Ultraviolet radiation: the sun's death ray

UV index

If you are not prepared to take measures for maximum possible UV-protection every single day, you can use the UV index to get an idea when to take UV-protection more seriously. Even though even a cloudy winter day at a high latitude can produce sun damage, not all days/locations are created equal -- some are much harsher on your skin then others. Enter the UV index. The UV index is an standard international measure of how strong the ultraviolet (UV) radiation from the sun is at a particular place on a particular day. You can find out about UV index at a given time/location in your area at weather.com or similar websites.

The UV index uses a roughly linear scale, approximately between 1 and 10. "Roughly linear" means that two hours of exposure at UV index 1 are approximately equivalent to one hour of exposure at UV index 2.

The US Environmental Protection Agency provides some basic recommendations for UV protection based on the UV index. Below is a brief summary of EPA recommendations (for more details, see EPA website).

• UV Index 0-2: Low danger to the average person. Wear sunglasses; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
• UV Index 3-5: Moderate risk of harm from unprotected sun exposure. Wear sunglasses and use sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense.
• UV Index 6-7: High risk of harm from unprotected sun exposure. Wear sunglasses and use sunscreen having SPF 15 or higher, cover the body with sun protective clothing and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 11:00 AM to 4:00 PM during summer in zones that observe daylight saving time).
• UV Index 8-10: Very high risk of harm from unprotected sun exposure. Same precautions as above, but take extra care - unprotected skin can burn quickly.
• UV Index 11 or higher: Extreme risk of harm from unprotected sun exposure. Take all precautions, including the following: wear sunglasses and use sunscreen, cover the body with a long-sleeve shirt and pants, wear a broad hat, and avoid the sun from two hours before to three hours after solar noon.

Keep in mind that UV index data, their interpretations and EPA recommendations have important limitations. In particular, the UV index is weighted more towards UVB frequencies and tends to underestimate UVA exposure. In other words, the UV index is more useful in assessing the risk of sunburn than long-term damage to the dermis and skin matrix that leads to wrinkles. In fact, low values of the UV index may, in certain situations, underestimate the impact of sun exposure on skin aging. Therefore, if you are concerned with skin aging you may want to be even more proactive than the EPA guidelines recommend.

UV exposure vs. season, time of day and latitude

Most people know that UV exposure is highest around noon and in early afternoon, in the summer and at low latitudes. Correspondingly, EPA recommends to reduce time in the sun between roughly 11:00 AM to 4:00 PM during summer in zones that observe daylight saving time. However, things are not as simple as they might seem. Indeed, UV intensity varies dramatically during the day as well as between seasons and across latitudes. (See the charts of radiation intensity vs time of day/year for UVA and UVB. For example, a sunny winter morning in Canada presents virtually no risk of sunburn, whereas a sunny summer noon in Egypt virtually guarantees it (assuming no sun protection). Yet, sunlight can contribute to skin aging even outside peak hours, summer season, or low latitudes. This is partly due to the fact that skin aging is caused predominantly by UVA. While both UVA and UVB rise towards midday, summer and lower latitudes, the changes are much less dramatic for UVA than for UVB. As a result, non-peak levels of UVA may be considerable compared to its peak levels (much more so than for UVB). Therefore, if you wish to minimize skin aging, you should expand your UV-protective measures beyond just midday, the summer and low latitudes.

Indirect sunlight

Much of UV radiation remains in the sunlight when it is reflected from the majority of common surfaces or refracted via clouds or water. When outside, don't assume that wearing a hat or staying in the shade fully protects you. Reflected sunlight may retain over a third of its UV rays. In particular, your UV exposure is much higher on the snow and water. Even beach sand reflects 20-30% of UV. Beach goers and skiers should be especially thorough regarding UV protection measures.

Clouds, except perhaps on a heavily gloomy day, cannot be relied upon for UV protection. A thin layer of clouds reduces UV intensity by only 20-40%. Water is a similarly weak protector: twelve inches of pool water would also reduce UV intensity by only 20-40%.

Clothing can be more or less protective, depending on thickness, color, fabric type and many other factors. Wet clothing is less protective than dry clothing - up to 50% of UV penetrates wet clothing.

Indoor UV light

You may have heard that glass blocks UV rays. It does block UVB quite well but often fails to block much of UVA. The degree of indoor UV exposure depends on light intensity, type of glass and other factors. (See our article on UV radiation indoors for more details and protection guidelines.)