Fire crews were called to a house in South Avenue, Southall after a suspected carbon monoxide leak.
Four adults and four children left the property before the arrival of the brigade, all suffering with effects of carbon monoxide poisoning. All eight were treated at the scene by London Ambulance Service crews.
Firefighters isolated the gas supply and ventilated the property before leaving the premises in the hands of the gas authority.
The Brigade was called at 1240 and the incident was over at 1413. Fire crews from Southall and Ealing fire stations, along with a fire rescue unit from Wembley Fire Station, attended the scene.