There is sufficient evidence that commercial PCB mixtures containing 60% chlorine by weight are carcinogenic in rats. Aroclor 1254 and otherlower chlorinated commercial PCB mixtures have a lower carcinogenic potential than the 60% chlorine mixtures. Although the evidence that PCBsare carcinogenic in rats is conclusive.
Human studies provide suggestive evidence that PCBs are carcinogenic. The carcinogenicity of PCBs in humans has been investigated inretrospective cohort mortality studies, which investigated cancer in exposed workers, and in casecontrol studies of environmental exposure thatexamined associations between serum or adipose tissue levels of PCBs and occurrence of cancer. Some of the mortality studies suggest thatoccupational exposures to PCBs were associated with cancer at several sites, particularly the liver, biliary tract, intestines, and skin(melanoma). There is no clear association between occupational exposures to PCBs and cancer in other tissues, including the brain,hematopoietic, and lymphatic. The hypothesis that environmental exposure to PCBs can cause breast cancer in humans is controversial and needs tobe further studied. A number of casecontrol studies have investigated possible associations between breast cancer and concentrations of PCBs inbreast tissue or blood in the general population. Breast adipose levels of total PCBs or individual congeners were increased in women withbreast cancer in some but not all studies. Other environmental exposure studies used serum PCB concentrations as the marker of exposure withblood samples taken after the diagnosis of breast cancer, or prospectively collected prior to diagnosis. None of the serum studies foundsignificantly different mean blood levels of PCBs in breast cancer cases and controls. There also were no significant associations between riskof breast cancer and serum PCBs in most of these studies, although some data suggest that risk may be increased in some subgroups ofpostmenopausal women. Many of the better designed studies were prospective, and none of the prospective studies found that PCBs were associatedwith the occurrence of breast cancer.
Department of Health and Human Services (DHHS) has stated that PCBs may reasonably be anticipated to be carcinogens. Both US EPA and theInternational Agency for Research on Cancer (IARC) have determined that PCBs are probably carcinogenic to humans.