared at the site has been downgraded to level 2, which means that it now only affects the plant, and that it is now safe for nearby residents to go outside safely.
As reported by broadcaster CBC a yellow cloud was visible above the Vale complex.
"The cloud is dissipating. The levels that we registered throughout the event were very low," said McPhee, adding that Vale has not received reports of any adverse effects outside the plant.
Nitrogen dioxide gas affects the respiratory system and can be fatal when consumed in high doses.