Six people were checked by ambulance staff after a substance believed to be CS spray or similar was let off in a pub.
The incident took place in a basement at The Moon Under Water in Wolverhampton, the fire service said.
Fire crews, sent to the scene at about 20:35 GMT on Tuesday, said there had been "a strong chemical smell" that was "effectively" dealt with.
No-one was seriously hurt, the fire service said.
The gas was released in a stairwell from the ground floor to the basement that leads to toilets, it stated.
A substance believed to be CS spray or similar was let off and a cordon was put in place while the area was made safe, West Midlands Police said.
The fire service said the pub had been evacuated by staff before it arrived.
After investigations, in which no smells or abnormal readings were detected, crews ventilated the premises.
The six members of the public who had complained of breathing difficulties and nausea were checked by the ambulance service and their conditions improved with fresh air, the fire service said.
A controlled release of about 800 customers from the Grand Theatre opposite on Lichfield Street, who had been to watch a performance of Fame, was conducted by police to safely move people out of the cordon area, it added.
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