The UV index was created for the general public and unfortunately is not considered a sensitive enough scale for most XP families. However, in the absence of a personal UV meter, it can be used to help guage the amount of protection and exposure risk when having to go outdoors during the day. The UV index is highest around noon in summer and lowest at sunset in winter. It usually achieves a reading of zero within half an hour before sunset or after sunrise. A zero index tends to correspond to a reading of a few tens of uWatts/cm2 on a UV irradiance meter.
How is the UV index different from a personal UV irradiance meter? The answer is that the UV index actually gives a reading of the biological effective UV, which means it tells us how much UV energy is damaging the skin. On the other hand, most personal UV meters display the amount of UV detected by a photodiode detector, which is not matched to the way our skin absorbs the UV energy. For example, when the index is 1 it is ten times less damaging to the skin than when the index is 10. The same kind of thinking cannot be used with an irradiance meter. A reading of 100 uWatts/cm2 on an irradiance meter may or may not be ten times safer than a reading of 1000 uWatts/cm2. If the UV index had resolution between readings of 0 and 1, it might be a better tool for XP patients to determine the amount of protection afforded by the atmosphere near sunset. For now and until we learn more, the best approach to avoiding UV from the sun is to wait until sunset or use proper protective gear.
How does the UV index work? We said the UV index is a biological effective UV reading. Secifically this means that it is a measure of the sunburn intensity of the UV spectrum at a given spot on the globe. It is calculated using a measurement of the ozone thickness, acquired from two orbiting NOAA satellites. Computer models are then used to determine how much UV makes it to the Earth’s surface, while taking altitude, latitude, and cloud cover into account. This information is then adjusted for how UV interacts with human skin in terms of each wavelength’s ability to produce a sunburn. The result is a single number that represents the biological effective UV for sunburn. It is scaled down to become a number typically between 0 and 10. The larger the number, the greater the sunburn effects of the sun. In some places it can even be higher than 10. This is usually denoted as 10+. Click here for a more technical description and example calculation of the UV index
How does the UV index vary throughout the world? The UV index is useful to gain insight into how UV varies throughout the world. The picture below was a projection of what the UV index would be on New Years Eve, 2000, the height of summer in the southern hemisphere. As you can see, the index was extremely high in Peru , while it was less than one in Alaska during the same season.
Click here for a forecast of the UV index in your location or, View a snapshot UV index that is updated frequently during the day.