Sun and skin – a dangerous combination

Key text

What is in sunlight?

The sun is an immense nuclear reactor. As well as producing heat and light, it also emits other types of electromagnetic radiation. We cannot detect much of this radiation without special instruments. This can be a problem, because sometimes the ‘hidden’ components of sunlight can damage living things.

Fortunately, the Earth’s atmosphere filters out much of the more dangerous solar radiation. But some gets through – mainly in the ultraviolet (UV) band (Box 1: The ultraviolet family).

The ultraviolet radiation in sunlight causes skin cancer

Ultraviolet radiation doesn’t get very far into our bodies. It mainly affects our skin and eyes. Even though we can’t feel it – just as we can’t feel an X-ray – it can damage us. The higher the dose, the greater the likelihood of damage.

The most serious effect of sunlight on skin is the generation of cancer. There are three main types of skin cancer, classified according to the type of skin cell affected (Box 2: Types of skin cancer). The majority of skin cancers are not lethal.

Cancer is often brought about by exposure to a mutagen, which can change (or mutate) the genes. Ultraviolet radiation is a well-known mutagen. It promotes a chemical change in the DNA, which may manifest itself as an error or misprint in the information encoded in a gene (Box 3: The molecular mechanisms of skin cancer).

A mutation can take many years to show up as a symptom. Most skin cancers are found in people over 40 years old, but the actual damage that caused the cancer occurred when these people were children. In the intervening decades, the altered DNA has remained, and the misprint in the gene has been copied into more and more cells.

Sunlight also causes other damage

Sunlight doesn’t just cause cancer. It can also cause painful sunburn, which usually shows up the day after exposure. Sunlight also ages skin. Sun-weathered skin becomes leathery and loses its softness and lustre. Other blemishes, such as moles and solar keratoses, can also form. These are unsightly but they are not usually considered dangerous.

The eyes are also affected by radiation from the sun. Minor damage will show up in the formation of a pterygium, which can be removed. Far more serious are cataracts, which can cause blindness.

Reactions to sunlight differ

People vary in how much ultraviolet exposure their bodies can tolerate before their skin is damaged or cancer occurs. Because dark-skinned people have greater amounts (and a different form) of melanin in their skin, they are much less sun-sensitive than fair-skinned people. Melanin absorbs ultraviolet radiation, like a sunblock. The cells beneath the melanin are therefore somewhat protected.

Exposure to sunlight causes the skin to produce more melanin in an attempt to protect itself. In pale-skinned races, this shows up as a tan. Dark-skinned people will also produce more melanin after time spent in sunlight.

In pale-skinned people, the maximum amount of melanin that the skin can make isn’t enough to protect against the doses of ultraviolet radiation that occur in and around the tropics. When people of European origin live in places that have much higher levels of year-round ultraviolet radiation than Europe, then their skin can never naturally protect itself. This is the problem facing most of Australia’s population.

The rate of skin cancer in Australia is very high

Australians have the highest rate of skin cancer in the world. With only 0.3 per cent of the world’s population, we manage to account for 6 per cent of all the lethal forms of skin cancer diagnosed globally. About 1200 people die each year in Australia as a result of skin cancer.

The skin cancer rate is likely to increase when many of the ‘baby-boomers’ who loved the sun in their youth enter their 50s and find that cancers have developed. In addition, ozone depletion in the stratosphere is increasing the amount of ultraviolet radiation reaching the ground over Australia, thereby worsening the risk.

In future, the skin cancer rate may fall as the understanding of the link between sunlight and cancer becomes more widespread and people protect themselves more.

Detecting and dealing with skin cancer

If caught early enough, most skin cancers can be removed with very little harm done (Box 4: Diagnosing skin cancer). However, the longer a cancer remains, the greater are the chances that it will spread.

Australian research

Because of the seriousness of the skin cancer problem in Australia, our researchers are among the world leaders in the field (Box 5: Australian research) .

