Although mercury does pass into breastmilk, the amount of mercury in breastmilk is not expected to be a problem under normal circumstances. A mother's contamination of blood with Mercury does result in transfer of Mercury to her baby during  pregnancy as in a known source of Minamata disease.  Environmental clean up remains essential. However-- mercury is poorly carried in breast milk. Risk posed by not breastfeeding are greater than the risk of minimal amounts of mercury in breast milk. If you are aware of being in a toxically high level of mercury and or lead environment seek medical evaluation. 

 Per Lawrence & Lawrence, in Breastfeeding: A Guide for the Medical Profession (2005, p. 417), “Acute exposures to methylmercury from industrial or environmental sources should be evaluated on a case-by-case basis, although it appears breastfeeding is safe.” A 2004 review on mercury and lead during breastfeeding (Dorea 2004) noted that “cows’ milk-based formulas pose a greater risk of infant exposure to neurotoxic substances than Breast MilK. For more information see: the carefully researched  piece by http://kellymom.com/bf/can-i-breastfeed/chemicals/mercury/  EVERYONE SHOULD BE VERY CAREFUL ABOUT DISINFORMATION WHICH IN ANY WAY DISCOURAGES BREAST FEEDING -- by almost every health parameter for children through adulthood breastfeeding exclusively for 6 months and co-feeding until age 2 is the best known way to improve the health.