The 0.8% ultraviolet B content of an ultraviolet A sunlamp induces 75% of cyclobutane pyrimidine dimers in human keratinocytes in vitro

Tanning lamps, emitting predominantly ultraviolet (UV) A, are used widely throughout the U.K. and other countries, but little is known about the long-term risks associated with their use, especially with respect to skin cancer. We have exposed normal human epidermal keratinocytes to a commercial tanning lamp and used the comet assay in association with DNA repair enzymes T4 endonuclease V and endonuclease III to investigate the relative yields of directly formed cyclobutane pyrimidine dimers (CPDs) and indirectly formed types of oxidative DNA damage. To put the risk of using tanning lamps into perspective, the sunbed used in this study (five Philips Performance 80W-R UVA tubes at a distance of 35 cm) was found to be approximately 0.7 times as potent at inducing CPDs as U.K. natural sunlight around noon on a fine summer day. This compares with a relative risk for CPD induction and erythema of 0.8 and 0.7 times, respectively, calculated from the relevant action spectra of tanning lamps and British noontime sunlight. To determine the relative contribution of UVB and UVA to the induction of CPDs and oxidative DNA damage, we modified the spectral output of the tanning lamps with a series of Schott WG UVB filters. The induction of CPDs was more dependent on the UVB component of the sunbed than oxidative types of damage. Schott WG UVB filters with 50% transmission at 305 nm reduced the yield of T4 endonuclease V sites by 42% while there was only a 17% decrease in the yield of endonuclease III sites. CPD induction was not completely abolished after irradiation through WG335 and WG345 nm filters despite there being no detectable UVB. From these data, it was estimated that, although the tanning lamps emitted only 0.8% of their total output in the UVB range, these wavelengths were responsible for the induction of over 75% of CPDs and 50% of the oxidative damage to DNA.