How to prevent skin cancer

Public education campaigns have made most Australians aware of sun damage. We rank as one of the best countries in the world in terms of how we deal with the problem. Obviously, keeping out of the sun is the best prevention. Next comes sun-proof clothing, then sunblocks and sunscreens (Box 6: Sunblocks and sunscreens). Finally, a careful check of your skin will help catch any potential cancers as early as possible.

Box 1. The ultraviolet family

UV is normally used to describe radiation with wavelengths between about 100 and 400 nanometres. (A nanometre is one-millionth of a millimetre.) As with all electromagnetic radiation, the shorter the wavelength the greater the energy carried.

The ultraviolet family can be divided into three parts:

UV-A (315-400 nanometres) has the longest wavelengths of the family and is the least damaging. However, it does cause sunburn and has been implicated in causing sun-induced premature ageing of skin and some cancers. This is the form of ultraviolet produced in most solariums.

UV-B (280-315 nanometres) can cause skin cancer and eye damage. It also causes sunburn. Radiation with a wavelength close to 280 nanometres is strongly absorbed by proteins, altering and often damaging their function. In this way, UV-B can reduce the immune response and it also interferes with photosynthesis in some crop plants. A very small amount of exposure to UV-B is necessary to produce vitamin D in human skin.

UV-C (100-280 nanometres) is the most dangerous member of the family. Wavelengths around 260 nanometres are absorbed by DNA and so nearly all life forms are irreparably damaged by this radiation.

The good news is that the stratospheric ozone layer absorbs all UV-C, the most deadly form, and even a thinned ozone layer is unlikely to let much through.

The intact ozone layer does, however, let through some UV-A, especially when the sun is high in the sky, and a very small amount of UV-B. The proportion of both of these reaching ground level will increase with ozone loss.

Many species have some natural protection against UV-A. Scientists are examining some of these protective compounds with a view towards developing better sunscreens for human use.

Box 2. Types of skin cancer

• Basal cell carcinomas account for about 80 per cent of all skin cancer cases. At first they look like small, pinkish raised areas. They are most common on areas that receive the greatest exposure to ultraviolet radiation – the face, ears and neck. They are not painful, but they grow and eventually start to look like an ulcer, because they develop a crater in the middle. However, they don’t spread into the rest of the body and so are not usually lethal. If left in place they can end up looking very unsightly but they are easy to remove.

• Squamous cell carcinomas account for about 10 to 20 per cent of skin cancers. These are more dangerous than basal cell carcinomas. They grow faster and can spread beyond the skin and kill. They typically appear on the face, ears, the top of a bald head and also on hands and arms. They are more common in men than women. They look like sores, sometimes oozing fluid or even blood and often with crusts on them. They may feel tender. They can grow quite fast – especially those on the lip and ear.

• Melanocytes are the melanin-producing cells and they are scattered along the bottom of the epidermis. Every basal cell is attached to a melanocyte. There is one melanocyte per 10 to 20 basal cells. (Melanocytes also lie in the hair roots, where they give their pigment to the hair, thereby determining its colour.) Melanin produced in the melanocytes is delivered to the basal skin cells.

  Paler-skinned people do not have fewer melanocytes than black people, but simply produce less melanin and a different kind of melanin per melanocyte. Ultraviolet radiation stimulates the enzyme that makes melanin.

  Melanomas are the deadliest skin cancers of all. They account for only about 5 per cent of cases but cause 80 per cent of skin cancer deaths. Melanomas can occur anywhere on the body. The important signs are either a new mole, a recent change in the size, shape or colour of an existing freckle or mole, or bleeding or tenderness of a mole. As it grows, a melanoma will eventually push beyond the epidermis of the skin, and then can spread throughout the body. But if spotted early, it can be easily removed before it spreads, and the prognosis is then good. If left untreated, a malignant melanoma is usually fatal.

