The breast milk biomarker: samples provide clues to exposures. (Science Selections).

Since the early 1950s, environmental chemicals and their metabolites have been detected in human breast milk throughout the world. Research has focused primarily on organochlorine pesticides, polychlorinated biphenyls, and dioxins, while metals, solvents, and other chemicals have received scant attention. A few countries, notably Germany and Sweden, have ongoing breast-milk monitoring programs, but data from most countries come from small, scattered investigations. Taken as a whole, however, the data point to the pervasiveness of chemical exposure and hint at regional variations and trends over time. These data form the basis of the current review of chemical contaminants in breast milk by Gina Solomon, a senior scientist with the Natural Resources Defense Council in San Francisco, California, and Pilar Weiss, a graduate student researcher in the School of Public Health at the University of California, Berkeley. [EHP 110: A339-347]

Because most studies were designed to stand alone, cross-study comparisons are challenging. The authors indicate that donor selection, sample processing, and other research methods vary considerably from one study to the next, as do the reported data. The greatest difficulty arises from inconsistent analyses, particularly for chemical classes with multiple members. For example, dioxins and furans include 17 chemical species, but only one dioxin, 2,3,7,8-TCDD, is nearly always measured.

Despite the challenges, Solomon and Weiss describe regional and time trends for several contaminants. Based on their review, the presence and concentrations of breast-milk contaminants are affected by global, regional, and local use of chemicals, and individual variability arises from age, diet, number of children, and duration of breastfeeding. Although local or regional use often predicts contaminant levels, some chemicals are detectable in breast milk even in its absence. As a case in point, studies in the mid-1980s showed measurable amounts of chlordane in breast milk samples from Finnish women although the chemical was never used in Finland and was heavily restricted in surrounding countries. The exposure was ultimately linked to consumption of fish from the Baltic Sea, a finding that also demonstrates that diet is a common route of exposure.

Solomon and Weiss's review indicates that levels of several contaminants, including chlordane, DDT, and hexachlorobenzene, have declined over time. These decreases are apparently linked to bans or restrictions on the use of those particular chemicals, and the only chemical class that appears to be increasing is polybrominated diphenyl ethers (PBDEs). However, the authors caution that these trends are based on incomplete data and only apply to the narrow sampling of chemicals for which breast milk has been tested.

The authors conclude that information gaps could be remedied by international breast milk monitoring programs using a standard methodology. Such programs would yield reliable data to demonstrate the scope of chemical exposure and to assess variations by location and population. These data could spur research into the health effects of detected chemicals, prompt action on limiting or eliminating exposure, and, over time, serve as a measure of pollution control success. However, such research needs to be sensitive to the importance of breastfeeding. Ample data demonstrate both short-term and long-term benefits to breastfeeding, note Solomon and Weiss, and any risk due to environmental contaminants in breast milk is overshadowed by these benefits.