Human milk almost certainly contains more dioxins, PCBs and other persistent chemicals compared to cow's milk or formula because people eat a varied diet which likely exposes them to these substances. However, the benefits of breastmilk (nutritional and immunological protection) far exceed the small risks from chemicals in the milk. Quantitative assessments of the benefits of breastfeeding have estimated that breastfeeding provides greatly improved mortality and morbidity rates compared to those rates for formula-fed infants in all socio-economic strata. In developed countries, reductions in mortality rates in breast-fed infants have been estimated to range from about 2500 to 5000 per million when compared to formula-fed infants. Generally, increased estimated cancer risks based on chemicals in ingested breastmilk are far lower (usually not higher than several 100 per million). Therefore, in almost all circumstances, breastfeeding is highly advantageous for infant health and well-being dispite the presence of chemical contaminants in human milk. Please see a chapter I authored on this subject: Chapter 5 "Transport of Organic Chemicals to Breastmilk: Tetrachloroethene Case Study", in Environmental Toxicology and Pharmacology of Human Development,1997, S. Kacew and G. Lambert, Eds, Taylor and Francis Publ. Judy Schreiber, Ph.D., New York State Dept of Health. I can answer specific questions by email.