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ST. LOUIS (KSDK) - Authorities say the strong odor many people reported downtown Saturday is harmless.

A cloud of Mercaptan was released somewhere in Illinois, a fire department spokesman said, and it passed through the downtown area. Mercaptan is part of the chemical added to natural gas and propane that makes it easier to detect a gas link, since both fuel sources are typically colorless and odorless.

Though a material safety data sheet lists Mercaptan as highly toxic, the St. Louis fire department assured NewsChannel 5 that there were no dangerous levels in St. Louis.

Authorities took readings inside seven different buildings and found that they were all at safe levels.

No evacuations were needed.