Organochlorinated compounds were investigated worldwide after the first report in 1951 of the presence of dichlorodiphenyltrichloroethane (DDT) in the milk of healthy U.S. women (Laug et al., 1951). Polychlorinated biphenyls (PCBs), hexachlorobenzene, dieldrin, and heptachlor epoxide have also been identified in milk from women with known heavy exposures. In a study of hundreds of subjects at delivery in the Great Lakes region of New York State in 1978, PCB was not detectable in the cord blood of the infants or in the milk of the mothers, except when the women illegally ate contaminated lake fish at least once a week (Lawrence, 1989). In general, PCB levels in milk decline over the period of lactation and with the number of children nursed (Rogan et al., 1986) if there are no new exposures during this period.
The levels of DDT and other insecticides in human milk vary with the weight of the mother, the number of children, the duration of lactation, and the residence and occupation of the mother (Rogan and Gladen, 1985; Wilson et al., 1973). Insecticide levels in human milk tend to be higher than those in cow's milk, because humans are at the top of the food chain. Levels are higher at the end of a single nursing because the fat content of the milk is increased at that time.
In a study of 858 children from birth to 5 years of age whose mother's milk contained PCBs and dichlorodiphenyl ethane (DDE, a metabolite of DDT) after a heavy exposure, there was no evidence of change in growth rate or general health, nor were there adverse effects on weight gain or differences in the number of physician visits for illness. However, DDE in the human milk was associated with a shorter duration of breastfeeding (Gladen et al., 1988; Rogan et al., 1987). These children remain under surveillance.
Agent Orange (2,3,7,8-tetrachlorodibenzo-p-dioxin), the best known of the dioxins, has been found in human milk in pooled samples from women with known exposures to high levels in Vietnam. There is no evidence that the U.S. population at large is at risk (Schecter and Gasiewicz, 1987; Schecter et al., 1986, 1987).
Heavy metals—such as lead, mercury, arsenic, and cadmium—are found in higher concentrations in certain water supplies, cow's milk, and reconstituted formula than in human milk (Dabeka et al., 1986; Jensen, 1983). Thus, breastfed infants are exposed to lower amounts than their formula-fed counterparts are. Whenever there has been an exposure or a woman has been found to have elevated mercury or lead levels, the infant's serum and the milk levels should be checked (Perkins and Oski, 1976). In the case of lead poisoning, evidence suggests that maternal serum levels under 40 mg/dl are not associated with elevated lead values in the milk (Dillon et al., 1974). Lead levels in milk are lower than would be predicted from maternal serum levels (Wolff, 1983). Based on a single case study, Ryu and colleagues (1978) state that loss of lead from the body can be expected in breastfed infants exposed to lead in utero if their lead intake is less than 5 µg/kg of body weight per day.