4. Practical use of the UV Index 
 

UV Index and its modification by clouds and altitude
As mentioned in section 2 the UV irradiance at any given place is affected by clouds and depends on the altitude above sea level. If, at a certain altitude, UVI0 represents the UV index for a cloud-free sky the following equation may be used to calculate the UV index, UVI ,  for a cloudy sky and at a different altitude: 

                                           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. 

 
Skin types 
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 sun 
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.
 

Protection of the skin
The skin is best protected by clothes. A shirt, a hat and trousers offer the best protection. UV-transparent clothes ought to be marked clearly. Skin parts that are not protected by clothes should be protected with a sunscreen containing UV-B and UV-A filters. During the first exposures to the sun a sunscreen with a Sun Protection Factor (SPF) of about 30 is recommended. Particular care should be exercised with babies and small children. Note that the protective effect of sunscreens depends not only on the quality of the sunscreen but also on a correct application of the sunscreen. Most people apply too little and not often enough. A sunscreen with an SPF of at least 15 must be generously applied every 2 hours to be protective. It should be applied well before exposure to the sun and it should be reapplied after a swim. If sunscreens are properly used they can protect from sunburn, skin cancer and skin ageing.
 

How to choose and use Sunscreens 
The Sun Protection Factor (SPF) stated on sunscreens indicates how efficient the product is. The higher  the SPF, the more efficient the sunscreen. According to the evidence of damage caused to the skin by UVA radiation and because a standardised method to evaluate the UVA filtration of sunscreens does not yet exist, it is also recommended to check the presence of UVA filters in the product, which is generally printed on the packaging. In all cases, sunscreens should not be used in order to prolong time of exposure but rather to limit the damage caused by sun exposure. That is why sunscreens should often be applied to parts of the skin that are not covered by clothing, especially to sensitive areas like the nose, the ears, the neck, the upper back, the top of the feet, etc. To choose the most appropriate sunscreen, users could refer to table 3 which indicates the SPF values according to the different skin types and to the UV Index levels. 
 

 
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.
 

Protection of the eye
The eyes can be protected by sunglasses containing UV-B and UV-A filters. According to the E.C. directive , 89/686/CEE , producers should put forward the category of protection of the lenses against visible and UV light. For general use a category of 3 is recommended, for high risk activities like mountaineering or boating, a category of 4 is recommended. Otherwise, because of the lateral exposure, it is recommended to wear wrap-around sunglasses. Such advice is especially important for children because the transmittance of UV radiation through the eye is higher for children than for adults – a child’s retina is less protected. The rationale is that sunglasses without UV protecting filters should not be used.

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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