Regardless of what you think about ozone depletion and CFCs, ultraviolet radiation (UVR) is very real. So are its effects. Even if we should see no long term increase in UVR, most places on earth receive more than enough to cause real problems if you often go outside without protecting yourself.
Perhaps the best-known consequences of excessive UVR exposure is erythema, or sunburn. Sunburns can be mild or severe. If you've had a severe sunburn, you'll not likely forget it. Such cases are marked by bright pink or even scarlet-colored skin, swelling, blistering, and exquisite pain. An extremely severe case may also be accompanied by nausea, fever or chills, and tachycardia (a racing heart beat). Because of water lost through the skin, sunburns can also lead to dehydration.
Actually, the painful symptoms of sunburn are caused more by the body's response to UVR skin damage than by the damage itself. Although no one really understands the whole process, UVR damage apparently triggers an increase of several chemical substances, including prostaglandins and histamines. Both substances contribute to inflammation. Whole body exposure can also lead to increased levels of serum interleukin-1 and interleukin-6, which could partly account for some of the symptoms associated with an extremely severe sunburn.
Sunburn is primarily caused by the UVR wavelengths between about 295 and 320 nanometers (nm). Wavelengths in this range are known as UV-B.1 However, UVR between 320 and 400 nmcalled UV-Acan also give you a burn. UV-A is less energetic than UV-B, but it can penetrate the top layers of skin, damaging the lowest level. It is also absorbed less efficiently by the atmosphere. Consequently, the ratio of UV-A to UV-B will increase as the sun gets lower in the sky, and its contribution to sunburn will be relatively high in the early morning and late afternoon.
Of course, sunburn is not the only effect of UVR. One effect of long-term exposure is sun-damaged skineven in the absence of sunburn.
Much of what was once attributed to aging is now known to be caused by sun damage. Old age can bring about roughness, fine wrinkling, and looseness of the skin. Sun-exposed skin, however, is also marked by coarse wrinkling and elastosis, which gives the skin a pebbly, yellowed quality. Both wrinkling and elastosis are caused by damage to elastic fibers in the lowest level of skin, the dermis.
In addition to these effects, sun-exposed skin is also prone to irregular hyperpigmentation and depigmentation, and actinic keratoseswhich are rough, red patches of precancerous skin cells.
Another long-term effect of UVR exposure is the formation of cataracts. A cataract is any change in the structure of the lens that leads to a loss of transparency.
Cataracts have been associated with many risk factors, including smoking, diabetes, steroids, episodes of severe dehydration, andof courseUVR exposure.
As with skin damage, cataract formation is associated more with chronic exposure than acute exposurewhich is not to say that acute exposure is recommended. Indeed, acute overexposure can lead to permanent or temporary blindness.
One particularly excruciating result of acute overexposure of the eyes is keratoconjunctivitis, or snow blindness. Snow blindness is essentially a sunburn on the surface of the eye (i.e. the cornea and conjunctiva).
Symptoms include redness of the eyes and a gritty feeling, which progresses to pain and an inability to tolerate any kind of light. The pain has been compared to rubbing sandpaper across one's eyes. Fortunately, snow blindness is usually only temporary.
Sun and snow is an ideal combination for getting snow blindness. Snow is an outstanding UVR reflector, and the combination of direct and reflected sunlight is a double whammy for unprotected eyes. Skiers should thus be careful to protect their eyes when they hit the slopes.
Surfers should also be careful. Reflected light from the water can have the same effect as reflected light from snow.
Three kinds of skin cancer have been associated with UVR exposure: basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and melanoma.
By far the most common form of skin cancer is BCC, which makes up 75 to 90 percent of all skin cancers. It is strongly linked with sun exposure, and is rarely found on skin surfaces not exposed to the sun. The only exceptions usually involve arsenic or radiation exposure, or complications from tattoos, scars, burns, or vaccinations.
Fortunately, BCC does not metastasize (except in some AIDS patients) and is slow growing. Nonetheless, if it is left untreated, it can damage or destroy underlying tissue and cause disfigurement.
The next most common kind of skin cancer is SCC, accounting for about 20 percent of all cases. Like BCC, SCC most often occurs on sun-exposed skin. It can also occur in scar tissue, infections and ulcerations, areas of previous radiation exposure, areas of chronic irritation, and non-healing wounds.
Although less common than BCC, SCC is a more serious matter because it can metastasize. About 95 percent of all BCC can be cured if treated early. Nonetheless, SCC claims the lives of as many as 2,000 people a year.
Both BCC and SCC are thought to result from chronic UVR exposure rather than one or more acute episodes. Malignant melanoma, however, the most deadly form of skin cancer, seems to be most common in white people who have had intermittent sunburnsespecially in childhood or adolescence. Indeed, some scientists believe that one painful sunburn in children 15 or under can triple their odds of getting melanoma later on.
