Feature

UV -- A Burning Issue

Despite considerable publicity, confusion remains about the dangers of ultraviolet radiation and how to avoid those dangers.

By Janine Griffin

The thinning of the ozone layer and increasing public awareness of the dangers of unprotected sunbathing have heightened interest in the effects of ultraviolet (UV) radiation. Most UV radiation from the sun is blocked by the earth's atmosphere, but that which does get through has the potential to do serious harm to human health, particularly in the southern hemisphere where UV levels are higher than in the northern hemisphere. As the ozone layer continues to thin, UV damage can be expected to increase, but how dangerous is UV, and how can we protect ourselves from its effects?

Ultraviolet radiation is the form of electromagnetic radiation with wavelengths between the violet region of the visible light spectrum and the x-ray region. By far the most common source of UV radiation is the sun, but other sources include electric arc welding, "black light" lamps, and germicidal lamps used in hospitals.

There are three types of UV -- longer wavelength UVA, medium wavelength UVB and shorter wavelength UVC. Of the three, the UVC band is by far the most potentially harmful, followed by the UVB and UVA bands. However, as all UVC and the shorter UVB wavelengths radiated by the sun are blocked by the atmosphere, it is the longer UVB wavelengths and UVA that are of most concern. UVA is far more abundant than UVB, but the small amount of UVB present poses the greatest risk to human health.

Because ultraviolet radiation penetrates only about three millimetres into the body, it is the eyes and the skin that are most at risk. UV produces photochemical changes in tissue, most of which are damaging, according to the National Radiation Laboratory. At low rates of exposure, though, some effects are reduced by the natural replacement of damaged tissue.

UV Health Effects

Many people consider a tan a sign of good health, and some even see it as a shield against skin cancer. In order to acquire that tan, exposure to potentially harmful UV is necessary; a tan is a sign that the skin has been damaged.

UV sets up a sort of defensive response in human skin. UVB causes the production of melanin, the dark pigment that produces a tan and protects the body from further damage from radiation. UVA acts on melanin already present in the skin to give a light, short-lasting tan.

Moderate exposure to UV produces tanning and thickening of the skin -- overexposure produces more serious consequences. These include skin cancer, eye problems and immune suppression.

The most well-known consequence of overexposure to UV is sunburn, or erythema, which is caused by the dilation of blood vessels beneath the skin. Erythema is produced mainly by UVB, although UVA does play a minor role. Sunburn, although painful, is short-lasting; repeated sunburn, though, can have more serious consequences.

The public is aware of the risk of skin cancer. Cancer Society campaigns and numerous sunscreen advertisements have seen to that. The bulk of the attention has focused on melanoma -- the most deadly of the skin cancers -- and when people talk about "skin cancer" they are more than likely talking about melanoma.

More than 1,000 New Zealanders develop melanoma each year, reports the Cancer Society, and thousands more develop other skin cancers. Early detection of skin cancer leads to a cure in 99% of cases.

UVB produces skin cancer more readily than UVA, says the National Radiation Laboratory, and there is evidence to suggest that repeated overexposures which result in sunburn can lead to cancer.

A working party of the Australian National Health and Medical Research Council (NHMRC) reports that childhood exposure to UV is important in the development of skin cancer. Children's skin is very sensitive to UV, and the results of repeated childhood sunburn may not become apparent for 30-40 years. Solar keratoses, the early warning signs of skin cancer, are less common in British immigrants to Australia than in Australian-born people. Immigrants experience less UV exposure early in life than their Australian counterparts, and so have a lower risk of skin cancer.

Solar keratoses, or sunspots, are not cancerous themselves but warn the bearer that they are prone to skin cancer. These red, flattish, scaly areas may sting if scratched, and appear and disappear irregularly. They appear on areas not usually protected by clothing and are most common in middle-aged or older people.

Basal cell carcinoma (BCC) is the most common type of skin cancer but the least dangerous. It appears as a lump or red, scaly area, and as it grows it may form sores that bleed when scratched. BCCs grow slowly and appear on the head, neck and upper trunk. It is almost always curable if diagnosed and treated early. Squamous cell carcinomas (SCCs) are a more dangerous, but treatable type, similar to BCCs.

Melanoma is the most dangerous of the skin cancers and will spread to other parts of the body if left untreated. Melanoma appears as a freckle or mole that changes colour or shape, or sometimes as a new spot. It usually has a smudgy irregular outline and more than one colour, and grows over weeks or months anywhere on the body. Early detection leads to cure in 95% of cases.

The best way to prevent UV damage to the skin is to stay out of the sun. People can be sunburnt through glass, says Rob Coppell of the National Radiation Laboratory, but the sunburning strength is something like a tenth of what it would be outdoors. UVBs, the main UV wavelengths responsible for sunburn, do not penetrate ordinary window glass. UVA can and does cause sunburn, but this is extremely unusual.

