82 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS An attempt is made below to present a step by step analysis and clinical classification of tha photosensitivities in accordance with the following scheme: Reactions to Sunlight I. Normal reaction (sunburn) II. Hypersensitivity reactions A. Nonspecific hypersensitivity (e.g., lupus erythematosus) B. True photosensitivity 1. Phototoxicity, quantitative hypersensitivity 2. Photoallergy, qualitative hypersensitivity (altered reactivity) NORMAL REACTION TO SUNLIGHT The normal photobiologic response of the skin, when exposed to ultraviolet light of a specific wavelength, is erythema ("sunburn"). This must be stressed, since sunburn is sometimes thought to be analo- gous to a thermal (infrared) burn, but they are not the same. The terms "sunburn spectrum" and "sunburn radiation" are used to describe those ultraviolet light wavelengths which produce erythema in normal human skin. The appearance of minimal perceptible erythema is the most ac- curate method for determining the erythemic response and is graphically represented by a curve (Fig. 1), known as the "action spectrum curve for erythema." There is a maximum at about 2500 A, falling to a mini- mum at 2800 A, reaching a very sharp maximum at 2967 A, then falling sharply and nearly reaching zero at 3200 A. Ordinary window glass prevents transmission of wave lengths below 3200 A and, therefore, will prevent sunburn. Frequently, the wavelengths transmitted to the earth are not shorter than 3200 A, and in this case sunlight has no sun- burn producing power. An approximate average curve for the spectral distribution of sun- light is shown in Fig. 2 ("S"). The wavelength distribution is between 2900 and 18,500 A. The intensity and spectral distribution of sunlight varies with the season of the year and with varying atmospheric condi- tions, including water vapor, smoke, and clouds. The sun's rays are most intense in late spring and early summer when the sun is at its zenith (summer solstice). At this time the sun is directly overhead and its rays are perpendicular to the earth's surface, thus traveling their shortest distance to earth, with the least filtering effect of the atmos- phere. It is during this season, naturally, that most skin reactions are observed.

i•HOTOSE NSITiV ITY 83 As expected, the normal reaction of the skin to sunlight in the sunburn spectrum varies with the intensity and length of time of exposure. Finally, some individual factors are involved: Thus, dark skinned races react less to sunlight than light skinned people. Blondes are more susceptible to the action of the sun's rays than brunettes. There is also a variation in individual tolerance to sunlight, due to unknown factors. •oo •90 8O •7o Z 6O ¸ -- (..95O r'F40 -- •3o 24 O0 ACTION SPECTRU! ERYTHEMA PRODUCTION I I I I I I 26 O0 2800 .300 0 WAVE- LENGTH 320 0 Figure 1. Action spectrum for erythema [after Blum (2)] With the above factors in mind, one may then observe certain bio- logic effects. (a) Production of erythema: The degree of erythema and the latent period before its appearance vary according to the intensity and duration of the irradiation. Excessive irradiation may result in blistering of the skin. At 2536 A, maximal reddening ordinarily occurs in about six hours and may last for a few days. At 2967 A, maximal reddening usually occurs in twenty-four hours and persists longer. (b) Pigmentation: Pigmentation is usually observed much later than erythema. The degree and rate of pigmentation vary with the amount of radiation. The primary changes that lead to pigmentation are the same as those which cause erythema, and the wavelengths that result in pigmentation correspond to those that cause erythema how-