The sun emits radiation over a broad spectrum of wavelengths: light that is visible, infrared that you feel as heat, and ultraviolet that you don’t see or feel.

UV has a shorter wavelength and a higher energy component than visible light. There are three types of solar UV radiation, UVA, UVB and UVC. Except for the latter, which is blocked by the atmosphere, they affect human health both positively and negatively.

UVA is needed by humans for synthesis of vitamin D, necessary for good health. However, overexposure to UVA has been associated with skin aging and skin cancers, suppression of the immune system, and eye damage, such as cataracts.

UVB is the most destructive form of UV radiation because it causes photochemical damage to cellular DNA. Harmful effects from it also include erythema (sunburn), cataracts and skin cancers, which may not show up until years later.

The sun is highest in the sky around noon. At that time, UV levels are the greatest because the sun’s rays have the shortest distance to travel through the atmosphere. In early morning or late afternoon, the sun’s rays pass through more atmosphere at an angle, so intensity is reduced at the surface.

The sun’s angle also varies with the seasons. UV intensity is higher in the summer. The sun’s rays are also stronger at lower latitudes where the sun is more directly overhead. The atmosphere is naturally thinner in the tropics compared to the mid and higher latitudes, so there is less to absorb the UV radiation as it passes through. At higher latitudes, the sun is lower in the sky, thus exposing those areas to less UV light.

UV intensity increases in direct proportion to altitude because there is less atmosphere to absorb it. As a result, the chances of damaging your eyes and skin increases at higher altitudes. So when skiing, for example, you receive a double dose, one from the altitude and one from UV reflected off snow.

Cloud cover blocks some UV, but not completely. Clouds do reflect some of the sun’s heat, so depending on how thick the cloud cover, it is possible to burn on a cloudy day, even if it doesn’t feel warm.

UV can be created artificially using special lamps. A common use of these lamps is in tanning beds. The World Health Organization published an in-depth report titled “Exposure to Artificial UV Radiation and Skin Cancer” in which it stated, “There was a prominent and consistent increase in risk for melanoma [a deadly form of skin cancer] in people who first used indoor tanning facilities in their twenties or teen years.” In other words, limit or avoid the use of tanning beds. If a tan is important to you, understand the risks to your health.

Surfaces like snow, sand, pavement, and water reflect much of the UV radiation that reaches them. For instance, because of this effect, you can burn your face even when wearing a sunhat.

Ozone naturally occurs in the stratosphere and greatly affects the amount of UV radiation reaching the Earth’s surface. Ozone is a colorless, odorless gas and has three oxygen atoms (O3) as opposed to the oxygen in our air. That oxygen, which makes up about 20 percent of the air, contains only two atoms (O2). Without this ozone layer above the atmosphere, UV hitting the surface of the Earth could be very dangerous to humans. Some UV that does get through the ozone layer is absorbed in the lower atmosphere. Even so, enough reaches us to cause health issues. These problems particularly impact people who spend a lot of unprotected time outdoors.

The amount of UV the ozone layer absorbs varies depending on the time of year and other natural events. Also, the ozone layer has become thinner due to consumer products like refrigerants (CFCs). Although some of these chemicals are being phased out, the ozone layer is not predicted to heal to pre-1980 levels for many years. One of the most destructive effects on the layer is from methane, a gas produced both in nature and also due to industrial activity. There is concern that thinning and holes in the layer could increase the prevalence of skin cancers.

In summary, it is important to protect yourself from the dangers of excessive UV exposure: don’t burn your skin; avoid tanning beds; use a sunscreen that blocks UVA and UVB; cover up or seek shade; avoid the sun during peak times and use extra caution around reflected sunlight. Most sunglasses block UV, so use them to safeguard your eyes.

Those who would like to ask me questions about any other science topic may e-mail inquiries to

Marcus Goodkind of Tuckasegee, a retired aerospace engineer, worked with the National Aeronautics and Space Administration as a manager at Kennedy Space Center on all the manned programs from Mercury to Shuttle, including Apollo 11, the first manned lunar landing.