Sierra Club of/du Canada

Linkage of Pesticides and Breast Cancer through Lactation Studies

Important Public Health Issues

Peggy Land

In July ’97, much research implicating environmental factors was presented at the first World Conference on Breast Cancer in Kingston. In Ottawa, July ’99 more evidence was presented at the second conference. However, like tobacco and lung cancer, the challenging issues of establishing absolute proof, and the economic implications, have combined to make policy-makers very cautious. Meanwhile, The Canadian Cancer Society says that in 1999, one in nine Canadian women has a lifetime risk of developing breast cancer. So we need more than ever to know which risk factors are avoidable and which behaviors are protective.

Some risk factors such as family history of breast cancer and alcohol intake, for example, are already well established. The importance of other possible factors such as breast-feeding history and exposure to environmental toxins, however, have been more difficult to establish. A review of the literature on these two possible factors, with commonly used organochlorine pesticides as the environmental toxins being examined reveals linkage between the two, with far-reaching implications for women to consider. In this regional municipality of Ottawa Carleton, breast-feeding and encouragement to seek alternatives to domestic-use pesticides are actively promoted by the Department of Health. This paper suggests that a possible result, in common to both practices, could be a protective effect from breast cancer:

Laboratory evidence using experimental animals has shown that some organochlorine pesticides and organochlorine pesticide contaminants can cause or promote growth of mammary tumors. These chemicals can also damage human reproductive systems. According to the International Agency for Research on Cancer (IARC), World Health Organization, since 1993 there has been "sufficient" evidence that pesticides Chlordane and DDT, and dioxins and related components cause cancer in humans and/or animals. (1) There is evidence as well that Atrazine, beta-HCH, Chlordane, Dieldrin, 2,4-D and 2,4,5-T,(1) and DDT(2), (3), can cause malignant mammary tumors in rodents. As well, all of the preceeding plus seventy-eight other pesticides are considered to be known or suspected estrogen-like hormone disrupters (4) (5), (6), (7), (8). Such chemicals can permanently damage human endocrine, reproductive and central nervous systems (5). Atrazine, Endosulphan and Lindane, all still used in Ontario, have been linked specifically with breast cancer. (9).

Bio-accumulation and long term persistence in human tissues of some toxic organo-chlorine-type pesticides and organochlorine pesticide components is well documented. Such widely used pesticides as DDT/DDE, Lindane, Atrazine, Dieldrin, Chlordane, Mirex and b-HCH are fat-soluble and have accumulated up the food chain into our (breast and other sites)fat (1), (10), (11), (12), (13), (16) and breast milk (14), (15), (17), (18). Of this list, only DDDT, Dieldrin and Mirex and Chlordane have been banned in Canada, but all are considered to be at least "possibly" carcinogenic, and some are "probably" carcinogenic (19). The IARC has confirmed that the dioxin 2,3,7,8- TCDD is a known carcinogen (20). In addition, the dioxin contaminants of 2,4,-D and 2,4,5-T are (as are all dioxins) fat-soluble, "highly persistent, bio-accumulative and toxic" (21). And although 2,4,5-T has been banned in Canada, 2,4-D is a very widely used herbicide for agricultural and domestic purposes, and is a suspected estrogen-mimicker. It can it can be tracked into our homes before it has degraded and persist as household carpet dust for as much as a year. (22,23,24)

Women with breast cancer tend to have higher levels of some organochlorine pesticides including DDT, Dieldrin, Mirex (74) and b-HCH, in their blood and breast fat, than those without breast cancer. (4), (16), (18), (25) (26), (27). Also, it has recently been reported that when some estrogen-mimicking organochlorine pesticides are present in human breast cells, cancerous growth is accelerated, and so it is now believed that there is sufficient evidence to say that such (bio-accumulated) environmental estrogen is a contributing risk factor to breast cancer (28).

