All gas appliances for heating and for production of hot water can produce carbon monoxide (CO) in some circumstances which are potentially fatal. Their safe use depends on the unit's extractive properties for combustion gases. Three factors that can contribute to a dangerous situation are: (1) incorrect installation: when the heater is placed in an area of less that the minimum required volume and deprived of ventilation; (2) incorrect use: when the unit is in prolonged use, despite being designed only for intermittent use; and (3) lack of maintenance: over time the unit's heat exchanger become progressively blocked, reducing the quality of combustion and increasing the amount of CO produced.
When carbon-based fuels such as natural gas, coal, wood, oil and petroleum are burned in an adequate supply of oxygen, the carbon dioxide gas (CO2) is given off. In the absence of sufficient oxygen, combustion is incomplete and carbon monoxide (CO) is produced instead. When breathed into the body, CO reacts with haemoglobin in the red blood cells, forming a compound that prevents the haemoglobin transporting oxygen to body tissues and the brain. Breathing in even small amounts of carbon monoxide can cause headaches, sickness and vomiting. With higher exposure the victim feels unable to move and can lose consciousness. Just two or three percent of CO in the air is enough to kill someone in half a minute.
Carbon monoxide poisoning from heating appliances accounts for the largest number of poisoning admissions in some hospitals. It is mostly a winter and domestic phenomenon. In Belgium it kills 150 people each year; 50 alone are in Brussels, where another 500 a year are poisoned badly enough to need hospital treatment, which may include sessions in a pressurized oxygen chamber. The 24-hour poisons hotline in Brussels received around 1,000 calls each work relating to CO poisoning. Immigrants and young people, who tend to live in poorly equipped and maintained properties built before 1920, are most at risk. More recently at risk, however, are people in homes with improved insulation features, such as double-glazing, shutters and roof cladding, that reduce the fresh air supply and inhibit combustion gases from dispersing. Weak wind and temperature inversions increase daily risks.