Thirty people evacuated from a block of flats on Bramlands Close in Clapham Junction this morning, while firefighters investigated a suspected carbon monoxide leak.
A woman from the affected flat which is on the first floor, was treated at the scene by the London Ambulance Service suffering effects of carbon monoxide.
Firefighters used detection and monitoring equipment to sweep the eighteen floor building and the ground floor plant room was ventilated before the scene was handed to the building management and the gas authority.
Two fire engines from Battersea and Clapham fire stations attended the incident.
The Brigade was called at 1026 and the incident was over at 1331.