The emergency situation this morning at Vale's Copper Cliff smelter in Sudbury, Ont., was declared after two gases combined to form a toxic mist during a cleaning at an acid cooling tower.

A joint investigation is underway to determine what exactly happened.

The gas combined nitrous oxide and nitrous dioxide, a combination with the potential to cause respiratory issues. No other injuries have been reported. Erik White of CBC News tweeted the highest reading of nitrous oxide outside of the smelter were below the limit.

However, Vale, the mining company responsible for the incident, said there were four minor injuries reported last night due to exposure to the acid but were unrelated to this morning's repairs.

The emergency was called in an abundance of caution before being downgraded to a Level 1 emergency, officials said.

"This was a precaution, but an essential one," Vale mining company's general manager of production services Jody Kuzenko said at a news conference. "We don't believe there was ever a risk to the community."

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The yellow plume visible Thursday morning above the Vale mining complex in Sudbury, Ont., occurred during maintenance at an acid cooling tower, officials said. (Erik White/CBC News)

The situation was downgraded to a Level 1 emergency this morning, allowing roads to open and people to go outside. The downgrade means the situation was contained, and police and fire officials have cleared the area.

"We sincerely regret the concern that was raised," Kuzenko said.

The noxious gas emergency began with loud sirens at around 6 a.m. Trevor Bain, the local paramedic and fire chief, said trucks arrived at 6:12 a.m. to address the situation.

The company, which confirmed nitrogen dioxide was released into the air earlier in the day, said it was reassuring to know emergency response efforts worked so efficiently.

"The system worked in terms of sounding the horn, notifying the community, " said Vale spokesman Cory McPhee, who described the situation as one of caution. "You can't take chances with types of events." 

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Jody Kuzenko, Vale's general manager of production services, said the company does not believe there was ever a risk to the community during the gas leak. (Erik White/CBC News)

The company said the gas release occurred during planned maintenance at its acid plant. It remains unclear how the nitrogen dioxide escaped from the smelter. The gas affects the respiratory system and can be lethal in large doses.

"As we do a post-mortem and find out exactly what happened, we'll know more about the levels that were released, but at this point in time, it doesn't appear that there was any danger to anyone in the community." 

Earlier in the day, a Level 3 emergency was declared, and people were told to stay indoors with their doors and windows closed.

A yellow plume was visible above the Vale complex, roads around the smelter were closed and cars were being directed away from the area.