4.6. Strengths and limitations
Several authors have proposed different potency equivalency
factors using data from studies in different species, strains and routes
of administration. The PEF used in this paper, i.e. the one proposed by
the EPAQS, 1999, was based on intrapulmonary carcinogenicity in
rats, as were also other approaches widely used in the literature, such
as the modified PEF values given by Nisbet and Lagoy (1992).
However, some of the recent PEF methodologies proposed (e.g. Larsen
and Larsen, 1998), which contain PEF values for high potency
carcinogens, are based on studies using various routes of administration
(oral, pulmonary and skin application). Therefore a certain level
of uncertainty is introduced in the PEF methodology to allow for the
extrapolation from routes of exposure other than inhalation and the
extrapolation from different species to humans. This uncertainty is
reflected in the fact that the PEF values of high potency carcinogens
vary by more than 10-fold depending on the chosen methodology,
which greatly affects the contribution of individual compounds to the
carcinogenic potential of the PAH mixture, as discussed in Section 4.5.