(CNN) A leak in a container holding hazardous chemicals created a massive chemical spill early Thursday and sent 37 people to the hospital, including seven in critical condition, said authorities in Beach Park, Illinois.

One of the patients in critical condition is a firefighter.

A preliminary investigation found that the containers, which held anhydrous ammonia, were being towed by a tractor on Green Bay Road in Beach Park, a village north of Chicago, Sgt. Christopher Covelli of the Lake County Sheriff's Office said.

Anhydrous ammonia, a colorless gas with pungent fumes, can cause unconsciousness and even death when inhaled. The chemical is commonly used by farmers as an agricultural fertilizer, according to the Centers for Disease Control and Prevention

The spill created a plume of chemical smoke, Covelli said.

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