86 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS sense ultraviolet sunburn which is, as heat burn, a damage to living tissue. Both heat and ultraviolet radiation cause reddening of the skin by vasodilation, called heat erythema and ultraviolet erythema, respec- tively. There is a great difference between these erythemas. Heat erythema is an almost immediate reaction to heating skin to 38-42 øC. It is a normal physiologic reaction to counteract overheating of the skin. When the heating is stopped, the heat erythema does not immediately disappear but lasts for about 1-2 hours. Ultraviolet erythema appears several hours after damage is done to the skin the period of latency depends on the ultraviolet dose. This delay is the time necessary for substances formed in the epidermis by the ultraviolet light to diffuse to the dermis where the blood vessels are located (1). After a minimum erythema dose (MED), the latency is 8-10 hours. With 8 times the MED the latency can decrease to 1-2 hours. After very strong irradia- tion accompanied by strong heating of the skin, a heat erythema may change into an ultraviolet erythema (UV-E) without perceptible latency of the latter. UV-E's last longer, again depending on the dose. After a dose of more than 3 MED's a UV-E persists for several days, while 24 hours after such a dose even an edema may be present for some hours. Still higher ultraviolet doses may cause severe edema for days while the UV-E may last for months, although it may become imperceptible when increased pigmentation tans the skin. In contrast, only in infrequent pathological cases can a heat erythema last for 24 hours or more. Such a heat erythema can be distinguished from an average UV-E because the former is patchy while the latter is homogeneously red. The MED is the smallest dose which still damages the skin. It is the quantity which expresses the sensitivity of a skin to irradiation. An MED exists for each wavelength of the spectrum. Figure 1 shows the action spectra of the skin, viz, the MED's for all wavelengths from 250 nm to about 550 nm (2). The skin is most sensitive to the wavelengths 250 and 297 nm in the short wave ultraviolet range but much less sensi- tive to longer wavelengths. However, "long-wave" UV-E's accom- panied several days later by increased pigmentation can be elicited. Wavelengths longer than 300 nm penetrate deeper into the skin than shortwave UV. They provoke an early vascular re. action, presumably by direct action on the blood vessels in the dermis (3). Sunlight on the earth's surface contains no radiation at 250 nm be- cause of absorption in the ozone layer in the higher atmosphere. In sun-

BIOLOGIC PROBLEMS CONCERNING SUNSCREENS 87 tog MED in Wsec/cm 2 -4 -3 -2 -1 0 +1 - 3 300 400 500 wavelength in nm Figure 1. Action spectra for MED's on normal human skin. The lines are tentative inter- polations between the points measured. 1, early vascular reactions 2, for unpigmented skins ,3, for pigmented skins and daylight, therefore, the skin sensitivity maximum at 297 nm is the primary cause of sunburn. This wavelength is present in sunlight only at very low intensities which depend on many factors such as the sun's altitude, the purity of the air, etc. The intensity increases sharply with longer wavelengths, which explains why sunlight sunburn can be caused by a somewhat longer wavelength than 297 nm, eg, by 306 rim. Near this wavelength climatological sunlight measurements have greater importance than near 297 rim. Mercury arc lamps are generally used in the laboratory to measure MED's. The lines at 297 and 302 nm in the emission spectra are the principal causes of UV-E. In cases of very sensitive skins mercury arc erythemas may partially be caused by wavelengths of 248-265 rim. An MED-erythema should be examined about eight hours after the irradiation. It disappears 12-15 hours after the irradiation. Great diversity exists between the values of MED's measured in different laboratories. However, any weak UV-E indicates relative differences in skin reactivity to ultraviolet. The MED 300 is extremely variable not only in different persons but also in different parts of the body. The MED 300 of one person