James A. Hathaway and David H. Sliney
Ultraviolet radiation is that portion of the electromagnetic spectrum between visible light (about 400 nm) and the lower limit of ionizing radiation (about 100 nm). The energy of ultraviolet radiation photons increases as the wavelength decreases. The ultraviolet spectrum is divided into the following three bands: from 315 to 400 nm, it is called UV-A; from 280 to 315 nm, it is designated UV-B; and from 100 to 280 nm, it is referred to as UV-C.1
Natural sunlight includes biologically significant amounts of energy in the UV-A and UV-B bands. The upper atmosphere filters out the UV-C radiation, although there is concern about UV-C exposure in areas where the ozone layer is absent. Employees who work in the natural environment incur the greatest occupational exposure to ultraviolet radiation. Examples of such occupations include farmers and other agricultural and forestry workers, fishermen, outdoor construction workers, landscapers, and lifeguards.
Altitude, latitude, time of year, time of day, and the thickness of the ozone layer affect UV strength. Levels are higher at higher altitudes, closer to the equator, and during summer. For latitudes between the Tropic of Cancer and the Tropic of Capricorn, levels are moderate to extreme year-round. UV strength is also increased between 10 a.m. and 4 p.m. Levels are also higher where the ozone layer is thinner. While exposure is highest under clear skies, cloud cover provides only a limited reduction in exposure. Exposure is also increased when ultraviolet radiation reflects off water, snow, or sand.
The most common exposure to significant levels of nonnatural ultraviolet radiation occurs among welders. Other workers may receive exposure from sources such as gas discharge lamps and carbon arcs.2 Low-pressure mercury vapor lamps are used to control microorganisms in operating rooms, to control bacterial growth in meat, to prevent contamination in biological laboratories, to reduce airborne bacterial levels in air ducts, and to eliminate coliform bacteria in drinking water. High-pressure mercury vapor lamps are used for photochemical reactions and to identify minerals. High-pressure xenon arcs and carbon arcs have a broad spectrum of radiation, including visible and ultraviolet radiant energy. They are used as high-intensity light sources, such as searchlights, as well as in the printing industry.3
Workers with occupational exposure also have nonoccupational exposure, including exposure during outdoor activities and possibly through artificial (indoor) tanning.
Measurement is not a significant issue when using many of these exposure sources since it is already known that they emit harmful levels of ultraviolet radiation. In these cases, skin and eye protection is required. However, if direct measurement of ultraviolet radiation is needed, several UV meters are commercially available. Care should be taken to ensure that the UV meter is effective in the wavelength range of the source. UV detection devices include photodiodes, certain photomultipliers, and vacuum photodiodes.
Selective filters are frequently used to isolate the part of the UV spectrum under study. Frequent calibration of meters is often necessary with heavy use. No single instrument perfectly matches the biological hazard action spectrum of ultraviolet radiation; therefore, it is necessary to calibrate meters to several wavelengths to evaluate broad-spectrum sources of UV.4 Some recently developed detectors are remarkably well matched to the action spectrum.
The American Conference of Governmental Industrial Hygienists (ACGIH) has published exposure limits called threshold limit values (TLVs®) for ultraviolet radiation.5 To protect against photokeratitis effects on unprotected eyes from UV radiation in the 320–400 nm range, total irradiance should not exceed 1.0 mW/cm2 for periods >10 seconds (~16 mm) or >1.0 J/cm2 for exposures less than ~10 seconds. Exposure limits for other wavelengths vary significantly by wavelength and duration of exposure. The tables in the ACGIH TLV® guide should be used to determine exposure limits for a particular set of conditions. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) also adopted these limits with minor modifications in the UV-A region.6
Ultraviolet irradiation of 7-dehydrocholesterol in the skin produces previtamin D3, which is converted to vitamin D3 (cholecalciferol). This vitamin and some vitamin D3 from the diet are converted to circulating vitamin D.7 Vitamin D is essential for the regulation of metabolism of bone minerals. Some level of ambient ultraviolet radiation below the TLV® appears to be necessary to maintain good health. Low levels of vitamin D result in increased risks of numerous health problems, including rickets and osteomalacia.7 Several studies also suggest that ultraviolet radiation can reduce the risks of some types of cancer, although this has not been shown in all studies.8-14
Another normal physiologic reaction to ultraviolet radiation is the tanning of the skin. Tanning provides some protection against UV exposure, with a sun protection factor (SPF) of 1.5–4.15 (SPF is a measure of UV-B protection, the factor by which the time required for unprotected skin to become sunburned is increased.) Individuals with darker skin may also have greater protection than those with fair skin.15 Exposure limits can be increased for tanned individuals.5 However, tanning will not result from exposures below the TLV®, and growing evidence suggests that a risk of skin cancer still exists even with careful tanning.6
The penetration of ultraviolet radiation into human tissue is very limited. As a consequence, adverse health effects have historically been thought to be limited to acute and chronic skin and eye damage. The response of biologic tissue to UV radiation is highly dependent on the depth of absorption, which in turn is wavelength specific. The wavelengths of concern will be discussed along with each type of tissue damage.16 Some studies have demonstrated that UV radiation can suppress the immune response in the skin and may cause some systemic immunosuppression.17,18 These effects may play a role in the development of skin cancer, and it has been speculated that they could alter host response to infectious diseases.
