Chlorine when used to disinfect water can react with certain organic matter and produce hundreds of chemical by-products. These chemical by-products are know as DBP’s. Two major classes of DBP’s, trihalomethanes (THMs) and haloacetic acids (HAA), make up the bulk of these by-products.
Research has linked these chemicals to cancer in lab animals and seems to be a factor worth considering regarding breast cancer. In my opinion the endocrine inhibitor factor of chlorine could be a contributing element.