GRANBY -- "I heard the detonation of the fire, like a really big bang," said Tommy Forand, who works nearby. "We have big windows in the office so I saw everything."

Thick, black smoke was visible for kilometres, but while the fire raged for hours no other buildings were in danger of catching fire, because the distance to other structures was too far.

It took several hours to put out the flames, and when the work was done a building lay in ruins.

Firefighters were worried the smoke could have been toxic, and they told residents they may have to evacuate their homes and head to emergency shelter at nearby schools if the wind started blowing smoke toward a residential area.

The plant makes styrofoam and had sulfuric acid, benzene, ammonia and other dangerous products on hand. However before any evacuation was ordered, workers at the plant were able to inform authorities that the smoke was not toxic.

"We prepared to make an evacuation and the city has the way to take care of people, but we didn't need to do [it]. We had a plan prepared for that. We started our plan, but we didn't need to use it," said Granby Fire Chief Pierre Lacombe.

When the blast started the fire about a dozen employees were inside Aldex Chemicals.

"Nobody was injured, everyone got out safely," said police spokesperson Guy Rousseau.

Officials from the provincial environment ministry rushed to the scene to ensure the smoke was not dangerous. Air tests confirmed that the air was safe.

"I can confirm there's no chemicals that would affect the population in the area. So we're sure of that," said Christian Blanchette.

There remain concerns that runoff from the firefighting efforts could affect the nearby Yamaska river, so officials will be testing soil and water sources to ensure everything is safe.

On Thursday work turned to removing barrels of dangerous chemicals from the factory, with the company's owner trying to convince providers to take them back for storage.

Failing that the materials will be brought to a secure location.

However the roof of one building collapsed, and debris from that section will have to be hauled away before the containers of hazardous material can be transported to a safe facility.

The president of the company told CTV Montreal he was shocked by the disaster, and is very glad that none of his employees, some of whom he hired decades ago, were hurt.

The cause of the explosion is not known.