The typical acute condition of the eye caused by UV exposure is photokeratitis of the cornea, commonly called welder’s flash, arc eye, or flash burn. It is caused by UV radiation below 315 nm, usually from unprotected exposure to a welding arc. Between 2 and 24 hours after exposure, the worker experiences severe pain, redness, photophobia, and spasm of the eyelids if the exposure was severe. The condition usually clears up in 1–5 days, depending on the severity of the exposure. Healing is usually complete, and there is no residual injury.19
Ultraviolet radiation, especially in the UV-C and UV-B bands, produces erythema of the skin (sunburn). If the exposure is more severe, edema and blistering will result. Although UV radiation above 315 nm is less efficient at producing erythema, there is sufficient energy in the UV-A band from tropical sunlight to produce sunburn even if the UV-B is filtered out. The maximum effective wavelength for producing sunburn is 300–307 nm in sunlight and at shorter wavelengths in artificial light. The National Institute for the Occupational Safety and Health (NIOSH) criteria document on ultraviolet radiation includes an extensive discussion of the histological and cytological changes in the skin induced by acute UV exposure.20 These effects include an inflammatory response with edema, lymphocytic infiltrates, capillary leakage, and evidence of local dermal cell damage. The response to UV radiation is photochemical. A short-duration phase occurs in 1–2 hours; a later phase appears after 2–10 hours and may last for several days. The intensity and duration of the burn are proportional to the dose and wavelength of UV radiation; it is also related to the individual’s skin pigmentation. The latent period to erythema becomes shorter with more intense exposures.21
An additional concern with acute exposure is the potential for photosensitivity reactions. Photosensitizing agents may have biological action spectra in the UV-A range, and they can act either systemically or locally. A number of medications (systemic photosensitizers) can predispose an individual to photosensitivity. Drugs such as sulfonamides, sulfonylureas, chlorothiazides, phenothiazines, and tetracyclines are also well-known sensitizers. Occupational exposure to certain chemicals that may remain on the skin (local, or contact, sensitizers) can also act synergistically with UV radiation to produce erythema at much lower dose of UV radiation than would ordinarily be required. Coal tar products are well known for their photosensitizing properties. When combined with UV exposure, severe irritation and blistering can result.22 Individuals with certain underlying diseases or a genetic predisposition may be unusually sensitive to the acute effects of UV radiation. Examples of conditions caused or aggravated by acute exposure to UV radiation include solar urticaria, polymorphous light eruption, the porphyrias, and systemic lupus erythematosus. Numerous other skin conditions can also be aggravated by the UV radiation from sunlight or artificial sources.
Ultraviolet radiation in the 295–400 nm range can cause photochemically induced opacities of the lens of the eye. The most effective wavelengths are 295–325 nm.23 Radiation above 315 nm also causes cataracts in experimental animals. Some authorities have long theorized that ambient exposure to ultraviolet radiation is the primary cause of cataract development in older persons, and this has been demonstrated in some epidemiology studies.24-26 Potential effects on other ocular structures are not as well understood, but UV exposure may contribute to macular and retinal degeneration.27,28 UV-B radiation may also be a risk factor for the development of pterygium.29
Chronic exposure to ultraviolet radiation results in accelerated aging of the skin and an increased risk of skin cancer.2,30 Prolonged exposure causes loss of elasticity, resulting in wrinkles. Actinic keratoses may form in the epidermis and are precursors to squamous cell carcinoma.
