U-What? The UV Index Explained

Chehreh Hessami, BA&Sc | Aug 12, 2014 | Health UV Sun

Ultraviolet, or UV radiation is part of the electromagnetic spectrum that is emitted from the sun. It has wavelengths shorter than visible light, making it invisible to the naked eye. Although invisible, too much UV radiation exposure in a short amount of time can cause suntans as well as sunburns.

Ultraviolet radiation is composed of UVA, UVB and UVC radiation, classified based on their length. Measured in nanometres, or billionths of a meter, UVA rays are the longest (between 320 nm and 400 nm) and UVC rays are the shortest (100 nm to 290 nm). The energy, and skin-damaging capacity of UV light is inversely proportional to its wavelength, meaning the shorter, UVC wavelengths are more harmful.

Luckily, the ozone layer filters out most of these shorter wavelengths, and the majority of the UV radiation that reaches our skin is composed of UVA and UVB. Despite the fact that the ozone layer provides this beneficial filtering effect, the remaining UV rays that do manage to get through the Earth’s atmosphere can still damage the skin. This is why it is important to apply sunscreen that provides “broad spectrum” coverage, protecting against both UVA and UVB radiation.

Since the strength of UV radiation is dependent on the amount of filtering provided by the atmosphere, it can vary depending on the location and time of day. Checking the local UV Index can help determine the risk factors throughout the day. The UV Index was first developed by Environment Canada to help the general public determine their risk of sunburn throughout the day, and to effectively protect against the harmful consequences of overexposure to UV light. The higher the UV index, on a scale from 0 to 11+, the higher the risk of harm from unprotected sun exposure

The total dose of UV radiation reaching the earth’s surface depends on many factors, and therefore the potential damage to human skin and tissues does, too. The UV index is calculated based on the forecast ozone levels, the effects of cloud filtering, and the elevation above sea level. Completely overcast skies can prevent a good portion of the UV radiation from reaching ground level, however, even on a cloudy day, enough UV rays can get through to harm the skin

Here are the different UV Index ranges, as described on the UV Canada app. When UV levels reach 3 (Moderate), sun protection is required:

Values of 11 or more are very rare in Canada, however, the UV index can reach 12 or higher in the tropics and southern US.

These guidelines are suggested for the average skin type in average conditions. For those with more sensitive skin - Type I (always burns, never tans) or Type II (usually burns, minimally tans) - it may be necessary to take precautionary measure even when the UV Index is “low”. It is important to get to know your skin type and determine your own personal sun safety needs.

For every skin type, checking the UV index should be a part of your daily routine. Get in the habit of looking up your local UV index before heading outside for the day, to ensure you are protecting your skin properly. Check out the useful “Time to Burn” feature on the UV US and UV Canada Apps. Available for free download on the Apple Store and on Google Play)

Source: Calculating the UV Index - United States Environmental Protection Agency

UV Index information courtesy of UV Canada App Ver 2.1.2