Study of ultraviolet radiation and genotoxic effects of natural sunlight in relation to skin cancer in Saudi Arabia.

Sunlight-related biological effects such as skin cancer, sunburning, and synthesis of vitamin D in the body have been found to be very low in people in the midregion of Saudi Arabia. The present studies were undertaken to measure the sunburning-carcinogenic ultraviolet light (UV) radiation (UV-B) in natural sunlight in the city of Riyadh (25 degrees north latitude). The average noontime incidence of UV-B, measured with a sunburn UV-meter, was found to be between 1.9 and 3.4 sun units/hr in the months of March to May. Concomitantly with the UV measurements, a bioassay using a wild-type and an excision repair-deficient diploid strain of the yeast, Saccharomyces cerevisiae, was carried out to detect both lethal and genotoxic effects (mutation and mitotic gene conversion) of the sunlight. Exposure of the yeast cells to sunlight 30 to 180 min resulted in a significant level of cell death and a dose-dependent induction of mutations and mitotic gene conversion. The use of a Mylar filter cutting off virtually all of the wavelengths below 312 nm greatly reduced the lethal and genotoxic effects of sunlight. The results of UV measurements and biological studies suggest that an appreciable amount of potentially carcinogenic short UV wavelengths is present in sunlight in the Riyadh area. Therefore, factors other than the lack of biologically significant UV radiation in sunlight appear more likely to be responsible for the reduced incidence of sunburning and skin cancer in this geographical area.