Dangers of Ultraviolet Radiation

Background information on Radiation

Radiation is a form of energy transport. Electromagnetic energy (radiation) is composed of individual "packets" or "particles" of energy called photons. A photon is the smallest amount of radiation energy that can exist, i.e., photons cannot be broken down. Photons are classified by the amount of energy they carry. Some of the different types of radiation you may have heard of include X-ray, ultraviolet, visible, infrared, and microwave among others. The list was given in order of decreasing photon energy, i.e., an ultraviolet photon carries more energy than a visible photon, which carries more energy than an infrared photon, etc. Natural radiation is emitted (or given off) by all objects. The hotter an object, the more energetic photons it emits. For example, the hot sun emits ultraviolet, visible, and infrared photons (billions upon countless billions of individual photons), while the colder objects on Earth (the ground, oceans, trees, you) do not emit ultraviolet or visible photons, only infrared photons. Our eyes cannot see infrared photons, such as those emitted by relatively cold objects on the Earth (includes emission from ground surface, human beings, etc.), but we can see visible photons coming from the Sun. In fact most of the radiation emitted by the Sun is in the form of visible photons. However, the Sun also emits significant radiation energy in the form of ultaviolet photons that cannot be seen by our eyes, but can be dangerous to us.

Some of you may be more familar with using the electromagnetic wavelength to define different types of radiation. I thought the photon concept would be easier for most students. In terms of wavelength, ultraviolet radiation has shorter wavelengths than visible radiation, which have shorter wavelengths than infrared. Ultraviolet range of wavelength (0.01-0.4 micrometers); Visible range of wavelength (0.4-0.7 micrometers); Infrared range of wavelengths (0.7-100 micrometes). You will not have to know this wavelength information for the exam. What I would like you to know is that a single photon of ultraviolet radiation carries more energy than a single photon of visible radiation which carries more energy than a single photon of infrared radiation.

An object absorbs radiation energy by absorbing photons. This is how energy is transferred by radiation. One object emits or gives off radiation energy (photon by photon) and that energy is delivered to another object when that object absorbs radiation energy (photon by photon). Ultraviolet photons have enough energy to dissociate molecules, visible and infrared photons do not. For example, when ozone (O3) absorbs ultraviolet radaition, the molecule is split leaving (O2) and (O). Of more importance to us, ultraviolet photons have enough energy to damage or destroy DNA, visible and infrared photons do not. When we absorb visible or infrared photons the energy carried by the photon goes into heating us up, but will not permanently damage our cells like ultraviolet photons can. See this WORD document summarizing uv radiation.

Each of the classes of radiation is defined over a range of photon energies. For example, within visible light, a "blue" photon carries more energy than a "red" photon. In the same way ultraviolet radiation is further divided into 3 categories:

1. UV-C (wavelengths 0.2-0.29 micrometers)
• most energetic and potentially damaging to cells
• virtually all UV-C from Sun is absorbed by O3 and O2 in the stratosphere (and below) and does not reach the surface
2. UV-B (wavelengths 0.29-0.32 micrometers)
• Most UV-B from the Sun is absorbed by O3 in the stratosphere, but a significant portion does penetrate to the ground
• Linked to about 90% of skin cancers
• Also responsible for sunburns, eye cataracts, photoaging (wrinkling of skin), suppression of the immune system, and other problems in humans
• Reduces growth and health of ocean phytoplankton, damages early development in some fish, shrimp, and amphibians
• Damages and stresses many land plants including some staple food crops
3. UV-A (wavelengths 0.32-0.40 micrometers)
• Least energetic type of ultraviolet photon
• "Tanning Rays"
• But long-term exposure can lead to skin damage, such as photoaging (wrinkling or "leathery" skin) and perhaps skin cancer (not proven)

Effects of UV exposure on skin

The epidermis or outermost layer of skin is itself composed of three distinguishable layers: a protective outer layer of dead skin cells that are constantly being shed, a middle layer of living skin cells which migrate upward and eventually become part of the outermost protective layer, and finally cells which produce the new skin cells (see figure 2). The possible effects of UV radiation on the skin depends on where the photons are absorbed:

1. (*)If UV-B absorbed by outer dead skin cells, no problem
2. IF UV-B absorbed by live migrating skin cells, they can be damaged (sunburned) if cells have insufficient melanin. Melanin is a "tan" pigment in skin cells that can absorb UV radiation before it is able to reach the nucleus or otherwise damage the cell. Note that a sunburn is skin damage and indicates dangerous exposure to ultraviolet radiation. Specifically, the ultraviolet radiation damages and kills skin cells. As your body senses these dead cells, the immune system is activated to clean up the mess. It increases blood flow in the affected areas, opening up capillary walls so that white blood cells can come in and remove the damaged cells. The increased blood flow makes your skin warm and red. The damaged cells and subsequent swelling activate pain receptors.
3. IF UV-B absorbed by the cells responsible for producing new skin cells, several things can occur

