Sun Safety: The UV Index

Ultraviolet (UV) radiation is a form of radiation that is emitted by the sun and artificial sources, such as tanning beds, some types of lasers and some halogen, fluorescent and in candescent lightning. It has shorter wavelengths than visible light, so they are invisible to humans. Although some insects can see them, such as bumblebees. UV radiation is classified into three different divisions, which are UV-A, UV-B and UV-C.

UV-A radiation is the least affected by the atmosphere, so the UV radiation reaching the Earth’s surface is primarily composed of UV-A radiation and a small UV-B component. It is also the primary radiation used in tanning beds. UV-A radiation can cause damage to your skin, resulting in a tan. It causes almost all forms of skin aging, including wrinkles. It can also damage the collagen and elastin in the skin.

UV-B radiation makes up a small percentage of the UV rays from the sun, but it Is very high energy. UV-B radiation does not penetrate as deeply as UV-A radiation, but it can wreak havoc on the top layers of your skin and create sunburn. It damages skin cells and causes DNA mutations that can eventually lead to melanoma and other types of skin cancers. UV-B radiation from the sun can also cause cataracts, which proteins in your eye get damaged. The proteins start to collect pigments that cloud your vision. Thankfully, most of these rays are absorbed by ozone in the Earth’s atmosphere.

UV-C rays are the most harmful and are almost completely absorbed by our atmosphere.

The amount of radiation from the sun that hits the Earth’s surface depends on several factors. Those factors are:

• Stratospheric Ozone Layer: The amount of UV rays the ozone layer absorbs varies depending on the time of year and other natural events. Additionally, the ozone layer is thinner than it used to be due to ozone depleting chemicals used in industry and consumer products.
• Time of Day: The sun is highest in the sky around Noon. At this time, the sun’s rays have the least distance to travel through the atmosphere and UV-B levels are at their highest. In the early morning and late afternoon, the sun’s rays pass through the atmosphere at an angle and their intensity is greatly reduced.
• Time of Year: The sun’s angle varies with the seasons, causing the intensity of UV rays to change. UV intensity tends to be the highest in the summer.
• Latitude: The sun’s rays are strongest at the Equator, where the sun is most directly overhead, and UV rays must travel the least distance through the atmosphere. Ozone is also naturally thinner in the tropics compared to the mid- and high latitudes, so there is less ozone to absorb the UV radiation as it passes through the atmosphere. At higher latitudes, the sun is lower in the sky, so the UV rays must travel a greater distance through ozone-rich portions of the atmosphere, and in turn, expose those latitudes to less UV radiation.
• Altitude: UV intensity increases with altitude because there is less atmosphere to absorb the damaging rays. As a result, your chance of damaging your eyes and skin increases at higher altitudes.
• Weather Conditions: Cloud cover reduces UV levels, but not completely. Depending on the thickness of the cloud cover, it is possible to burn your skin on a cloudy day, even if it does not feel warm.
• Reflection: Surfaces like snow, sand, pavement and water reflect much of the UV radiation that reaches them. Because of this reflection, UV intensity can be deceptively high even in shaded areas.

The UV Index can take into account all of these factors to produce a scale to indicate the amount of radiation from the sun that will impact humans on Earth’s surface. The scale is as follows:

• A number of 0 to 2 means there is Low UV radiation, and no protection is needed. You can safely stay outside using minimal sun protection.
• A number of 3 to 7 means Moderate to High UV radiation and protection is needed. Seek shade during late morning through mid-afternoon. When outside, generously apply broad-spectrum SPF-15 or higher sunscreen on exposed skin and wear protective clothing, a wide-brimmed hat, and sunglasses.
• A number of 8 or higher means Very High to Extreme UV radiation and extra protection is needed. Be careful outside, especially during late morning through mid-afternoon. If your shadow is shorter than you, seek shade and wear protective clothing, a wide-brimmed hat, and sunglasses and generously apply a minimum of SPF-15, broad-spectrum sunscreen on exposed skin.

When it comes to UV radiation, remember the “Shadow Rule.” If your shadow is taller than you are, your UV exposure is likely to be lower. If your shadow is shorter than you are, you are being exposed to higher levels of UV radiation. Seek shade and protect your skin and eyes.
 
Sources: CDC, NASA, EPA, FDA, MD Anderson Cancer Center
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Image: UV index chart infographic, safety scale of sun exposure risk. (Sudowoodo via Shutterstock