OZ3. The Danger of UV to Living Things

Ozone GSS book cover

Chapter 3

{ Ozone Contents }

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.

I. 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.

Family and dog on a beach
Beaches are a common spot to soak up the Sun’s rays—including UV.
(Photo by John Pickle.)

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.

II. Identifying Melanoma

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.

Assymetrical mole
(A) Asymmetry A symmetrical mole has matching sides. An asymmetrical mole has sides that are different from each other, showing an irregularity. Watch for moles developing asymmetrically.

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.

Mole with border irregularity
(B) Border irregularity Safe moles have sharply defined borders. A physician should examine a mole with notched, jagged or blurred edges.

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.

Mole with uneven color
(C) Color Healthy moles have an even color. Moles that are changing and becoming cancerous develop an uneven color with red, black or blue specks in the mole.
Mole growing in size
(D) Diameter Have a physician examine any mole greater than 6 millimeters (about the size of a pencil eraser) or one that is growing in size.

OZ3.1. Investigation:
Just How Fast Is Melanoma Increasing

You are a student volunteer helping a senator to prepare for a Congressional Committee meeting on public health hazards. You have some information on melanoma in the U.S. including the number of people in the U.S. that have been diagnosed with melanoma, and how many people died from melanoma in each year since 1973. 

The senator you are helping wants to educate her colleagues in Congress concerning the facts on melanoma skin cancer. Your job is to figure out how to present the information in the form of a graph so that someone can tell at a glance if the number of melanoma cases and deaths is increasing or decreasing. Prepare a sheet of graph paper with the appropriate scales and labels, so all the information is clear and easy to read. 

(Hint: Scientists who study disease patterns commonly use two measures to detect whether a disease is increasing or decreasing: 

• incidence: the number of cases diagnosed each year per 100,000 people.

• mortality: the number of individuals who die of a given disease each year per 100,000 people.)

After plotting the data, get together with the rest of the staff to study the evidence and discuss specific statements that the senator might make that would be supported by the evidence. 

Melanoma cases in the United States, 1974-1998
All Races, Females and Males

Table of Melanoma cases in the United States 1974-1998

Questions to Consider

  1. Is the number of melanoma cases each year increasing or decreasing? 
  2. Is the number of deaths due to melanoma increasing or decreasing? 
  3. Are the number of cases and the number of deaths changing at the same pace? Is one changing faster than the other?
  4. What do these data tell you about melanoma trends in the U.S.?
  5. Based on these trends, about how many people do you predict will contract melanoma in 2005? About how many people will die from it?
  6. What factors could cause the incidence of the disease to decrease?
  7. What factors could cause the incidence of the disease to increase?
  8. What percentage of people in the US with melanoma died of it in the years 1975, 1980, 1985, 1990, and 1995?
  9. What does this tell you about how treatment and detection of melanoma have changed in the past 15 years?
  10. In planning the Senator’s presentation, what additional information would you want to provide about the probable causes of melanoma?
  11. What legislation do you think the Senator might want to support based on this information?

III. 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.

IV. How Can a Person Avoid Getting Skin Cancer?

The Sun at different positions in the sky
The best way to avoid skin problems is to avoid the Sun when it is high in the sky.  Learn how the Sun moves across the sky in your area. (Photo by John Pickle.)

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. 

V. 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%.

Examples of skin cancer

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: 

Question 3.1  
What would you say about the difference in incidence rate of melanoma among the people of Norway compared to Italy?

Question 3.2  
How do you think the incidence rate of melanoma might compare among Polynesians and Canadians?

Question 3.3  
Which group do you think would be more likely to have a higher incidence of melanoma, rural farm workers or urban office workers? Why?

Question 3.4  
Would people in a high socioeconomic group be more or less likely to get melanoma than those in a lower socioeconomic one? Why?


OZ3.2. Investigation:
Sunscreens and Clothing

Determine how effective sunscreens and clothes are to filtering out ultraviolet energy.

Family on the beach...silhouettes

Perhaps you know someone who likes to lay for hours in the sunlight. Many light-skinned people want to get a “tan” during the summer.

