German authorities temporarily closed the airport in the northern city of Hamburg and evacuated hundreds of passengers after people reported an unusual smell, respiratory ailments, and watering eyes.
Some flights were diverted to other airports after the incident on February 12 and hundreds of passengers had to wait outside the terminals in freezing temperatures.
Firefighters designated special areas outside the airport building where physicians were examining those affected.
"The airport was completely closed" at around 12:30 p.m. local time, said Maik Lewerenz of the federal police office at the airport said.
He said the airport was reopened at 1:45 p.m.
"There was an odor problem in the central air-traffic control area, where passengers and hand luggage are checked," he said.
"Some people complained of stinging eyes and coughing," he said, adding that "the cause is unknown so far".
The Bild newspaper reported that an unknown substance was thought to have entered the hall though the air-conditioning system and that fire brigade experts were seeking to identify the substance.