Nursing infants may be exposed to lead from breast milk, but relatively few data exist with which to evaluate and quantify this relationship. This route of exposure constitutes a potential infant hazard from mothers with current ongoing exposure to lead as well as from mothers who have been exposed previously due to the redistribution of cumulative maternal bone lead stores. We studied the relationship between maternal breast milk lead and infant blood lead levels among 255 mother-infant pairs exclusively or partially breast-feeding through 1 month of age in Mexico City. A rigorous, well-validated technique was used to collect, prepare, and analyze the samples of breast milk to minimize the potential for environmental contamination and maximize the percent recovery of lead. Umbilical cord and maternal blood lead were measured at delivery; 1 month after delivery (± 5 days) maternal blood, bone, and breast milk and infant blood lead levels were obtained. Levels of lead at 1 month postpartum were, for breast milk, 0.3-8.0 µg/L (mean ± SD, 1.5 ± 1.2); maternal blood lead, 2.9-29.9 µg/dL (mean ± SD, 9.4 ± 4.5); and infant blood lead, 1.0-23.1 µg/dL (mean ± SD, 5.5 ± 3.0). Infant blood lead at 1 month postpartum was significantly correlated with umbilical cord (Spearman correlation coefficient r_S = 0.40, p < 0.0001) and maternal (r_S = 0.42, p < 0.0001) blood lead at delivery and with maternal blood (r_S = 0.67, p < 0.0001), patella (r_S = 0.19, p = 0.004), and breast milk (r_S = 0.32, p < 0.0001) lead at 1 month postpartum. Adjusting for cord blood lead, infant weight change, and reported breast-feeding status, a difference of approximately 2 µg/L (ppb; from the midpoint of the lowest quartile to the midpoint of the highest quartile) breast milk lead was associated with a 0.82 µg/dL increase in blood lead for breast-feeding infants at 1 month of age. Breast milk lead accounted for 12% of the variance of infant blood lead levels, whereas maternal blood lead accounted for 30%. Although these levels of lead in breast milk were low, they clearly have a strong influence on infant blood lead levels over and above the influence of maternal blood lead. Additional information on the lead content of dietary alternatives and interactions with other nutritional factors should be considered. However, because human milk is the best and most complete nutritional source for young infants, breast-feeding should be encouraged because the absolute values of the effects are small within this range of lead concentrations. Key words: blood lead, breast milk lead, breast-feeding, KXRF bone lead, lactation. Environ Health Perspect 112:1381-1385 (2004). [Online 11 May 2004]

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Address correspondence to A.S. Ettinger, Harvard School of Public Health, Landmark Center, East 3-110-A, 401 Park Dr., Boston, MA 02215 USA. Telephone: (617) 384-8968. Fax: (617) 384-8994. E-mail: rease@channing.harvard.edu. Address reprint requests to H. Hu, Harvard School of Public Health, Landmark Center East 3-110-A, 401 Park Dr., Boston, MA 02215 USA.

This study was supported by a National Institute of Environmental Health Sciences (NIEHS) grant P42-ES05947, Superfund Basic Research Program NIEHS R01-ES07821, NIEHS Center Grant 2 P30-ES 00002, and NIEHS T32-ES07069 NRSA Training Grant; and by Consejo Nacional de Ciencia y Tecnología (CONACyT) grant 4150M9405 and Consejo de Estudios para la Restauración y Valoración Ambiental (CONSERVA), Department of Federal District, México. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS.

The authors declare they have no competing financial interests.

Received 28 July 2003; accepted 10 May 2004.