Ultraviolet Application
Applications of Ultraviolet
The intensity of UV radiation reaching the skin is highest with a high-power lamp positioned close to the patient with the radiation beam perpendicular to the skin's surface. Penetration is deepest for UV radiation with the highest intensity, longest wavelength, and lowest frequency.
UVA penetrates farthest and reaches through several millimeters of skin, whereas UVB and UVC penetrate less deeply and are almost entirely absorbed in the superficial epidermal layers. The penetration of UV radiation is also less deep if the skin is thicker or darker.
Erythema, or redness of the skin as a result of dilation of the superficial blood vessels caused by the release of histamines, is one of the most common and obvious effects of exposure to UV radiation.
Erythema is produced primarily in response to UVB exposure, or in response to UVA exposure after drug sensitization. Without drug sensitization, UVA is 100 to 1000 times less potent in inducing erythema than UVB. With sensitization, the erythemal efficacy of UVA is similar to that of UVB alone, with less risk of overexposure or burning.
The precise mechanism of UV-induced erythema is unknown; however, it is known that this effect is mediated by prostaglandin release from the epidermis and that it may be related to the DNA-damaging effects of UV radiation. The severity of erythema, which can produce blistering, tissue burning, and pain, and the risk of cell damage are the primary factors limiting the intensity and duration of UV exposure that can be used clinically.
Because patients vary in their degree of erythemal response to UV, a minimal erythemal dose (MED) is determined for each patient before initiating treatment with UV radiation. Progressively higher doses of UV radiation are generally needed during a course of UV treatment due to skin changes from tanning and hyperplasia. In the laboratory setting UVC in adequate doses can be bactericidal. UVC radiation is used to kill bacteria in food and, in one small study, UVC was found to be as effective as standard hospital cleaners in removing pathogens from hospital surfaces. One clinical study also found that UVC radiation may help reduce bacterial load in open wounds and improve wound healing.
When applying UV radiation for therapeutic purposes, one must first determine the individual patient's sensitivity to UV radiation. This varies widely among individuals and can be affected by skin pigmentation, age, prior exposure to UV radiation, and use of sensitizing medications. For example, even for Caucasians, there can be a fourfold to sixfold variation in minimal erythemal dose. Sensitivity to UV radiation is assessed using dosimetry procedures described in the next section. Because the response to UV radiation can vary significantly with even slightly different frequencies of radiation, the same lamp must be used for assessing an individual's sensitivity and for all subsequent treatments. For example, the skin is 100 times more sensitive to UV with a wavelength of 300 nm than to UV with a wavelength of 320 nm. If the lamp must be changed, the individual's response to the new lamp must be assessed before it is used for treatment. Reassessment is also necessary if there is a long gap between treatments because lamp output intensity decreases with prolonged use and skin tanning and hyperplasia decreases over prolonged periods. Once the individual's responsiveness to a particular UV lamp has been determined, the treatment dose can be selected to produce the desired erythemal response.