Also, indoor workers who vacation in the sun and get occasional burns are more likely to end up with melanoma than those who work in the sun.
Other possible factors include chemical carcinogens, viruses, and immune deficiencies.
Melanoma is easily cured if treated early. Once it spreads to the lymph nodes, the survival rate drops dramatically.
Finally, excessive UVR can also produce immune system deficiencies. Indeed, the development of skin cancers may well beat least in parta result of immune system damage.
For example, in one study, skin cancers were induced in mice by exposing them to UVR. When these skin cancers were transplanted into normal, genetically identical mice, most were rejected by the new host's immune system. However, when transplanted to mice that had been subjected to a short course of UVR exposure, the tumors grew and eventually killed them.
Similarly, another study showed that after human subjects had undergone twelve 30-minute exposures to artificial UVR in a commercial tanning bed, the functions of T cells and Natural Killer cells (which play a role in fighting viral infections and are cytotoxic to some tumor cells) were negatively affected.
How does UVR exposure affect immunity? One way is by the damage it does to Langerhans cells. Langerhans cells, which make up about 4-7 percent of the cells in the epidermis, are responsible for communicating with T cells and initiating a response to foreign invaders. UVR can damage these cells so that they can no longer perform that function. Instead, a suppressive response may be initiated, which actually prevents an immune response against the invader.
UVR is not all bad. For one thing, it assists in the production of vitamin D in skin cells. This vitamin D is absorbed by the body and then used in the up-take of calcium from the intestinal tract. This vitamin is essential for the growth and development of healthy bones. Fortunately, brief exposure to sunlight on a regular basis is enough to produce all the vitamin D most people need. The vitamin can also be obtained from dietary sources.
UVR is also useful for treating psoriasis and alopecia areata. But such treatment may also increase the risk of skin cancer, and should be undertaken only under the supervision of a knowledgeable physician.
An important part of protecting yourself from excessive UVR exposure is to recognize that UVR intensity can vary as a result of many different factors, including:
Time of day. UVR is most intense when the sun is highest in the sky. That's because it has to pass through less of the atmosphere to reach you.
Time of year. Because the sun is highest in summer, UVR is also at its most intenseother things being equal.
Latitude. You'll burn much faster in Hawaii than in Maine.
Elevation. Because ozone blocks out UV-B whether it's at ground level or in the stratosphere, you'll generally burn faster on top of a mountain than at its base.
Reflective surfaces. Reflective surfaces like sand, water, and especially snow, can greatly increase your UVR exposure, leading to burns. Indeed, reflected UVR can give you a burn even when you're sitting under an umbrella.
Stratospheric ozone. Observations have shown that stratospheric ozone can fluctuate dramatically in a relatively brief time. In addition to the sharp increase Mims noted in his article, he has also observed periods of extremely low ozone (as low as 230 Dobson units), along with correspondingly high UVR levels. This occurred in June, 1993 and is occurring again this summer.
So, what can you actually do to protect yourself from excessive UVR exposure?
1. If possible, avoid going outside during the midday hour, when solar UVR reaches its peak intensity. (Remember, midday is not necessarily at 12 p.m. It's when the sun is highest in the sky.)
2. If you do go outside, protect your eyes by wearing a hat and sunglasses. Studies have shown that using both of these is very effective in reducing UVR reaching the eyesand in reducing risk of cataracts.
3. Protect your skin by wearing a sunscreen that provides both UV-A and UV-B protection, and has a sun protective factor (SPF) of 15 or more.2 Sunscreen products are available with very high SPF ratings45 and up. But the chemicals used in these products are more concentrated than they are at the lower ratings, and may cause skin irritation for some people.
You should also know that some chemicals can cause an allergic reactioneither by themselves or in combination with UVR. One such chemical is para-aminobenzoic acid (PABA), which was used in some of the earliest sunscreens developed. Because so many people are sensitive to PABA, its popularity and use has declined greatly in recent years.
Excessive exposure to UVR can have many undesirable consequences. But by taking a few sensible precautions, you can protect yourself and still have plenty of fun in the sun.
1Actually the UV-B spectral region includes wavelengths from 280 to 320 nm. But very little UVR reaches the ground at wavelengths shorter than 295 nm. Return to document
2SPF is a ratio that tells you how much energy it takes to produce a minimal sunburn through a sunscreen product compared to how much energy it takes to produce the same sunburn without the sunscreen. Thus, if you normally get a minimal burn in 20 minutes, it would take you 15 times as long, or five hours, to get one using a sunscreen with an SPF of 15. Of course, these numbers are generalizations, and will vary with several conditions, including skin type. Also, SPF refers only to UV-B. UV-A protection is rated by the percentage of UV-A that the sunscreen blocks. Return to document