  Melanomas may occur on areas of the body that don’t receive regular exposure to the sun. It is thought that they are caused by brief intense exposure to ultraviolet radiation for short periods – especially during childhood or adolescence. People most at risk seem to be those in indoor occupations as adults, with little time in the sun, who spend a short time each year exposed to the sun and get sunburnt. They recover after their holiday but may develop a melanoma 20 or more years later.

The bad news

Australia – particularly the northern parts – is the world’s skin cancer capital. Statistically speaking, an Australian has a far greater risk than almost anyone else of developing one of the non-melanoma cancers at some stage in life. Skin cancer is the most common cause of cancer death in white people aged 25 to 40 in Australia.

Related site

• Skin cancer and melanoma (Queensland Institute of Medical Research, Australia)

Box 3. The molecular mechanisms of skin cancer

When present in a high enough concentration, the p53 protein causes a cell to ‘self-destruct’. This protein is normally produced in greater amounts after exposure to ultraviolet radiation. But after the radiation damages the gene for p53, the p53 protein that the gene makes is slightly altered and doesn’t function correctly. The defective version of the protein cannot make a damaged cell die.

The gene for p53 only needs a very small change for a defective protein to be produced. This tiny error will be perpetuated whenever the cell divides. Eventually, many cells with the incorrect gene will exist.

The cells with the incorrect gene won’t die when exposed to too much ultraviolet radiation. Instead they will reproduce, stimulated to do so because they find the normal cells around them dead. With each subsequent heavy exposure to too much sunlight, more normal cells will commit suicide, and the colony of cells with the damaged gene will multiply all the more. This is the beginning of a tumour.

So, by inducing cells carrying the normal p53 gene to kill themselves off, sunlight favours the proliferation of p53-mutated cells. Ultraviolet radiation is thus responsible for the two key steps in cancer generation: mutation and tumour promotion. The p53 gene and its protein are involved in the development of basal cell carcinoma and squamous cell carcinoma, but not melanoma. A similar gene, p16, and its protein are thought to be involved in the development of melanoma, but these studies are preliminary.

Related site

• p53: The guardian of the genome (Howard Hughes Medical Institute, USA)
• DNA and genes (Back to basics, Australian Academy of Science)

Box 4. Diagnosing skin cancer

Suspected basal cell or squamous cell carcinomas need to be removed by laser or cauterising. Melanomas, being much more serious, require surgical excision.

Of the three cancer types, the diagnosis of melanoma is clearly the most important.

The danger signs in moles or pigmented spots are:

Asymmetry of the mark
Border is irregular, unlike a normal mole
Colour is variable (could be blue, grey or pink as well as brown and black)
Diameter is often greater than 5 millimetres.

At first, melanomas are flat and they grow outwards (radially). At this stage they cannot send out metastases (clumps of cancer cells) into the rest of the body, so they are not lethal. Later, they develop a raised part in the middle. This shows that they are now enlarging upwards and downwards – and thus spreading beneath the skin into the lymphatic and blood systems. They can therefore metastasise.

Diagnosis of melanoma is usually confirmed by a biopsy – a piece of the suspected cancer is taken and checked in the lab. If positive, the whole area must then be cut out, ensuring that no cancer cells remain lower down in the skin. Even when the removed area is free of remaining melanoma cells it does not mean that microscopic clumps of cancerous cells may not have been released. These may grow and show up in later life. (Recent Australian research may enable melanoma diagnosis to be made without the need for a biopsy, see Box 5.)

Once a melanoma has been detected and removed, the affected person must remain vigilant because people who have had one melanoma are quite likely to develop another.

Related site

• The next four pages could save your life (The Cancer Council of Western Australia)

Box 5. Australian research

1. CSIRO scientist Dr Mark Berman and his colleagues have developed a form of microscopy to diagnose melanomas without needing a biopsy of the material. The Image Analysis Group at CSIRO Mathematical and Information Sciences has developed an instrument for computer diagnosis based upon analysing pictures of the skin.

