Two people were taken to hospital after a substance believed to be CS gas was set off at a bar.
West Midlands Ambulance Service was called to Nu Bar in Kidderminster, Worcestershire, at about 01:30 BST on Saturday.
A 29-year-old man from the town was arrested on suspicion of possessing a weapon for the discharge of a noxious liquid or gas and assault.
He has been bailed until 28 April, West Mercia Police said.
The ambulance service said paramedics had assessed four people who had mild symptoms and two were discharged.
The other two, who were taken to hospital with non-serious injuries thought to be caused by inhaling the substance, have been released, police said.