The known health effects of UV  from WHO  
http://www.who.int/uv/faq/uvhealtfac/en/print.html   downloaded March 2010
Do UVA and UVB affect the body in different ways?
The skin
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.
The eye
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. {actually fairly low} 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
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
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
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
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.
What are the effects of UV on the skin?
The truth about a suntan
There is no such thing as a healthy tan! The skin produces a dark-coloured pigment, melanin, as a shield against further damage from UV radiation. The darkening provides some protection against sunburn: a dark tan on a white skin offers a sun protection factor of between 2 and 4. However, it is no defence against long-term UV damage such as skin cancer. A suntan may be cosmetically desirable, but in fact it is nothing but a sign that your skin has been damaged and has attempted to protect itself.
There are two distinct types of tanning reaction: immediate pigment darkening occurs, where the pigment called melanin already present in the skin darkens on exposure to UVA. This immediate tan begins to fade within a few hours after cessation of exposure. Delayed tanning happens over a period of about three days, when new melanin is produced and distributed between the upper skin cells. This tan can persist for several weeks.
Sunburn
High doses of UV radiation kill most of the skin cells in the upper skin layer, and cells that are not killed get damaged. In its mildest form, sunburn consists of a reddening of the skin called erythema. This appears shortly after UV radiation exposure and reaches a maximum intensity between 8 and 24 hours. It then fades over the course of a few days. However, a strong sunburn may cause the skin to blister and peel, which is not only painful but also leaves the very white and new skin underneath unprotected and even more prone to UV damage.
Photosensitivity
A small percentage of the population have a skin condition that makes them particularly sensitive to the sun's UV rays. A minimal dose of UV radiation is sufficient to trigger an allergic reaction leading to rash or severe sunburn. Photosensitivity is often associated with the use of certain medications, including some non-steroidal anti-inflammatory drugs and painkillers, tranquillizers, oral anti-diabetics, antibiotics and antidepressants. If you take any medication on a regular basis, please refer to the package insert for the product or consult your health care provider about possible photosensitivity reactions. Some foods and cosmetic products like fragrances and soaps may also contain ingredients that cause or aggravate the photosensitivity condition.
The wrinkle factor
Sun exposure promotes the ageing of your skin due to a combination of several factors. UVB stimulates cell proliferation in the outer layer of the skin. As more and more cells are produced the epidermis thickens. UVA penetrating into the deeper skin layers disturbs the connective tissue: the skin gradually loses its elasticity. Wrinkles, sags and bags are the common result of this loss of elasticity. A phenomenon often seen in elderly people is the localized over-production of melanin resulting in dark patches or liver spots. Furthermore, the sun's rays dry out your skin making it coarse and leathery.
Non-melanoma skin cancers
In contrast to malignant melanoma, the non-melanoma skin cancers basal and squamous cell carcinoma are not usually fatal but surgical treatment can be painful and result in scars. Non-melanoma skin cancers are most frequent on those parts of the body that are commonly exposed to the sun such as ears, face, neck and forearms. The finding that they are more frequent in outdoor than in indoor workers suggests that the accumulated life-time exposure to UV radiation plays a major role in the development of non-melanoma skin cancers.
Basal cell carcinomas are the most common type of skin cancer. Their reported incidence has drastically increased over the past two decades and continues to rise. They usually appear as a red lump or scaly area, however, no clear precursors have been identified. They grow slowly, rarely spread to other parts of the body (metastasize) and can be removed surgically.
Squamous cell carcinomas are the second most common form of skin cancer. They appear as a thickened red scaly spot at body sites most often exposed to UV radiation. As they sometimes metastasize they are more dangerous than basal cell carcinomas. However, they also tend to be slow growing and can usually be removed surgically before they become a serious risk.
Melanoma skin cancers
Malignant melanoma is the rarest but most dangerous type of skin cancer. It is one of the most common cancers among 20- to 35-year-olds, especially in Australia and New Zealand. All forms of skin cancer have been on the increase over the past twenty years, however, the highest rate of increase worldwide is noted for malignant melanoma.
Malignant melanoma may arise as a new mole or as changes in colour, shape, size or sensation of an old spot, freckle or mole. Melanomas tend to have an irregular outline and a patchy colouring. Itching is another common symptom but is also found in normal moles. If recognized and treated early, the chances of survival are good. If untreated, the tumour can develop rapidly, and cancer cells can spread to other parts of the body.
The causes of malignant melanoma are not fully understood. Exposure during childhood is thought to be more important than exposure later in life. Tumour development may be linked to occasional exposure to short periods of intense sunlight, such as at weekends or on holiday. The higher incidence of malignant melanoma in indoor workers compared to outdoor workers supports that notion.
