Firefighters were called to a smell of chemicals at a health club on Cabot Square in Canary Wharf.
A mix of chemicals had caused high levels of fumes and vapour in the building. One man and one woman were treated at the scene by London Ambulance Service crews.
Station Commander Dave Hill, who was at the scene, said: “Crews carried out a sweep of the building and found elevated readings.
"Firefighters ventilated the building and monitored the levels of fumes until there were no elevated readings.
“Around 900 people were evacuated from the building and a 25m cordon was put in place as a precaution."
The Brigade was called at 0908 and the incident was over for firefighters by 1224. Two fire engines Millwall and Poplar fire stations and two fire rescue units from Bethnal Green and Euston fire stations were at the scene.