Police officers were called to a Northampton youth club last night after reports a group outside had been sprayed in the face with an unidentified substance.
Officers were called to the Spring Boroughs youth club in Tower Street, Northampton, at about 8.10pm.
On arrival, it was established 10 people had been exposed to the substance and were complaining of irritation to their eyes and faces.
It is believed the four offenders used an alkaline based irritant which quickly evaporated once sprayed.
Staff from Northamptonshire Fire and Rescue Service and East Midlands Ambulance Service also attended the scene.
Three people were dry-decontaminated and all attended Northampton General Hospital and have since been discharged, not requiring any further treatment.
The offenders left the scene in a small black vehicle.
Following the incident, a 19 year old man has been arrested on suspicion of actual bodily harm.
Witness or anyone with information about this incident can contact police on 101 or Crimestoppers anonymously on 0800 555 111.
Alternatively you can use their anonymous online form at www.crimestoppers-uk.org