Carbon monoxide: Families in Ammanford and Llangollen poisoned
A family, including a six-month-old baby from Carmarthenshire, and an elderly couple from Llangollen, Denbighshire, were rescued after being poisoned in their homes.
They were all treated but the fire service stressed the importance of getting boilers checked.
The gas is known as the silent killer because it cannot be seen or smelt.
Fire crews from Ammanford were called to the property in Betws on Monday after the property's CO alarm was activated.
The baby, two adults and two children aged under four were taken to hospital by ambulance suffering from mild carbon monoxide poisoning.
Jason Cadman, Mid and West Wales crew manager, said: "The combi boiler that was responsible for this incident was less than three-years-old. It is very important not to become complacent even with new boilers, as this incident shows they can still pose a danger."
Station manager Mark Davies added: "I cannot emphasis enough the importance of installing a carbon monoxide detector and ensuring you get your appliances checked and serviced regularly."Recovering
In a separate incident earlier this month, pensioners John Lightfoot, 84, and his wife Beryl, 79, were found collapsed at their farmhouse in Llangollen.
The air vents to their home had been blocked by heavy snow and exhaust fumes from a power generator engulfed the ground floor.
They were discovered unconscious on the floor by their son, Gwyn, but an ambulance was unable to drive through the snow to reach them.
Six rescue workers and a doctor were towed by a tractor to get them to the farm.
The couple are now recovering.
Symptoms of carbon monoxide poisoning include headaches, dizziness, nausea, breathlessness and losing consciousness.