Hamburg airport in Germany was briefly evacuated and at least 50 people were injured by an unknown substance believed to have been in the air conditioning system.
Both terminals was cleared and flights were halted after people complained of eye irritation and coughs. Hundreds of passengers waited outside as officials tried to establish the cause.
The airport reopened about an hour later and flight operations resumed.
The alarm was raised after reports of a strong smell in the airport and fire crews set up treatment centres for people complaining of irritation.
An unknown substance was later found in the security check area, Reuters quoted a police spokeswoman as saying.