Estimating UV's human cancer risk.

Since 1985, the Antarctic's seasonal ozone hole has riveted attention on the importance of stratospheric ozone -- that natural filter of the sun's cancer-causing ultraviolet (UV) light. But Jan C. Van der Leun's concern over ozone thinning began even earlier, when modelers were calculating the threat posed by supersonic jets. Though researchers could model how much UV would seep through a thinner ozone layer, Van der Leun knew they couldn't knowledgeably predict human skin's response. So he launched what would become a 20-year investigation to do just that at University Hospital Utrecht in the Netherlands, using the hairless mouse as a human surrogate. In the October HEALTH PHYSICS, he and colleague Frank R. de Gruijl describe the culmination of that effort.

It takes about the same UV dose to induce sunburn in humans as it does in hairless mice. What's vulnerable to carcinogenic transformation, however, is not the surface skin, but the proliferating cells below, de Gruijl explains. So he and Van der Leun fine-tuned data on UV's skin carcinogenicity -- its action spectrum -- in this mouse to account for the longer path UV rays must traverse through human skin.

This adjustment for the long-wavelength (A) portion of the UV spectrum now indicates that UV-A offers five times the human cancer risk seen in the mouse action spectrum. The finding supports earlier warnings about the cancer threat posed by tanning salons that use lamps emitting UV-A (SN: 5/3/86, p.281). In the 1980s, many salons switched to such lamps when cancer concerns erupted over shorter, UV-B wavelengths emitted by the sun and conventional sunlamps. The new human action spectrum now indicates that the longer UV-A exposures needed to create a tan would pose about the same cancer risk as a UV-B tanning dose.

The new human UV action spectrum also indicates that when it comes to a permanent thinning of Earth's ozone, each 1 percent drop in stratospheric ozone could increase the incidence of nonmelanoma skin cancers by 2 percent.