There’s been a lot of publicity about sunbeds in recent months. I used one once to see if it helped me to increase my vitamin D. Indeed it did. Sunbeds can also help give you a tan. But at what cost? They also increase your risk of sunburn, and skin cancer – and possibly other unknown effects. The trouble is that the spectrum of radiation that they emit is quite different from sunlight, which our skins have evolved to cope with. The UV-A component of sun beds (wavelengths from 315-400 nm), which is the part that makes your skin wrinkle, and which may also be implicated in melanoma, is several times brighter than ever occurs in natural sunlight. But the most scary thing about it is that there is a lot more dangerous UV-B radiation (wavelengths from 280-315 nm) that has been implicated in dna-damage and all forms of skin cancer.
The graph below compares the UV part spectrum of sunlight at mid-latitudes (45S) for midday in summer and winter with that from a typical sunbed. Note that it’s plotted with a logarithmic scale on the y-axis so you can see the differences in the UV-B region where intensities are much lower than in the UV-A region. The grey curve shows the relative importance of these wavelengths in causing sunburn. The curve is called the “erythemal weighting function” (“erythema” is just a fancy word for “skin reddening”). The shortest wavelengths shown are far more damaging than the longer wavelengths.
The graph below shows what happens when you weight the spectra above by multiplying them with this erythemal weighting function. The total erythemally-weighted UV is just the area under each of these curves, which is usually expressed in terms of the UV Index (UVI). The areas under the curve for winter sunlight corresponds to a UVI of about 1. The areas under the curves for summer sunlight and the sunbed are about the same, and correspond to a UVI of about 12. But the distribution of energy with wavelength is dramatically different. Who knows how our bodies are supposed to cope with such abuse?