Devon and Somerset Fire and Rescue Service said crews were sent to a property in Hill View, Ottery St Mary, after a CO detector alarm was activated.
The family of seven was initially treated at the scene by paramedics before being taking to hospital.
Crews said the CO reading at the house was “higher than the safe level”.
Carbon monoxide is often referred to as a “silent killer” because people cannot see, smell, touch or hear it.
If inhaled, it prevents red blood cells from carrying oxygen and can often be fatal.
CO POISONING SYMPTOMS
- Nausea and vomiting
- Stomach pain
- Sore throat and dry cough
- Breathing difficulties
- Loss of consciousness
CO is caused by the incomplete burning of fuels, such as coal, wood, petrol and oil and the Department of Health estimates it is responsible for about 50 deaths a year.
The gas is believed to have caused the deaths of four people and four dogs in two separate incidents in Cornwall in February.
Last week a family of three and their pet dog were found dead in their park home in Camborne, where CO levels considered to be 20 times the fatal dose were recorded, while a woman and three dogs died at a house in St Ives earlier in the month.
The Royal Society for the Prevention of Accidents said all fuel-burning appliances should only be installed by a Gas Safe engineer and should be serviced every year.