July 31, 2003

PCBs in Farmed Salmon: PCBs cause cancer

PCBs cause cancer

There is broad agreement from multiple government agencies that PCBs are likely to cause cancer in people (ATSDR 2000). While agencies use different labels to describe the human cancer risk, ranging from “probable” or “probably,” to “reasonably anticipated,” the message is the same -- PCBs are expected to cause cancer in people.

Too few well-designed human studies have been conducted for these agencies to conclude whether PCBs are definite human carcinogens, but the data from laboratory animals are strong. Scientists from General Electric published a study in 1998 showing that all four commercial PCB mixtures tested, called Aroclors, caused cancer in rats (Mayes et al. 1998). Each commercial mixture was shown to cause liver tumors in females and thyroid tumors in male rats. Other studies have also shown that PCBs cause liver tumors (EPA 1996). One study, conducted by the National Cancer Institute (NCI) in 1978, found that a commercial PCB mixture caused a rare type of stomach tumor (gastric adenocarcinoma) in 6 of 144 exposed rats. This finding was statistically significant, since this type of tumor was found in only 1 of 3548 control male and female rats at NCI (IRIS 1997).

Most of the human cancer studies for PCBs are in exposed workers, or comparisons of PCB levels in people with and without certain types of cancer. Some of these studies show that workers exposed to PCBs may be more likely to die from cancers of the liver, biliary tract, intestines and skin. Less clear are associations with cancers of the brain, blood and lymphatic systems. Two studies have found that patients with non-Hodgkin’s lymphoma have higher levels of PCBs compared to healthy matched controls (ATSDR 2000).

Overall, the available human studies have limitations that prevent scientists from drawing definite conclusions about PCB exposure and cancer risk in people. For example, the human studies are often too small to detect statistically significant effects. Also, exposures to other factors that can alter cancer risk, such as smoking, obesity, alcohol consumption and other chemical exposures, can obscure potential PCB effects. For these and other reasons, EPA has concluded that the available human studies “are most appropriately viewed as inconclusive. Limited studies that produce inconclusive findings for cancer in humans do not mean that PCBs are safe” (EPA 2002).

Occupational and dietary mixtures of PCBs are not the same because PCBs can change in the environment. In general, the types of PCBs that bioaccumulate in fish and other animal products are more carcinogenic than commercial mixtures encountered by workers. As a result, people who ingest PCB-contaminated fish or other animal may be exposed to PCB mixtures that are even more toxic than the PCB mixtures contacted by workers and released into the environment (EPA 2002).

Notes: 1. The Environmental Protection Agency (EPA) considers PCBs to be “probable” human carcinogens, the International Agency for Research on Cancer (IARC) classifies PCBs as “probably” carcinogenic to people, and the National Toxicology Program refers to PCBs as “reasonably anticipated” human carcinogens.


  1. Agency for Toxic Substances and Disease Registry (ATSDR). 2000. Toxicological profile for polychlorinated biphenyls (Arochlors -1260, -1254, -1248, -1242, -1232, -1221, and -1016 (update). Available online at http://www.atsdr.cdc.gov/toxprofiles/tp7.html.
  2. Environmental Protection Agency (EPA). 1996. PCBs: Cancer Dose-Response Assessment and Applications to Environmental Mixtures. National Center for Environmental Assessment, Office of Research and Development. EPA/600/P-96/00.
  3. Environmental Protection Agency (EPA). 2002. Health effects of PCBs. Available online at http://www.epa.gov/opptintr/pcb/effects.html.
  4. Integrated Risk Information System (IRIS). 1997. Polychlorinated biphenyls (PCBs) [CAS Number 1336-36-3]. Available online at http://toxnet.nlm.nih.gov/.
  5. Mayes, BA., McConnell, EE., Neal, BH., Brunner, MJ., Hamilton, SB., Sullivan, TM., Peters, AC., Ryan, MJ., Toft, JD., Singer, AW., Brown, JF, Jr.., Menton, RG and Moore, JA. 1998. Comparative carcinogenicity in Sprague-Dawley rats of the polychlorinated biphenyl mixtures Aroclors 1016, 1242, 1254, and 1260. Toxicol Sci 41 (1): 62-76.