Firefighters were called to a suspected carbon monoxide leak at a shop on Gallions Road in Charlton.
Firefighters wearing breathing apparatus have used specialist equipment to carry out a systematic sweep of the building and found elevated readings of carbon monoxide. Crews have carried out further sweeps to check for no further elevated readings. They have also completed a ventilation of the building.
Around 35 people left the building before firefighters arrived and crews have assisted colleagues from the London Ambulance Service to treat a number of patients at the scene.
The Brigade was called at 0835 and Control Officers mobilised two fire engines and two fire rescue units to the scene. The incident was over for firefighters by 1350.