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

Introduction
10. Once a NOAEL (or LOAEL) has been identified, safety factors,
also termed uncertainty factors, are used to account for inter species
differences and susceptible groups in humans in the data from which
the air quality standard or guideline is set. These provide a margin of
safety between the NOAEL / LOAEL derived from measurements of a
small number of humans or animals and permissible levels of exposure
for the whole population. Expert judgement is used to determine the
number and size of safety factors to be applied. For the metals covered
in this report, three human health end points (lung cancer, irritation /
inflammation and hypersensitivity) were thought to have specific
implication for the applications of safety factors. These considerations
are outlined in Section 1.4.2.
11. Where effects arise as a result of prolonged exposure, it is appropriate
to adjust the exposure data to allow for continuous exposure in
ambient air. An additional factor may therefore need to be applied
where the original study from which the NOAEL / LOAEL was
derived was not for an entire lifetime, for example from occupational
data where people were only exposed at work.
12. The idea that complete protection can be offered by a standard implies
that there is a threshold of effect for the pollutant being considered –
a level below which normal health is not affected. Although for many
substances the concept of a threshold seems to be broadly true, there
are some chemicals for which no threshold of effect exists. This group
includes genotoxic carcinogens. These compounds are known to cause
cancer as a result of interacting with DNA – the material inside the
nucleus of cells that carries genetic information.
13. For substances, such as genotoxic carcinogens, where no threshold has
been observed for adverse effects, EPAQS uses a similar approach to
that it has taken to substances with a threshold. The Panel identifies a
starting point at which it believes the risks associated with that
exposure are exceedingly small. Safety factors are then applied to this
value in the usual way. A broader discussion of the approach used by
EPAQS and other organisations in setting air quality guidelines for
chemical carcinogens can be found at Appendix 1.
14. An alternative approach to that adopted by EPAQS, and one used by
the WHO and some other countries is based on quantitative risk
assessment. This approach seeks to extrapolate the occupational data
to lower concentrations and therefore to quantify the additional risk of
cancer at concentrations likely to occur in the environment. There are
many ways in which this extrapolation can be made depending upon
the assumed mechanism of carcinogenesis. It should be noted that
quantitative risk estimates should not be regarded as being equivalent
to the true cancer risk, but represent plausible upper bounds which
may vary widely according to the assumptions on which they are based
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