"Two complete sweeps of the airport building were carried out jointly by firefighters and police officers both wearing protective equipment," the London Fire Brigade said in a statement. "No elevated readings were found and the building was ventilated, searched and declared safe."
Authorities believe the incident might have been caused by a CS gas spray -- a type of tear gas -- but are still investigating. "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 canister," a statement says.
"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."
The airport was evacuated after a number of passengers reported feeling unwell, police said.
Firefighters received reports of the chemical incident about 4 p.m. (11 a.m. ET), the brigade said. By 7:25 p.m., the airport had reopened.
Officials haven't said what caused the incident.
The city's ambulance service said it had treated 27 patients at the scene for difficulty breathing. Two were taken to the hospital, the service said.
Alistair Snowie, who spoke to CNN on the phone from a medical tent where he was being treated, said he was waiting in line to check in for his flight home to Stirling, Scotland, when everyone started coughing.
"It all happened very quickly," he said. "The alarm didn't go off at first, but we all just ran. Nobody knew what was going on."
Hours later, Snowie said he was still wondering what made him start coughing.
"We still don't know what we breathed in," he said. "Don't feel too great."
London City Airport focuses on business travelers heading to destinations in the UK, Europe and the United States. It's in east London.
Far smaller than Heathrow and Gatwick, the major airports in London, the airport serves more than 40 destinations, according to its Facebook page
Evacuated passengers shared stories of frustration and concern on social media.
Milly Blunt posted photos of passengers waiting outside. The evacuation, she said, canceled her flight home to visit family in Edinburgh, Scotland.
Boxer David Haye said he wouldn't be able to make it to a fight in Scotland.
"City Airport got evacuated when everyone started coughing uncontrollably," he said on Twitter.