• (*)These cells can be stimluated to produce more melanin to help protect cells. Fair skinned people should limit their first exposure of the year to allow melanin to be produced (takes about 48 hours for enough melanin to be produced).
• Cells can be damaged to the point where they can no longer reproduce new skin cell properly (DNA damage)
• (*)Often the body can repair DNA damage but the ability to repair declines with age and repeated exposure to UV radiation
• Otherwise skin cancer (uncontrolled growth of mutant cells)
• Most common of all cancers
• Often takes 20 years or more to notice
• Non-melonoma is most common type and rarely spreads. May appear as small fleshy bumps, raised warty growth, or red scaly patch. Most common on head, face, lips, or neck, of fair skinned people. Lesions can be removed.
• Melonoma is very serious and will spread beyond skin. Once this occurs it is usually too late. Melonoma accounts for only 5% of all skin cancers, but 95% of all skin cancer deaths. Most often appears on trunk of fair-skinned men and lower legs of fair-skinned women. Although less common in darker skinned people, melanoma can develop on palms of hands, soles of feet, or in mouth.

Melonoma can be detected early (before it spreads) using the "ABCD" self-examination:

• A - Asymetry
• B - Border irregularity
• C - Color variation
• D - Diameter > 0.25 inches (size of a number 2 pencil eraser)

Notes on UV protection

Most suncreens contain chemicals which absorb UV-B photons, though experts suggest selecting one that also absorbs UV-A photons.

SPF (Sun Protection Factor) is a measure of how much longer a person can safely stay in sun with sunscreen compared to without sunscreen. For example SPF-15 means you are safe 15X longer. If you normally burn in 10 minutes without sunscreen, you would last 150 minutes with sunscreen.

Although fair skinned people are more susceptible to skin cancer, even dark skinned people who don't burn can develop skin cancer and should use sunscreens when appropriate.

Lying under an umbrella at the beach or on a boat is not enough protection from the UV because UV photons are reflected from sand (50%) and water (>90%), so that even if you are sheltered from direct uv radiation, you are still exposed to reflected uv radiation (see figure 3).

High altitude or mountain locations are more dangerous than a location at sea level because UV photons have less atmosphere to travel through. At 10,000 ft, you are exposed to 40% more UV photons than at sea level. This is because the UV photons which get through the stratosphere can be absorbed or reflected back up by air molecules before they reach the surface of the Earth. In other words, although ozone in the stratosphere absorbs the majority of ultraviolet photons headed for the surface of the Earth, some ultraviolet photons get though the stratosphere. Those that do still need to make their way though the rest of the atmosphere, which becomes increasingly dense as one approaches sea level. At higher altitudes, the UV radiation that has made it through the stratosphere is better able to penetrate the thin atmosphere.

The sun angle is a very important factor. If the sun is directly overhead, the sun angle is 0° and when the sun is setting on the horizon, the sun angle is 90°. The closer the sun is to straight up (or the smaller the sun angle), the greater the UV exposure.

• In general, sun angle is smaller near the Equator (tropics) becoming larger toward the poles.
• Outside the tropics, sun angle is much smaller in summer compared to winter
• Minimum sun angle in northern hemisphere occurs on June 21
• Maximum sun angle in northern hemisphere occurs on December 21
• During a day, sun angle is smallest at solar noon

UV exposure does not depend on air temperature, i.e., you can get burned on a cold day just as easily as you can get burned on a hot day (if the sun angle is the same).

Skiers are often unpleasently surprised by sunburns for several reasons: (1) They think they can't get burned because it may be cold; (2) Snow reflects UV photons; (3) Mountain location.

Clouds reflect UV radiation, but thin clouds (especially cirrus) allow enough UV radiation to get through so burning can occur.

WARNING: Residents of Tucson are exposed to very high levels of ultraviolet radiation. Skin cancer rates in Southern Arizona are among the highest in the United States. While some of this may be related to Tucsonans spending more time outdoors, there are several climatological factors that account for the relatively high ultraviolet exposure for those living in Tucson:

• Lots of sunshine (lack of cloud cover)
• Relatively low latititude (more direct rays from the sun compared with most of rest of country).
• Relatively high altitude (2,500 ft above sea level)
• Clean atmosphere with low relative humidity. The visibility here is often 50 miles or more, which allows UV photons to pass right through. In more polluted places, aerosols block the passage of UV photons. In addition, aerosol particles swell (get larger) as relative humidity increases, and thus in humid environments, aerosols are able to block (or reflect away) more uv photons.

To alert people to the dangers of UV exposure, the weather service issues a UV Index forecast each day for each major city. The index ranges from 1-15 (15 being most UV exposure) and is valid for noon-time conditions. Calculation of the index considers:

• Sun angle at noon (depends on latitude and time of year)
• Current amount of ozone overhead
• Cloud cover
• Concentration and size of aersols in the atmosphere (these can reflect or absorb UV photons)