88 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS can be ten times or more that of another the face may be eight times less sensitive than the back, the forearm four times less sensitive than the back. In addition, these values may be different in different seasons. In the summer, for example, sensitivities may be much lower than in the winter, because of habituation to sunlight. HABITUATION OF THE SKIN TO SUNBURN Habituation follows medium strong UV-E's, e.g., after more than 2-3 MED 300. Such a UV-E is an inflammatory reaction of the skin to re- pair the damage. The mitotic rate of epidermal cells increases, so that the epidermis and its horny layer become thicker. Melanocytes in the basal layer of the epidermis produce more melanin granules which are deposited in the basal cells. These melanin granules break up into finer particles which migrate with the epidermal cells to the surface of the skin. The most superficial cells of the horny layer finally fall off with the finely divided melanin. The horny layer absorbs short-wave ultraviolet strongly but melanin also absorbs long-wave ultraviolet and visible light. In the course of 4-7 days the increased thickness of the horny layer and in- creased melanin content of the epidermis makes the skin less sensitive to irradiation. Habituation after medium strong UV-E's is, therefore, the natural reaction of the skin against repetition of irradiation damage by sunlight. Excessive sunburn (more than six times the MED 300 in one dose) leads to peeling of the horny layer after one or two weeks and conse- quent loss of protective power. For that reason strong sunburning in a single dose does not add to habituation. The process of habituation is not equally well developed in all persons. There are people in whom it does not work well and others in whom it does not work at all. A quite arbitrary classification of people in their reaction to sunlight is the following: 1st group: 2nd group: 3rd group: the insensitives, with good habituation and pigmenta- tion the sensitives, with bad habituation and no pigmenta-. tion the diseased, with a pathological skin reaction to sun- light.

BIOLOGIC PROBLEMS CONCERNING SUNSCREENS 8• In Holland those belonging to the first group will acquire 1-3 MED 30(J, in 1-3 hours, at first exposure of the trunk in the summer sun. This does not harm their skin. Such doses repeated on the following days will gradually provoke a red-brown tan, and after a week subjects easily tolerate 8-10 hours sunlight a day, even at the Mediterranean. They may reach this result by habituation. An initial exposure of eight hours, for example, may even do harm to their skins. Persons of the second group acquire 4-10 MED 306, at the first ex- posure of one hour. This may cause a disagreeable sunburn in the evening. When peeling does not interfere, repeated exposures to such doses may, by habituation, gradually decrease the sunburn. But these persons can never expose themselves as long to the sun as those in the first group. Moreover, they remain as red as lobsters without much tanning. A range of sensitivities exists between these two groups. These people may all use sunscreens but with quite different aims. Insensitive subjects do not require a high screening factor. To tan they need a medium-strong UV-E. The sunscreen they use will be some oil that does not screen out the short-wave ultraviolet (unless they have to endure very long exposures before habituation has started). Their oil protects the skin against drying and keeps it smooth. Sensitive sub- jects, however, seek real protection against ultraviolet in order to endure longer exposures without disagreeable sunburn. THE SCREENING FACTOR OF A SUNSCREEN The screening factor (SF) of a sunscreen is the number indicating how much longer exposure to sunlight is needed to acquire a MED. It is 1 when no protection exists to 306 nm, 2-4 for moderate protection, and more than 4 for good protection. The basis for measurement of an SF in the laboratory is the MED which requires the shortest exposure time. The SF is a physical quantity it is related to the percentage of the light passing into the skin through the layer of the sunscreen. Initially, SF's were calculated from the transmission (at 300 nm, for example) of a suitable dilution of a sunscreen, as measured in a spectrophotometer. In that case the thick- ness of the sunscreen layer on the skin was assumed to be, for example, 0.01 mm. In this way SF's of many sunscreens were determined. Their value is limited since SF's measured on the skin are usually much lower (4). The principal reason for this difference between transmission mea- surements and skin tests is the thinner layer of sunscreen on the skin as