When the Earth was unprotected by the ozone layer, the full strength of the Sun’s ultraviolet energy prevented living things from existing outside of the protection of water.
After the ozone layer formed, life was able to evolve on land. Although the ozone absorbed most of the Sun’s ultraviolet energy, some managed to penetrate to the surface. The result was that all living things on our planet evolved some protective structures, such as feathers, scales, or in the case of people, a dark pigment in the skin called melanin.
The thinning of the ozone layer that has been observed over the South Pole, and more recently over the North Pole as well, will allow more UV energy to penetrate to the Earth’s surface. As the amount of ultraviolet energy increases, the defenses that plants and animals have developed to protect them from ultraviolet energy will be tested.
Increased ultraviolet energy may pose hazards to life on Earth, starting with the dangers posed directly to humans: skin cancer, cataracts, and reduced immunity to disease. It is possible to protect ourselves from exposure to ultraviolet energy, but unlike humans, plants and animals are not able to protect themselves by putting on sunglasses, hats and sunscreen. We can predict how increased exposure to ultraviolet energy will affect plants and animals, including the tiny living things that exist in the very top layer of the world’s oceans, where ultraviolet energy is able to penetrate.
The Effect of UV Energy on Humans
Researchers are currently focusing on three problems that increased exposure to ultraviolet energy may cause to people: cataracts, reduced immunity, and skin cancer. Cataracts are a clouding of the clear lens of the eye, which can cause blindness. Common in older people, cataracts can be treated by replacing the cloudy lens with artificial materials. You can reduce your risk of cataracts by wearing eyeglasses with glass lenses or coatings that absorb ultraviolet energy.
Your immune system protects your body from viruses and bacteria that can cause disease. Certain cells within your body identify the invading organisms and absorb or destroy them. The devastating effect of AIDS occurs when the disease attacks the immune system itself, making the body vulnerable to a wide variety of serious illnesses. Scientists suspect that increased ultraviolet energy may also reduce the effectiveness of the immune system.
A great deal of attention in recent years has been focused on the role of ultraviolet energy in causing skin cancer. Ultraviolet energy is able to penetrate the skin, pierce cell walls, and damage DNA within the cell. DNA is a complex, spiral-shaped molecule that tells the cell how and when to reproduce. If damaged, it can give the cell incorrect instructions, producing cancerous tumors rather than normal skin.
Most skin cancers can be cured if found early. However, an especially dangerous form of skin cancer, called melanoma, is very difficult to stop if not detected and removed early. Although at one time melanoma was rare, the number of cases is growing faster than all other cancers except lung cancer. The reasons for the increase are not certain, but many doctors suspect our sun-loving life-styles. We live and work indoors, so our skin builds up very little protection against the sun’s rays. On weekends we rush out to the beach, exposing the DNA in our skin to increased ultraviolet energy. In the next two investigations you will have an opportunity to examine some of the data about the incidence of melanoma, and experiment with the effectiveness of various kinds of sunscreen, which may help prevent it.
Dermatologists have identified the warning signs to spot moles that may turn cancerous. Moles are harmless skin growths that may be flat or protruding. They vary in color from pink flesh tones to dark brown or black. The majority of people are born without moles, and few people older than 70 or 80 years old have moles. However, usually in childhood and adolescence, everyone develops moles; some more than others. No one knows what causes moles or what their function is. However, the number of moles appears to depend on one’s genetic make-up.
Most are perfectly harmless. However, some moles, due to overexposure to the Sun and its UV radiation, may turn cancerous. Monthly examinations of your moles using the following ABCD rules on this page will help you identify problem moles at an early stage. Early identification of melanoma is critical for effective treatment. If any of your moles shows signs of change in any of the following categories, see your physician immediately.
What Are Your Chances of Getting Melanoma?
The work you have been doing — plotting, examining, and understanding disease patterns — is the science of epidemiology, from the word epidemic. An epidemic is a large outbreak of a disease, as when nearly everyone in school gets the flu—that’s a flu epidemic. An epidemiologist is a person who tries to understand the patterns of a disease in populations.
