Five people were taken to hospital with suspected carbon monoxide poisoning when fumes from a leaking boiler infected a block of flats in Surbiton.
Residents in flats in Tolworth Rise North called emergency services after they started to feel ill in the early hours of Sunday morning.
Twelve people were evacuated as firefighters swept and ventilated the premises, identifying gases from a heater, used to heat the water in the flats, as the source.
A spokesman from London Fire Brigade said: "Fire fighters arrived at the scene at 12.48am on Sunday morning after calls from residents complaining of feeling ill.
"Twelve people were evacuated and four taken to hospital with suspect carbon monoxide poisoning. The source of the fumes appears to be a leak from boiler."
He advised residents to buy carbon monoxide detectors saying the fumes could be deadly.
A spokeswoman for LAS said: "We were called about 11.30pm on Sunday, July 15, and sent two ambulance crews, a hazardous areas response team and duty officer staff.
"We treated five patients. Three adults and two teenagers were all taken to Kingston hospital feeling unwell, dizzy and nauseous."