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Pesticides and Herbicides: Evidence

Leaner postmenopausal women with high levels of fat tissue pesticide residues have increased rates of breast cancer.

Researchers examined levels of sixteen different pesticides, as well as total environmental estrogen level, in tissue from women who had been diagnosed with breast cancer as well as women who had never had the disease. Increased rates of the disease were associated with higher total xenoestrogen level, as well as with the levels of the individual pesticides aldrin and lindane [Ibarluzea et al., 2004]. Aldrin is a potent pesticide that was banned for use on agricultural products in 1970, and as an anti-termite agent in 1987. Decreasing, but still observable levels of the main aldrin metabolite, dieldrin, remain in the bodies of approximately a quarter of the adult population of the United States [CDC, 2005].

Most people in the U.S. carry many pesticides in their bodies.

In the Third National Report on Human Exposure to Environmental Chemicals, released by the U.S. Centers for Disease Control in 2005 [CDC, 2005], people of all ages across the country were tested for the presence of 140 chemicals, including 38 pesticides, in their bodies. One chemical that was found ubiquitously was p,p-DDE, a breakdown product of DDT, a persistent organochlorine pesticide that was banned from use in the United States in 1973. Levels of p,p-DDE were higher in older people, as well as in Mexican Americans of all ages.

Early exposure to DDT is associated with increased risk of breast cancer.

Scientists interested in the long-term health effects of exposure to the banned pesticide DDT looked at blood DDT levels at the time that young women gave birth to children. These blood levels, along with information about the women’s own years of birth, served as markers of DDT exposures during their youth. Researchers then followed the women for the next 2 decades after they had given birth, noting cases of non-invasive or invasive breast cancer before the women reached age 50, and deaths from breast cancer before the age of 50 years. Exposure to DDT during childhood and early adolescence was associated with a five-fold increase in risk of developing breast cancer before the age of 50 [Cohn et al., 2007].

Risk of breast cancer increases with chemical use among young Hispanic farm workers.

Female Hispanic farm workers who worked with three particular pesticides (2,4-D, malathion, and chlordane) showed slight increases in risk for breast cancer, after all other demographic factors were considered. Women who worked with mushroom crops, but not grapes, were most likely to show an effect. More generally, when pesticide-related effects were found, they were associated with earlier (premenopausal) diagnosis of the disease. Interestingly, there was also a historic relationship, with women who were diagnosed between 1988-1994 showing the increased risk related to pesticide exposure, but not those diagnosed between 1994-2001 [Mills and Yang, 2005].

Dieldrin is an endocrine disruptor that affects breast tumor cell growth

Like many other pesticides found in the environment, dieldrin has been shown to be an endocrine disruptor, both by stimulating estrogen-regulated systems and by interfering with androgen-regulated systems. Addition of dieldrin to human breast cancer (MCF-7) cells in vitro stimulated their growth and proliferation [Andersen et al., 2002; Soto et al., 1994]. The exposure of normal (non-cancerous) human breast epithelial cells to mixtures of organochlorine pesticides, including dieldrin and aldrin, as well as DDT/DDE at levels found in the environment led to greater induction of cellular processes linked to cancer than exposures to any of the chemicals individually [Valeron et al., 2009].

There is a relationship between serum levels of the organochlorine pesticide dieldrin and breast cancer incidence.

A relationship between body burden of dieldrin, an organochlorine pesticide, and breast cancer incidence was demonstrated in a study of approximately 7,500 Danish women. During the 1970s these women had blood samples taken. Organochlorine compounds were detected in a majority of the samples from 240 women who were subsequently diagnosed with breast cancer and dieldrin was found in 78% of these samples. Women who had the highest levels of dieldrin years before cancer developed had at least a doubled risk of breast cancer compared to women with the lowest levels. A study of organochlorines in Long Island, found no such association between dieldrin and increased breast cancer risk. However, dieldrin levels were measured near the time of breast cancer diagnosis and therefore did not take into account the levels at the time of exposure and during the critical periods of breast development. [Hoyer et al, 1998; Gammon et al., 2002].

Mixtures of low levels of pesticides enhance human breast cancer cell proliferation.

Real-world exposures to pesticides and herbicides are not isolated events, but rather result in many different chemicals being present in the body simultaneously. In some instances, these chemicals work together in different ways to increase their effects. A mixture of four persistent organochlorine pesticides found in human tissues were shown to have an additive effect on the induction of cell proliferation of human breast cancer cells (MCF-7). Further analysis demonstrated that there were synergistic effects even when each component was present at levels at which effects on cell proliferation are not observed [Payne et al., 2001].

Prenatal exposures to the pesticide atrazine alters mammary gland development in rats.

Prenatal exposure to the pesticide atrazine delays development of the rat mammary gland in puberty, widening the window of sensitivity to breast carcinogens.   Similarly, exposure of rats late in pregnancy to a mixture of metabolites of atrazine also leads to persistent changes in mammary gland development in her pups that were exposed during gestation.  These abnormalities in the prenatally exposed persist into adulthood [Enoch et al., 2007].