Photo by John Pickle

But in addition to the increased production of melanin in the skin known as tanning, exposure to the ultraviolet energy from the Sun can cause burning of the skin and DNA damage associated with skin cancer. 

There are numerous products available—creams, lotions, or gels—which claim to offer protection from the Sun’s harmful UV radiation. They advertise a Sun protection factor (SPF). How well do they really work? Are SPF ratings accurate? Would a regular lotion that makes no SPF claim offer any protection?

People who tan often have tan lines that outline where clothing was worn while in the Sun. So clothing must stop UV, but how much protection do they provide compared to sunscreens?

To determine how effective sunscreens and clothes are at filtering out UV energy, you can expose sunprint paper to sunlight. The sunprint paper changes color when exposed to the UV energy in sunlight. Just smear some sunscreen on a strip of clear acetate and tape the acetate over the sunprint paper. Cloth can be tested by just placing it on the sunprint paper. After a few minutes, you’ll see how well you are being protected from the Sun’s UV.


  • sunprint paper
  • various sunscreens (different brands and SPFs)
  • swatches of different cloth (jeans, cotton T-shirt, sweatshirts)
  • acetate sheet for overhead projectors
  • cellophane or plastic tape
  • glass stirring rod
  • manila folder
  • water bowl
  • paper towels
  • markers

Strategies for Investigation

1. Plan your experiment. Look at the collection of sunscreens and cloth swatches, and decide on four or five for your experiment. If you select sunscreens to test, design your experiment to test a single variable—such as SPF, or brand of sunscreen, or thickness of the sunscreen layer. For clothing, variables to test might be color, thickness, fabric, weight, or wetness (some people use T-shirts while swimming). Write a hypothesis, stating how you think the variable you selected will affect UV energy from the Sun. Be sure all other variables are controlled (kept constant) in your experiment.

2. Prepare your samples. For sunscreens, smear a layer of each kind onto a separate strip of acetate. To make uniform layers of sunscreen:

  • Cut a strip of acetate, about 2 cm by 5 cm.
  • Tape the acetate to a table on three sides.
  • Squeeze a bead of sunscreen on an end of the strip.
  • Use a glass rod to smear the sunscreen down the strip, using a single, smooth motion. The sunscreen thickness should be the same as the thickness of the tape.

For cloth swatches, cut the pieces so they all fit onto one sheet of sunprint paper.
For testing the effect of water on the clothing’s protection, place a piece of acetate between the wet cloth and the sunprint paper. Wet swatches will lie on the acetate while dry swatches must be taped to the acetate.
Of course, if using acetate, you should run one test just to see if the acetate has any sunscreen effect.

3. Tape the samples on sunprint paper. Working in subdued light, to not expose sunprint paper to UV energy before the experiment is ready, place your samples on the sunprint paper and tape in place. Keep them in an opaque folder, box, or bag to prevent exposure to UV. Label the sunprint paper with the brand and SPF of each sunscreen sample or cloth type, color, and thickness.

4. Expose the samples on sunprint paper. Take the folder outdoors, open it, and place the sunprint paper in the Sun. Expose it for 1-2 minutes if the sunlight is bright, and up to 5-6 minutes if the sky is a bit overcast. Weight the folder around the edges with rocks if it is windy. Put the samples back in your folder, bag or box immediately after exposure.

5. Develop the sunprint paper. Indoors, in subdued light, remove the samples from the sunprint paper (being sure they are labeled correctly). Rinse the exposed paper in a bowl of water for at least one minute to develop and fix the color change, then remove it and lay it on a table to dry.

Results and Conclusions

1. Compare the sun print papers and describe the results. 

2. Was your hypothesis supported or contradicted by the results?
Give an explanation for why your hypothesis was supported or contradicted.

3. Can you advise people about Sun protection based on this experiment?
Can you advise people about what brand is best? What SPF provides significant protection from the Sun? If you tested the effect of different thicknesses of sunscreen, what advice would you give someone about how to put on sunscreen so it offers protection? If you tested clothing, would you recommend a fair-skin person to use that clothing if outdoors all day?