Fortunately, lactation, or breast feeding, is an effective way to flush out organochlorine toxins such as pesticides which have been accumulating in breast fat and breast milk, (17), (29), (30), (31), making it now the most contaminated of all human food. (70). Concentrations, decline on successive feedings (17), (32), (70) Despite concerns about this unloading effect on the baby, the benefits of breast-feeding still outweigh the risks, including the tendency of breast-fed babies to have fewer bouts of infectious diseases (70), to be less likely to develop breast cancer (34), (35) and to live longer (30), (36). It has long been known that cholostrum in breast milk has potent immmune-system properties and this may be why it is even now, still so good for our babies. But how long will it be until women are told not to give their milk to their babies because it is simply too contaminated? Breast milk of women in the High Arctic who eat a diet high in animal fat has unfortunately become seriously contaminated, and may have already exceeded limits which justify giving it to their children. (71)

If lactation has a protective effect against breast cancer because of this unloading effect, at least temporarily, then of the thirty studies found comparing breast-feeding to non-breast-feeding women, most could be expected to show less breast cancer in the breast-feeding group. Indeed, most do, showing that the protective effect tends to be strongest in women of pre-menopausal age. (17, 29, 33, 34, 37-53). A recent study showed a higher protection rate in women over 50 (72). Other studies show no clear association (54-63) and none could be found indicating an inverse relationship. It has also been it noted by researchers that the inconclusive studies were largely confined to post-menopausal women, and most failed to precisely define lactation (64), or failed to examine the total duration of lactation or included few women with prolonged lactation (51), or used a wide variety of methodologies for reporting lactation history (42, 63). Also of note is that the inconclusive studies were on average done ten years before the studies which show protective effects of breast-feeding. Certainly, though, the majority of studies indicate that breast-feeding is protective against breast cancer mainly in women of premenopausal age. With no evidence to the contrary, why then is this not a recognized protective factor by the Canadian Cancer Society?

Researchers have suggested three possible mechanisms to explain the apparent protective effect of breast-feeding against breast cancer. They are not mutually exclusive. The first theory is that lactation suppresses production of ovarian estrogen, considered to be essential for the initiation or growth of some types of breast cancer (14, 38, 42, 48, 51, 53, 65, 66). The second is that physiological changes in the cells of the mammary ducts, on reaching functional maturity with lactation, are in some way then able to confer some protection against the development of breast cancer ( 37, 38, 42, 48, 51, 63, 71). The third theory goes further in suggesting that lactation unloads breast tissue of carcinogens including highly suspect organochlorine (and estrogenic) pesticides and is therefore protective ( 25, 32, 38, 48, 63,). This would explain why functionally mature (and toxin-unloaded) breast tissue is less likely to become cancerous. This third theory is supported by animal research (66, 67, 68), and also by a study which showed that women who had had babies and who were found to have Mirex in their breast tissue were more likely to develop breast cancer if they had not breast fed.(74)

In summary, the literature supports that exposure to certain organochlorine pesticides, especially those with estrogen-mimicking abilities could be a risk factor for the development of breast cancer. Studies on lactation have varied greatly in design making it difficult to establish exact predictions of degree of protection, but despite this, it appears that lactation does have a protective effect in women up to menopause and possibly beyond. The connection between these two factors is that lactation is an effective way to unload breast tissue from chemical carcinogens including highly suspect organochlorine estrogen-mimicking pesticides. It is up to decision makers and educators to acknowledge that these two factors are important for women to know about, and also that estrogen-mimicking chemicals in our environment should, as in the U.S., be officially declared toxic. Recently the Federal Commissioner for the Environment and Sustainability, Brian Emmet, reported that the vast majority of pesticides have not been re-evaluated for safety by current standards. (73). Following this came the announcement that all pesticides would be re-evaluated for their effects on children, but not until the year 2006. Unfortunately, by then, mothers’ milk may be fit only for toxic waste disposal facilities.

Abstract: Linkage between Pesticides and Breast Cancer through Lactation Studies

With high odds of developing breast cancer (one in nine in Canada), women need to know how to better their chances for prevention. An extensive search of the literature supports that exposure to commonly used organochlorine pesticides, especially those which are persistent and estrogen-mimickers, should be avoided as a preventative measure, and that breast-feeding is protective against breast cancer up to menopause and possibly beyond. Studies on pesticides show complex and indirect linkage with breast cancer. Lactation studies vary greatly in method and design, but do indicate that lactation is an effective way to flush out toxins from breast tissue. Fortunately, breast-fed babies still tend to be healthier than their formula-fed peers. Lactation’s protective effect on breast cancer may well be due to the achievement of functional maturity combined with the subsequent flush-out of toxins including highly suspect organochlorine, and estrogen-mimicking pesticides. Avoidance of unnecessary use of pesticides, and promotion of lactation are important public health issues, and persistent estrogenic chemicals should be classed as toxic in Canada, as they are already in the U.S.

