Practical use of the UV Index
UV Index and its modification by clouds
UVI = UVI0 x CMF x (1+ 0.08 x dH)
where CMF is a so-called Cloud Modification Factor (a number between 0 and 1 – see Table 1) and ?H is the difference in altitude (in km) from the reference altitude of UVI0 . Table 1 shows CMFs for different cloud types and different cloud cover.
The harmful effects of UV radiation depend not only on the received UV dose but also on the sensitivity of the individual. Human skin is often classified into four main groups according to the skin’s ability to tan. This classification is shown in Table 2 which also gives the approximate dose (in J/m2) required to obtain a reddening of the skin (1 MED). Thus 1 MED varies for different skin types.
|The sunburn time is the maximum time one can stay unprotected in the sun without receiving a sunburn. Sunburn times can be calculated for each skin type from the UV Index and the value of 1 MED for each skin type. As an example, Figure 3|
Figure2: Sunburn times in minutes for skin types I, II, III and IV and 1 MED according to DIN-5050 calculated for clear sky days
|shows the sunburn times in minutes for different UV Index values and MEDs defined by DIN-5050 (shown in Table 2). It is important to point out that the value of 1 MED is not a precisely determined number for each skin type. Dermatological studies have shown that within one of the skin types the value of 1 MED may differ depending on the disposition of individuals. To further describe this phenomenon sophisticated regional studies of the photosensitivity of populations are needed.|
|Exposed to the
The skin and the eyes are the organs with the highest exposure to the sun’s ultraviolet rays. Although hair and nails are well exposed they are less important from a medical point of view. Exposure to solar UV radiation may result in acute and chronic health effects for the skin, the eye and the immune system. Acute effects of UV exposure include erythema (sunburn) of the skin and photokeratitis (welder’s flash, snowblindness) of the eye. Chronic skin effects are skin cancer and premature ageing of the skin while chronic effects to the eye include cataract, pterygium and droplet keratopathy. While UV-B radiation mainly causes erythema and various skin cancers UV-A radiation has a pronounced effect on the subcutaneous tissue and can alter the structure of collagen and elastin fibres and hence accelerate ageing of the skin. It is important to understand that the skin has a capacity to adapt to UV radiation by producing melanin (tan) that protects against UV exposure. The human eye does not have such a capacity.
How to choose
and use Sunscreens
|In addition to skin type, possible cutaneous
or ocular reactions could modify the efficiency of protective measures.
Such photosensitivity reactions may be caused by a number of external or
internal agents. Use of some drugs like psoralens, porphyrins, coal tar,
antibiotics or various kinds of anti-inflammatory agents, antimicrobial
products, fragrances, plants etc. can result in erythema at lower UV doses.
A simple guide for application of UV protective
tools for different values of the UV Index and for the most photosensitive
skin (skin type I and babies) and for more tolerant skin type III is given
by Table 4. This guide only presents a rough indication and is an
example of how the public could be addressed in a leaflet.
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