Hamburg airport in Germany was briefly evacuated and at least 50 people were hurt by an irritant gas that was released in the air.
Both terminals was cleared and flights halted after people complained of eye irritation and breathing problems.
Firefighters later found an empty cartridge, thought to have contained pepper spray, at the scene.
They say a “prankster” may have released the pepper spray, which then spread through the ventilation system.
Pepper spray is legal, and readily available, in Germany.
The alarm was raised on Sunday after reports of a strong smell in the airport.
Fire crews set up treatment centers for people complaining of irritation.
Hundreds of passengers waited outside as airport terminals were evacuated and officials tried to establish the cause.
The airport reopened about an hour later and flight operations resumed.
Officials say they are not treating the incident as a terrorist act.