4. What additional experiments do you think should be done to prepare a full report about the effectiveness of different types of Sun protection?

VI. Cataracts and Sunglasses

Kids holding balls...as seen with normal vision

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. 

How the world would look with normal vision…

 (Photo courtesy of the National Eye Institute.)

Kids holding balls as seen by person with cataracts

and how the world would look with cataracts. 

(Photo courtesy of the National Eye Institute.)

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%.

VII. Reducing Your Risk of Eye Damage from the Sun

  1. Wear UV-blocking sunglasses (or eyeglasses) when you will be outdoors for more than 20 minutes. 
  2. Avoid peak Sun exposure times. The worst time period is between 10 a.m. and 3 p.m., when the Sun is highest in the sky. 
  3. Avoid glare. The worst environments are high altitudes and areas affected by highly reflecting surfaces (snow, water, sand, and concrete). 

Sunglasses can prevent much of the damage caused by the Sun’s UV. (Photo by John Pickle.)


OZ3.3. Investigation:
Sunglasses and UV Protection

persons eye

When a healthy eye encounters bright light, the pupil contracts to cause less light to enter the eye. The pupil, appearing as the black center in the colored part of the eye, is actually a hole in the iris, and it is the iris that causes the pupil to change size. In dim light, the pupil gets bigger (dilates) to let more light into the eye. Wearing dark sunglasses decreases the amount of sunlight entering the eye, so the pupil responds by dilating.

Diagram:  before and after pupil is  dilated

If the sunglasses decrease the visible light but do not block UV, then significantly more UV enters the eye compared to when not wearing sunglasses. Using a darkened room as a simulation of the amount of light entering your eyes while wearing sunglasses, find the minimum amount of UV that must be blocked by sunglasses to ensure that increased levels of UV are not entering your eyes.


Ruler with millimeter increments

Strategies for Investigation

  1. Students work in pairs. In a darkened room carefully hold the ruler close to your partner’s eye. Measure the diameter of the pupil. Switch so each student may make a measurement.
  2. Repeat the procedure, but turn on bright lights to contract your pupils as much possible. Do not make the lighting uncomfortable for the people. If the day is sunny and warm enough, you might want to do this outside.
  3. Calculate the area of the pupil in both pictures. Assume that the pupil is a perfect circle.
  4. Find the ratio of the area of the dilated pupils to that of the constricted pupils. This is an estimate of the amount of UV that must be blocked by a pair of sunglasses.

Results and Conclusions

  1. What is the minimum percentage of UV that must be blocked so your sunglasses are not enhancing the damaging effects of UV on your eyes? As an example, if the area of the pupils were twice as large when dilated compared to contracted, the sunglasses would need to block 50% of the UV in order to compensate for the enlarged pupils. If the pupils grew three times in area when sunglasses were put on, then 67% of the UV would need to be blocked. 
  2. What assumptions are you making in order to draw this conclusion? Discuss the strengths and weaknesses of these assumptions.
  3. How could you modify your experiment to eliminate these assumptions? If you can carry out a new experiment, compare the results of the two.
  4. Can you design an experiment that would relate the SPF of sunscreen to the UV-blocking ability of sunglasses? This experiment could be used to test the rating of your sunglasses! Can this be done mathematically without the experiment?
  5. Do you feel your sunglasses adequately protect you? If you wear untinted eyeglasses, would you benefit from a lens coating that blocks UV?
  6. If the amount of UV reaching the ground increased 4% due to thinning of the ozone layer, what is the minimum UV-blocking required to keep the harm to your eyes at the same level as before? Remember unprotected eyes are being harmed at the current levels of UV radiation.

VIII. 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.

Graph of UVa,  b, and c penetration in the atmosphere
Amount of UV spectrum entering the top of the atmosphere and reaching the ground. (Courtesy of NASA.)

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. 

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.

Cloudy day
Photo by John Pickle.

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. 

