Water used to douse the Sunday morning blaze has mixed with the plant's chemicals, touching off fears that nearby Lake Ontario could become contaminated.
Initial Environment Ministry tests showed xylene, a possible carcinogen, had been released into the air at 5 percent the acceptable level, said spokesman Gord Haugh.
"A breathable amount (is what) we'd be concerned about," said Haugh. "At this time, it's not dangerous for anyone in the area."
Fumes from xylene can cause dizziness, nausea and hallucinations. The chemical, used in refining of high octane gasoline, can kill if it builds to a concentration of 10,000 parts per million in the air.
Ministry officials monitored the air for toxins and wind shifts throughout the day on Monday using a special unit.
"I've asked for that unit to be out there for at least a week to monitor the air for people in the community," said Environment Minister Dan Newman, who said the ministry is also analyzing the soil and water from the runoff and lake.
Meanwhile, 90,000 liters of water used to douse the fire, which began for unknown reasons at the U.S.E. Hickson Products plant, were vacuumed and disposed of.
Of great concern was keeping several tanks -- some holding solvents and mineral oils -- smothered with foam and water to stop them from exploding.
The fire's high temperature did destroy some chemicals which couldn't withstand the heat, said Haugh.
The fire marshal is investigating the cause of the fire, which appears to have been contained before its smoke reached residential areas.