Guidelines for Metals and Metalloids in Ambient ... - ARCHIVE: Defra

Expert Panel on Air Quality Standards
A3
The quantitative risk assessment approach
314. The data from occupational exposures to chemical carcinogens tell us
that at a certain (high) exposure concentration there is a particular
level of additional risk for exposed workers. The quantitative risk
assessment approach seeks to extrapolate the occupational data to
lower concentrations and therefore to quantify the additional risk of
cancer at environmentally meaningful concentrations. There are many
ways in which this extrapolation can be made depending upon the
assumed mechanism of carcinogenesis. This can lead to very different
estimates of risk at low exposure concentrations, which is one of the
weaknesses of the Quantitative Risk Assessment approach. In practice
the simplest assumption and that used by the WHO is that the risk of
cancer is zero only at zero concentration of that compound and
increases proportionately with concentration. This is referred to as a
linear model without threshold (i.e. there is no level below which no
effect is thought to occur).
315. This method leads to an estimate of risk referred to as the incremental
unit risk estimate which is defined by WHO as “the additional lifetime
cancer risk occurring in a hypothetical population in which all individuals
are exposed continuously from birth throughout their lifetimes to a
concentration of 1 µg/m 3 of the agent in the air they breathe”.
316. The WHO is frank about the limitations to this approach (WHO, 2000)
and one of its concerns shared by the Department of Health’s
Committee on Carcinogenicity is that the method can give a spurious
impression of precision when extrapolating data which already have
inherent uncertainties over orders of magnitude in concentration. The
WHO (2000) point out that the quantitative risk estimates should not
be regarded as being equivalent to the true cancer risk, but to represent
plausible upper bounds which may vary widely according to the
assumptions on which they are based. Nonetheless, the method is very
widely used and, for example, for benzene, yields an excess lifetime risk
of leukaemia at an air concentration of 1 µg/m 3 of 6 x 10 -6 (WHO, 2000)
– i.e. 6 in 1 million, meaning that six people in every million exposed to
benzene at that concentration will develop leukaemia in a lifetime. The
WHO does not directly utilise this unit risk factor to recommend a
numerical air quality guideline but calculates the concentrations of
airborne benzene associated with particular levels of excess lifetime
risk. In the case of benzene, the excess lifetime risk of one in 10,000
occurs at a concentration of 17 µg/m 3 , a lifetime risk of one in 100,000
at 1.7 µg/m 3 and a lifetime risk of one in 1 million at 0.17 µg/m 3 .
317. The quantitative risk assessment method uses the same occupational
cancer data as a starting point and inherently very similar assumptions
to the EPAQS method. However, unlike the EPAQS method it does not
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