36
3.4. ANALYSES OF CONGENER TOXICITY
McFarland and Clarke (1989) explain how toxicity of some PCB congeners is
correlated with induction of mixed-function oxidases. Some congeners are described
as phenobarbital-type inducers, others as 3-methylcholanthrene-type inducers, and
some as having mixed inducing properties. The latter two groups most resemble
2,3,7,8-tetrachlorodibenzo-p-dioxin in structure and toxicity. Based on potential for
toxicity (some forms of toxicity, for example, neurotoxicity, may not be well represented)
and frequency of occurrence in environmental samples, 36 congeners of highest
concern were identified and classified (see table 3–3).
Table 3–3. PCB congeners of highest concern
Highest toxicity High toxicity Abundant in Potential
and abundance
and abundance environment for toxicity
ab c d
3–MC-type inducers: PB-type inducers: 18: 2,5,2'–TrCB 37: 3,4,4'–TrCB
77: 3,4,3',4'–TeCB 87: 2,3,4,2',5'–PeCB 44: 2,3,2',5'–TeCB 81: 3,4,5,4'–TeCB
126: 3,4,5,3',4'–PeCB 99: 2,4,5,2',4'–PeCB 49: 2,4,2',5'–TeCB 114: 2,3,4,5,4'–PeCB
169: 3,4,5,3',4',5'–HxCB 101: 2,4,5,2',5'–PeCB 52: 2,5,2',5'–TeCB 119: 2,4,6,3',4'–PeCB
153: 2,4,5,2',4',5'–HxCB 70: 2,5,3',4'–TeCB 123: 3,4,5,2',4'–PeCB
Mixed-type inducers: 180: 2,3,4,5,2',4',5'–HpCB 74: 2,4,5,4'–TeCB 157: 2,3,4,3',4',5'–HxCB
105: 2,3,4,3',4'–PeCB 183: 2,3,4,6,2',4',5'–HpCB 151: 2,3,5,6,2',5'–HxCB 158: 2,3,4,3',4',6'–HxCB
118: 2,4,5,3',4'–PeCB 194: 2,3,4,5,2',3',4',5'–OCB 177: 2,3,5,6,2',3',4'–HpCB 167: 2,4,5,3',4',5'–HxCB
128: 2,3,4,2',3',4'–HxCB 187: 2,3,5,6,2',4',5'–HpCB 168: 2,4,6,3',4',5'–HxCB
138: 2,3,4,2',4',5'–HxCB 201: 2,3,4,5,2',3',5',6'–OCB 189: 2,3,4,5,3',4',5'–HpCB
156: 2,3,4,5,3',4'–HxCB
170: 2,3,4,5,2',3',4'–HpCB
Pure 3-methylcholanthrene-type inducers and mixed-type inducers reported frequently in environmental samples.
a
Phenobarbital-type inducers reported frequently in environmental samples.
b
Weak inducers or noninducers reported frequently in environmental samples.
c
Mixed-type inducers not reported frequently in environmental samples, but toxicologically active.
d
Source: Adapted from McFarland and Clarke (1989).
U.S. EPA (1991) examined 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 could not be characterized by chlorine
content alone. Before adopting toxic equivalence factors (TEFs) for PCB congeners, it
was recommended to define other classes of PCBs and identify the mechanisms
involved. Criteria for developing TEFs were listed as (1) a demonstrated need, (2) a
well defined group of chemicals, (3) a broad base of toxicological data, (4) consistency