Three types of skin cancer have been associated with UV radiation—basal cell carcinoma, squamous cell carcinoma, and melanoma. Squamous cell carcinomas are related to the cumulative dose of ultraviolet radiation to the skin. They occur primarily on sun-exposed areas of the body, and it has been known for many years that persons in outdoor occupations are at a higher risk for these cancers.31,32 Basal cell carcinoma risk is more dependent on exposure patterns, and melanoma risk is even more so with intermittent exposure being the most dangerous.33 Although UV radiation is important in the pathogenesis of melanoma, cumulative exposure to UV does not appear to be the primary factor. Nor has occupational exposure to UV radiation been correlated with the incidence of melanoma. Several studies have found that childhood exposures to sunlight that resulted in severe blistering sunburns were predictive of malignant melanoma risk.34,35 The fact that melanoma is more common on the trunk than on commonly sun-exposed areas such as the face, back of the neck, and hands supports this theory. Melanoma is also rare on the buttocks and on women’s breasts, which are typically covered during sunbathing.
Fair-skinned people have a higher risk of developing skin cancers than darker ones. Individuals of Celtic origin are also at greater risk. Genetic factors that affect DNA repair, such as xeroderma pigmentosum, can greatly increase the risk of skin cancer, and immunosuppression may also be an important factor. Kidney transplant patients have been identified as being at greater risk of developing skin cancer.36
Research on the effects of UV radiation on the immune system has led to a new field of research called photoimmunology. In animal studies it was found that many UV-B-induced cancers were highly antigenic and were rejected when transplanted into normal syngenic animals. The UV radiation not only induced the tumors but also lead to systemic T-lymphocyte-mediated immunosuppression, which reduced the host animal’s ability to reject the tumor cells. This has raised the possibility that UV radiation-induced immunosuppression might also lead to a reduced response to infectious diseases.24,25
Detailed discussion of the treatment of chronic effects of UV radiation is beyond the scope of this book. The most significant acute effects are UV burns of the eye and skin (i.e., sunburn).
Ultraviolet burns of the eye (photokeratitis) should be examined with a slit lamp using fluorescein stain. Diffuse punctate staining of the corneas will be seen within the palpebral fissure. Both eyes should be patched, and cycloplegic agents should be used. Local anesthetics should not be prescribed. Recovery is usually complete in 24 hours.37
Sunburn can range from mild to severe. Aspirin or other nonsteroidal anti-inflammatory agents may be useful for fever and pain. Corticosteroids may be required for severe reactions.
Medical surveillance has generally not been recommended for workers who are exposed to ultraviolet radiation. It would not be useful for acute effects on the skin or eye and has not been demonstrated to be of particular value for chronic effects. Nonmelanotic skin cancer and ocular cataracts both have relatively long latencies (time from exposure to the earliest manifestations of disease) in most people. Most of the diseases attributable to occupational exposure would probably not be seen until after retirement; even if detected sooner, it would still be many years after the initial exposure. At best, medical surveillance might be expected to help detect some basal and squamous cell carcinomas when they are small and therefore more easily treated. It would also be possible to treat precursor lesions, such as actinic keratoses.
Periodic examinations of the skin have been suggested for high-risk groups such as coal tar workers with concurrent outdoor exposure. Skin cancers that are hard to see, such as those on the back of the neck or ears, could be detected earlier through this type of examination.
Early detection of cataracts for workers with ultraviolet exposure has not been specifically investigated. Studies on workers exposed to lasers or microwave radiation have attempted to assess early changes in the lens of the eye. These studies did not identify early changes that would be useful from a medical surveillance perspective.38,39
Prevention of exposure to man-made sources of ultraviolet radiation is accomplished through a combination of engineering controls and personal protective equipment. Typical controls include the use of opaque shields or curtains when welding to eliminate exposure to coworkers. UV radiation used for germicidal purposes can usually be installed in ducts or recessed areas so that exposure to individuals in the same room is eliminated. Individuals performing operations such as welding use welding helmets and protective clothing.40
For outdoor work, simple measures such as long-sleeved shirts, broad-brimmed hats, canopies, and awnings provide significant protection. Sunscreens with an adequate SPF and UV-A protection should be used on exposed parts of the body. The wearing of tinted glasses is also recommended for protection from UV exposure, and is especially important for occupations near water, sand, or snow, where ambient exposures are amplified.27,41 Eyeglasses with lenses made of glass normally provide substantial UV-B protection without special tinting, but tinted plastic lenses do not necessarily stop UV exposure. Plastic lenses usually state what degree of UV protection they provide, and this should be specifically checked. Sunglasses that block wavelengths below 400 nm are widely available. Lenses should meet ANSI Z80.3 blocking requirements. Tinted lenses should also be of a wraparound design or have side shields or large temples to block peripheral exposure of the eye.27,41 Contact lenses that absorb significant amounts of UV radiation may provide additional protection.27 Polarized lenses may be helpful to reduce reflected glare, but do not enhance UV protection.
Outside of work, individuals should also follow safe sun practices and avoid indoor tanning. Behavioral counseling is recommended by the US Preventive Services Task Force (USPSTF).42