Screening Out UV

Most of the risk from UV occurs outdoors. Sensible use of sunscreens and protective clothing is essential to minimising UV damage while outdoors.

It is possible to be sunburnt through clothing. Clothing does offers a degree of protection against UV, but some fabrics are better than others. The amount of protection depends on the weave density, but not the colour. What gets through the gaps between the fibres is what's important, says Coppell, not the colour on the fibres.

The tighter the weave, the better the protection. Tightly woven cotton or linen polyester offer more protection than loosely woven muslin clothing. Some natural fibres, such as cotton, allow more UV through when wet.

Clothing that is opaque to light will be opaque to UV. Therefore, holding a garment up to the light will give some idea of how effective it will be as a shield against UV. The more light that passes through the garment, the less protection it is likely to offer.

Parts of the body not covered by clothing, or covered by clothing that doesn't offer adequate protection from UV, should be protected by a sunscreen. Consumer recommends using sunscreens on all exposed parts of the body while outdoors, even if in the shade, and during long summer car trips where exposure to UVA through glass may be sufficient to cause sunburn.

Sunscreens block out about 96% of UV rays, but the small amount allowed through will eventually cause sunburn. Constant applications of sunscreen will protect the wearer for about 2-3 hours, says the Cancer Society. The Society recommends the use of broad spectrum sunscreens, those that protect against both UVB and UVA, rather than the UVB-only sunscreens.

The Sun Protection Factor (SPF) of a sunscreen tells how much protection it provides. The higher the number, the greater the protection. For example, if you burn after 10 minutes without protection, a sunscreen of SPF15 will protect you for about 15 times longer, or two and a half hours. Consumer recommends people buy the least expensive sunblock which suits them -- that way they will be less reluctant to apply large amounts. About 30 ml of sunblock over the whole body will give the SPF protection on the label.

Sunscreens need time to soak into the skin, and should be aplied about 30 minutes before going outdoors. They also wear off, sweat off and can be rubbed off, indicating the need for regular applications.

Sunblocks such as zinc cream and titanium dioxide act as a total barrier to UV, and if they are applied thickly, skin will not burn or tan. However, says the Cancer Society, sunblocks stop people perspiring and should only be used on small areas vulnerable to overexposure such as the lips, the ears or nose.

The eyes are also vulnerable to UV radiation. The cornea, the outermost part of the eye, absorbs UVB and UVC and transmits UVA; the lens absorbs the UVA transmitted by the cornea. Furthermore, UV light is known to be harmful to the retina, the inner layer of the eye which receives the image formed by the lens and transmits it to the brain through the optic nerve.

Overexposure to UVB and UVC causes keratoconjunctivitis, a painful inflammation of the cornea similar to having grit in the eye. Electric arc welding is a source of UVC which can damage the eye, and keratoconjunctivitis is commonly known as "arc eye" or "welder's flash". "Snow blindness" is similar, although it is caused by solar UVB reflected by snow. Symptoms usually disappear in a few days.

Protective glasses which screen all UV wavelengths are the best protection against UV damage. The Cancer Society's ideal sunglasses totally block UV radiation, eliminate reflected light rays, wrap around the eyes and fit tightly on the brow, and have durable, flexible frames. Furthermore, they should be a colour or tint which doesn't interfere with recognising objects and colours.

Some sunglasses offer protection against infrared rays or claim an SPF factor of 100 or 500. Such statements are misleading, as there is no scientific proof that infrared is damaging to the eyes, and SPF factors apply to sun-screens, not sunglasses.

A less well-known property of UV is its effects on the immune system. UV causes two types of changes in the immune system. The first is limited to the skin itself, but the second affects the immune system as a whole. These effects have been well studied in mice, but there is little information on the effects on the human immune system.

It has been known for some time that prolonged UV exposure leads to the development of skin tumours in mice. UV-induced tumours grow in irradiated mice but are usually rejected when transplanted into normal, non-irradiated mice, indicating that the irradiated mice possess a reduced immune response.

The reasons for this apparent suppression of the immune system are not clear, but seem to involve the Langerhans cells in the skin, which are important in starting immune responses. Human skin exposed to UV shows reductions in Langerhans cell numbers, and therefore immunity. People exposed for one hour per day over a two-week period showed significant changes in immunity which were still apparent two weeks after treatment ceased.

Studies on the effect of UV on the human immune system are only in their early stages, and whether long-term immune suppression occurs is not known. But if further studies show that immunity is indeed impaired by overexposure to UV, then skin cancer and eye problems won't be the only reasons for staying out of the sun.

Janine Griffin is a freelance journalist specialising in science issues.