Fifteen people have been treated for breathing difficulties after an unknown substance was sprayed inside three business premises.
Emergency services were called to The Arcade in Upper Gornal, Dudley, shortly after 19:00 BST on Tuesday.
One person was taken to hospital, but none have suffered any lasting effects, West Midlands Police said.
Forensics experts have been examining the scene to establish the nature of the substance.
West Midlands Ambulance Service said it was first called to Pizza Hut before it became clear the substance had also been sprayed in Tesco and McDonald’s.
Fourteen people were treated by paramedics and discharged at the scene while one man was taken to Russells Hall Hospital for further assessment.
Police have urged anyone with information to come forward.