A man has been arrested after a "potential hazardous materials incident" trigged an evacuation of Terminal 4.
Specialist officers attended the scene where 21 people required medical treatment following "mass hysteria".
Emergency services recovered a canister believed to hold CS spray - otherwise known as tear gas - during a search of the airport.
The Metropolitan Police said in a statement: "It is thought that this substance caused a reaction to those within the airport."
A man has been arrested after a 'potential hazardous materials incident' trigged an evacuation of Terminal 4
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A 57-year-old man had been arrested on suspicion of possession of a firearm (CS spray) and causing a public nuisance.
The suspect remains in police custody and officers say the incident is not being treated as terrorism related.
Police closed Terminal 4 while emergency services dealt with reports of a "possible hazardous materials incident."
London Ambulance Service confirmed it treated 21 patients, with one taken to hospital while the others were discharged at the scene.
Last night, the check-in area inside Terminal 4 was evacuated "as a precaution whilst firefighters conducted operations", London Fire Brigade said in a statement.
It continued: "Specialist crews were deployed to carry out an assessment of the scene.
"No elevated levels of hazardous materials were discovered by the brigade."
A spokesperson for the Metropolitan Police said yesterday: "On Monday September 8 at 4.56pm, police were called to a potential hazardous materials incident at Heathrow airport, Terminal 4.
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"Specialist officers attended the scene alongside the London Fire Brigade and London Ambulance Service and conducted a thorough search of the area. Around 20 people reported an injury.
"None were deemed to be life-threatening or life-changing. Inquiries are ongoing."
A Heathrow Airport spokeswoman said yesterday evening: “Emergency services have confirmed Terminal 4 is safe to reopen and we are doing everything we can to ensure all flights to depart as planned today.
"We are very sorry for the disruption caused, the safety and security of our passengers and colleagues is our number one priority.
Emergency services recovered a canister believed to hold CS spray - otherwise known as tear gas - during a search of the airport
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"We encourage passengers to check with their airline for the latest information about their flight this evening and our colleagues will be on hand into the night to assist.”