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.