Sunscreens, skin photobiology, and skin cancer: The need for UVA protection and evaluation of efficacy

Sunscreens are ultraviolet radiation (UVR)-absorbing chemicals that attenua te the amount and nature of UVR reaching viable cells in the skin. They are selected and tested for their ability to prevent erythema. No sunscreen pr events photodamage, as it has been demonstrated that suberythemal doses of UVR cause a variety of molecular changes (including DNA damage) in these ce lls. Furthermore, the spectrum of UVR reaching viable cells is altered by t opically applied sunscreen. In this review, the basic aspects of sunscreens and skin photobiology are reviewed briefly. Although there can be no quest ion concerning the efficacy of sunscreens for the prevention of erythema, q uestions remain because of the possible cumulative effects of chronic suber ythemal doses and the increased exposure of skin cells to longer UVR wavele ngths. The current major issue surrounding sunscreens involves their abilit y to protect skin cells against the effects of UVA radiation. These UVA eff ects may be direct damage (base oxidations) or effects on the skin immune s ystem, yet there is no uniformly accepted method for the evaluation of UVA protection. This review is focused primarily on the latter topic covering a ction spectra that implicate the need for UVA protection. In addition, in v ivo and in vitro methods proposed for the evaluation of candidate sunscreen formulations of UVA protective ability are reviewed. Finally, revisions in the terminology used to describe the protection afforded by sunscreens are suggested. It is proposed that SPF ("sun" protection factor) be renamed "s unburn" protection factor and that "critical wavelength" be designated "lon g wave index."