The harmful effects from exposure to ultraviolet (UV) radiation can be classified as acute or chronic. The acute effects of UV-A and UV-B exposure are both short-lived and reversible. These effects include mainly sunburn (or erythema) and tanning (or pigment darkening). The chronic effects of UV exposure can be much more serious, even life threatening, and include premature aging of the skin, suppression of the immune system, damage to the eyes, and skin cancer.
Exposure to UV radiation in the form of sunlight is overwhelmingly the most important etiologic factor for the development of nonmelanoma skin cancer. The wavelengths of 290 to 320 nm (UVB), and to a lesser extent, 320 to 400 nm (UVA) have been implicated. The amount of exposure, the timing of exposure, and the skin type being exposed determine the rate of carcinogenesis. Light-skinned people develop skin cancer much more readily at a given rate of sun exposure than dark-skinned individuals, who are protected by the melanin pigment in their skin. As can be expected, nonmelanoma skin cancers occur most frequently on skin sites that are most often exposed to the sun.
Squamous cell carcinoma usually occurs in the sun-exposed areas of head and neck. Persons with outdoor occupations, such as sailors and farmers, have a higher incidence of skin cancers than those with indoor occupations. Epidemiologic studies worldwide suggest that UV radiation is the most important etiologic agent for skin cancers. The incidence of nonmelanoma skin cancers directly correlates with the proximity to the equator. A quantitative association has been observed between lifetime sun exposure and the risk of developing nonmelanoma skin cancers. In outdoor workers, the most common sites for skin cancers are on the head, neck, and dorsum of the hands, which are the sites of maximal chronic sun exposure. Chronic UV exposure has changed from occupational to a more recreational pattern, and younger and younger persons are being diagnosed with skin cancers. Recent studies have further delineated risk factors for the development of UV light-induced skin cancers. There is an increased incidence of skin cancer in Australia as compared with Scandinavian countries, where the population has similar skin but UV light exposure is different. In addition, there is a higher incidence of skin cancer in albinos than in normally pigmented persons in Africa.
The risk for basal cell carcinoma is increased in light-skinned people, in those who freckled or suffered a severe sunburn in childhood. A large increase in the risk for basal cell carcinoma is seen in individuals with increased sun exposure in childhood and adolescence. This relationship was strongest among individuals who burned rather than tanned upon sun exposure. Another study suggested that intense sun exposure delivered intermittently to poorly tanning light-skinned individuals carried more risk than the same dose of sun exposure delivered more continuously over the same total period of time.
The risk for squamous cell carcinoma is also strongly related to UV radiation. Again, it occurs more frequently in light-skinned individuals and is related directly to the amount of solar radiation they receive. Sailors, farmers, and others with outdoor occupations have a higher incidence of squamous cell carcinoma than those with indoor occupations. The temporal relationship to occupational solar exposure was further delineated by a recent study that found a strong trend toward increased risk with increasing occupational solar exposure in the 10 years prior to diagnosis of squamous cell carcinoma. Other forms of UV radiation, such as the one used in conjunction with psoralens for the treatment of psoriasis, also increase the risk of skin cancers, especially that of squamous cell carcinoma.
There is considerable environmental concern about the depletion of the ozone layer by certain chemicals. The ozone layer acts as a strong barrier in absorbing a major portion of UV radiation and preventing it from reaching the earth. The chemicals responsible for the depletion of the ozone layer include chlorofluorocarbons, which are found in aerosol sprays and refrigerators. It is thought that the depletion of ozone over the next few years will rapidly increase the incidence of skin cancers.
Persons with light complexions have a higher chance of developing skin cancers than those with darker skin, who are protected from solar damage by the melanin pigment in the skin. Cutaneous malignancies are rare in dark-skinned persons, but African albinos have a high incidence of skin cancers in sun-exposed areas.
UV light may influence the development and progression of skin cancers by affecting the host immune system. The classic work of Kripke and colleagues showed that UV-induced cancers in mice are highly antigenic and that most are rejected by the host’s immune system after transplantation into a normal, genetically identical animal. However, the primary host in whom the tumor was induced by UV light becomes tolerant to the tumor and allows its rapid growth. Kripke’s work indicates the development of suppressor factors and cells that suppress the host’s immune system and prevent rejection of UV-induced skin cancer. These observations have not yet been confirmed in humans, but it is likely that UV light affects Langerhans cells in human skin, which may alter the host immune system, allowing the development and progression of skin cancer. Recent studies have noted an intriguing finding that may elaborate on Kripke’s work. A tumor-suppressor protein, designated p53, is present in very small amounts for very short periods of time in normal keratinocytes, such that it is normally undetectable. Long-lived mutants of p53, which may have lost the tumor-suppressor activity, have been detected in sun-damaged epidermis adjacent to basal cell carcinomas, which also contain the mutant p53. This suggests that UV damage to keratinocytes may result in mutation of p53 to an ineffective form, allowing carcinogenesis to proceed. Mutant p53 has also been seen in squamous cell carcinomas, suggesting that the above model may have wide application.
