J. Cosmet. Sci., 71, 217–225 (July/August 2020) 217 Comparison of Skin Photoprotection by Pigmentation and Sunscreens ANTONY R. YOUNG , St John’s Institute of Dermatology, King’s College London, London, United Kingdom (A.R.Y.) Synopsis Melanin, in people with naturally pigmented skins, offers a high level of photoprotection against the adverse molecular and clinical effects of solar ultraviolet radiation but, in contrast, has a modest inhibitory effect on vitamin D synthesis. Tanning in those with light skin offers relatively modest photoprotection. Sunscreens have the potential to offer high levels of protection in people who lack melanin. In theory, sunscreens can give protection comparable with that of deeply pigmented skin. This depends on the labeled sun protection factor (SPF) which in turn depends on how well the sunscreen is applied. In most cases, this will not achieve the desired SPF. The threshold dose for vitamin D synthesis is much lower than that for sunburn, such that vitamin D synthesis is still possible with sunscreen application. INTRODUCTION The adverse clinical effects of solar ultraviolet radiation (UVR ~ 295–400 nm) on human skin are well documented. Solar range UVR also has effects on the skin (1) and blood (2,3) transcriptomes. The only fully established benefi t is vitamin D syn- thesis by UVB (280–315 nm) that is much more important than vitamin D intake from food sources. There is, however, increasing evidence of other benefi ts from solar exposure, such as a reduction in blood pressure (4). Cutaneous responses to sunlight are highly dependent on the Fitzpatrick skin type (FST) that ranges from I (very light) to VI (very dark) thus, FST I is much more prone to sunburn (erythema) and skin cancer than FST VI. The most probable reason for this is the amount of melanin in the epidermis. In other words, melanin is a very effective natural sunscreen. The presence of melanin is either constitutive or facultative the former being the natural baseline level present in habitually sun-protected skin (i.e., buttock), whereas the latter is tanning in response to solar or other UVR exposure. Facultative pigmentation is also FST dependent. Most photobiological research on human skin in vivo has been carried out on lighter FSTs ranging from I (e.g., Celtic) to IV (e.g., Mediterranean). Relatively little has been carried out on FST V (e.g., Indian subcontinent) and VI (e.g., African) (5). The most widely used Address all correspondence to antony.young@kcl.ac.uk

JOURNAL OF COSMETIC SCIENCE 218 acute endpoint to determine personal UVR sensitivity is erythema (i.e., sunburn), in particular the minimal erythema dose (MED). This is the lowest physical UVR dose (J/cm2) that gives just perceptible skin redness about 24 h after exposure. Melanocytes are dendritic cells that reside in the basal layer of the epidermis. They syn- thesize melanin that is transferred to epidermal keratinocytes that are the majority cell type. This process is enhanced (i.e., tanning) when the skin is exposed to UVR. Melanin is mostly concentrated in the lower epidermis and also presents as caps over nuclei of basal layer keratinocytes. Several studies have attempted to enhance the tanning response with a view to improving photoprotection. Some have used sunscreen formulations that include 5-methoxypsoralen (5-MOP), which is a natural photosensitizing ingredient in bergamot (Citrus bergamia) oil (6). However, this approach is not viable because 5-MOP is a photocarcinogen (7). An important factor in tanning is the binding of α-melanocyte–stimulating hormone (α-MSH) to the melanocortin 1 receptor on melanocytes. Another approach has been to generate synthetic analogues of α-MSH to enhance tanning (8). The re is a melanin gradient with terrestrial UVB in indigenous populations. Melanin decreased as humans in prehistory dispersed away from tropical latitudes, and one driving factor for this loss is thought to be the need to maintain adequate vitamin D synthesis (9,10). Mass migration and travel in relatively recent history has meant people’s skin color no longer matches the solar conditions under which it evolved. The need for photo- protection in normal healthy people depends on the FST, the intensity (irradiance) of the sun, and the time spent outdoors with uncovered skin. The World Health Organization has promoted the ultraviolet index (UVI) as means of estimating the sunburning capacity of solar UVR at any given location. The categories are low (1–2), moderate (3–5), high (6–7), very high (8–10), and extreme (11+). Photoprotection is recommended for light skin when the UVI reaches 3, but this has raised some controversy (11). SUN SCREENS Sun screens are topical formulations that contain synthetic organic UVR fi lters and/or mineral and/or organic nanoparticles (12). They work by absorbing solar UVR and also by some scattering in the case of pigments. Ideally, they should remain on the surface of the skin, but there is increasing evidence that they can penetrate into the systemic circulation because they or their metabolites can be detected in blood and urine (13). Sunscreen effi cacy is measured by their ability to prevent erythema ~24 h after exposure to solar simulated radiation (SSR). The index for this is the sun protection factor (SPF) when the test prod- uct is applied at 2 mg/cm2 skin. Essentially, this is a ratio of the MED with and without sunscreen application (14). SSR does not contain visible light (400–700 nm), and sun- screens are not designed to protect in this region. It has been suggested the laboratory- determined SPF may underestimate protection against erythema because of not taking visible radiation into account (15). COM PARISONS OF PHOTOPROTECTION BY MELANIN AND SUNSCREENS Mel anin, especially constitutive melanin, has the advantage of being always present. Fur- thermore, its presence increases after exposure to sunlight. Sunscreens, on the other hand,