Peggy Land, B.Sc. P.T.,
Co-chair with Region of Ottawa -Carleton Councillor Dianne Holmes: Working Group on Health Dangers of Urban-Use Pesticides,
Director of Campaign for Pesticide Reduction, Sierra Club of Canada


(1) National Library of Medicine, Registry of the Toxic Effects of Chemical Substances, Bethesda, MD, 1993, cited in J Thornton: Chlorine, human health and the environment: the breast cancer warning,Greenpeace report l993.

(2) A Robison et al: DDT supports the growth of an estrogen-responsive tumor, Toxicology Letter27:1985

(3) JD Scribner et al: DDT acceleration of mammary gland tumors induced in the male Sprague-Dawley rat by 2-acetamidophenanthrene, Carcinogenesis 2: 1235-9

(4) P. Rainey: Pesticide linked to increased breast cancer risk, Dec.3, 1998, Reuters News ( note: refers to Dieldrin study by researchers at the Copenhagen Centre for Prospective Population Studies Dec. 1998)

(5) T Colborne, and C. Clement: Chemically-induced alterations in sexual and functional development: the wildlife/human connection. Princeton, New Jersey: Princeton Scientific Publishing Co.1992

(6) B Hilleman: Concerns broaden over chlorine and chlorinated hydrocarbons, Chemical and Engineering News 71 16: 11-20, 1993

(7) Toxics that tamper with harmones, World Wildlife Fund, Wildlife Toxicology Program publication: Eagle’s Eye: Summer,1995. Also see website at

(8) Seventh biennial report on Great Lakes water quality, International Joint Commission: 1994

(9) H L Bradlow et al : Effects of pesticides on the ratio of 16a/2-Hydroxyestrone: a biological marker of breast cancer risk, Environmental Health Perspective, 103: Oct. l995.

(10) J Mes et al: Organochlorine residues in adipose tissue of Canadians, Bulletin of Environmental Contamination and Toxicology 45: 681-688, 1991

(11) J Mes et al: Polychlorinated biphenyl and other chlorinated hydrocarbon residues in adipose tissue of Canadians, Bulletin of Environmental Contamination and Toxicology 28: 97-104, 1982

(12) D Davis et al: Medical Hypothesis: Xeno-estrogens as preventable causes of breast cancer, Environmental Health Perspectives: 101: 5, 1993

(13) FW Kutz et al: Organochlorine pesticides and polychlorinated biphenals in human adipose tissue, Review Environmental Contaminants Toxicology: 120: 1-87, 1991

(14) J Westin and E Richter: The Breast Cancer Anomaly, Annals of New York Academy of Sciences,609: 269-279, 1990.

(15) D Davis and J Mes: Comparison of some specific polychlorinated compounds in breast milk of the general and indigenous Canadian population, Bulletin of Environmental Contamination and Toxicology 39: 743-749, l987

(16) E Dewailly et al: High Organochlorine Body Burden in Women with Estrogen Receptor-Positive Breast Cancer, Journal of the National Cancer Institute: 86, 3, l994

(17) AA Jensen et al: Chemical contaminants in human milk, Boca Raton, Fla: CRC Press Inc., 1991

(18) F Falk et al: Pesticides and PCB residues in human breast lipids and their relation to breast cancer, Archives of Environmental Health 47: 143-146, 1992

(19) International Agency for Research on Cancer (IARC), World health Organization categorization 1994, cited in the Ontario Task Force Report on the Primary Prevention of Cancer, 1996

(20) E Haley: Dioxin classified as a known human carcinogen in Alternatives: 23, 3, 2, Summer 1997. -

(21) Canadian Environmental Protection Act, Priority Substance List : Dioxins and Furans, l990

(22) K Solomon, University of Guelph Centre for Toxicology: memo to Bill Strugnell, Parks and Operations, City of Gloucester, March l9,1996

(23) L Armstrong, editor: Connections, Women's Network on Health and the Environment, 2 3, March 1996

