Justification of an Air Quality Standard for Benzene

26. The Panel accept that benzene 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 thus have taken the view that an air quality standard can be set. In recommending a Standard, we have considered the evidence concerning risks of leukaemia in workers since there are no useful data available on the effects in humans of the exposure to the low benzene concentrations found in the ambient air. We consider that great uncertainties surround estimates of exposure in such cohorts, which make 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. The Panel have therefore adopted the pragmatic approach of recommending a target Standard that is as low as is reasonably practicable.
27. Consideration of the evidence has led the Panel to conclude that the increased risk of leukaemia in cohorts of workers exposed to 500 ppb of benzene over a working lifetime would be too small to detect in any feasible study. 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 500 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 leukaemia, young children and those with impaired defence mechanisms, who might be unduly sensitive, the Panel recommend that a further safety factor be applied. In the absence of any scientific evidence on interindividual differences we have chosen a factor of 10, analogous to factors used in regulatory toxicology for noncarcinogens. We thus arrive at a recommendation for an Air Quality Standard of 5 ppb, measured as a running annual average.
28. In recommending a running annual average of 5 ppb the Panel recognise that the current average concentrations of benzene to which the general public are exposed in the United Kingdom's air (which rarely exceed this concentration) present an exceedingly small risk to health.
29. Since benzene is a genotoxic carcinogen and since, in principle, exposure to such substances should be kept as low as practicable, the Panel further recommend a target Standard of 1 ppb running annual average. We recommend that the Government set a date by which this target Standard be achieved. Achievement of this Standard will ensure that ambient air is no longer the main source of individual exposure, even for non-smokers.