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could not be characterized by chlorine content alone. Chlorine content was formerly regarded by some
scientists as correlated with cancer risk. Recently, however, Aroclor 1254 was found to be a more
potent liver tumor inducer than 1260, which was only slightly more potent than 1242 (Brunner et al.,
1996). This suggests that both the number and position of chlorines are a more useful indicator of
cancer potency than total chlorine content.
US EPA designated three reference points for each range (see Table 4): a "high risk" point based on
studies of Aroclor 1260 and 1254, which give the highest observed potencies; a "low risk" point, based
on the study of Aroclor 1242; and a "lowest risk" point, based on the study of Aroclor 1016. The "high
risk" point is used for exposure pathways associated with environmental processes that tend to increase
risk, including early life exposure. The "low risk" point is used for exposure pathways resulting in
decreased risk. The "lowest risk" point is used in cases where congener or isomer analyses verify that
congeners with more than four chlorines comprise less than one-half percent of total PCBs.
Table 4.
Tiers of human potency and slope estimates for environmental mixtures
(from US EPA, 1996)
HIGH RISK AND PERSISTENCE
Central Upper-bound
ED10 LED10 slope slope Criteria for use
0.086 0.067 1.0 2.0 Food chain exposure
Sediment or soil ingestion
Dust or aerosol inhalation
Dermal exposure, if an absorption factor has been
applied to reduce the external dose
Presence of dioxin-like, tumor- promoting, or
persistent congeners in other media
Early-life exposure (all pathways and mixtures)
LOW RISK AND PERSISTENCE
Central Upper-bound
ED10 LED10 slope slope Criteria for use
0.38 0.27 0.3 0.4 Ingestion of water-soluble congeners
Inhalation of evaporated congeners
Dermal exposure, if no absorption factor has
been applied to reduce the external dose
LOWEST RISK AND PERSISTENCE
Central Upper-bound
ED10 LED10 slope slope Criteria for use
2.4 1.4 0.04 0.07 Congener or isomer analyses verify that
congeners with more than 4 chlorines comprise
less than 1/2% of total PCBs
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