Since medical researchers do not know enough about cancer it is impossible to tell who will and who will not develop melanoma. However, epidemiologists use statistical information to identify those factors that tend to increase the chances a particular individual will develop melanoma. It makes good sense for you to understand the risk factors so you can take action to reduce that risk.
There are some risk factors you can do nothing about. They are genetic traits, and the more of them you have the more you should be on your guard. If you have a light complexion and thin skin, freckles, light hair color, blue or green eyes, and have moles, you are at an increased risk of getting melanoma. People with red hair are especially susceptible.
But the risk will be reduced if you eliminate or reduce the other risk factors that are under your control. The main one is exposure to the Sun. “Cover up or stay in the shade!” should be the rule for those with several of the high-risk characteristics. Even for those with none of the high-risk characteristics, the danger is increased if they have had one or more severe sunburn events in the past, or if they were frequently overexposed to the Sun as a child.
How Can a Person Avoid Getting Skin Cancer?
People with black hair, brown skin, no freckles, and brown eyes — have lower chances of getting melanoma than those with lighter complexions, but anyone can develop melanoma. Darker pigmentation means that those people have more melanin than people with lighter pigmentation. Melanin is the pigment that protects the skin by absorbing the harmful solar rays before they get to the DNA. If you haven’t got much melanin you need to find another way to protect yourself from the harmful rays of the Sun.
It is a very good idea to use sunscreen or to wear protective clothing when you are in the Sun, especially between 10 a.m. and 3 p.m., which is when sunlight passes through a smaller thickness of the atmosphere, so less of the UV radiation is filtered out.
Most likely you have noticed the Sun Protection Factor (SPF) on sunscreen bottles. This number tells you how much longer you can stay out in the Sun after applying the sunscreen without burning, than you would be able to stay out with no protection. For example, it is probably not a good idea for fair-skinned people to be in the Sun for more than 15 minutes without sunscreen. A sunscreen with SPF 20 would allow fair-skinned people to sun safely for up to 5 hours.
In case you will be swimming or sweating, it is important to use a water-resistant sunscreen. Also, it is good to use a “broad-spectrum” sunscreen. This is the kind that screens out the most ultraviolet radiation. Remember that the “tanning rays” are dangerous too.
In addition to avoiding the Sun, you should regularly check your skin for spots that look like small mole-like growths, which grow and becomes discolored and oddly shaped. They may feel a little sore or they may not. If you notice any one of these symptoms, you should have a doctor examine you. Remember, in the early stages, skin cancers are often very easily removed; and catching it early can prevent it from spreading.
Other Forms of Skin Cancer
Although melanoma is the deadliest form of skin cancer, there are other forms you should be aware of, which if detected and treated early have a cure rate of nearly 95%. The two most common types of skin cancer are basal cell and squamous cell carcinomas. Basal cell carcinomas usually appear as fleshy bumps or nodules generally on the head and neck, but can grow on any part of the body. These slow growths rarely spread, but they can penetrate to the bone, causing damage. Squamous cell carcinomas are typically red, scaly patches or nodules. These can grow into large masses and spread to other parts of the body.
There are other forms of skin damage caused by long-term exposure to UV radiation, with the primary being premature aging where the skin thickens and becomes leathery and wrinkled. This is a gradual process, but as with all skin problems resulting from UV exposure, it can be avoided by following the list of precautions mentioned earlier.
Based on what you know about personal characteristics and skin cancer risk, answer these questions:
Cataracts and Sunglasses
Cataracts, which result when the eye lens whitens and becomes opaque, cause nearly half of all new cases of blindness in the United States each year. It appears cataracts are a result of a lifetime of cumulative Sun exposure, beginning in childhood. There is no particular age where exposure is particularly risky. Doctors believe reducing exposure to sunlight and taking precautions to protect eyes could significantly reduce the risk of developing cataracts. People who have a lifetime of exposure to the Sun are four times more likely to have cataracts than those who have had less exposure to the Sun are. Just wearing UV-blocking glasses on a regular basis appears to reduce the risk of developing one form of cataracts by about 40%.
