ATMO336 - Fall 2005

Dangers of Ultraviolet Radiation

Background information on Radiation

Electromagnetic energy (radiation) is composed of individual "packets" or "particles" of energy called photons. 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, 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.

An object absorbs radiation energy by absorbing photons. 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.

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:

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

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

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)

Thus, you can see that 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, I should mention 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. A diagram will be drawn in class. The possible effects of UV radiation on the skin depends on where the photons are absorbed:

(*)If UV-B absorbed by outer dead skin cells, no problem

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

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.

The human body protects itself from UV radiation in three ways denoted by (*) above.

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)

If you notice any of these signs, you should get it checked out immediately. NOTE: lesions often develop on the trunk of the body, even in areas that were never exposed to the sun.

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.

WARNING: 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%).

WARNING: although fair skinned people are more susceptible to skin cancer, even dark skinned people who don't burn can develop skin cancer.

WARNING: 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.

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

A good rule of thumb is the shadow rule. If your shadow is shorter than your body, you need UV protection, and if your shadow is longer than your body, UV exposure is probably not extreme.

WARNING: 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.

WARNING: 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. 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.

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)

! Link to today's UV forecast for major cities