Sun beds and sunscreens - friends or foes?

When absorbed, a photon of a given energy has the same chemical and biological effect whether it comes from the sun or from an artificial, man-made source like a sun bed. If the spectrum and intensity of a sun bed are the same as those of the sun, the biological effects will be exactly the same. The main difference between radiation from sun and sun beds is that sun beds emit less visible light. This may affect the relative efficiency of photoreactivation, a process through which ultraviolet (UV) -induced DNA damage is repaired. Sun creams do not only reduce the intensity of the radiation reaching living skin tissue, but also changes the spectrum of the radiation, since they are mainly designed to reduce erythemogenesis, which is induced mainly by UVB radiation (280-320 nm). However, other biological effects, such as generation of melanomas, skin ageing and photosensitization associated with endogenous and exogenous dyes, may be caused also by UVA radiation (320-400nm) and even by visible light. Solar UV radiation is the main source of vitamin D in humans, and most sun beds are equally efficient vitamin D producers. However, both sun and sun beds can cause skin cancer. Notably, intense, intermittent exposures, yielding sunburns, are carcinogenic, while more regular, moderate exposures are less so. The positive effect of such exposures, mainly related to vitamin D photosynthesis, may be much more important than the negative effects. Considerations of skin colours of populations living at different latitudes, as well as modern experimental and epidemiological work, clearly urge us to focus more on the positive effects of UV radiation from sun and sun beds. The spectral region related to a given biological effect is given by the action spectrum of that particular effect. The action spectra of erythema, melanogenesis