Two adults and three children, all under the age of five, have been treated in hospital for carbon monoxide (CO) poisoning.
Cornwall Fire and Rescue was alerted after a CO detector alarm sounded at a house in Saltash on Tuesday evening.
Crews evacuated the house and the family was taken to hospital suffering from headaches and nausea.
The colourless and odourless gas reduces the blood's ability to carry oxygen and can be fatal.
Carbon monoxide is produced by incomplete burning of carbon-based fuels, including gas, oil, wood and coal.
The fire service said gas boilers, stoves, wood burners, open fires and heating appliances could pose a real danger if they were not properly maintained.
The five family members were taken to Derriford Hospital in Plymouth. South Western Ambulance Trust said their condition was not thought to be life threatening.
Cornwall fire service said crews had been called to several incidents of CO poisoning recently.
"Carbon monoxide poisoning can have tragic results, but there are steps you can take to keep you and your loved ones safe," spokesman Mark Pratten said.
He advised people to buy carbon monoxide detectors for their homes and, like smoke alarms, test them regularly.
According to the Department of Health, there are about 50 deaths a year from CO poisoning, 200 non-fatal poisonings that require hospital admission and 4,000 visits to accident and emergency departments. Children and older people are particularly at risk.
Sheila Merrill, a public health adviser at the Royal Society for the Prevention of Accidents, said carbon monoxide could be given off by burning all types of fossil fuels and could have "very severe" consequences.
"We're pleased that a carbon monoxide detector alerted the family in this case to the presence of carbon monoxide in their property.
"To prevent carbon monoxide poisoning, we recommend the regular servicing of all fuel-burning appliances, using Gas Safe registered engineers for gas appliances, good ventilation, and the regular cleaning of chimneys and flues."