   In collaboration with specialists from the Sydney Melanoma Unit and a medical instrumentation company, the scientists have put the knowledge of melanoma experts into a computer program. A special magnifying camera converts information from the skin into digital form, which the computer then analyses for possible melanoma, using its database compiled from the existing knowledge of experts. General practitioners without specialist knowledge will be able to use the system and come up with a faster and cheaper diagnosis.

   Related sites

   • CSIRO Mathematical and Information Sciences (Image Analysis Project)

   • Spot check (Australia Advances, CSIRO)

2. Dr Bill McCarthy, of the Royal Prince Alfred Hospital in Sydney, is investigating children’s moles. He has found that children in Townsville have six times more moles than children in Melbourne. Using meters to determine levels of ultraviolet radiation in the cities and individual monitors on children, he is precisely correlating doses of ultraviolet radiation with skin changes. The number of moles are known to be a good indicator of the risk of developing melanoma later in life.

   The work with children is especially important because it is when we are young that most of the sun damage is done. It pays to be careful at any time, but the ultraviolet radiation received in the first 20 years of life can be enough to ensure that a cancer develops 30 or 40 years later.

3. The Bureau of Meteorology now presents daily forecasts of the intensity of UV-B radiation. This is useful in helping people make decisions about appropriate activities and clothing. The regular announcement on weather bulletins also helps remind people of the ever-present nature of ultraviolet radiation.

But forecasting ultraviolet radiation is not easy. The calculation must take into account the time, date, latitude, altitude, atmospheric ozone concentration and the possibility of cloud or haze, because all these factors can affect the amount of ultraviolet radiation reaching the ground. The Bureau has worked with the Cooperative Research Centre for Southern Hemisphere Meteorology to develop a special global computer model to help predict ozone concentration (ozone absorbs ultraviolet radiation) and calculate the amount of ultraviolet radiation that will pass through the atmosphere to the ground. Forecasters can then correct for the amount of cloud expected at any location around midday.

The effectiveness of clouds in screening ultraviolet radiation is complex. It depends partly on the thickness, composition and type of cloud. Most clouds block some ultraviolet radiation, but some types can actually increase the ground level intensity by reflecting the sun’s rays. When the sun is high in the sky, ultraviolet radiation can still penetrate thick cloud.

Related site

• Bureau of Meterology, Australia

Box 6. Sunblocks and sunscreens

Sunblocks physically prevent sunlight from reaching the skin, mainly by reflecting it. They are usually a cream containing a metal such as zinc oxide or titanium dioxide. They are rarely used for whole body protection, being kept for regions of high exposure such as the nose.

Sunscreens are chemicals that absorb ultraviolet (UV) radiation. Most contain compounds called para amino benzoic acid (PABA) or benzophenone and its derivatives. Others may also contain esthers of cinnamate. Sunscreens are sold with a sun protection factor (SPF) number. The higher this number, the greater the power to block ultraviolet radiation. In theory, a person can spend longer in the sun with a sunscreen of a higher SPF rating.

There’s no doubt that, if properly and regularly applied, sunscreens prevent sunburn. But ultraviolet radiation does more than just burn the skin. Some controversy exists over whether sunscreens, as opposed to sunblocks, provide adequate protection against the effects of ultraviolet radiation on the immune system in the skin. (Most sunscreens absorb more UV-B than UV-A.) And the ultimate question of how effective they may be in stopping the long-term development of cancer has also been raised.

There are differences of opinion here. Some believe that sunscreens provide a false sense of security by encouraging people to spend longer in the sun. Without any burning it is easy to remain outside far longer than we normally would. But others say that it is better to be protected than to suffer sunburn.

Another line of argument about sunscreens relates to the chemistry of their constituents. Just how safe are the compounds in sunscreens – especially when exposed to light and ultraviolet radiation, both of which can cause chemical changes? In addition, a proportion of the active compound is absorbed by the skin. What effects might it have?

