What is the UV index?

Adopted in 1994 by the World Health Organisation (WHO) and the World Meteorological Organisation (WMO), the UV index, also known as the universal solar radiation index, is a standard international indicator that informs us every day of the degree of intensity of the UV radiation emitted by the sun that reaches the Earth's surface in the middle of the day, when the sun's elevation is at its maximum, and at a given location.

An instruction not to be confused with thesun protection factor (SPF) indicated on the packaging of sun care products.

This parameter thus allows us to understand and assess the risks posed by excessive exposure to ultraviolet rays for the skin (sunburn, skin cancer...), and not on the risks of eye damage (cataracts, corneal inflammation, eye cancer...), contrary to what one might think.

The UV index is communicated for a period of 2 to 4 hours around solar noon. This information is widely disseminated on the Internet, in the media or via weather applications for mobile phones in order to raise awareness among the population, and to encourage a change in sun exposure habits and behaviour.

The UV index is based on the erythemal action spectrum (skin reddening). It is presented on a scale without a ceiling, ranging from 1 to 11+ and depends on numerous factors:

• The latitude: The lower the latitude, the higher the UV index. Therefore, it is higher in equatorial regions than in temperate zones, due to the lower solar zenith angle than at high latitudes;

• The season: Its peak is reached at the summer solstice (around June 21 in the northern hemisphere and December 21 in the southern hemisphere);

• Solar Time:The UV index is stronger during the two hours before and after solar noon , when the sun is at its highest (short shadows). Depending on the geographical location and whether or not solar time is applied, solar noon falls between 11am and 4pm in the afternoon;

• Thealtitude: It increases by 2 to 5% every 1,000 metres despite the drop in temperature. In other words, high-altitude sites (mountains) receive more UV radiation than those located near sea level at low altitude, as the atmosphere is thinner and therefore absorbs less UV rays;

• The reflection of rays : Depending on the surface, UV rays are more or less reflected or scattered. For instance, fresh snow can reflect about 80% of UV rays, dry sand approximately 15%, and sea foam present on the surface of oceans around 10%;

• The cloud cover: Clouds can significantly reduce UV rays and visible solar radiation depending on their density, although they do not significantly absorb UV radiation. Indeed, if the cloud layer is thick then the UV index will be low; on the other hand, in the case of thinner clouds, the UV index will be high because the radiation is less filtered;

• The ozone layer: Its thickness determines the amount of UV radiation that reaches the ground, and can vary depending on the time of year and even during the day. Thus, a decrease (or increase) of 1% in stratospheric ozone leads to an increase (or decrease) of about 1.2% in the UV index. Indeed, ozone acts as a natural "absorber" of UVB radiation, just like nitrogen dioxide and sulphur dioxide.

Note: The UV index is never dependent on temperature: UV rays do not generate heat. It is possible to be exposed to UV rays in both summer and winter. Therefore, cold weather does not protect against a high dose of radiation

UV rays are categorised into five levels of risk:

• UV Index of 1 to 2: A UV index of 1 or 2 is a low UV index, meaning that it only poses a small risk of sunburn. However, individuals with a light phototype or sensitive skin may experience tingling or even irritation on exposed areas;

• UV Index of 3 to 5: A UV index of 3 to 5 is considered moderate;

• UV Index of 6 to 7: A UV index of 6 or 7 is high. UV rays can damage the skin and cause sunburn;

• UV Index of 8 to 10: A UV index of 8 to 10 is a very high UV index. If the skin is not protected, it will be damaged and can burn quickly;

• UV Index 11+ : A UV index higher than 11 is referred to as an extreme UV index. The risk of skin damage is then very high, thus it takes less time (a few minutes) to get sunburned.

In other words, the higher the UV index, the less time it takes to get a sunburn.

From the figures provided, it is thus possible to take precautionary measures to combat its harmful effects:

CAUTION! The recommendations given in relation to the UV index do not take into account the risks associated with UVA and phototype.

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• SASAKI H. & al. UV index does not predict ocular ultraviolet exposure. Translational Vision Science & Technology (2021).