A nitrogen dioxide leak at a smelter in Sudbury, Ont., had warning sirens blaring and emergency officials urging people nearby to stay indoors on Thursday morning.

No injuries were reported. The leak was contained within a few hours, but at the height of the emergency, police warned that nitrogen dioxide can be fatal in large doses.

Here's more about nitrogen dioxide (NO2) and its risks:

  • Nitrogen oxide is one of the one of the nitrogen oxides (NOx), a group of air pollutants produced from combustion processes.
  • NO2 is a common pollutant usually formed by burning fossil fuels, including coal, oil and gas. About 70 per cent of the gas emitted in Ontario comes from vehicles, the province said in 2012.  
  • It has a pungent and irritating odour and is a major component of smog. It irritates the lungs, decreases lung function and increases susceptibility to allergens for people with asthma, Health Canada says.
  • Extremely high-dose exposure can cause excess fluid in the lungs and lung injury, the U.S. Environmental Protection Agency said.
  • People usually have to worry about nitrogen dioxide over a long period of time at low levels — for instance, from gas stoves at home. 
  • Prolonged exposure can lead to coughing and wheezing.

In the case of the Vale mine emergency Thursday, authorities had been warning people to stay indoors with their houses sealed and air intakes like air conditioners turned off, but the warning was declared all clear at 8:26 a.m. ET and they were told they could go outdoors.

Emergency and Vale personnel say the wind speed and weather mean there's "no risk of additional substance travelling beyond company boundaries."

Officials were reminding people not to call 911 for information.