Dioxins

Of all toxic chemicals, dioxins may be the most widespread. The body fat of every human being, including every newborn, contains dioxins. Dioxins are formed by the incineration of products containing PVC, PCBs and other chlorinated compounds; as well as from industrial processes that use chlorine, such as pulp and paper manufacturing; and from the combustion of diesel and gasoline. Dioxins break down very slowly; they accumulate in fat of wildlife and bioaccumulate across the food chain.

Dioxins are known human carcinogens and endocrine (hormone) disruptors. One of the dioxins (2,3,7,8-tetra chlorodibenzo-para-dioxin [TCDD]) has been classified by IARC as a known human carcinogen. In 2000, the U.S. Environmental Protection Agency officially declared TCDD to be a known carcinogen.

People are exposed to dioxins primarily through consumption of animal products and human breast milk. Dioxin enters the food chain when vehicle exhaust or soot from incinerated chlorinated compounds falls on field crops later eaten by farm animals. It is then passed to humans through dairy and meat products.

There have been very few epidemiological studies examining possible effects of dioxin exposure and breast cancer risk. Most of the studies have been with fairly small numbers of women. Further, comparisons are made looking at dioxin levels at the time of diagnosis of breast cancer, not at earlier times when exposures might be influencing cancer initiation. Concentrations of dioxins in breast tissue may change dramatically over the reproductive span of a woman’s life. There is a substantial decrease in the amount of dioxin remaining in a woman’s breast fat tissue after she has breast fed because, unfortunately, the chemicals have been passed on to her newborn via breast milk. Although the presence of toxic chemicals in breast milk is potentially dangerous, the beneficial nutrients and immune system boosters that are transferred from mother to infant far outweigh the potential toxic transfers.

Not surprisingly, given these methodological issues, study results have been conflicting. However, a recent follow-up study on women exposed to dioxins during a chemical plant explosion in 1976 in Seveso, Italy makes a more compelling case for a connection between dioxin and breast cancer. Scientists analyzed blood samples taken and stored at the time of the explosion and correlated the results with subsequent cases of breast cancer. They found that a tenfold increase in TCDD levels was associated with more than twice the risk of breast cancer. Women who were children at the time of the accident are just beginning to reach the age when breast cancer is most likely to develop and researchers will continue to follow the Seveso women. They expect to find additional breast cancer cases.

Another study examined deaths from cancer among people who had worked in a chemical factory in which they were exposed to high levels of TCDD. There was no increase in overall mortality from cancer for female workers, although there was a significant increase in deaths from breast cancer among those who worked in high exposure regions of the factory.

A number of laboratory studies have demonstrated that when looking at later changes in mammary cancer rates, the timing of exposures to dioxins matters. Although exposing animals to dioxins in adulthood may not affect cancer rates, earlier exposures may have profound effects. Several studies have shown that administration of dioxin (especially TCDD) to pregnant rats leads to structural abnormalities in the development of their pups’ mammary tissues and higher incidence of tumors when the pups grow to adulthood.

See below for a table of air pollutants, including dioxins, linked to increased breast cancer risk. A PDF of this table is also available.

Learn more about policy and research recommendations to reduce exposure to air contaminants »

For references, see State of the Evidence 2008.