All organic matter contains carbon which has the chemical symbol C. When burnt in the presence of adequate air, fuels such as wood, peat, coal, oil and gas produce carbon dioxide in the chemical process. There is about 21% oxygen in air. The chemical symbol for oxygen is O though in air it exists as O₂. Carbon dioxide has the chemical formula CO₂. If there is inadequate ventilation and therefore inadequate oxygen, carbon monoxide (CO) is formed.
In the lungs, oxygen from inspired air ‘binds’ with haemoglobin, the iron containing molecule in red blood cells. When transported to the tissues, haemoglobin gives up oxygen and binds to the carbon dioxide formed by cellular metabolism. When this blood returns to the lungs, the carbon dioxide is given up, and exhaled, and oxygen again taken up. Arterial blood is bright red, venous blood is dark red. Normally, there is no problem with the uptake or release of either oxygen or carbon dioxide.
However, carbon monoxide has a much greater ‘affinity’ for haemoglobin than oxygen; and once bound to haemoglobin, carbon monoxide is released much more slowly than carbon dioxide would be. So, if someone remains in the presence of carbon monoxide, the levels of this in the blood will continue to rise unless action is taken.
Carbon monoxide is colourless, odourless and tasteless; we have no natural means of recognising its presence.
The symptoms of carbon monoxide poisoning depend on whether the exposure is acute or chronic, and the level of exposure. In acute poisoning the brain and nervous system, and the heart are earliest affected as these organs are most dependant on oxygen. Symptoms include headache, confusion, fast heart rate and arrhythmias, dizziness, fits and unconsciousness. Untreated, in the presence of continuing levels of CO, poisoning will be fatal. Carbon monoxide poisoning produces symptoms common to many other illnesses; it is a ‘great imitator’. Carbon monoxide is said to produce a ‘cherry red’ colouration; but this is not a reliable clinical sign in the living. Rather the diagnosis is made, or at least suggested, from the clinical history, thinking about the possibility, and confirmed by a measurement of CO in the blood.
The treatment is to remove the individual from the source of CO, and administering high concentrations of oxygen through a face mask. Associated symptoms are treated as necessary in the appropriate way. Above atmospheric concentrations of oxygen in a hyperbaric chamber have not been convincingly shown to be of benefit.
CO poisoning is dangerous because we cannot detect it, the initial symptoms are insidious and confusing, and proper treatment needs the condition to be actively thought of. About 40 people die yearly in the UK from CO poisoning.
Prevention is vital. To ensure adequate ventilation, rooms should not be hermetically sealed, gas barbecues should only be used out of doors, flues should be cleaned, and appliances regularly checked by an approved expert. Further, the fitting of a CO alarm is very strongly advised (in addition to a smoke alarm).
Robert Campbell is a retired surgeon.