In the United States, from the first case of breast cancer in the 1930s to today, a woman's risk of breast cancer increases by to one eighth (Gray et al.). The continuous increase in the rate of breast cancer has caused much concern not only among ordinary people but also among scientists. For decades, scientists have been working on the causes of breast cancer in order to find corresponding treatment methods. However, only about 25% of breast cancer cases have been explained (Brody et al.); to date, heredity, lifelong exposure to environmental estrogens (the female sex hormone), and dietary fat are the only known major causes (“Cover Story: Breast Cancer and Environment”). For the 75% of unexplained breast cancer cases, scientists, through various researches, have proposed several hypothetical causes of breast cancer, in which synthetic chemicals and environmental radiation are the main causes. Much evidence has linked breast cancer to environmental chemicals. . Since World War II, many endocrine disruptors (synthetic chemicals) have entered the environment, accumulated throughout the food chain, and ultimately entered the human body (Brody et al.). According to Gray et al., environmental chemicals are carcinogenic because they often interrupt hormonal regulatory pathways, particularly estrogen, and thus cause negative genetic variations. The experiments carried out by Gray et al. indicate that breast tissue synthesizes estrogen from local hormones (androgen hormones) using aromatase, the activity rate of which is abnormally higher in breast cancer tissue than in normal breast tissue. Theoretically, “estrogens promote the growth of cancer cells in vivo” (Mitra et al); the fact that women are more likely to...... middle of article ...... all causes of breast cancer. Public Health Reports 111. (1996): 494(14). Electronic library. Internet. January 16, 2014. “Cover story: Breast cancer and the environment.” Contemporary Women's Issues Database. May 1. 1994: 33+. Electronic library. Internet. January 16, 2014. Gray, Janet., Evans, Nacy., Taylor, Brynn., Rizzo, Jeanne RN., Walker, Marisa.. “State of the Evidence: The Link Between Breast Cancer and the Environment.” » International Journal of Occupational and Environmental Health. 1(2009):43. Electronic library. Internet. January 16, 2014. Greene, Gayle and Ratner, Vicki. "A Toxic Link to Breast Cancer? The Chemicals Around Us." The Nation June 20, 1994: 866+. Opposing viewpoints in context. Internet. January 16, 2014.Mitra, Amal K., Faruque, Fazlay S., Avis, Amanda L.. “Breast cancer and environmental risks: where is the link? ". Journal of Environmental Health 7(2004):24. Electronic library. Internet. January 16. 2014.