In the 1930s, attitudes toward ultraviolet (UV) light exposure began to change significantly within the medical profession. UV radiation had been promoted as healthful since the century's start, and particularly after the discovery of its role in vitamin-D metabolism. Increasingly, however, attention would focus on the risks of UV light exposure from sunlamps and sunbathing. During this time, the American Medical Association established guidelines for the approval of UV lamps and the appropriate therapeutic uses of phototherapy. The landmark experiments of Findlay and other researchers, in which malignant skin tumors were induced in rodents after exposure to UV lamps or sunlight, would lead to widespread recognition of the carcinogenicity of UV radiation. The role of sunlight in the etiology of skin cancer was increasingly mentioned in articles in popular magazines in the 1940s and 1950s. There was rapid growth of the sunscreen industry as well, although product efficacy remained highly variable. In the 1950s, interest developed in the use of 8-methoxypsoralen ("the suntan pill") and dihyroxyacetone ("suntan in a bottle"). In spite of the known risks of UV exposure and attempts by physicians and other health professionals to educate the public and modify behavior, suntanning has remained tenaciously popular. Today, excessive UV light exposure is recognized as the major cause of the approximately 1.3 million cases of skin cancer in the United States each year.