A man has been arrested after what emergency services described as a "potential hazardous materials incident" at Heathrow that saw Terminal 4 of the airport evacuated.
The check-in area of the terminal was also closed in the incident on Monday, after around 20 people reported injuries.
The Metropolitan Police confirmed in an update on Tuesday that a cannister of what is believed to be CS spray was discovered in a police search.
Police said the reported injuries were the result of irritation caused by the spray.
On Tuesday a 57-year-old man was arrested on suspicion of possession of a firearm (CS spray) and causing a public nuisance.
A Met spokesperson said: "On Monday, 8 September at 16:56hrs police were called to a potential hazardous materials incident at Heathrow Airport, Terminal Four.
"Specialist officers attended the scene alongside the London Fire Brigade and London Ambulance Service and conducted a thorough search of the area.
"Following the search, Met officers located a cannister of what is believed to be CS spray. It is thought that this substance caused a reaction to those within the airport.
"Around 20 people reported to paramedics after the (believed) CS spray caused irritation. It has been confirmed that the spray did not cause any life-changing/threatening injuries.
"Following urgent enquiries, a 57-year-old man was arrested on suspicion of possession of a firearm (CS spray) and causing a public nuisance on Tuesday, 9 September.
"He remains in police custody.
"The incident is not being treated as terrorism related. The investigation remains ongoing."
