A man has been arrested on suspicion of possession of CS gas and causing a public nuisance following the partial evacuation of Heathrow Airport on Monday night.
Hundreds of people were forced to leave Terminal 4 at about 17:00 BST on Monday, before being allowed back in three hours later.
No hazardous materials were found but the Metropolitan Police discovered a can of CS spray which it said “caused a reaction to those within the airport”. London Ambulance Service treated and discharged 20 people for “irritation”.
The incident is not being treated as terrorism related, Scotland Yard said, and the 57-year-old suspect remains in police custody.
In a statement, the Met said following a search officers had “located a cannister of what is believed to be CS spray”.
“Around 20 people reported to paramedics after the (believed) CS spray caused irritation,” the force said.
“It has been confirmed that the spray did not cause any life-changing/threatening injuries.”
CS gas or spray is a synthetic chemical which is an irritant and causes a burning sensation in the eyes and tearing.
It is used in the UK by police forces as a temporary incapacitant spray to subdue people who pose a risk.
An investigation is ongoing, police added.
On Monday, a Heathrow spokesperson said the airport reopened to passengers shortly after 20:00 and they were “very sorry for the disruption caused”.
Disruption to flights landing and departing from Terminal 4 appeared minimal during the evacuation, according to flight data.
