Three people have been taken to hospital with suspected carbon monoxide poisoning from a faulty boiler at a farmhouse in Morriston, Swansea.
Mid and West Wales Fire and Rescue Service said an elderly man and woman and boy aged 16 were treated by fire crews until an ambulance arrived.
A spokeswoman said the alarm was raised just before 05:30 GMT.
The householders were advised not to return to the property until the boiler had been checked, she said.
She added that the occupiers had previously reported sickness, believed to have been caused by carbon monoxide poisoning.
“On arrival, crews used gas monitoring equipment to confirm the presence of the gas, which was being omitted by a coal-fired boiler,” she said.
‘Extremely serious consequences’
“Two elderly occupiers, a male and a female, and one 16-year-old were given oxygen therapy by fire crews until the arrival of ambulance personnel.
“Two cats which were present in the property died as a result of the gas.”
Fire officer Alan Sherman, who attended the incident, said: “Please make sure all domestic heating appliances are checked and maintained on a regular basis.
“Faulty appliances can have extremely serious consequences as this incident shows.”