Gas concentration too low to harm public: Vale

The company's operations in Copper Cliff were in a planned maintenance period and washing the sulfiric acid reservoirs is a routine part of that maintenance, Kuzenko said.

When water interacts with the acid it creates NOx – an orange-yellow gas that can cause irritation to the eyes, nose, mouth and lungs at low levels, and serious health complications at high concentrations.

The cleaning procedure normally only creates a small, and manageable, amount of the gas, Kuzenko said. But for reasons the company cannot yet explain, the gas was in a higher concentration than normal, and eventually traveled beyond Vale's property.

“The release itself should not have happened, and the cause will be investigated,” Kuzenko said.

It was at that point, around 6 a.m. Thursday, that Vale sounded its alarms in Copper Cliff to alert the community about the noxious gas.

People were warned to stay in their homes, close their windows, and turn off any air intakes, including air conditioners.

By 8:26 a.m., the gas had dissipated to safer levels, and the company called off its level 3 warning.

Kuzenko said the highest reading at the perimeter of Vale's property found a NOx concentration of 0.5 parts per million.

The Ministry of Labour considers levels below three parts per million, over an eight-hour period, to be safe.

“We were well below the limits that create a risk to personal safety,” Kuzenko said.

Vale will collaborate with the United Steelworkers to investigate the incident, find out what the root causes were, and determine whether the emergency response was appropriate.

While Vale sounded its alarms at 6 a.m., nitrogen dioxide readings from the Ministry of the Environment, which are measured in downtown Sudbury, showed levels started to increase by 10 p.m. Wednesday, and reached a peak of 20 parts per billion by 2 a.m. Thursday.

Kuzenko said the Ministry of the Environment's timelines were inconsistent with Vale's own measurements of the gas.

In an incident unrelated to the NOx gas release, Kuzenko said four workers received minor injuries from exposure to sulfuric acid during planned maintenance procedures.

She said the workers were all treated and no one was seriously harmed.

When asked about evacuation procedures for Copper Cliff residents in case of an emergency, Kuzenko said Vale is working with the city to put a plan in place, but added an evacuation would not have been an appropriate response Thursday morning, since people were asked to avoid being outside.