The British news agency reports that over 500 people have been evacuated from the facility because of the scare.
The London Fire Brigade (LFB) has evacuated about 500 people from the east London airport at 16:00 BST after some passengers felt unwell, BBC reports.
According to the reports, the ambulance staff treated 26 patients at the scene and took another two to hospital. The situation seems to have been contained now as the airport was declared “safe” at about 19:00.
BBC quoted LFB as saying that “no elevated chemical readings were found” there.
A statement from fire service said: “Two complete sweeps of the airport building were carried out jointly by firefighters and police officers both wearing protective equipment.
“No elevated readings were found and the building was ventilated, searched and declared safe.”
An eyewitness Chris Daly, 35, from Southend, told the BBC he had just landed from BA flight from Glasgow when he heard the fire alarms.
“When we got into the airport terminal building we could hear the fire alarms going on, then there were announcements in three different languages saying this is a fire alarm and the crew were directing us at the baggage carousel to evacuate the building.
“We are now all standing on the tarmac under the wing of the airplane because it has started to rain.
“I can see some plane circling in the sky but nothing else has landed. No-one has really told us anything. We saw some fire crew going inside to do a sweep.
“I saw something on Twitter about it being a possible chemical incident. I’m not particularly worried because we are outside now.”