Article
Carbon monoxide Poisoning
Each year, in the UK alone, about 50 people die and many more are injured from carbon monoxide (CO) poisoning.
I have investigated a number of cases.
It is the most likely toxic substance that an individual will come into contact with at home or in work.
Domestic fires, boilers, car exhausts and barbeques are common sources. Inefficient domestic appliances, blocked flue provisions and inadequate ventilation are common causes.
Formation
CO which is colourless, odourless, tasteless and very toxic, is formed due to incomplete combustion of substances containing Carbon.
When fossil fuels (coal, oil, gas, wood, charcoal, etc), some of which are known as hydrocarbons, are burned completely or efficiently in air the principal products of combustion, i.e. water vapour and carbon dioxide are formed; neither is toxic.
In an efficient combustion process, with excess quantities of air/oxygen, the hydrogen combines with oxygen first because of its lower molecular weight to form water vapour. The carbon is then oxidised to CO which further combines with oxygen to form carbon dioxide and only trace elements (very small amounts) of CO remain.
If there is incomplete or inefficient combustion, say due to lack of sufficient air or temperature or turbulence or time, then not all of the CO is further oxidised to carbon dioxide and significant concentrations of CO can be present.
Each fuel has different combustion characteristics. If we consider natural gas (mostly methane) and it is burning as a pre-mixed flame, i.e. the gas and air have been mixed in correct quantities prior to arriving at the flame front, the resulting flame will be blue with traces of orange or yellow at the tip(s). If it is burning inefficiently then there will be greater orange or yellow and CO will be formed.
Poisoning
Under normal circumstances when a person is breathing in air the oxygen enters the blood stream to combine with haemoglobin, contained in the red blood cells, to form oxyhaemoglobin which conveys oxygen to the organs which need it, e.g. heart and brain.
When CO is inhaled it combines with the haemoglobin to form carboxyhaemoglobin which prevents the oxygen from getting to the heart and brain. Poisoning which can result in anything from feeling ill to death follows, depending upon the amount of CO in the atmosphere and duration of exposure. CO binds about 200 – 240 times more strongly with haemoglobin than does oxygen. The concentration levels of CO in the atmosphere which are sufficient to cause injury or death are very low, typically less than 0.1% or 1,000 parts per million (ppm).
Typical Levels of CO
Source |
CO parts per million (ppm) |
Fresh air |
0.06 – 0.5 |
Urban air |
1 – 50 |
Heavily trafficked intersection |
50 –100 |
Car exhaust |
30,000 – 80,000 |
Cigarette smoke |
20,000 – 60,000 |
Cigarette smoke filled room |
2 – 16 |
