Gas boilers, gas fires, cookers, water heaters and engines all burn carbon compounds such as natural gas, oil, wood or petrol. If an appliance is faulty, its flue is blocked or the room is poorly ventilated, there may not be enough oxygen and incomplete combustion happens.
This causes two main problems. First, carbon monoxide is made. It is poisonous, colourless and has no smell, so it can build up in a room without being noticed. Second, soot (carbon) is made. Soot coats the appliance, can block the flue so waste gases cannot escape, reduces efficiency and stains walls. In engines it leaves as black smoke that can cause breathing problems.
The risks are reduced if appliances are serviced every year by a qualified engineer, vents and flues are kept clear, and a carbon monoxide alarm is fitted. A yellow or orange flame instead of a blue one in a gas appliance is a warning sign. Incomplete combustion also wastes fuel, because less energy is released.