Pesticides and Breast Cancer: A Wake Up Call

by: Meriel Watts, PhD, a scientist with Pesticide Action Network (PAN) Asia and the Pacific, coordinator of PAN Aotearoa New Zealand, and author of Ethical Pesticide Policy: Beyond Risk Assessment

Incidence of breast cancer continues to rise in most countries. Even those countries that traditionally have been regarded as having relatively low rates, compared with countries like the USA and New Zealand-notably the developing countries of Asia-are beginning to catch up. Why? There is no easy answer. The factors involved in breast cancer genesis are many and complex, and still not fully understood. One potential factor, however, is frequently overlooked, and this is exposure to pesticides, which has escalated dramatically in developing countries over the last 20 years.

Pesticides and Breast Cancer: A Wake-Up Call examines available experimental and epidemiological evidence suggesting that pesticides may contribute to the puzzling growth of breast cancer. While this new book focuses on pesticides used in the Asia Pacific region, the information it contains applies to any country where they are employed. It identifies 98 pesticide active ingredients, one adjuvant and two contaminants of pesticide formulations that potentially affect the risk of breast cancer. The pesticides identified include insecticides, herbicides, fungicides and fumigants.

It would be nice to think of these substances as largely obsolete pesticides such as DDT, although the resurgence in use of this one in Africa is a real concern. But, in fact, the vast majority of pesticides covered in the book are still currently in use, particularly in developing countries. Some are also still widely used in the USA, Europe and New Zealand. These include organochlorines like lindane used for head lice control, the synthetic pyrethroids that we find in household fly sprays, the herbicide atrazine which contaminates ground and drinking water in the US, New Zealand and other countries, the aerially applied herbicide 2, 4-D renowned for its ability to drift off-target over long distances, and organophosphate insecticides. Some pesticides-such as chlorpyrifos and diazinon-are found in the meconium, or first feces, of newborn children indicating exposure while still in the womb. Others are contaminating breast milk. This does not mean breastfeeding should be replaced by bottle-feeding. Despite the residues, breastfeeding confers overwhelming benefits on both mother and child.

The evidence considered in A Wake Up Call includes epidemiological studies examining possible links between exposures to various pesticides and incidence of breast cancer; or body burdens of organochlorine insecticides and incidence. It also considers mammary tumors produced in laboratory trials. These are the commonly acknowledged indicators of breast cancer risk. In fact, this is as far as most reviews go, and other indicators of a pesticide's potential to cause breast cancer are generally overlooked.

The capacity of pesticides to disrupt hormones has been well documented. The role of estrogen in breast cancer is now well established and the potential role of estrogen-mimicking chemicals widely accepted. But what of the effects of pesticides on other hormones-androgens, progesterone, prolactin, melatonin and prostaglandins? These hormones also have a role to play in the development of breast cancer. And so may the pesticides that affect them.

In addition, there are a number of other mechanisms by which pesticides might influence the onset and progression of breast cancer: through their influence on the immune systems' Natural Killer T-cells, through interfering with cellular communication, or the function of enzymes on cell repair, or impeding the cell's natural ability to cause tumor cells to die off. And there is evidence that a number of pesticides can adversly affect these mechanisms, raising the risk of breast cancer.

While the available evidence does not prove that pesticides are causing breast cancer in humans today, it does provide proof that they have the potential to influence its onset and development, and thus to raise the risk.

So what do we do, while we wait for evidence to accumulate? Continue using these pesticides despite the warnings? Hope for the best while we wait for irrefutable proof? Or take a precautionary approach and drastically reduce the exposure of girls and women-especially pregnant women-because of the heightened vulnerability of the neonate to carcinogens and hormonally active chemicals?

We cannot rely solely on epidemiology to unravel the relationship between pesticides and breast cancer-not least because breast cancer can have such a long latency period, it may be virtually impossible to establish exposures in a meaningful way. The relevant exposures may have occurred very early in life or even at the fetal stage. The fetus is exquisitely sensitive to carcinogens and hormonally active substances. Recent studies covered in our report indicate that when pregnant rats are exposed to the fungicide vinclozolin, breast cancer incidence is elevated in each of the four succeeding generations (at least).

Table 2 (see below), adapted from the Silent Spring Institute^[1], condenses information on 216 different chemicals proven to cause mammary tumors in animals. Some of these are found in pesticides, and none has ever been regulated for its potential to induce breast cancer.

Our report calls for a systemic change in our thinking about pesticides in order to reduce the risk of breast cancer. Regulatory processes have to come to grips with the many different mechanisms that underlie breast cancer and how pesticides may influence these. The current paradigm in many nations allows use of suspect pesticides to continue until proof of human harm, sufficient to persuade the regulators and the courts of law, can be collected. This places the onus on scientists who work in the public interest to try to achieve the impossible. We can only begin to protect women from breast cancer when we reject this approach, applying the precautionary principle and requiring substitution of less harmful methods of pest, weed, and disease management in place of the suspect pesticides.