Ultraviolet Radiation and Exposure

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 (break chemical bonds between atoms), visible and infrared photons do not. For example, when ozone (O3) absorbs ultraviolet radiation, the molecule is split leaving (O2) and (O). Once the uv photon is absorbed by ozone, it no longer exists, and its energy has been delivered to the ozone molecule. This is how ozone prevents much of the potentially harmful uv radiation from reaching the Earth's surface. Concerning the effects of ultraviolet radiation for humans, 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 in the upper atmosphere 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)

Note that most indoor tanning facilities expose users to both UV-A and UV-B radiation and thus are generally not considered safe, e.g., Indoor Tanning Is Not Safe.

The figure below shows amount of ultraviolet radiation energy from the Sun that strikes the top of the Earth's atmosphere as well as the amount the reaches the ground surface. Almost no UVc reaches the ground surface, only about 10% of the total UVb from the Sun reaches the ground surface, and about 50% of the total UVa penetrates through the atmosphere to the ground. In other words, ozone in the stratosphere absorbs nearly 100% of the UVc photons from the Sun, it absorbs about 90% of the UVb photons from the Sun, and about 50% of the UVa photons from the Sun, which largely protects inhabitants at the Earth's surface from dangerous exposure to ultraviolet radiation.

The flux of ultraviolet radiation energy from the Sun. Top blue line shows the ultraviolet energy that strikes the top of Earth's atmosphere. Bottom line shows the ultraviolet radiation energy that strikes the Earth's surface at sea level. The bottom axis has the highest energy uv photons on the left and lower energy uv photons toward the right.

Ultraviolet radiation can be quite harmful to both humans and other inhabitants of the Earth. This is why we should be so concerned about any depletion of stratospheric ozone. But before moving on, it should be mentioned that some ultraviolet radiation is beneficial to us. It stimulates the body to produce vitamin D. In a place like Tucson, there is enough stray ultraviolet radiation (that reflected off other surfaces and not in the direct beam of the Sun) for plenty of vitamin D production. However, lack of ultraviolet exposure resulting in vitamin D deficiencies can be a problem for people living in high latitude regions, especially in winter. In addition, ultraviolet radiation may be responsible for random mutations in organisms, some of which lead to evolutionary speciation.

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 above). 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 since those cells are there for protection.
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. The melanin moves within the cytoplasm of the cell and will absorb uv photons before they are able to reach the nucleus. Fair skinned people should limit their first sun 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 protect the skin from exposure to UV-B photons, though experts suggest selecting one that also protects against UV-A photons. Sunscreens protect the skin by absorbing or reflecting ultraviolet photons, which prevents them from being absorbed by the skin and possibly leading to skin damage and cancer.

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 picture of uv reflection).

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 to the elevated ground surface.

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, the sun angle is smaller near the Equator (tropics) becoming larger toward the poles. Thus, in general potential exposure to uv radiation is greatest at low latitude locations and smallest at high latitude locations.
• Outside the tropics, sun angle is much smaller in summer compared to winter
  • Minimum sun angle in northern hemisphere occurs on June 21, which is the day of greatest potential uv exposure for all locations in the northern hemisphere north of the tropics. The closer the date to June 21, the greater the potential uv exposure.
  • Maximum sun angle in northern hemisphere occurs on December 21, which is the day of least potential uv exposure for all locations in the northern hemisphere north of the tropics. The closer the date to December 21, the smaller the potential uv exposure.
  • The dates are reversed for the southern hemisphere ... June 21 is the day of least potential uv exposure and December 21 is the day of greatest potential uv exposure.
• During a day, sun angle is smallest at solar noon, which is the exact halfway point between sunrise and sunset. Thus, on any given day, the closer to solar noon, the greater the potential ultraviolet exposure.

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 do reflect some UV radiation, but some UV radiation penetrates through clouds. The amount of UV radiation that gets through depends on the cloud type and cloud thickness. Thin clouds let a lot of UV radiation through. Thus, it is recommended that you apply sunscreen even on cloudy days. (Instuctor's Note: Some of the worst sunburns that I have experienced happened to me on cloudy days when I stupidly did not think to apply sunscreen).

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. Several factors account for this:

• 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)