• A 1996 study found liver tumors in female rats exposed to Aroclors 1260, 1254, 1242, and 1016, and in male rats exposed to 1260. These mixtures contain overlapping groups of congeners that, together, span the range of congeners most often found in environmental mixtures. Earlier studies found high, statistically significant incidences of liver tumors in rats ingesting Aroclor 1260 or Clophen A 60 (Kimbrough et al., 1975; Norback and Weltman, 1985; Schaeffer et al., 1984). Mechanistic studies are beginning to identify several congeners that have dioxin-like activity and may promote tumors by different modes of action. PCBs are absorbed through ingestion, inhalation, and dermal exposure, after which they are transported similarly through the circulation. This provides a reasonable basis for expecting similar internal effects from different routes of environmental exposure. Information on relative absorption rates suggests that differences in toxicity across exposure routes are small. The human studies are being updated; currently available evidence is inadequate, but suggestive.
• This may be a synopsis of the full weight-of-evidence narrative.

Oral Slope Factor: 2 per mg/kg-day (high risk and persistence, upper bound)
Extrapolation Method: Linear extrapolation below LED10s
Tumor site(s): Hepatic
Tumor type(s): Liver hepatocellular adenomas, carcinomas, cholangiomas, or cholangiocarcinomas (Brunner et al., 1996 Norback and Weltman, 1985)

Inhalation Unit Risk: 1 x10 ^-4 per µg/m³ (Low risk and persistence; upper-bound unit risk)
Extrapolation Method: Linear extrapolation below LED10s
Tumor site(s): Hepatic
Tumor type(s): Liver hepatocellular adenomas, carcinomas, cholangiomas, or cholangiocarcinomas (Brunner et al., 1996; Norback and Weltman, 1985)