Firefighters were called to reports of a smell of chemicals at a bank and offices on Wimbledon Hill Road in Wimbledon.
Twelve members of staff left the building before the Brigade arrived. They were all assessed at the scene and three men and a woman were taken to hospital by London Ambulance Service crews.
Firefighters carried out a sweep of the property but found no elevated readings. The cause of the incident is believed to be fumigation of the building which had taken place overnight.
The Brigade was called at 1231 and the incident was over for firefighters at 1422. Two fire engines from Mitcham and Wimbledon fire stations and two fire rescue units from Bethnal Green and Lewisham fire stations attended.