(24) M.Nishioka, et al: Measuring transport of lawn-applied herbicide acids from turf to home: correlation of dislodgeable 2,4-D turf residues with carpet dust and carpet surface residues, Environmental Science and Technology: 30, 14, Spring 1994

(25) M Wolff et al: Blood levels of organochlorine residues and risk of breast cancer, Journal of the National Cancer Institute: 85, 8, April 1993

(26) Mussalo-Rauhamaa et al: Occurence of beta-hexachlorocyclohexane in breast cancer patients, Cancer 66:2124-2128, 1990

(27) M Wasserman, et al: Organochlorine compounds in neoplastic and adjacent apparently normal breast tissue, Bulletin of Environmental Contamination and Toxicology 15:478-484,1976

(28) C Bennett, National Institute of Environmental Health Sciences, overview report . and M Shekhar, Wayne State University study report to the American Association for Cancer Research, San Diego April l977 in The Globe and Mail, April 16, 1977.: Repeated dieting may increase risk of some cancers, Chemicals stored in fat spur rapid growth and raise chances of mutation in cells.

(29) DB Thomas, EA Noonan: Breast cancer and prolonged lactation. The WHO Collaborative Study of Neoplasia and Steroid Contraceptives, International Journal of Epidemiology. 22 (4): 619-26,1993

(30) JW Frank: Breast-feeding in a polluted world: uncertain risks, clear benefits, Canadian Medical Association Journal: 1, 149, 1993

(31)WJ Rogan et al: PCB’s and DDE in human milk: effects of maternal factors and previous lactation. Am. J Public health 76;172-77,1986

(32) E Dewailly et al: Protective effect of breastfeeding on breast cancer and body burden of carcinogenic organochlorines, Journal National Cancer Institute: 86, 803, l994

(33) PA Newcomb et al: Lactation and a reduced risk of premenopausal breast cancer, New England Journal of Medicine, 330, 2, 1994

(34)R Dominguez et al: Family background, parity, breast feeding and cancer of the breast, Journal Revista De Sanida E Higiene Publica: (JC): 41, 64:7-80, 361-8,1990

(35)Freudenheim et al: Epidemiology 1994, as cited in ER Newton: Does breastfeeding protect women from cancer?, The Academy of Breastfeeding Medicine: News and Views: 2, 2, 1996.

(36) W Rogan et al: Should the presence of carcinogens in breast milk discourage breastfeeding? Regulatory Toxicology and Pharmacology: 13, 228-240 , 1991

(37) CP Yang: The influence of lactation, occupational exposure and postmenopausal hormone use on the incidence of breast cancer, Journal Diss Abst Int (B): 53(6):2813, 1992

(38) A McTiernan et al: Evidence for a protective effect of lactation on risk of breast cancer in young women. results from a case-control study, American Journal of Epidemiology: 124(3); 353-8,1986

(39) T Byers et al: Lactation and breast cancer. evidence for a negative association in premenopausal women, American Journal of Epidemiology, (JC)(3h3) 121(5):664-74, 1985

(40) A Ekbom et al: Breast-feeding and breast cancer, British Journal of Cancer: (JC:av4) 67 (4): 842-5, 1993

(41) JH Lubin et al: Risk factors for breast cancer in women from North Alberta, Canada, as related to age at diagnosis, J Natl Cancer Inst: 68: 211-17, 1982

(42) K.Katsouyanni et al: A case-control study of lactation and cancer of the breast, Br. Journal of Cancer: 73, 814-818, 1996

(43) L Rosero-Bixby et al: Reproductive history and breast cancer in a population of high fertility, Costa Rica, 1984-85. Int J Cancer: 40: 747-54, 1987

(44) SC Tao et al: Risk factors for breast cancer in Chinese women in Beijing. Int J Cancer: 42; 495-8, 1988

(45) JM Yaun et al: Risk factors for breast cancer in Chinese women of Shanghai, Cancer Res: 48; 1949-53, 1988

(46) V Siskind et al: Breast cancer and breastfeeding: results from and Australian case-control study, Am J epidemiol; 130: 229-36, 1989

(47) PM Layde et al: The independent association of parity, age at first full term pregnancy, and duration of breast feeding with the risk of breast cancer J Clin Epidemiol 42:963-73, 1989

