Human Health Reassessment of TCDD and Related Compounds (NRC, 2006) noted that the classification of DLCs as “carcinogenic to humans” vs. “likely to be carcinogenic to humans” is dependent on “the definition and interpretation of the specific criteria used for classification, with the explicit recognition that the true weight of evidence lies on a continuum with no bright line that easily distinguishes between these two categories.”


Bioaccumulation of Dioxins in Seafood Exposure to dioxins and DLCs occurs when fish consume aquatic invertebrates that come in direct contact with dioxin particles that settle in sediment; through direct absorption through the gills; or by eating contaminated sediment, insects, and smaller fish (Evans, 1991). Because of their lipophilic character, dioxins and DLCs are distributed to fatty tissues in fish, including the liver and gonads. Muscle tissue is less contaminated, depending on the fat content of the muscle, which is likely to be greater in the older, larger, and oily fish.


Adverse Health Effects TCDD is used as the reference congener as a measure of toxicity for all dioxin-like compounds. Adverse health effects associated with exposure to dioxins have been identified in populations exposed through unintended industrial releases. One of the largest population exposures to TCDD occurred from an unintended industrial release in Seveso, Italy. Those who were exposed to the highest doses, primarily children, exhibited chloracne (Mocarelli et al., 1999), a severe skin disease with acne-like lesions that occur mainly on the face and upper body. Other adverse health outcomes included an increased risk for cancer. When compared to the nonexposed general population, the exposed population did not show an increased overall cancer mortality, but did have a significant excess mortality risk for esophageal cancer in males and bone cancer in females among those who were exposed to the lowest doses (Bertazzi et al., 1997). The US EPA (2000a) concluded that the cancer data on the Seveso population was difficult to interpret because of the small number of cases, exposure classification problems, and limited follow-up.

In 1997, the International Agency for Research on Cancer (IARC) placed TCDD in a Group I (agents with sufficient evidence of carcinogenicity for humans) designation, but weaknesses and inconsistencies among the positive studies published have made this designation controversial (Cole et al., 2003). The US EPA (2000a) considers TCDD to be a human carcinogen and other DLCs likely carcinogens, based on epidemiological and animal studies. Although epidemiological evidence alone does not support a causal relationship between dioxin exposure and cancer, US EPA (2000a) describes TCDD as a non-genotoxic carcinogen and a potent tumor promoter.



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