A gas buildup at a residential Mt. Dennis building Tuesday afternoon could have been deadly, according to Toronto’s Deputy Fire Chief.

Emergency crews were called to the building on Emmett Avenue near Jane Street and Eglinton Avenue around 2:30 p.m. after receiving reports of a sulfer-like smell.

Toronto Fire made the decision to immediately evacuate the third floor of the building and told other residents to stay in their units.

According to Deputy Fire Chief Jim Jessop, preliminary information suggested that a cleaning agent was placed in the plumbing system on the 20th floor and somehow ended up pooling on the third floor.

“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.”

Jessop said that a number of people fainted as a result of the gas leak, but nobody required transportation to the hospital.

Police initially said that a dog had been found deceased in the building, but Toronto Fire later confirmed the animal was alive and recovering from the incident.

According to police, 25 buses were sent to the area in order to shelter residents who have been told to evacuate the building.

Students at York Humber High School, which is located within walking distance of Emmett Avenue, were advised to stay away from the building.

Paramedics said that five ambulances and two buses have been dispatched to the scene. Toronto Fire has also dispatched 14 trucks to the area.

Residents were forced out of the building for about two hours before being allowed to return to their homes.