(48) I Romieu et al: Breast cancer and lactation history in Mexican women, American Journal of Epidemiolgy,143,6:543-552, 1996

(49) C Chilvers: Breast feeding and risk of breast cancer in young women: United Kingdom Case-control Study Group, British Medical Journal, 307, July 1993

(50) CE Land et al: A case-control interview study of breast cancer among Japanese A-bomb survivors, I. Main effects. Cancer Causes Control; 5:157-65, 1994

(51)J Kelsey, E John: Lactation and the risk of breast cancer, The New England Journal of Medicine, 330,2, 1994

(52) J Kelsey et al: Reproductive factors and cancer of the breast: Epidemiology Review: 15:36-47, 1993

(53)T Byers et al: Lactation and breast cancer: evidence for a negative association in premenopausal women, American Journal Epidemiology: 121: 664-74, 1985

(54) A Kalche et al: Lactation and breast cancer, British Medical Journal: 280:223-24, 1981

(55) B MacMahon et al: Lactation and cancer of the breast: a summary of an international study. Bulletin World Health Organiaztion: 42:185-94, 1970

(56) G Brignone et al: A case-control study on breast cancer risk factors in a southern European population, International Journal Epidemiology: 130, 356-61, 1970

(57) MH Haring et al: Does breast-feeding protect against breast cancer? An epidemiological study. Ned Tijdschr Gneeskd: 136: 743-47, 1992

(58) AE Laing et al: Breast cancer risk factors in African-American women: the Howard University Tumor Registry experience, Journal National Medical Association: 85: 931-39, 1993

(59) L Brinton et al: Reproductive factors in the aetiology of breast cancer, British Journal Cancer: 47, 757-762, 1983

(60) SW Duffy et al: Risk factors for breast cancer: relevance to screening. Journal Epidemiology Community Health: 37, 127-131, 1983

(61) S.J.London et al: Lactation and Risk of Breast Cancer in a Cohort of US Women, (JC3h3) 132(1):17-26,1990

(62) G Kvale , I Heuch: Lactation and Cancer Risk: Is there a relation specific to breast cancer?: Journal of Epidemiology and Community Health-London (JC:ilp): 42, 1: 30-7, 1988

(63) KB Michels et al: Prospective assessment of breastfeeding and breast cancer incidence among 89 887 women, The Lancet: 347,431-436, 1996

(64) ER Newton: Does breastfeeding protect women from breast cancer?, ABM News and Views, Academy of Breastfeeding Medicine Newsletter: 2, 2, 1996

65) TJA Key and MC Pike: The role of estrogens and progestagens in the epidemiology and incidence and prevention of breast cancer, European Journal Cancer Clinical. Oncology: 24, 29-34

(66) BE Henderson et al: Endogenous hormones as a major factor of human cancer: Cancer Res: 42: 3232-9, 1982

(67) JL Kelsey and GS Berkowitz, Breast cancer epidemiology: Cancer Res, 48: 5615-231, 1988

(68) J Marchant, Chemical induction of breast rumors in mice of C57B1 strain: the influence of psuedo-pregnancy and lactation on induction by methylcholanthrene, British Journal Cancer : 15: 568-73, 1961

(69) C Brancifion et al, Chemically induced mammary tumors following unilateral excision of the nipples in pseudo-pregnant and lactating breeding BALB/c mice, British Journal Cancer: 16 232-7, 1962

(70) S Steingraber, in Protecting Public Health and the Environment: implementing the precautionary principle, Island Press,1999

(71) J Jensen et al (eds): Canadian Arctic Contaminants Assessment Report, Northern Contaminants Program, Indian and Northern Affairs Canada, 1997

(72) R Millikan et al, International Journal of Epidemiology 1999; 28;396-402

(73) B Emmet, Annual Report of Commissioner of the Environment and Sustainable Development, May 25, 1999. Cited in Globe and Mail, May 26, 1999: Ottawa faulted on chemicals, pesticides: Canadians at risk, report charges.

(74) K Moisich, Environmental Organochlorine Exposure and Post-Menopausal Breast Cancer Risk, Cancer Epidemiology, Biomarkers and Prevention, 1998, vol.7, p.181-188