Reducing Your Risk of Eye Damage from the Sun
Factors Affecting UV at Ground Level
The solar radiation reaching the top of the Earth’s atmosphere has the approximate proportions of 8% UV, 39% visible, and 53% infrared radiation. The UV spectrum is often divided into 3 “colors” or bands of wavelengths: the longest are called UV-A (just beyond visible violet in the spectrum); the shortest is UV-C; and the medium wavelength is UV-B. Even before the ozone layer began to thin, almost all of the UV-A, roughly 2% of the UV-B, and none of the UV-C would reach the ground. And though we tend to discuss the loss of ozone over the poles, ground-level UV is many times greater at the tropics compared to the poles due to the higher angle of the Sun in the sky during the hours surrounding noontime. But in any geographic region, there are additional factors that can increase the amount of UV reaching the ground.
Are you safe from UV radiation on cloudy days? As is the case with the majority of situations dealing with nature, the answer depends on several factors. If there are scattered clouds in the sky, a portion of the UV will be reflected from the sides of the clouds. If the clouds are not blocking the Sun directly, you may be getting an increased dose of UV radiation compared to a clear day.
In general, thin clouds don’t block much UV radiation from reaching the ground, but they do block infrared radiation, which is the portion of the solar spectrum that causes the skin to feel warm during sunbathing. Often, with these cooler temperatures, people tend to remain outdoors longer, often without adding sunscreen. Severe sunburns can be the result. When thick, dark gray clouds cover the sky, most of the incoming solar spectrum is reflected away from the ground, including up to 80% of the UV. The overall effect of thick clouds on blocking UV radiation from the Earth’s surface is the second most important factor after stratospheric ozone.
The amount of UV that does reach the ground can be reflected toward you, increasing your dosage of UV. Fresh snow can reflect up to 85% of the UV, which, if combined with increasing UV exposure with altitude (20% increase in UV at 3 km compared to sea level), can contribute to severe sunburns while skiing. Sand and light-colored concrete can reflect up to 20% of the UV reaching the ground, making sun bathing at the beach more dangerous. Water reflects less than 10%, with a large amount of UV penetrating into the water, allowing you to get sunburned while in the water. In fresh water, almost half of the UV penetrates to a depth of 30 cm, but in a swimming pool, where sediments and floating particles have been filtered from the water, over three quarters penetrates to 30 cm.
Can you get sunburned while driving in a car with the windows up? This depends on the glass. Untreated glass removes only 30% of UV-A but over 90% of the UV-B; however, UV-B is one thousand times more potent in causing sunburns than UV-A. The estimated SPF of untreated glass is 10. Treated glass can block most of the UV radiation, reaching SPF values of 120.
As you can see, there are many local factors that can increase or decrease the amount of UV you could receive throughout the day, and the UV index reported by national weather agencies cannot take into account this type of small-scale variability. You must be aware of your surroundings and take the proper precautions if you want to minimize the damage of UV to your health.
The Effect of Increased UV on Ocean Life
Ultraviolet energy penetrates into the upper layer of the ocean, where it affects tiny organisms known as plankton. There are two kinds of plankton: tiny plants called phytoplankton, and tiny animals, called zooplankton. UV damages both. Plankton are at the base of the food chain for the entire marine ecosystem. They are the food for krill, which are tiny shrimp-like animals. Some whales feed directly on krill, and they provide the diet for many kinds of fish. These fish, in turn, are food for larger fish, sea mammals, and birds.
