Around 40 revellers had to leave a bar when an unidentified gas caused them to cough and struggle for breath.
The incident happened last night at a cocktail bar called Dr Inks in Stockton-on-Tees, Cleveland.
An arcade containing several pubs was evacuated and cordoned off.
Police and fire crews have not yet identified the mystery substance.
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Drinkers described how the 'slow-moving' gas began to drift through the air, leaving people struggling to breathe. Eventually around 40 people were affected by coughing fits before crowds began to leave the bar.
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Police are analysing CCTV to establish where the gas came from and if anyone was responsible for releasing it.
Nobody was hospitalised although ambulance crews were sent to the scene.