Firefighters were called to a health club in London's Canary Wharf financial district on Friday after a mix of chemicals caused high levels of fumes and vapours.
About 900 people were evacuated. Photos and footage from the scene showed workers outside a high-rise building while crews handled the incident.
“Crews have carried out a sweep of the building and found elevated readings,” said Station Commander Dave Hill.
“Firefighters have ventilated the building and are monitoring the levels of fumes. We have evacuated around 900 people from the building as a precaution.”
Canary Wharf is home to multinationals such as Barclays, JP Morgan Chase and Deutsche Bank.
It is just a few miles from the Queen Elizabeth Olympic Park in east London, where 200 people were evacuated from the Aquatics Centre two weeks ago following a chlorine gas leak.