Sunbathing, as a serious recreation, is a behavioural phenomenon of relatively recent times. It became universally popular only in the present century, and brought with it the perennial dilemma: how do you to acquire an attractive tan, without the potential dangers caused by overexposure to the sun.
The dangers of excess are not a consequence of the heat or the light the sun provides, but of another invisible, and for the most part impalpable, ingredient: the short-wave energy called ultraviolet radiation.
It is similar in essence to visible light waves or radio waves, but unlike these two, ultraviolet radiation is harmful to human health and to our general environment, and it is streaming towards us from the sun continuously. The ozone layer, some 15 miles or so above our heads, gives some protection, but it only shields us from the very worst effects. By convention, ultraviolet radiation - or UV for short - is divided into three categories, each identified by its characteristic wavelength. UV-C is that of 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, is relatively harmless, but in the middle lies UV-B; less lethal than UV-C, but dangerous and potentially harmful nonetheless. It is UV-B radiation which streams through to us in increasing quantities during those periods when the ozone layer is weakened. The dangers of increased exposure to UV-B are threefold. First, it affects the skin, at the very least by causing accelerated ageing or wrinkling, or much more seriously by causing mutations which may lead to various forms of cancer.
Secondly, excessive exposure to UV-B radiation can cause the lenses of the eye to cloud up with cataracts, a condition which may lead to blindness if untreated. And there is also reason to believe that excessive exposure to UV radiation may affect the body's immune system, and thus its general ability to resist disease.
And, of course, it is UV-B which causes sunburn. Long before the days of 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.
And for sun-induced headaches there were infusions of a herb called feverfew, or tea brewed from a freshly-gathered bunch of thyme. But nowadays we try to concentrate on prevention rather than cure.