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Do UVA and UVB affect the body in different ways?
UVA activates melanin pigment already present in the upper skin cells. It creates a tan that appears quickly but is also lost quickly. Furthermore, UVA penetrates into the deeper skin layers, where connective tissue and blood vessels are affected. As a result the skin gradually loses its elasticity and starts to wrinkle. Therefore, large doses of UVA cause premature ageing. Furthermore, recent studies strongly suggest that it may enhance the development of skin cancers. The mechanisms of this UVA damage are not fully understood, but a popular hypothesis assumes that UVA increases oxidative stress in the cell.
UVB stimulates the production of new melanin, which leads to a heavy increase in the dark-coloured pigment within a few days. This tan may last a relatively long time. UVB also stimulates the cells to produce a thicker epidermis. Therefore, UVB is responsible both for the darkening and thickening of the outer cell layers – these reactions are the body’s defence against further UV damage.
However, higher doses of UVB cause sunburn which increases your likelihood of developing cancer. The exact mechanism of how UVB initiates or promotes cancer is not yet known. In people suffering from Xeroderma pigmentosum, a rare pigmentation disease, the ability to repair DNA damage caused by exposure to UV radiation is impaired. The much-increased rates of skin cancer in these patients suggest that direct UV damage of DNA may be the mechanism that links exposure to the development of cancer.
As with effects on the skin, the various incoming sun rays penetrate the eye to different depths. While UVB is fully absorbed by the cornea, UVA passes through these surface layers to the lens. Among adults only 1 per cent or less of incoming UV radiation reaches the retina because of the filter function of cornea and lens. In contrast visible light easily penetrates through to the retina, where it activates photoreceptors and starts the chain reaction of biochemical processes to produce a visible image.
The immune system
Most experiments to date have concentrated on UVB, as it appears to be more important than UVA in causing immunemodulation. However, recently the interest in the effects of UVA on the immune system has been growing. It is believed that UV radiation is absorbed by a molecule located in the skin. This leads to changes in the distribution and activity of some of the key molecular and cellular players of the immune system. An altered balance of the immune response through cells and antibodies may reduce the body’s ability to defend itself against certain diseases.
Are there beneficial effects of UV radiation?
The sun’s rays provide warmth and light that enhance your general feeling of well-being and stimulate blood circulation. Some UV radiation is essential to the body as it stimulates the production of vitamin D. Vitamin D has an important function in increasing calcium and phosphorus absorption from food and plays a crucial role in skeletal development, immune function and blood cell formation. There is no doubt that a little sunlight is good for you! But 5 to 15 minutes of casual sun exposure of hands, face and arms two to three times a week during the summer months is sufficient to keep your vitamin D levels high. Closer to the equator, where UV levels are higher, even shorter periods of exposure suffice.
Hence, for most people, vitamin D deficiency is unlikely. Possible exceptions are those who have very limited sun exposure such as the housebound elderly, or those with heavily pigmented skin who live in high-latitude countries where UV levels are relatively low. Recognising the need for vitamin D, many countries have introduced supplements into common food like flour, cereals and milk. Naturally occurring vitamin D is very rare in our diet, it is present mainly in fatty fish and cod liver oil.
UV radiation has been used to successfully treat a number of diseases, including rickets, psoriasis, eczema and jaundice. This therapeutic use cannot eliminate the negative side-effects of UV radiation but treatment takes place under medical supervision to ensure that the benefits outweigh the risks.
Rickets causes a child’s bones to soften because they are not getting enough calcium. One common reason for this is a lack of vitamin D. Vitamin D increases calcium absorption from food and transports the ion from the gut into the bone. UV radiation exposure stimulates the production of vitamin D. However, today, most people receive enough vitamin D from supplements in the food they eat.
Lupus vulgaris is tuberculosis of the skin. It used to be common especially in northern Europe during the winter. The disease produces large ulcers on the face and neck, which are difficult to cure and often leave bad scars. A Danish doctor called Neils Finzen developed a UVB lamp that was so successful in curing the disease that it won him the Nobel prize in 1903. Today, Lupus is very rare and is usually treated successfully with antibiotics.
Psoriasis is a disease which produces sores and scaling of the skin. It affects 2 to 3 per cent of the population and is probably an autoimmune disease; a disease where your immune system attacks your own cells. Among the treatments for psoriasis PUVA therapy is one of the most popular and successful. The patient is given a drug called psoralen to make the skin more sensitive to UV and is subsequently exposed to UVA radiation. This is repeated several times in the course of treatment. Unfortunately, PUVA treatment increases the patient’s risk of developing squamous cell carcinoma, the most common type of skin cancer.
Vitiligo is a patchy loss of skin pigmentation caused by destruction of the pigment-producing cells called melanocytes. It is probably an autoimmune disease and can be treated by PUVA therapy. In PUVA treatment, the patient is given a drug called psoralen to make the skin more sensitive to UV and is subsequently exposed to UVA radiation. The therapy is fairly successful but increases the patient’s risk of developing squamous cell carcinoma, the most common type of skin cancer.
Despite these important roles and medical applications, the harmful effects of exposure to UV radiation usually far outweigh its benefits. In addition to the well-known short-term effects of overexposure to the sun such as sunburn or allergic reactions, long-term effects pose a life-long hazard to your health. Overexposure to UV radiation affects your skin, your eyes and probably your immune system. Many people forget that the effects of exposure to UV radiation accumulate over a lifetime. Your sun exposure behaviour now determines your chances of developing skin cancer or cataracts later in life! Skin cancer incidence is strongly correlated with the duration and frequency of sun exposure.