Residents of southeast Sarnia were ordered to close their windows and stay indoors Friday following a benzene vapor leak at Imperial Oil.

Company officials said the release occurred shortly before 4 p.m. after a storage tank collapsed in an area of the plant near the St. Clair River.

Sarnia police issued a shelter-in-place order an hour later for residents south of Wellington Street and west of Mitton Street, advising them to stay indoors and to close all windows and air intakes.

Benzene is a component of gasoline and a known carcinogen.

An emergency CVECO Code 8 was issued, which notifies of a potential problem in Chemical Valley. A Code 6 followed, which calls for full traffic control in response to a toxic vapor release. Vidal Street was closed in both directions.

Ferguson said the Code 6 was a precaution. It was lifted at 8:47 p.m.

The cause of the tank collapse still is being investigated. Ferguson said company firefighters quickly contained the vapors by dousing the tank with foam, and remained on standby. No one was injured during the incident, she said.

Imperial Oil spokeswoman Julie Ferguson was not sure about the age of the tank and when maintenance most recently was done.

"We have a very good tank (and equipment inspection) program," she said.

Sarnia's emergency co-ordinator Cal Gardner said Sarnia's Emergency Management Primary Control Group met at police headquarters in response to the vapor release.

Gardner said Imperial Oil officials provided regular updates on reported benzene levels, which were detected as high as 0.4 parts per million. The Ontario Ministry of Labour recently set the occupational exposure limit at 1 part per million, based on an eight-hour work day.

"The readings are low," Gardner said.

At the time of the shelter-in-place order, southeast winds were reported at 11 kilometers per hour.

Jeffrey Friedland, St. Clair County Homeland Security Emergency Management Director, said his office was told of the leak and it notified the Marysville Fire Department, since the wind was blowing in that direction, and Port Huron city officials.

Marysville officials were given the option of using emergency management's equipment to monitor the air, but chose not to. He said if the leak posed a significant threat, residents would have been notified immediately by radio and TV broadcasts.

Friedland said there are hazardous gases throughout the county, whether they come from Sarnia or are being shipped through the county.

"We have gases all over this county," he said. "That's one of the reasons we do have a hazardous material response team - the volume of chemicals."

Sarnia residents were alerted by radio and an emergency TV broadcast. A Port Huron police dispatcher knew nothing about the release about 5:30 p.m. when notified by the Sarnia Observer newspaper.

"We haven't heard anything. Nobody has been called, and I've been here all day," dispatcher Shannon Rees said.