A number of people fainted as a result of a gas buildup in a Mt. Denis apartment on Tuesday afternoon that could have easily been deadly, Deputy Fire Chief Jim Jesssop says.

Emergency crews were first called to the building on Emmett Avenue near Jane Street and Eglinton Avenue at around 2:30 p.m. to investigate a sulphur-like smell.

Jessop said that once crews arrived on scene, a decision was quickly made to elevate the call to a Level 3 Hazmat call, which is the highest possible response.

He said that the third floor of the building was then immediately evacuated and residents in other parts of the building were told to shelter in place.

At that time, firefighters began ventilating the building and working to determine the source of the gas buildup.

Jessop said that preliminary information suggests that some type of cleaning agent was deposited into the plumbing system on the 20th floor and “for whatever reason ended up pooling on the third floor.”

He said that’s where hydrogen sulphate readings approaching 100 parts per million were recorded.

“We are very, very, very fortunate that we didn’t have serious injuries,” he said. “These are readings that candidly we have not seen in a long, long time.”

Police initially said that a dog was found deceased inside the building, however Jessop said that the animal was just knocked unconscious as a result of being exposed to the gas and was revived upon being taken outside.

Paramedics said that five ambulances and two ambulance buses were dispatched to the scene to help assess residents, though nobody was actually transported to hospital.

Residents on the third floor who were forced out of the building due to the gas weren’t let back in for about two hours.