MISSISSAUGA, Ont. -- Four firefighters were injured Wednesday following an explosion and wall collapse during a massive blaze at a warehouse in Mississauga, Ont.

Fire Chief John McDougall said one of the four men, who was taken to a Toronto hospital, was in critical but stable condition on Wednesday afternoon.

McDougall said two others suffered broken bones when something exploded as they were entering the building on Wednesday morning, while the fourth man was treated for smoke inhalation.

McDougall said he spoke to one of the men in hospital and said the firefighter told him he had "no idea how far he was thrown" by the blast.

While unable to give personal information on the men, McDougall said all four are "seasoned" and experienced firefighters at the mid-career level, and that all had been conscious and talking in hospital.

Mississauga Mayor Hazel McCallion said firefighters had no idea what they were up against when they arrived at the scene because the building was vacant the last time it was inspected.

"We don't know of any occupancy permit that was granted by the city for the building to be occupied with the type of material," McCallion said.

"The explosions caused the building to collapse and therefore seriously injured our firefighters," she said.

McDougall said the firefighters didn't have any information on what was in the building and were faced with an explosion.

"An explosion is a very, very serious matter," he said, adding that the injuries three of the firefighters suffered were "blast injuries."

Peel police Const. Thomas Ruttan said the warehouse was a storage facility for imported goods and that butane lighters and aerosol cans of insect spray stored there caused explosions inside.

While there were no concerns about toxicity in the smoke coming from the building, Ruttan said people in the area were advised to avoid breathing in smoke if possible.

McDougall said while the fire had been contained Wednesday afternoon, crews were still on the scene, putting out hot spots.

Huge clouds of black smoke and numerous large fireballs lit the pre-dawn sky at the scene of the blaze just north of Pearson International Airport.

Witnesses reported hearing small explosions and a series of loud popping sounds after the fire began before 5 a.m.

Airport officials said flights were not affected by the thick smoke across the area but there were numerous delays involving street traffic, commuter trains and local transit service.

The cause of the fire is under investigation.