• 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)
Note: Cancer potency of PCB mixtures is determined by a tiered approach that depends on exposure matrix and route, type of congener present, and lifestage. See IRIS Summary for specific factors used to select appropriate slope factor. High risk and persistence: 2 per mg/kg-day; Low risk and persistence: 0.4 per mg/kg-day; Lowest risk and persistence: 0.07 per mg/kg-day.

Inhalation Unit Risk: 1 x 10 ^-4 per µg/m³ (Applies to inhalation of evaporated congeners)
Extrapolation Method: Linear extrapolation below LED10; route-to-route extrapolation from the oral slope factor
Tumor site(s): Hepatic
Tumor type(s): Liver hepatocellular adenomas, carcinomas, cholangiomas, or cholangiocarcinomas (Brunner et al., 1996; Norback and Weltman, 1985)
Note: For inhalation of an aerosol or dust contaminated with PCBs, see the IRIS Summary for information on the appropriate inhalation unit risk.