A man and a woman from Devon have been taken to hospital with carbon monoxide poisoning.
Fire crews were called to Tiverton Road in Cullompton at 11 o'clock last night. They detected the gas in the end terrace house, isolated the mains and ventilated the property.
Neighbouring homes were found to be free of carbon monoxide.
Cornwall Fire Service is warning about the dangers of carbon monoxide poisoning after three people died at their caravan home in Camborne at the weekend.
John and Audrey Cook, their daughter Maureen and their pet dog, died within minutes - after inhaling fumes from a faulty heater.
Fire officers say having gas appliances checked every year and fitting a carbon monoxide alarm could save lives.