Justification of an Air Quality Standard for Carbon Monoxide

18. It is clear from the foregoing that the highest exposures in the ambient outdoor air likely to be experienced by non-smokers will occur from peak concentrations close to heavy motor traffic, especially during cold still conditions in winter. Regular smokers are unlikely ever to be affected by such conditions, since their blood levels of carboxyhaemoglobin are already higher than can be reached by breathing polluted outdoor air (see Table 1), and therefore the recommendation for an Air Quality Standard for carbon monoxide is intended to limit exposure of non-smokers. After reviewing the large amount of evidence on the health effects of carbon monoxide, the Panel have concluded that the people most susceptible to exposure to carbon monoxide are those with angina and disease of the coronary arteries. The evidence suggests that the lowest level of carboxyhaemoglobin at which effects can be detected in such people lies between 3 and 4%. The Panel have therefore concluded that ambient atmospheric concentrations of carbon monoxide should be such that the concentration of carboxyhaemoglobin in the blood of people breathing that air over a prolonged period should not exceed 2.5%, thus allowing a safety margin.
19. It is possible to relate blood carboxyhaemoglobin levels to atmospheric concentrations of carbon monoxide by use of mathematical formulae. This relationship shows that carboxyhaemoglobin concentrations would be kept below 2.5% when breathing the following concentrations of carbon monoxide at maximum levels of activity:

    10 ppm for 8 hours
    25 ppm for 1 hour
    50 ppm for 30 minutes
    87 ppm for 15 minutes.

20. It is apparent from the results of air monitoring referred to above that an 8-hour standard provides the tightest control. The Panel therefore recommend an Air Quality Standard for carbon monoxide in the United Kingdom of 10 ppm. In order to ensure that maximum concentrations are not missed by arbitary averaging periods, we recommend that a running ² 8-hour average be used for the Standard.