SKIN CARE: SUN AND SOLARIUM DAMAGE – VARIABLES
The total amount of ITV radiation reaching the skin at any given time is influenced by such factors as the season, time of day, latitude, altitude, and conditions of the immediate atmosphere, such as the presence of rain or clouds, and the degree of pollution.
These variations are related both to the scattering of the UV radiation in the atmosphere, and to the distance the radiation must travel through the atmosphere. This distance depends on the position of the sun. Hence, the lower the angle of the sun above the horizon, the greater the atmospheric distance which must be crossed. Therefore, at noon in summer, with the sun almost directly overhead, the UV radiation is the most intense. Of the total UV radiation received from the sun on a midsummer’s day, about 66 per cent of the energy is received between 10 a.m. and 2 p.m.
The ozone layer present in the atmosphere also plays a major role in the absorption of this radiation. The thickness of the ozone layer varies, there being, for instance, a thinner layer in the tropics than elsewhere. As a result the intensity of UV radiation is approximately 15 per cent greater in the tropics than in other places. The ozone layer thickness also varies with the seasons (except in places close to the Equator), It is thickest in the winter and thinnest in the summer, which is why the effects of sunlight are more intense during the summer months.
At high altitudes the effects of UV radiation are greater, because there is less atmosphere to absorb the sun’s rays. Furthermore, in the presence of snow this is augmented by reflection. Contrary to a commonly held belief, water does not reflect but in fact absorbs UV radiation, so that one may become burnt even under the surface of water. Similarly, on an overcast or foggy day. considerable UV radiation will be transmitted by scattering.
Careful studies have shown that in Australia, in the midday summer sun, the earliest onset of burning for those with fair untanned skin, will occur, on average after 12 minutes. In the United States, under similar circumstances, the average time is 20 minutes. Other controlled studies have shown that various environmental factors play an important contributing role in determining the extent of the damage produced by UV radiation. These include wind, temperature and humidity. It has been convincingly shown that a combination of wind, heat and humidity will enhance the injurious effects of this radiation; on their own. however, these factors have no observable effect.
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