Skin cancers are the most commonly occurring cancers in terms of incidence in the world. There are different types of skin cancer including the nonmelanoma skin cancers, basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), and melanoma. Exposure to UV radiation is thought to be an important factor in each of these cancers as it induces DNA damage, however the types of exposure necessary to cause the different types of skin cancer may vary. For the nonmelanoma skin cancers, cumulative sun exposure is believed to be important, whereas for melanoma the intermittent exposure hypothesis has been postulated. This hypothesis proposes that infrequent intense exposure of unacclimatized skin to sunlight is related to the increasing incidence of melanoma and is more important than chronic sun exposure
What are some health effects of exposure to UV radiation?
Some UV exposure is essential for good health. It stimulates vitamin D production in the body. In medical practice, one example is UV lamps can be used for treating psoriasis (a condition causing itchy, scaly red patches on the skin).
Excessive exposure to ultraviolet radiation is associated with different types of skin cancer, sunburn, accelerated skin aging, as well as cataracts and other eye diseases. The severity of the effect depends on the wavelength, intensity, and duration of exposure.
Effect on the skin due to exposure to UV radiation
The shortwave UV radiation (UV-C) poses the maximum risk. The sun emits UV-C but it is absorbed in the ozone layer of the atmosphere before reaching the earth. Therefore, UV-C from the sun does not affect people. Some man-made UV sources also emit UV-C. However, the regulations concerning such sources restrict the UV-C intensity to a minimal level and may have requirements to install special guards or shields and interlocks to prevent exposure to the UV.
The medium wave UV (UV-B) causes skin burns, erythema (reddening of the skin) and darkening of the skin. Prolonged exposures increase the risk of skin cancer.
Longwave UV radiation (UV-A) accounts for up to 95% of the UV radiation that reaches the earth’s surface. Although UV-A is less intense than UV-B, it is more prevalent and can penetrate deeper into the skin layers, affecting the connective tissue and blood vessels, which results in premature aging.
Certain chemicals and medications act as photosensitizing agents and enhance the effect of UV radiation from sunlight or other sources. Such agents include thiazide diuretics (drugs which cause excessive urine production), drugs used in the treatment of high blood pressure, certain antibiotics (tetracyclines, sulfonamides), cosmetics, and thiazine tranquilizers.
These are just a few examples; this is not intended to be a comprehensive list. However, it is important to know that these photosensitizing effects can occur in case people are exposed to UV radiation at work. For example, an inexperienced welder, who was taking a phenothiazine anti-depressant drug, suffered damage in both eyes in the part of the retina that absorbs short wavelength light (bilateral maculopathy). He began complaining of eye problems a day after he was arc welding for two minutes without wearing any eye protection. This damage, that fortunately was reversible after several months, occurred because the drug he was taking sensitized him to the UV radiation to which he was exposed.
Various plants such as carrot, celery, dill, fig, lemon and some types of weeds are known to cause photosensitivity. Exposure to fluids from these plants, especially if crushed, followed by exposure to sunlight can cause dermatitis. Citrus fruit handlers and vegetable harvesters, gardeners, florists and bartenders are at risk for experiencing dermatitis following exposure to certain plants and then to sunlight (phytophotodermatitis).
Coal tar and creosote are examples of photosensitizing agents in the workplace.
Effects of repeated exposures (chronic effects) include skin aging and skin cancer. There is a strong causal link between skin cancer and prolonged exposure to solar UV and from artificial sources.
Effect on the eyes due to exposure to UV radiation
The eyes are particularly sensitive to UV radiation. Even a short exposure of a few seconds can result in a painful, but temporary condition known as photokeratitis and conjunctivitis. Photokeratitis is a painful condition caused by the inflammation of the cornea of the eye. The eye waters and vision is blurred. Conjunctivitis is the inflammation of the conjunctiva (the membrane that covers the inside of the eyelids and the sclera, the white part of the eyeball); which becomes swollen and produces a watery discharge. It causes discomfort rather than pain and does not usually affect vision.
Examples of eye disorders resulting from UV exposure include “flash burn”, “ground-glass eyeball”, “welder’s flash” and “snow blindness” – depending on the source of the UV light causing the injury. The symptoms are the pain, discomfort similar to the feeling of sand in the eye and an aversion to bright light.
The eyes are most sensitive to UV radiation from 210 nm to 320 nm (UV-C and UV-B). Maximum absorption by the cornea occurs around 280 nm. Absorption of UV-A in the lens may be a factor in producing cataract (a clouding of the lens in the eye).