We're not going to go into the symptoms of carbon monoxide poisoning. You can find a good article on the nhs.uk web site. This is the link: http://www.nhs.uk/conditions/carbon-monoxide-poisoning/Pages/Introduction.aspx
Just to avoid any confusion: Carbon monoxide and carbon dioxide are very different. Carbon monoxide is the unwanted poisonous gas released as a by-product of bad combustion. Carbon dioxide is the gas we breathe out all day and all night.
Carbon monoxide is a poisonous gas which has no colour, taste or smell. It's normally a product of combustion of some sort and burning anything containing carbon can release this gas. People normally associate it with gas appliances or car engines but oil boilers, wood stoves and open fires can produce it, so can barbecues, candles, or silently smouldering furnishings where a match or cigarette has been dropped. Just about anything which burns can produce carbon monoxide. Unlike a wood stove, however, there may be very little obvious smell if flue product from a gas appliance spills into a room.
Carbon monoxide is also combustible; it can be burnt. In the UK, before the introduction of Natural Gas (then called North Sea Gas) in the 1970s, mains gas was a different product produced by heating coal. It was called Town Gas and contained a proportion of carbon monoxide, so the piped gas itself was poisonous. A gas leak could kill you by poisoning. It didn't need to be ignited!
When we breathe, oxygen molecules in air attach to haemoglobin molecules in our red blood cells. This enables the blood to carry oxygen to the cells of the body. Unfortunately, our haemoglobin "prefers" to connect to carbon monoxide molecules, preventing the oxygen from attaching. Because of this, if carbon monoxide is present as only a tiny proportion of the air we breathe, it can still preferentially bond to enough of our haemoglobin to starve the body of oxygen.
Babies and small children are particularly susceptible to carbon monoxide poisoning, as are pregnant women.
Normal combustion of gas in a boiler should produce very little carbon monoxide and the tiny amount which is produced should pass harmlessly out of the building with the flue gases. Normal combustion is also called clean combustion and generally produces a blue flame. There are sometimes tinges of red and mauve colours produced by metallic dust particles passing through the flame. These are harmless. If an old appliance is tapped sharply, the flames may temporarily turn bright orange, the colour of sodium street lights. Though it looks strange, this is harmless too and will disappear within a minute or less. The orange colour is caused mainly by iron (from rust) passing through the flames but, in open-flued appliances, some of the colour could come from sodium in the salt which comes from our skin and the skin of our pets. Sodium is a constituent of common salt.
In gas appliances, flames which are pale yellow may indicate a risk of poisoning. They are usually produced by incomplete combustion and a shortage of oxygen to the flame. This is most common in open-flued appliances which have not been recently serviced. (Open-flued appliances are chimney-type appliances.) The yellow is produced by soot particles glowing in the flame. Though soot is black, if it is heated up in a zone in a flame where there is no free oxygen, it glows yellow/white. That's how candle flames work. If these pale yellow flames touch a cooler surface, like the heat exchanger, they may deposit carbon as soot. The soot can build up and block the flue gas pathways through the heat exchanger. These pale yellow flames then also release carbon monoxide. With the heat exchanger blocked, the poisonous carbon monoxide cannot pass into the flue and instead spills out into the room.
If you see pale yellow flames in your gas boiler you should switch it off. Leave it switched off until it has been checked and passed as safe by a Gas Safe Registered engineer. However, even some blue flames produce too much carbon monoxide. To be safe, it's important to have your boiler or fire serviced every year by a Gas Safe Registered engineer.
The yellow flames produced by candles are something of a special case. If good quality candles are burnt in completely still air they produce very little carbon monoxide. When the flame is established and stable, there is a pool of liquid paraffin wax at the base of the visible wick. The liquid passes up through the wick and is evaporated by the heat of the flame. The highly combustible wax vapour starts to burn, producing soot particles (soot is a form of carbon). There is not enough oxygen in the middle of the flame for the soot particles to be completely burnt to harmless carbon dioxide. The heat of the flame makes these soot particles glow almost white, producing the light.
Huge numbers of glowing soot particles are produced and these pass out to the edge of the flame where there is an almost invisible zone called the mantle. In the mantle there is much more oxygen available and the carbon atoms in the soot rapidly combine with oxygen. First they attach to one atom of oxygen, producing carbon monoxide, then almost instantly to another, producing carbon dioxide. (Mono in monoxide meaning one oxygen atom; di in dioxide meaning two oxygen atoms.) If the flames are stable, in still air, very little carbon monoxide escapes the flame.
However, if the flame is disturbed and unstable (the old term was guttering), soot particles and carbon monoxide molecules are flicked out of the flame. Unstable candle flames in a confined space present a risk of carbon monoxide poisoning. You can sometimes see the soot particles coming off an unstable candle flame in the form of a streak of dark smoke. Over hundreds of years, the soot from unstable candle flames has turned the inside of many ancient churches black. In such a huge space however, the risk of carbon monoxide poisoning from candles was negligible.
We would always advise that an electronic carbon monoxide alarm is fitted in any room with an open-flued heating appliance. This also applies to open fires and log burners. Carbon monoxide alarms are freely available for under £20. They usually last for about 5 years and should then be replaced.
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