REFLECTIONS ON HEAT

It is the prevailing doctrine that the rays of the sun that are deleterious to the skin lie in the ultraviolet (UV) region (Hausser & Vahle, 1927; Cleaver, 1970). Experimental photobiologists like Blum (1959), Magnus (1976) and Forbes (1978) have established that sunburning rays, 280-315 nm (UV-B), are photocarcinogenic. UV-A (315-400 nm), although requiring vastly higher doses than UV-B, can produce similar changes. It can induce erythema (Kaidbey & Kligman, 1979) and damage blood vessels (Kumakiri, Hashimoto & Willis, 1977; Gilchrest et al, 1982), and in murine skin it has induced elastosis (Berger, Tsambaos & Kaase, 1980). UV-A also produces papillomas which may progress to squamous cell cancers (Forbes, Davies & Cole, 1982). Finally, UV-A augments the sunburning (Willis, Kligman & Epstein, 1973) and carcinogenic effects of UV-B (Willis, Menter & Whyte, 1981). However, terrestrial sunlight is polychromatic, extending beyond the UV region into the visible (400-700 nm), infra-red (700-1,000,000 nm) and ultimately radiowaves. The latter can probably be ignored with regard to skin but visible radiation (i.e. light), seemingly innocuous, cannot. Deleterious effects ranging from phototoxic reactions (Kaidbey & Kligman, 1978) to DNA crosslinks (Gantt et al, 1979) and tumour enhancement (Griffin et al, 1955) have been reported. Light also causes solar urticaria in susceptible persons (Harber & Bickers, 1981). The effect of light in preventing or augmenting the injurious skin effects of UV-B and UV-A has yet to be investigated. Curiously infra-red radiation (IR), which comprises about 40",, of the irradiance reaching the earth's surface (Koller, 1965), has been largely ignored by experimental photobiologists. This neglect may stem from theoretical considerations which deter experimentation. It is a basic tenet that photochemical reactions are not heat dependent. Temperature is of no importance in the production of excited states. However, this ignores the fact that biochemical reactions are strongly heat dependent. Products of photochemical reactions might readily be amplified by heat. It is in this sense that the effects of heat could produce a marked effect on ultraviolet injury.

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