Justification of an Air Quality Standard for 1,3-Butadiene

23. The Panel accept that 1,3-butadiene is a genotoxic carcinogen and that therefore no absolutely safe exposure level can be defined. Nevertheless, for practical purposes we believe that a concentration may be proposed at which the risks are exceedingly small and unlikely to be detectable by any practicable method. We have taken the view, therefore, that an Air Quality Standard can be set. In recommending a Standard, we have considered evidence concerning the risks of lymphomas and leukaemias in cohorts of workers exposed to 1,3-butadiene, since there are no useful data available on the effects on humans or animals of the very low concentrations found in the ambient air. Considerable uncertainties surround the estimates of exposure in such cohorts, making the accurate extrapolation of risk from high occupational to low ambient exposure impossible and which give to such formal quantitative risk assessments a misleading appearance of precision. This general view has also been taken by the Department of Health's Committee on Carcinogenicity.
24. Consideration of the evidence has led the Panel to conclude that it is unlikely that increased risks oflymphomas and leukaemias would be detectable by any practicable means in cohorts of workers exposed to 1,000 ppb of 1,3-butadiene over a working lifetime. In order to take account of the difference between working lifetime (approximately 77,000 hours) and chronological life (about 660,000 hours), the figure of 1,000 ppb has been divided by 10. Further, since it is reasonable to suppose that the population includes people, such as those exposed to other causes of lymphomas and leukaemias, and individuals who might be particularly sensitive to 1,3-butadiene, the Panel have recommended that a further safety factor be applied. In the absence of any evidence on interindividual differences we have chosen a factor of 10, analogous to factors used for this purpose in regulatory toxicology for non-carcinogens. We thus arrive at a value of 10 ppb.
25. The Panel noted the weaknesses of the exposure data in epidemiological studies of persons exposed in industry. We therefore reviewed the studies in animals on the carcinogenicity of 1,3-butadiene. It is clear that the recent laboratory data provide good evidence that 1,3-butadiene is a genotoxic carcinogen. Further, the biochemical pathways by which it is activated also exist in humans. The Panel were unanimous in agreeing that the levels of genotoxic carcinogens in the environment should not be allowed to rise.
    We also noted that the concentrations of 1,3-butadiene measured in urban air of the United Kingdom have not exceeded 1 ppb as a running annual average. We therefore recommend 1 ppb, measured as a running annual average, as the Standard, and are of the view that, at this concentration, any risks to the health of the population are exceedingly small.
26. The Panel debated the principle, discussed in the Benzene report, that concentrations of genotoxic carcinogens should be subject to a Standard that is as low as practicable. In the case of benzene this had led to the recommendation of a target Standard. We have adopted a different approach for 1,3-butadiene for the following reasons: (a) by recommending a Standard at a value close to the current levels we have incorporated an additional ten fold safety margin below the figure derived from human epidemiology; (b) as we have noted, there are uncertainties inherent in the human data; (c) there are, as yet, only limited monitoring data; and (d) the data are insufficient to determine what effects recent pollution controls will have on the environmental levels of 1,3-butadiene. For these reasons we believe that the Standard, and the need for a target Standard, should be reviewed by the Panel within five years. In the interim, as a precautionary measure, we recommend that the Government's programme of air pollution controls should aim to ensure that, at any one site, the Standard is not exceeded and that there is an overall decline in measured 1,3-butadiene concentrations.