A family of four, including two children, are being treated in hospital after carbon monoxide poisoning.
South Western Ambulance Service was called to a property in Canworthy Water, Launceston at about 02:00 GMT after the occupants felt unwell.
The fire service was also alerted and a gas detector indicated high readings of the poisonous gas.
Cornwall Fire and Rescue Service ventilated the property with a fan and fitted two smoke detectors.
The condition of the family is not known.