Long before sunblock, or even calamine lotion, our ancestors had numerous folk remedies for soothing sun-seared skin.
The medieval favourite was an ointment made by boiling ivy twigs in butter; later concoctions included preparations made from aloe and mallow roots, as well as a soap made of chicory and honey.
All were intended to prevent or repair the damage done to human skin by solar ultraviolet radiation. The radiant energy from the sun is spread over a wide spectrum, ranging in wavelength from about from 0.1 millionths of a metre - or micrometre - to 3 micrometres.
Most of the radiation reaching Earth is in the visible part of this spectrum - light we can actually see; its wavelength is roughly in the band from 0.4 to 0.8 micrometres. Some solar radiation is of longer wavelength, comprising infra-red radiation and radiant heat. But a significant proportion of solar energy is in the form of ultra-violet radiation - with a wavelength slightly shorter than that of visible light.
Both suntan and sunburn are caused by radiation in this part of the spectrum.
It is radiation that we cannot see, being outside the range to which our eyes react, but it has a palpable effect upon our skin. It ranges in wavelength from about 0.1 micrometres to 0.4 micrometres and is divided, by convention, into three categories, each identified by its characteristic wavelength.
UV-C is the very shortest wavelength, and if it were to reach the surface of the Earth, which it would if the ozone layer did not exist, it would be lethal to all living things upon the planet.
The longest UV wavelength, UV-A, was until the early 1990s thought to be comparatively harmless, but is now known to be almost as dangerous as the intermediate category, UV-B. This latter, although less lethal than UV-C, is dangerous and potentially very harmful.
In recent years, Met Eireann's Sunburn Forecast, published by the media, allows potential sunbathers, or those who might otherwise be occupationally or recreation ally vulnerable, to regulate sensibly their exposure to the sun.
Apart altogether from periodic man-induced depletion, the amount of ozone in the upper atmosphere varies, both seasonally and day to day; in general, high pressure results in less ozone overhead, and vice-versa.
The forecasters combine measurements of current ozone levels in the atmosphere over Ireland with the weather predictions from their daily computer models of the atmosphere, and the expected cloudiness, to arrive at an estimate of UV intensity, and hence an estimate of the maximum time that any individual should allow themselves to cavort in the midday sun.