The sun puts out three different types of rays. The first are the infrared rays which warm, and the second are the visible rays which provide the light by which we see. The third type of radiation from the sun is related to skin cancer and premature ageing. These rays are called ‘ultraviolet’ because technically they are invisible. Ultraviolet (UV) rays consist of three different types, ultraviolet A rays, ultraviolet В rays and ultraviolet С rays. Ultraviolet С rays are not particularly important to this discussion because they are trapped by the ozone layer. However, ultraviolet С rays could become a problem for us if the ozone layer continues to be depleted.
Both ultraviolet A and ultraviolet В rays can penetrate the skin and cause sunburn, ageing and cancer. Ultraviolet A rays primarily cause photoageing (ageing caused by sun exposure), while ultraviolet В rays are responsible for sunburn and skin cancer.
Ultraviolet A rays penetrate the skin readily and cause deeper damage to the skin’s structural fibres than ultraviolet В rays. Ultraviolet A rays are prevalent throughout the day, whereas ultraviolet В rays are most abundant between 10.00 a.m. and 3.00 p.m. Furthermore, ultraviolet A rays shine all year round, and are as abundant in winter as they are in summer. Ultraviolet В rays, on the other hand, are reduced in the winter months when the sun sits lower in the sky. In winter, the ozone layer ‘catches’ most of the ultraviolet В rays. Another important difference is that ultraviolet A rays are very strong, regardless of where you live. Conversely, ultraviolet В rays are most intense near the equator.
We know that ultraviolet light is a major cause of skin cancer because:
– Skin cancers become more common the closer one moves to the equator. The incidence of skin cancer in northern Australia is far greater than in Tasmania.
– Skin cancer is more common in outdoor workers and tends to occur on sun-exposed parts of the body such as the face and backs of hands.
– History of sun exposure, especially sunburn, correlates with skin cancer.
– Regular use of sunscreens in certain populations decreases the incidence of skin cancer.
Fair-skinned people are more susceptible to skin cancer than those with olive or dark skin because they produce less protective pigment to absorb the ultraviolet rays. It also seems that sun exposure early in life, particularly the first twenty years, is most critical. Studies have demonstrated that migrants coming to Australia after the age of twenty have a lower incidence of skin cancer than people of the same racial background who were born in Australia.
Skin cancers do not occur immediately after sun exposure. Rather, there is a lag phase of between twenty and twenty-five years before the lesions become obvious. Other factors, such as hereditary and probably viral influences, are also important in the development of skin cancer, especially melanoma (potentially deadly tumour).
There are many misconceptions about the benefits of sun exposure. For example, some people believe that a suntan protects the skin against skin cancer. The opposite is true – sun-tanning promotes skin cancer. It may also be thought that a suntan makes one feel and look better. This is only a state of mind. As fashions change, and models become paler, a suntan will no longer be considered desirable. Another point to make here is that the sun is necessary to stimulate vitamin D production, which is necessary for bone growth. However, only a tiny amount of sun exposure is necessary, that is, ten to twenty minutes a day.