I have sent this to Melissa but am posting to the entire list in case others
are interested. Here's an excerpt from a 1994 LEAVEN article, "Pesticides
and Breastfeeding" by Betty Crase, in which she describes a study by W.
Rogan, who compared the risks of contaminants in breast milk with the risks
of formula feeding:

"Estimated loss of life expectancy from cancer from human milk exposure to
the contaminants studied was less than three days; in contrast, the decrease
in life expectancy from excess postneonatal mortality in infants not
breastfed compared to the increase in life expectancy in breastfed infants
is about seventy days. There was not only no advantage to avoiding
breastfeeding, there was a disadvantage. Breastfed children, even those at
the extreme doses of contaminants considered to be in human milk, had a net
lower risk of death and longer life expectancy."

It's a terrific article and can be found in LEAVEN, Vol. 30 No. 3, May-June
1994, pp. 37-40. The reference cited in LEAVEN for this study is:

Rogan, W. J. et al. Should the presence of carcinogens in breast milk
discourage breast feeding? Reg Toxicol Pharmacol 1991; 13:228-40.

Kate Pennington, LLLL, Newcastle, Maine (with thanks to Margery W. & Diane
W. for earlier help in tracking down information!)