At least 25 people were treated on scene and two were hospitalized in a reported chemical incident at London City Airport, the fire brigade said.
"Three fire engines and a range of specialist appliances have been called to reports of a chemical incident," the brigade said in a statement. "Around 500 members of the public and staff have been evacuated and there are reports of a number of passengers feeling unwell."
Authorities were called to the scene around 4pm local time.
Firefighters and police conducted two complete sweeps of the building, and the airport was declared safe and was reopened.
The officers, wearing protective equipment, did not detect elevated readings and the building was ventilated, the fire brigade said.
The London City Airport later tweeted that officers found what is believed to be CS gas spray and are investigating whether it the incident was caused by an accidental discharge.
Full evacuation @LondonCityAir after alarm
More Fire Brigade arriving at London City