Breast cancer incidence rates have increased substantially over the past several decades in most Westernized countries, with great variation in incidence rates between countries (Kamangar et al. 2006). Given such patterns of breast cancer incidence and the fact that established risk factors for breast cancer explain less than half of all incident cases (Madigan et al. 1995), it has been widely suggested that the environment may play an important role in breast cancer etiology. Several classes of environmental exposures have been investigated in relation to breast cancer risk, with sources of exposure as diverse as the proposed mechanisms linking exposure to breast cancer risk.