The body’s own sunscreen, melanin, is undoubtedly effective when present in large enough amounts, as is attested by the fact that black people have a rate of skin cancer at least 10 times lower than white people. However, until instant melanin is available over the counter, white people in countries like Australia will always need additional protection because their skin simply cannot manufacture enough melanin.

The medical establishment still strongly supports the use of sunscreens and there is no doubt that if you have to remain in the strong sun you are better off with one than without one. A sunblock is probably better still. Under consideration is whether the total effectiveness of sunscreens can be improved and whether any risks can be minimised.

Related sites

• Ultraviolet radiation reference (Australian Radiation Protection and Nuclear Safety Agency)

• Sunscreens and photoprotection (e-Medicine.com)

• Sunscreens (The Electronic Textbook of Dermatology, The Internet Dermatology Society, USA)

Activities

• Resources for schools (SunSmart, Australia)
  Provides a number of activities including:
  • SunSmart detective code – students decode a message about skin cancer.
  • SunSmart language activity – a wordsearch and fill-in-the-gap activity.
  • UV radiation postcard design activity – students design and plan to distribute a postcard.
  Teachers notes are also available.

• Behind the News (Australian Broadcasting Corporation)
  • Cancer dangers (22 November 2005) – students appreciate that risk-taking can lead to cancer and look at lifestyle changes to reduce the incidence of cancer. Activity sheets are available.

• BioRAP – Biological Research for Animals and People (USA)
  • Sun and skin – includes 4 lesson plans relating to skin: (Healthy skin; UV and the ozone; In the lab and; Skin cancer)

• Science NetLinks (American Association for the Advancement of Science)
  • The behaviour and health connection – students explore how personal behaviour can affect health, especially the health of your skin.
  • Sun and skin – learn about the damaging effects of sunburns and tanning, and how sunscreens work to protect us from the sun’s harmful UV rays.

• Daily Lesson Plan (New York Times, USA)
  • That healthy glow? – students create posters related to tanning and skin cancer.

• Association of the British Pharmaceutical Industry (UK)
  • Skin structure and function – provides information and then asks a series of questions about skin structure and function.

Activity 1. Protecting skin from the sun

1. How do sunscreens work?

2. How long before going out in the sun should you apply sunscreen?

3. What protection level of sunscreen should you use?

4. How often should you reapply your sunscreen?

5. What does the sun protection factor (SPF) number on a sunscreen mean?

6. What precautions should be taken with chemical sunscreens?

7. Are there any potential disadvantages of using chemical sunscreens?

8. What products other than a chemical sunscreen can you use to protect your skin from the sun?

9. What is the name of the skin pigment that gives protection from ultraviolet (UV) radiation?

10. Where is this pigment produced?

11. What happens during the tanning process?

12. Do you know of any benefits of small doses of ultraviolet radiaiton?

(Activity used with permission of the NSW Cancer Council.)

Teachers notes

1. Sunscreens can be either physical or chemical.
   • Physical sunscreens (eg, zinc oxide) form an opaque film over the area where they are applied so that ultraviolet radiation is reflected away from the skin. These preparations are called 'total blocks' because they prevent both UV-A and UV-B from hitting the skin.

   • Chemical sunscreens form a film on the skin which absorbs ultraviolet radiation.

2. Sunscreens should be applied to clean dry skin at least 15-20 minutes prior to sun exposure.

3. Authorities now recommend that everyone should use 15+ sunscreen.

4. Re-apply sunscreen every 2 hours or every hour if you are sweating a lot. Re-apply after swimming.

5. The sun protection factor numbers range from 4 to 15+ and is a measure based on how long normal, unprotected skin takes to burn when exposed to a dose of artificial sunlight. On average this is about 10 minutes. In theory, if 15+ sunscreen is applied, you are able to stay in the sun for about 150 minutes (15 × 10) before you start to burn. Because the SPF is measured in a laboratory, it is not an accurate guide to the degree of protection a sunscreen will give at different times of day and of the year. A sunscreen of SPF 15 filters approximately 94 per cent of the UV-B rays.

