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The range of potency values is summarized in Table 2. It is based primarily on the range for Aroclors
1260, 1254, 1242, and 1016 in female Sprague-Dawley rats (Brunner et al., 1996), but considers the
other available studies also.
Table 2. Range of human potency and slope estimates (from US EPA, 1996a)
ED10
(mg/kg-d)
LED10 a
(mg/kg-d)
Central slope b
(mg/kg-d) -1
Upper-bound slope c
(mg/kg-d) -1
Highest observed potency 0.086 0.046 1.2 2.2
Lowest observed potency 2.4 1.4 0.04 0.07
a 95% lower bound on ED10.
b Computed as 0.10/ED10 and rounded to one significant digit.
c Computed as 0.10/LED10 and rounded to one significant digit.
The new upper-bound slopes are lower than the previous estimate of 7.7 per mg/kg-d average lifetime
exposure (U.S. EPA, 1988). The previous estimate was derived from female rats in the Norback and
Weltman (1985) study; the new estimate from the same study is 2.2 per mg/kg-d. This difference is
attributable to three factors, each responsible for reducing the slope by approximately one-third: a rat
liver tumor reevaluation (Moore et al., 1994), use of the new cross-species scaling factor (U.S. EPA,
1996b), and not using a time-weighted average dose. The difference between the highest observed
new upper-bound slope (2.2 per mg/kg-d) and the lowest (0.07 per mg/kg-d) is entirely attributable to
the availability of tests on several commercial mixtures (Brunner et al., 1996). This 30-fold range in
potency reflects differences in commercial mixture composition, as reflected in Table 3.
Table 3.
Typical composition (%) of some commercial Aroclor PCB mixtures
(from US EPA, 1996a)
1016 1242 1248 1254 1260
Mono-CBs 2 1 — — —
Di-CBs 19 13 1 — —
Tri-CBs 57 45 21 1 —
Tetra-CBs 22 31 49 15 —
Penta-CBs — 10 27 53 12
Hexa-CBs — — 2 26 42
Hepta-CBs — — — 4 38
Octa-CBs — — — — 7
Nona-CBs — — — — 1
Deca-CB — — — — —
U.S. EPA (1991) examined the toxic effects, including cancer, of four structural classes: dioxin-like
PCBs, ortho-substituted PCBs, hydroxylated metabolites, and sulfonated metabolites. Different
mechanisms were discussed for dioxin-like and other PCBs. It was concluded that congener toxicity
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