Evaluates the carcinogenic risks to humans posed by exposure to
polychlorinated dibenzo-"para"-dioxins (PCDDs) and polychlorinated
dibenzofurans (PCDFs). PCDDs are formed as inadvertent by-products,
sometimes in combination with PCDFs, during the production of
chlorophenols and chlorophenoxy herbicides, and have been detected
as contaminants in these products. PCDDs and PCDFs may also be
produced in thermal processes such as incineration and metal
processing and in the bleaching of paper pulp with free chlorine.
Of the PCDDs: 2 3 7 8-tetrachlorodibenzo-"para"-dioxin (2 3 7
8-TCDD), or, dioxin, has attracted the greatest concern. PCDDs and
PCDFs are ubiquitous in soil, sediment, and air, persist in the
environment, and accumulate in animal fat. Excluding occupational
and accidental exposures, most human exposure to these compounds
occurs from the consumption of meat, milk, eggs, fish, and related
products. Occupational exposures at higher levels have occurred
since the 1940s as a result of the production and use of
chlorophenols and chlorophenoxy herbicides and, for PCDFs, in metal
production and recycling. Even higher exposures have occurred in
sporadic industrial accidents and following incidents of rice oil
contamination. The evaluation, which considered abundant human and
animal carcinogenicity data found strong evidence from
epidemiological studies in humans that exposure to 2 3 7 8-TCDD
produces increased risks for all cancer combined, rather than for
any specific site, suggesting that 2 3 7 8-TCDD is an unprecedented
multi-site carcinogen with no single site predominating. Citing
data from animal studies and other lines of evidence, the monograph
concludes that 2 3 7 8-TCDD is carcinogenic to humans. Other
polychlorinated dibenzo-"para"-dioxins and dibenzo-"para"-dioxin
could not be classified as to their carcinogenicity to humans. For
PCDFs, the evaluation considered evidence from two large poisoning
incidents involving rice oil contamination in Japan and Taiwan.
Although excessive mortality from liver cancer was observed in
long-term follow-up of the Japanese cases, the report cited other
factors, including a high prevalence of chronic hepatitis B
infection in the geographical area concerned, as possible
explanations. Evidence of carcinogenicity to human was judged
inadequate. In the absence of convincing data from experimental
animals, PCDFs could not be classified as to their carcinogenicity
to humans.
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