Epidemiologic studies of cataract suggest that associated risk factors may include personal characteristics, such as age, sex, race, occupation, and educational status; ocular characteristics, such as iris color; and diabetes mellitus, hypertension, drug exposure, smoking, and sunlight exposure. Animal studies and observational epidemiologic studies suggest that deficiencies in vitamins C and E, carotenoids, and the trace elements zinc and selenium also may be associated with the development of the two diseases, especially cataract. Although surgical treatment to remove cataract is very effective, cataract surgery carries risks, as does any other surgery. Therefore, many research efforts focus on preventing or slowing cataract development, as well as on determining the causes of cataract formation.