The Effects of Carbon Monoxide on Human Health

10. Unlike many toxic gases, carbon monoxide is both colourless and odourless and life-threatening concentrations can be breathed without giving any warning to the victim. The first sign of severe poisoning is loss of consciousness and further inhalation of high concentrations readily leads to death. These effects are due to the interference of carbon monoxide with the processes whereby oxygen is taken up by the body and utilised in the cells in the body. It does this both by interfering with transport of oxygen by red cells in the blood (by the formation of carboxyhaemoglobin, which substantially reduces the ability of the red cell to carry oxygen) and also by blocking essential biochemical reactions in cells. In those people who recover from accidental or deliberate poisoning by carbon monoxide, brain damage of a greater or lesser degree due to lack of oxygen is a common sequel.
11. The formation of carboxyhaemoglobin in the blood of people exposed to carbon monoxide has proved a convenient method of monitoring exposure and relating this to possible health effects. The amount present in the blood depends on both the level and duration of exposure, as well as on the rate and depth of breathing. Thus someone exercising, and so breathing more rapidly and deeply, will have higher levels than someone resting but exposed to the same concentration. Carboxyhaemoglobin levels are measured by analysis of blood or, indirectly, by measuring carbon monoxide in exhaled breath. These are expressed as a percentage of total haemoglobin. In normal unexposed people, levels below 1% are found. Non-smokers, exposed at rest to concentrations in the air of 25-50 ppm might be expected to show carboxyhaemoglobin levels of 2-3% after several hours. In practice most people are subjected to varying concentrations as they move about in streets and in and out of buildings, and they do not necessarily approach equilibrium values during the course of the day. Figure 4 illustrates calculated blood levels in response to varying concentrations of carbon monoxide in the air breathed. Smokers may have levels of 4% to as high as 15%,depending on the numbers of cigarettes smoked. Uptake of carbon monoxide from multiple sources, such as smoking and traffic, is not additive. Smokers who already have a carboxyhaemoglobin level above the equilibrium value that would be reached by breathing air in their surroundings are liable to act as "sources", breathing out carbon monoxide rather than absorbing more.
12. The Panel were primarily concerned with setting a Standard which protects against possible harmful effects on health of moderate environmental exposure, rather than with the consequences of accidental exposure to very high concentrations. We have considered evidence linking carbon monoxide to possible effects on the heart and on the brain, since these two organs are crucially dependent upon a high rate of oxygen consumption and might therefore be affected by any impairment of the mechanisms of oxygen delivery.

Effects on the heart

13. The general population includes many adults with disease of the coronary arteries which limits thesupply of blood to the muscle of the heart. Such people often develop chest pain, angina, when exerting themselves and are at increased risk of having heart attacks. They are therefore likely to be at particular risk if their oxygen transport is impaired, for example by carbon monoxide. Well conducted studies have shown that levels of carboxyhaemoglobin of 3-4% shorten duration of exercise needed to induce changes in the electrocardiogram record and to induce angina pain. Small changes have been detected at levels of carboxyhaemoglobin as low at 2%. It has been suggested that the increased risk of hardening of the arteries, arteriosclerosis, in cigarette smokers is due to their raised levels of carboxyhaemoglobin.
14. In healthy people, some experiments on volunteers have shown that inhalation of carbonmonoxide can lead to a reduction in their capacity for maximal exertion; the higher the carboxyhaemoglobin level, the greater the reduction. However, this effect is small and unlikely to be of concern save for competing athletes, and a recent study of persons with carboxyhaemoglobin levels rising to 6-9% has not demonstrated any, significant effect on exercise performance.

Effects on the brain

15. Blood flow to the brain can, in normal circumstances, increase as a reflex response to reduction in oxygen supply due to carbon monoxide exposure. However, it is unlikely that this mechanism would be effective in people whose brain blood supply has been impaired, for example by narrowing of the arteries due to arteriosclerosis. Studies of brain function in volunteers exposed to carbon monoxide have shown subtle changes when carboxyhaemoblogin levels exceed about 5%. These changes have involved functions which require sustained attention or performance, such as hand-eye -coordination and detection of infrequent events.
16. Studies on developing animals have shown that exposure to high concentrations of carbon monoxide, giving rise to levels of 15-25% carboxyhaemoglobin, have caused delays in behavioural development and impaired brain function.

People at special risk from carbon monoxide

17. From the above, it is likely that people who already have a disease affecting the delivery of oxygen to heart or brain are likely to be at particular risk if these delivery systems are further impaired by carbon monoxide. That this is the case in people with angina is well established, but it is probable that others with severe heart and lung disease and with anaemia will also be unduly susceptible. The foetus, young infants and the elderly may also be susceptible. The decreased birth weight of babies born to mothers who smoke has been attributed to transfer of carbon monoxide across the placenta and a consequent reduced foetal oxygen supply. Studies have predicted significant changes in foetal oxygenation as a result of maternal carboxyhaemoglobin levels of about 5%. Such levels would be attained as a result of prolonged exposure to 30 ppm of carbon monoxide. The main known effects of carbon monoxide, in relation to concentration of carboxyhaemoglobin in the blood, are shown in Table 4.  

Table 4 Human health effects of exposure to carbon monoxide

Blood carboxyhaemoglobin levels (%)	Observed health effects
2.5 - 4.0	Decreased short-term maximal exercise duration in young healthy men
2.7 - 5.1	Decreased exercise duration due to increased chest pain (angina) in patients with ischaemic heart disease
2.0 - 20.0	Equivocal effects on visual perception, audition, motor and sensorimotor performance, vigilance, and other measures of neurobehavioural performance
4.0 - 33.0	Decreased maximal oxygen consumption with short-term strenuous exercise in young healthy men
20 - 30	Throbbing headache
30 - 50	Dizziness, nausea, weakness, collapse
over 50	Unconsciousness and death