About 500 people were taken from the east London airport at 16:00 BST after some passengers felt unwell, London Fire Brigade (LFB) said.
Ambulance staff treated 26 patients at the scene, taking two to hospital.
The airport was declared "safe" at about 19:00. LFB said "no elevated chemical readings" had been found.
The airport said it had since reopened, although passengers have been warned to expect disruption as flights will be staggered.
In a statement, the 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."
