Preliminary investigation showed a tractor was towing containers holding anhydrous ammonia when the containers leaked, causing toxic plumes of the chemical to be released into the air, according to the Lake County sheriff's office, Illinois.
"At this point, all transports are non-life-threatening," the sheriff's office tweeted.
Law enforcement and firefighters have carried out a door-by-door check of residents in the immediate area in Beach Park neighborhood, some 70 km north of downtown Chicago, to ensure those closest to the scene are safe.
All local schools have been closed on Thursday as a precaution, said authorities.
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