Police were called shortly after 16:00hrs on Friday, 21 October to London City Airport to reports of a number of passengers and staff feeling unwell.
Officers, the London Ambulance Service and London Fire Brigade attended.
A number of people were treated at the scene.
The airport was evacuated and was reopened at approximately 19:00hrs.
A subsequent search of the airport led to the discovery of what is believed to be a CS gas or spray. Whilst the cause of the incident has not yet been confirmed, officers are investigating if it was the result of an accidental discharge of this cannister.
At this early stage officers are investigating whether it may have been discarded by a passenger prior to check-in.
There have been no arrests and enquiries continue.