6. Don't use sunscreens after the use-by date and don't expect them to replace sensible sun avoidance. To avoid any skin allergy or irritation from the sunscreen, do a patch-test on a small area on the arm before applying it to the whole body. Sunscreens are only one component of skin protection and should be used in conjuction with wearing protective clothing and avoiding the midday sun.

7. A few researchers are opponents of sunscreen use for the following reasons: People using sunscreens tend to stay longer in the sun because they do not get a sunburn – they develop a false sense of security. Most chemical sunscreens absorb UV-B radiation but let most of the UV-A rays through. UV-A rays penetrate deeper into the skin and are strongly absorbed by melanocytes which are involved in sun tanning and melanoma formation. The long-term effects of regular sunscreen application are unknown, but no long-term side effects have been clearly demonstrated.

8. Loose-fitting long sleeved shirts and long pants of tightly woven fabrics are effective protection. Broad-brimmed or legionnaires hats protect the face and neck from the sun.

9. Melanin is the dark skin pigment that absorbs UV and is thus a natural sunscreen.

10. Melanin is produced by melanocytes located in the lower epidermis of the skin.

11. Melanin production by melanocytes is stimulated by exposure to sunlight. As more melanin is produced, the skin darkens.

12. About 75 per cent of the body's vitamin D supply is generated by exposure to UV-B rays.

Activity 2. Absorption of ultraviolet radiation by the atmosphere

1. Suggest the approximate time of day when the sun is in position A, position B, and position C.

2. Draw a circle with a radius of 4 centimetres to represent the Earth.

   Draw a second circle around the first (this time with a radius of 4.5 centimetres), to represent the atmosphere around the Earth.

   Draw three lines from the same point on the Earth's surface to represent the sun's rays when it is in position A, position B and position C.

3. Look at your diagram and determine the time of day that light from the sun travels through the least thickness of atmosphere.

4. Why are people advised to keep out of sunlight near the middle of the day?

(Activity and illustration adapted with permission of the NSW Cancer Council.)

Teachers notes

1. Position A of the sun approximates 12 noon, position B approximates mid-morning and position C approximates early morning. (You might point out to students that the diagram implies that the sun moves around the Earth and that this is incorrect. The diagram is a simple way of showing three different positions of the sun.)

2. 

3. The light from the sun travels through the least thickness of atmosphere at 12 noon.
   

4. At noon, sunlight travels through less atmosphere so less ultraviolet radiation is absorbed. People are advised to keep out of sunlight near the middle of the day because this is when more ultraviolet radiation strikes the Earth.

   You might like to discuss the position of the sun at your latitude at various times of the year. Students should understand that the time of year (as well as the time of day) has an influence on the thickness of atmosphere through which sunlight travels.

Further reading

Useful sites

A good introduction to skin and how it is affected by the sun's ultraviolet radiation.
http://www.howstuffworks.com/sunscreen.htm

Bad news on skin cancer (The Why Files, USA)

In-depth explanation of the connection between sun and cancer: intro to UV, forms and photos of skin cancer and the ozone layer. Also sets the groundwork for a look at epidemiological skin cancer studies by explaining meta-analysis, confounding variables and experimental bias.
http://whyfiles.org/173skin_cancer/index.html

About skin cancer (The Cancer Council Australia)

Provides information about the types, causes and treatment of skin cancer.
http://www.cancer.org.au/cancersmartlifestyle/SunSmart/Aboutskincancer.htm

