Ultraviolet sunscreens are increasingly used to protect from ultraviolet radiation and skin cancer. They are often added to cosmetics and other products for light stability. However, many of the chemicals in sunscreens are estrogenic, and can stimulate breast cancer cell growth. The chemicals benzophenone-3 (Bp-3), homosalate (HMS), 4-methyl-benzylidene camphor (4-MBC), octyl-methoxycinnamate (OMC), and octyl-dimethyl-PABA (OD-PABA), increased breast cancer cell growth in studies done on rats. The chemical butyl-methoxydibenzoylmethane (B-MDM) did not. Rigorous studies on the estrogenicity of chemicals used in sunscreens and cosmetics are needed to determine their safety.