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 skins 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 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.
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
Exposed to the
sun
The skin and the eyes are the organs
with the highest exposure to the suns 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 (welders
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 childs 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.