AS a cosmetic concept, the suntan is a relatively recent cultural phenomenon that became popular among the fair skinned races only in the present century.
In bygone times, a tan was associated with outdoor manual work, and was therefore something any member of the civilised classes would carefully avoid.
Recently, however, Victorian wisdom is coming to the fore again not for social or cosmetic reasons, but because there is a growing realisation that excessive exposure to the sun can be injurious to your health, and sometimes even lethal.
It is in this context that Met Eireann or the Meteorological Service to those who missed the change of name some months ago recently introduced a Sunburn Forecast Service, intended to allow potential sunbathers, or those who might otherwise be occupationally or recreationally vulnerable, regulate sensibly their exposure to the sun. It began on May 1st and will continue through September. The new facility will provide guidance as to how long, given the prevailing weather conditions, the average person can allow himself or her self to be exposed directly to the sun without the risk of burning.
The health problems associated with sunshine are not a consequence of the heat or the light that it provides, but of another invisible, and for most of the time impalpable, ingredient the short wave energy called ultraviolet radiation.
We are protected from the worst excesses of this ultraviolet radiation by the now famous ozone layer some 13 miles above our heads. The layer's condition, however, has been causing great concern of late, as man made substances provoke chemical reactions that tend to weaken it, and even when it is in the best of health, the ozone layer does not protect us from all the harmful effect of ultraviolet radiation.
UV radiation, as it is called, is divided by convention 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, it would be lethal to all living things. Fortunately, it is completely absorbed by the ozone in the upper atmosphere.
The longest UV wavelength, on the other hand the so called UV-A is relatively harmless. Exposure to it affects the pigmentation of the skin to produce a suntan.
But in the middle lies UV-B less lethal than UV-C, but dangerous and potentially harmful nonetheless and it is with this that the new warning service primarily concerns itself.
Excessive exposure to UV-B results in sunburn by causing a dilation of the blood capillaries in the cells beneath the skin, and a consequent redness and generation of heat. More importantly, however, this can lead, in certain circumstances, to mutations that result in various forms of skin cancer.
There are some 6,500 cases of skin cancer in the Republic every year, and the most serious form, the often lethal malignant melanoma, accounts for some 5 per cent of all new diagnoses annually. The high levels of recent years are undoubtedly linked to more people taking foreign holidays, but the fact that many sufferers have never been outside the country indicates that indigenous UV-B cannot be ignored.
Met Eireann's new warning facility, introduced in co-operation with the Irish Cancer Society, is aimed at encouraging people to use the sunshine sensibly.
The strength of the UV-B radiation reaching the surface of the earth at any given time depends mainly on ozone concentrations in the upper atmosphere the less ozone, the more UV-B that gets through. The amount of ozone, in turn, varies seasonally, often for reasons related to its destruction by man made chemicals. It also varies from day to day, depending on the pressure pattern. In general, high pressure over Ireland results in less ozone overhead, and vice versa.
Crucial to the success of the technique of predicting UV-B levels is an ongoing awareness by the forecasters of background ozone levels over Ireland. This is obtained from measurements carried out at Mace Head in Co Galway by staff from UCG, and from readings at Malin Head, Co Donegal, and Cahirciyeen, Co Kerry, obtained by Met Eireann.
The forecasters combine the output from their daily computer models of the atmosphere with their knowledge of current average ozone levels, and expected cloudiness, to arrive at an estimate of the maximum time the average person ought to spend exposed to the sun that day in order not to suffer sunburn. The sunburn information is included in forecasts provided daily to the media, and on the Weather dial telephone facilities. In the event of high risk conditions later in the summer, warnings will also be broadcast with the regular forecasts on radio and TV.
The forecasters emphasise that the times quoted which would typically range from more than 90 minutes in a low risk situation to less than 45 minutes at a time of high risk relate to a person of average skin type. People with sensitive skin should halve the times quoted, while more swarthy individuals might safely risk exposure for, say, twice as long.
Users of the information should also bear in mind that UV-B penetration is at a maximum around noon, when the sun is highest in the sky and not in the afternoon when the temperature may be higher. At midday, the sun's rays enjoy their shortest route through the atmosphere.