A recent study reports that if the amount of ozone in the stratosphere over the mid-latitudes were to diminish by 16%, the numbers of phytoplankton would decrease by about 20%. In addition to its devastating effect on plankton, increased ultraviolet energy can destroy the floating eggs and larvae of many marine organisms, sharply reducing the population of these species. Thus, a further reduction of the Earth’s ozone shield would severely impact the entire marine food chain, reducing the amount of food we obtain from the sea, and imperiling endangered ocean species.
The Effect of Increased UV on Coral Reefs
Coral reefs are home to thousands of different kinds of marine life. The tiny animals that create the coral are very sensitive to changes in their environment. When those animals are under stress the coral turns white. Bleached coral is being found all over the world. In some cases water at higher temperature is blamed. In 1991 Pamela Hallock-Muller, a marine biologist from the University of South Florida, made the connection between ozone depletion and the bleaching. Warm water normally sweeps across Florida’s reefs in late summer and bleaching occurs. After a month or two cool water comes in and the corals return to their natural colors. But starting in March, 1991, and extending through July—months before the water was at it warmest—Pamela Hallock-Muller found that the corals were turning white.
The scientist suspected that the corals were reacting to an increase in ultraviolet radiation due to a reduction in the ozone overhead. She was making a connection with the massive eruption of Mount Pinatubo in the Philippines the previous year. “A big volcano like Mount Pinatubo puts lots of ash into the stratosphere,” she said. “That causes more deterioration of the ozone and allows more ultraviolet radiation in.” She found support for her idea in her records. Major bleaching of the corals also occurred in 1983 and 1987, just after major volcanic eruptions.
Peter Glynn performed experiments with coral and ultraviolet radiation at the University of Miami. Using large tanks, he exposed coral to higher than normal levels of ultraviolet light and showed that some of the coral was severely damaged. The corals growing in similar tanks without the ultraviolet light were unharmed.
In the early days of mining, the danger of gaseous poisoning was ever-present. The miners needed an early warning sign so they would have time to get out before they were overcome. Many of them carried canaries in small cages. The birds were very sensitive to the poison gases. Their collapse signaled that danger was present. Pamela Hallock-Muller said, “People have compared coral to canaries in a mine, and they could very well be signaling changes in natural cycles.”
Scientists are by nature skeptical. They need to suspend judgement until they are firmly convinced by the evidence. They should take this approach because the scientific endeavor is a systematic search for ideas that describe the world as it actually is. Scientific knowledge is built on a long series of discoveries and a chain of logic. Should the results of an experiment prove to be mistaken or the chain of reasoning weak, other conclusions that depended on those results or reasoning will be equally faulty.
a. Hugh Ellsaesser of the Lawrence Livermore National Laboratory has pointed out that ultraviolet energy can vary by hundreds of percent from day to day because of clouds and other factors. He says that an average increase of a few percent has little meaning.
b. In the 1920’s it became fashionable to be suntanned. Doctors tend to associate melanoma with sunburns in teenage years but the disease is most often diagnosed in 40-year old people. This leads many researchers on skin cancer to believe that it takes from 25 to 30 years for melanoma to develop. If this is true, why was there no large increase in melanoma in the 1940’s among the Sun-worshipers of the 1920s?
c. Does preventing sunburn really reduce the risk of melanoma? Do sunscreens live up to the claims of the manufacturers?
These questions could be answered by experiments. But ethical and practical considerations rule out experimenting on humans. In other experimental work mice and rats are often used as substitutes for humans, but, since they do not get melanoma, virtually no experiments have been done to substantiate the protective role of sunscreens against cancer.
d. In the Investigation on page 25, the statistics for melanoma cases begin with 1974. Since that time the number of older Americans has been increasing. Could this be a factor in the increase in melanoma? Explain.
Although enough scientific evidence may be lacking to satisfy you or other skeptical scientists, the correlation between exposure to sunlight and skin cancer should be taken as a warning. It is prudent for people at risk to take precautions. It is easy to guard against excessive ultraviolet radiation. Cover up, rather than expose sensitive skin to the direct rays of the Sun in the middle of the day. The informed person is the person who can take the most appropriate action to keep healthy.