Four children and two adults have been taken to the Royal Cornwall Hospital at Treliske suffering from suspected carbon monoxide poisoning at their home.
All six, from the St Stephen area of St Austell, were given oxygen in the back of ambulances before being taken to hospital.
The clear odourless gas can prove deadly and usually comes from faulty gas appliances or poor ventilation.
Ambulance workers who went into the house to treat the occupants had to be checked and cleared of contamination.
The fire brigade is understood to have taken measurements of carbon monoxide inside the property and passed that information on to the hospital.