In 2009, the World Health Organization declared tanning beds to be carcinogens (JW Dermatol Jul 30 2009), based largely on epidemiologic evidence. The tanning bed industry countered, using marketing practices remarkably like those employed by tobacco manufacturers (J Am Acad Dermatol 2010; 62:685). Solar-radiation–related mutation is primarily attributed to effects of ultraviolet B (UVB) radiation. The tanning industry claimed that the UVA1 wavelengths used in indoor tanning are safer.
In a recent study, investigators sought molecular evidence that the UVA1 radiation emitted by tanning beds also causes mutations in DNA. They exposed small areas of the skin of 12 healthy volunteers (Fitzpatrick sun-reactive skin types I and II, i.e. fair skin types) to UVA1 or UVB radiation at doses that produced comparable levels of erythema.
In biopsy samples taken from the irradiated skin sites immediately after exposure, cyclobutane thymine dimers (which lead to UVB signature mutations in skin cancers) were present primarily in the basal (the lowest) layer of the epidermis (of the skin) in the UVA1 (ultraviolet light emitted by tanning beds)-exposed sites, but primarily in the upper epidermis in UVB-irradiated sites. UVB produced more than 4 times as many cyclobutane thymine dimers as UVA1 and also produced 6-4 photoproducts, the other major type of DNA mutation observed after UV radiation. UVA1 irradiation did not produce 6-4 photoproducts. Both UVA1 and UVB activated repair mechanisms able to partially repair damaged DNA, with comparable repair rates.
Comment: This evidence demonstrates that UVB and UVA1 have distinct effects on keratinocytes. Whereas the major site of UVB damage is the upper epidermis, most UVA1 (the ultraviolet light of tanning bed) damage showed up in the lower dermis (lower part of the skin) — a particular concern because epidermal stem cells, which may be the targets cells of nonmelanoma skin cancers, are thought to reside in that location. These observations provide molecular evidence that the wavelengths of ultraviolet radiation emitted by tanning beds cause skin cancer, a finding we can use to counsel patients about the consequences of such devices. The findings also suggest that broad-spectrum sunscreens that protect against both UVA and UVB wavelengths will more effectively prevent skin cancer than those that block UVB alone. (Craig A Elmets. MD)
Published in Journal Watch Dermatology November 4, 2011