Detectives are searching for a container and its mysterious contents following an altercation in Burnley.
Police were called to a disturbance on Harold Street in the town at shortly before 7-30pm yesterday.
During the altercation a 42-year-old man suffered an injury believed to have been caused by an unknown substance which was thrown at him.
He was taken to hospital for treatment and has now recovered from the effects, although he remains in hospital for a suspected fractured cheekbone.
While dealing with the victim, a number of emergency service personnel and hospital staff also required treatment for the effects of the substance but these symptoms subsided quickly and none required hospital admission.
Two men aged 26 and 29 from Burnley have been arrested and enquiries are on-going.
Det. Insp. Tim McDermott, of East CID, said: “We are investigating the disturbance and our enquiries are on-going at this time.
"I would like to reassure local residents that there is no evidence of any ongoing threat to the wider public.
"However, at this stage, whatever substance was used and the container it was in remain outstanding.
"I would therefore urge anyone who finds any container in the area of Harold Street that they are concerned about to leave it in situ and contact police. There will be a high visibility police presence in the area.
“If you have any information about this incident, please call us on 101 quoting incident reference 1511 of June 20th or independent charity Crimestoppers on 0800 555 111.”