Australian Broadcasting Corporation

• UVA light may cause skin cancer too (News in Science, 24 March 2004)
  Reports on research that has found rays of ultraviolet-A light may play a larger role in causing skin cancer than was previously thought.
  http://www.abc.net.au/science/news/enviro/EnviroRepublish_1072063.htm

• Skin cancer (Health Matters)
  This site contains links to a fact file about skin cancer, news and transcripts from ABC sources of articles concerned with skin cancer and links to non-ABC sites containing information about skin cancer.
  http://www.abc.net.au/health/regions/library/skincancer.htm

• Skin cancer and sunscreen
  Transcript of The Health Report (13 September 1999) covering an Australian study on the efficacy of sunscreens in protecting against skin cancer.
  http://www.abc.net.au/rn/talks/8.30/helthrpt/stories/s51601.htm

Ultraviolet radiation: Global Solar UV Index (World Health Organization)

This fact sheet summarises the major health concerns associated with sun exposure.
http://www.who.int/mediacentre/factsheets/fs271/en/

National UV index forecast (Bureau of Meteorology, Australia)

Forecasts of the daily UV index of Australian cities are given.
http://www.bom.gov.au/products/uvindex_national.shtml

Glossary

cataracts. Cloudiness in the lens of the eye, or the covering capsule of the lens, dimming the eyesight, producing distorted images, and eventually causing blindness. Cataracts can have several causes, one of which is prolonged exposure to too much sunlight.

DNA (deoxyribonucleic acid). The nucleic acid forming the genetic material of all organisms with the exception of some viruses which have RNA. DNA is present in the nucleus and other organelles such as mitochondria and chloroplasts.

electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA's Observatorium, USA).

gene. The basic unit of inheritance. A gene is a segment of DNA that specifies the structure of a protein or an RNA molecule.

melanin. A dark pigment produced by cells called melanocytes in the skin. It absorbs ultraviolet radiation and is thus a natural sunscreen. Exposure to ultraviolet radiation stimulates more melanin production, resulting in a tan. However, getting a tan entails the risk of skin damage. Dark-skinned people naturally produce much more melanin than fair-skinned types and so are less liable to experience sunburn or skin cancer.

moles. Areas of pigmentation, darker than the surrounding skin, and often raised. Harmless moles are usually less than about 5 millimetres across and have well-defined edges. A change to a mole or the appearance of a new one could indicate cancer. The number of moles on a person seems to be an indication of their exposure to the sun.

mutagen. Any agent (such as a chemical substance or radiation) that induces mutations by permanently altering the genes or chromosomes.

ozone. Ozone (O₃) is a form of oxygen. It is a colourless gas that has a very pungent odour. It exists naturally at low concentrations in the stratosphere where it absorbs ultraviolet radiation. In the troposphere it exists naturally at extremely low concentrations. But these concentrations increase when sunlight acts on various gases, coming mainly from vehicle exhausts, and ozone then becomes a pollutant in the troposphere. Ozone is a highly corrosive gas and is poisonous to most organisms. At concentrations as low as 0.00001 per cent (or 10 parts per hundred million) it can irritate the membranes lining the nose, throat and airways and can trigger or exacerbate asthma attacks.

pterygium. A small non-cancerous growth in the eye, arising from the outer layer of the eyeball. It is associated with long-term exposure to sunlight. Most pterygia are easily removed.

solar keratoses. (Also known as sunspots.) These are harmless areas of slightly changed pigmentation, not as distinct as freckles or moles. Although not cancerous, they are a sign of skin damage by ultraviolet radiation. Actinic keratoses are more scaly growths which may develop into cancer.

ultraviolet (UV). A form of electromagnetic radiation. Ultraviolet radiation has shorter wavelengths than visible light and is therefore carries more energy. It is divided into three broad categories: A, B and C. UV-A has the longest wavelength and is the least damaging form, although sufficient exposure will cause sunburn. UV-B damages proteins in unprotected organisms and can cause cancer, while UV-C is extremely dangerous because it can cause mutations in DNA.