WHITING | Residents throughout Lake County contacted their local police, fire and environmental agencies Wednesday night and Thursday morning complaining about a gas odor in the air.

After several hours, the odor was identified as a light hydrocarbon gas that had leaked from a unit of Whiting's BP Refinery.

During the startup of the Process Unit 11 pipestill at the BP Whiting Refinery, a heat exchanger developed a leak, which resulted in a light hydrocarbon being released to an internal refinery cooling water system, said refinery spokesman Tom Keilman.

The leak occurred Wednesday evening. It was isolated Thursday morning after the refinery began receiving complaints of an odor as a northwest wind carried the gas from the lakefront refinery toward inland areas.

"In the level associated with the leak, it is not dangerous," Keilman said. "It dissipated relatively quickly. The leak has been stopped."

Keilman identified the light hydrocarbon gas as intermediate virgin naphtha, and said Hammond Environmental Director Ron Novak was notified of the situation.

However, Novak said it was the Hammond Department of Environmental Management that just after noon pinpointed the refinery as the source of the odor emission, and then contacted the refinery's Environmental Superintendent Linda Wilson.

"We called them," he said. "They didn't call us. If they had, it would have eliminated activity by us and given our Fire Department the information it needed to answer calls about the odor."

Munster officials also reported some calls Wednesday night, from The Times among them, but knew of no cause for the odor.

Early Thursday, switchboards at 911 centers throughout Lake County lit up with a host of calls reporting the smell of natural gas.

The reports sent NIPSCO crews scrambling to see if any of their operations were the cause of the odor, according to NIPSCO spokesman Michael Suggs

Municipalities from throughout Northwest Indiana received calls, as did some Illinois communities. About 75 complaints were received by the city of Gary.

Similar complaints came in November and spanned from Crete to Crown Point. Officials were not able to determine the nature of the odor at that time.

-- Times Staff Writers Lauri Harvey Keagle and Deborah Laverty contributed to this report.