J. $oc. Cosmet. Chem. 29 31-44 (1978) Ohanges in sunburn and mechanisms of protection B. E. JOHNSON Department of Dermatology, University of Dundee, Scotland Synopsis A review and critical discussion is presented of the reactions of skin to sunlight and artificial sources of ultraviolet radiation. Particular attention is paid to the mechanisms involved in sunburn but mention is also made of premature ageing and skin cancer. Ultraviolet erythema is the most extensively studied reaction and it is possible that the primary molecular target for this and the other reactions is either DNA or lysosome membrane lipid. However, no definite conclusions may be drawn. Nor is it established at which level in the skin, the epidermis, where the most prominent histologic change, the appearance of sunburn cells, is found, or the dermis in which vasodilatation occurs, this primary target may be. Physio- logical protection against ultraviolet radiation is afforded by melanin pigment, the proteins of the horny layer and urocanic acid, but the mechanisms are poorly understood as is the possible involvement of naturally occurring anti-oxidants. INTRODUCTION The skin is essentially a functional interface between a living organism and its environ- ment, reacting to the various stimuli in that environment. Terrestrial solar radiation or 'sunlight' is a major component of the human environment and this paper is primarily concerned with the reactions of normal, lightly pigmented human skin to a single exposure to this radiation. These reactions, commonly known as sunburn, are a complex of inflammatory processes, the mechanisms involved being still largely unresolved. Some mention will be made of various abnormal skin reactions to sunlight, collectively termed the photodermatoses, and, for completion, the long-term effects of repeated exposures to high intensity sunlight will be briefly discussed. For more detailed discussions of the whole subject matter, the reader is advised to consult the reviews of Blum (1), Kimmig and Wiskemann (2), Johnson, Daniels and Magnus (3) and Magnus (4). Encyclopaedic accounts can be found in the proceedings edited by Urbach (5) and Fitzpatrick (6). SUNBURN The gross skin changes seen in sunburn are familiar to most lightly pigmented individuals and even Negroid types may experience discomfort as a result of injudicious exposure to early summer sunlight in the northern United States of America. In its mildest form, the reaction consists of a transient reddening, or erythema, of the exposed skin which may take some hours to develop and may fade rapidly. With a very slight increase in exposure time, a more intense erythema is produced, becoming visible during the exposure (7) reaching a peak of intensity 24 h or so later and persisting for up to 20 d (8). In individuals with a tendency to tan, hyperpigmentation begins to develop by 72 h after the exposure. With longer exposures, erythema may be accompanied by oedema, itching and pain. 0037-9832/78/0100-0031 $02.00 ¸ 1978 Society of Cosmetic Chemists of Great Britain 31

32 B. E. Johnson Blistering may occur in which case, any developing suntan will be lost in the blistered area. Even with a mild sunburn, the outer layers of the stratum corneum tend to be sloughed in sheets rather than as the normal minute scales. When larger areas of the body are exposed, the severe local reactions may be accompanied by nausea, headaches and muscle weakness and medical treatment may be necessary. In already tanned skin, in Asian caucasoids and in the less heavily pigmented negroids, the intensity of the inflammation produced by a given exposure dose is less than that in fair-skinned peoples and even in the latter, slight variations in degree of pigmentation produce differences in the intensity of reaction to sunlight (9). In pigmented skin, an immediate hyperpig- menration can be seen in exposed areas, a phenomenon known as immediate pigment darkening (IPD) which appears to result from an oxidation reaction in melanin already present in the outer layers of the skin but in a bleached form. WAVELENGTH DEPENDENCE Neither infrared radiation (beyond 700 nm) nor visible (390-700 nm) produces the characteristic sunburn reaction. Window glass, acting as a cut-off filter at 315-320 nm, prevents the reaction and this sets the upper limit for effective radiation at around 315 nm. The lower limit is set by the absorption of short wavelength ultraviolet radiation (uvr) by stratospheric ozone, the lowest value of wavelength recorded at the earth's surface being around 285 nm. It would seem that natural sunburn is produced by uvr between 280 and 315 nm, a minute fraction of the total solar spectrum. Because this wavelength region is so important for photobiology, the more conventional partition of the uv spectrum into far-uv (180-290 nm), and near-uv (290-390 nm) has been modified (10) into uv-A (315-390 nm), uv-B (280-315 nm) and uv-C (180-280 nm) a classification which has found considerable usage in dermatological literature. Artificial sources are essential for controlled studies of sunburn and other biologic effects of uvr. Low pressure mercury arcs emit principally the 254-nm line and this uv-C radiation does produce a sunburn-like reaction in human skin. There may be an additive or augmenting action of uv-A on the natural sunburn reaction (11, 12) and uv-B, uv-A and visible radiation are separately or collectively involved in the abnormal reactions seen in the various photodermatoses. There are significant differences between the erythema produced by exposure to uv-C, uv-B and natural sunlight (7, 8, 13). The uv-C erythema is less intense and has a shorter duration than that produced by uv-B. While intense irritation and epidermal damage may be caused, the severe lesions, involving damage to the dermis, which may develop with uv-B irradiation cannot be elicited with uv-C. On the other hand, the uv-B erythema has a shorter duration than that produced by sunlight and the latent period for its develop- ment is relatively prolonged. Thus the natural sunburn response is not necessarily pro- duced by exposure to a narrow band of uvr wavelengths and the reports of Hausser (14) that uv-A erythema, although requiring a very high exposure dose, may be observed very soon after the exposure and persists, and of Willis, Kligman and Epstein (11) and Parrish et al. (12) that the dose requirement for uv-B erythema may be reduced by exposure to uv-A, appear to have considerable relevance to the problem of the natural sunburn. Detailed studies of the wavelength dependency for uvr erythema began with the work of Hausser and Vahle (15). An action spectrum for the just perceptible erythema in upper back skin of lightly pigmented individuals is shown in Fig. I. This was plotted from data obtained by Dr John Anderson and myself at Dundee, using a small grating irradiation