Watch out for early symptoms of malignant melanoma
If you have a mole, freckle or spot you are concerned about, go and see a dermatologist! It may well not be a melanoma; however, if it is a melanoma, it is crucial that it is recognized and treated early.
Check regularly for any mole that grows, changes shape or colour, is inflamed or itchy, weeps or bleeds
What are the effects of UV on the eye?
The eye occupies less than 2 per cent of the whole body surface area, but it represents the sole organ system to allow the penetration of visible light deep into the human body. During human evolution a number of mechanisms have evolved to protect this very sensitive organ against harmful effects of the sun's rays:
The eye is recessed within the anatomy of the head and shielded well by the brow ridge, the eyebrows and the eyelashes. However, these anatomical adaptations are of limited use in UV protection under extreme conditions such as sunbed use or strong ground reflection from snow, water and sand.
Constriction of the pupil, closure of the eyelids and the squinting reflex minimize the penetration of the sun's rays into the eye. These mechanisms are activated by bright visible light and not by UV radiation – but on a cloudy day UV radiation exposure may still be high. Therefore, the effectiveness of these natural defences in protecting against UV damage is limited.
Photokeratitis and photoconjunctivitis
Photokeratitis is an inflammation of the cornea, while photoconjunctivitis refers to an inflammation of the conjunctiva, the membrane that lines the inside of the eyelids and eye socket. These inflammatory reactions may be compared to a sunburn of the very sensitive skin-like tissues of the eyeball and eyelids and usually appear within a few hours of exposure. Photokeratitis and photoconjunctivitis can be very painful, however, they are reversible and do not seem to result in any long-term damage to the eye or vision.
An extreme form of photokeratitis is snow blindness. It sometimes occurs in skiers and climbers who experience extreme UV levels due to high altitude conditions and very strong ground reflection – fresh snow can reflect up to 80 per cent of incident UV radiation. These extreme UV levels kill the outer cells of the eyeball leading to blindness. Snow blindness is very painful when the dead cells are being shed. In the majority of cases new cells grow quickly and vision is restored within a few days. Very severe snow blindness may involve complications such as chronic irritations or tearing.
Pterygium
This growth of the conjunctiva on the surface of the eye is a common cosmetic blemish and is probably linked to prolonged UV exposure. Pterygium may extend over the centre of the cornea and thereby reduce vision. It also has a tendency to become inflamed. Even though it can be removed by surgery, the outgrowth tends to recur.
Cataracts
Cataracts are the leading cause of blindness in the world. Proteins in the eye's lens unravel, tangle and accumulate pigments which cloud the lens and eventually lead to blindness. Even though cataracts appear to different degrees in most individuals as they age, they appear to be enhanced by exposure to UVB. Cataracts can be surgically removed and an artificial lens or other means of optical correction can restore vision.
Every year some 16 million people in the world suffer from blindness due to a loss of transparency in the lens. WHO estimates suggest that up to 20 per cent of cataracts may be caused by overexposure to UV radiation and are therefore avoidable.
Cancer of the eye
Current scientific evidence suggests that different forms of eye cancer may be associated with life-long exposure to the sun. Melanoma is the most frequent malignant cancer of the eyeball and sometimes requires surgical removal. A common location for basal cell carcinoma is on the eyelids.
Does UV interact with the immune system?
Sunlight exposure can precede the onset of recurrent eruptions of cold sores. UVB radiation appears to reduce the effectiveness of the immune system – in the case of cold sores it can no longer keep the virus Herpes simplex under control which results in re-activation of the infection. A study in the United States looked into the effect of sunscreen application on the incidence of cold sores. Of 38 patients, who recurrently suffer from Herpes simplex infections, 27 developed cold sores after exposure to UV radiation. In contrast, after the application of protective sunscreen, none of the patients developed cold sores. Therefore, beyond limiting skin-damaging effects, sunscreen may be effective in the prevention of sunlight-induced recurrent eruptions.
Research in recent years has provided increasing evidence that exposure to environmental UV levels can alter the activity and distribution of some of the cells responsible for triggering immune responses in humans. As a consequence sun exposure may enhance risk of infection or reduce the body's defences against skin cancer. Especially in countries of the developing world, high UV levels may decrease the effectiveness of vaccinations.
It has also been suggested that UV radiation may promote cancer in two distinct ways: by directly inducing DNA damage and by weakening the immune system. To date little research has been undertaken to describe the potential influence of immunomodulation on the development of cancer.
I am dark-skinned – do I still need to be careful?
Yes, you do. Compared to fair-skinned people, dark-skinned people have a much lower risk of developing melanoma or non-melanoma skin cancers. They do not normally have to apply sunscreens and can safely tolerate relatively high levels of UV radiation without getting burnt. But – regardless of skin colour - the risk of eye damage and of harmful effects on the immune system remains.