Clouds can both enhance or inhibit the amount of UV reaching the ground.  Know when to protect yourself.

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.

Desert with sand dunes

Sand and light-colored concrete can reflect up to 20% of the UV reaching the ground, making sun bathing at the beach more dangerous. 

Highly reflective surfaces such as sand can enhance UV damage to the skin.

(Photo by John Pickle.)

Sail surfer

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.

Water sports allow for increased UV exposure.
(Photo by John Pickle.)

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.

IX. 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. 

Elephant seal

Elephant seals in the Antarctica survive on fish that feed on the plankton.

(Photo courtesy of the National Science Foundation.)

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.

X. 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. 

Coral reef

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.

Photo of coral reef courtesy of the 
National Oceanic and Atmospheric Administration (NOAA).

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.” 

Question 3.5 
Do you think this report of the bleaching of coral is sufficient evidence to declare that the reduction of ozone in the mid-latitudes is causing damage to coral reefs? How would you evaluate this information? What additional experiments do you think might test the theory further? 


OZ3.4. Investigation:
The Effect of Increased UV on Plants

Compare the growth of plants under normal outdoor conditions, under a grow light, and under light with the same wattage, but increased ultraviolet energy.


Photosynthesis is the process that plants use to produce food for themselves. Studies in Sweden and in the United States have shown that ultraviolet energy destroys the chlorophyll molecules that plants need to capture light energy for photosynthesis. That’s how UV damages photosynthesis. Other studies suggest that UV energy reduces the insect resistance of many plants. Some plants have evolved defenses against damage by ultraviolet energy. Some can produce large amounts of pigments that absorb the radiation; others block out sunlight with waxy leaves or fine, fuzzy hairs.

There are plants that develop enzymes that undo the DNA damage. However, these defenses are not present in all vegetation. Many of the plants sensitive to ultraviolet energy are ones we depend on for food.

The purpose of this investigation is to compare the growth of plants under normal outdoor conditions, under a grow light, and under light with the same wattage, but increased ultraviolet energy.


  • 4 potted plants of the same variety (pea, bean, clover, alfalfa, cucumber, or soy bean)
  • 1 suntanning lamp or bulb (which is not a “growlight”)
  • 2 growlight lamps or bulbs of the same wattage as the suntanning lamp
  • 1 suntanning lamp or bulb with wattage as low as possible, preferably 30% or less of the wattage of the growlight
  • water
  • graph paper and pencil

Strategies for Investigation

1. Measure the area of a few sample leaves on each of the plants and calculate the average leaf area. Also record the total number of leaves on each plant. (Leaf area can be measured by putting a piece of graph paper under each leaf and tracing its outline.)

2. Place one plant in a spot where it can get direct sunlight and where it will be away from the lamps. It will serve as the “control” plant.

3. Place the other three plants indoors, one illuminated solely by the growlight, the second solely by the tanning lamp of equal wattage as the growlight, and the third illuminated by the growlight and the low-wattage suntanning light. The lamps for all three plants should be an equal distance above the soil in each pot. Turn off the lamps each day at about sundown and then turn them on again the next morning. (If this is not convenient put the control plant in the same light conditions as the experimental plants when their lamps are turned off.)

4. Make a prediction. Do you think the number and size of leaves will be affected by the kind of light? If so, how? If not, why not?

5. Leave the plants for several weeks, watering as needed. Make sure each plant has equal watering.

6. After several weeks, measure the average leaf area once again.


Make a bar graph to show the effects of the different lighting conditions on the average leaf area for all four plants. What conclusions can you draw from your experiment? What did you find out?


Discuss whether or not your experiment indicates that increased UV energy could affect agriculture. If you anticipate problems, how do you think their effects could be avoided?

Will our crops be affected as the amount of UV radiation changes? 
(Photo by John Pickle.)

XI. Being Skeptical

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.

Question 3.6 
Consider the following statements by skeptical scientists, who have questioned the UV/melanoma connection. 

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.

XII. Being Safe


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.

See Staying current for this chapter.