German authorities said the leak of a corrosive substance through the air conditioning system at Hamburg airport today was not an attack, adding that police were investigating the incident that forced the brief closure of the facility.

Firefighters and ambulances were rushed to the scene after some 50 people at one terminal complained of eye irritation and breathing difficulties.

"I want to explicitly deny ... that this was a terrorist attack. As far as we know it was at no period considered a terrorist attack," Hamburg fire department spokesman Torsten Wessely told a news conference in the northern city.

"A pepper spray-like canister has been found during our investigative searches which might be the source," he added.

He said the substance was spread through the facility's central air conditioning system.

Lightly dressed passengers were rushed outside the terminal in almost freezing temperatures, leaving their coats and jackets behind in the hurry. They were allowed back in shortly after.

A spokeswoman for the airport said flights resumed a couple of hours later.

According to live flight tracking website, flightradar24.com, some flights were diverted to other airports after the incident.

Maik Lewerenz of the federal police office at the airport said "a bad smell has spread in the security area, where passengers and their carry-on luggage are checked.

"A number of people complained about eye and respiratory irritation," he said.

Hamburg airport is Germany's fifth largest, catering for just over one million passengers a month.