Police were called to “a potential hazardous materials incident” at Heathrow Airport
Confusion remains over how 21 people were injured at Heathrow airport on Monday night, when police were called to “a potential hazardous materials incident”.
A medical source told the PA news agency “there was a substance” and the injuries were not caused by people rushing from the scene but London Fire Brigade (LFB) insisted “no elevated levels of hazardous materials were discovered”.
The BBC initially reported being told by a police source it was a “mass hysteria” event but removed that detail from later updates. Mass hysteria, or mass psychogenic illness (MPI), is a social phenomenon where a group of people experiences physical symptoms of illness, such as fainting or nausea, without an identifiable organic or external cause. It is often triggered by stress, anxiety, a perceived threat, or cultural factors and spreads through a population via social contagion
Even though LFB found no elevated levels, common triggers for such alerts include:
- Chemical spills (such as cleaning products, fuel, or industrial chemicals transported through airports).
- Aerosol releases (pepper spray, deodorants, disinfectants) which can irritate the eyes or respiratory system.
- Food-related reactions (strong-smelling food, allergens, dry ice in shipping containers).
Environmental/technical factors
- Ventilation failures can lead to a buildup of fumes or low oxygen, triggering symptoms.
- Dust or construction work in terminals has previously caused coughing or breathing issues in enclosed spaces
The check-in area inside Terminal 4 was evacuated “as a precaution whilst firefighters conducted operations”, LFB said in a statement. It went on: “Specialist crews were deployed to carry out an assessment of the scene. No elevated levels of hazardous materials were discovered by the brigade.”
A London Ambulance Service spokesperson said a total of 21 patients were treated. One was taken to hospital while the others were discharged at the scene.
A spokesperson for the Metropolitan Police said: “On Monday September 8 at 4.56pm, police were called to a potential hazardous materials incident at Heathrow airport, Terminal 4.
“Specialist officers attended the scene alongside the London Fire Brigade and London Ambulance Service and conducted a thorough search of the area. No trace of any adverse substance was found.
“Around 20 people reported an injury. None were deemed to be life-threatening or life-changing. Inquiries are ongoing.”
At 8.27pm, the airport said on X that emergency services had confirmed the terminal was “safe to reopen” and it was doing “everything we can to ensure flights depart as planned today”.
The LFB said it was in the process of standing down its response, and that the cause of the incident remained under investigation. Earlier on Monday evening, the airport had advised passengers not to travel to Terminal 4, while National Rail Enquiries said trains were unable to call at the terminal because of the incident.
Transport Secretary Heidi Alexander said she was monitoring updates.
According to the airport’s website, flights from Terminal 4 were largely unaffected – with only a few delays expected. Footage posted on social media showed passengers with their luggage waiting outside the airport, with some wrapped in foil blankets.
London’s Heathrow is the UK’s busiest airport.
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