Brian L. Diffey

Determination of sun protection factors (SPFs) using the ‘gold standard’ in vivo method (ISO 24444) is known to be associated with appreciable uncertainty because of experimental variability. Consequently, several in vitro methods have been evaluated, that show promising accuracy, reproducibility and correlation with the in vivo ISO 24444 method. As a way of eliminating the uncertainties associated with both in vivo and in vitro methods, a proposal is made here to dispense with the concept of the SPF and instead to introduce a binary system for labelling the protection afforded by topical sunscreens. Phototesting can be either in vivo or in vitro, with in vivo phototesting requiring only a single area of sunscreen‐protected skin to be irradiated. Alternatively, a validated in vitro method could be employed to determine the threshold protection required for assigning products to the appropriate binary category.

In response to the recent tsunami of misinformation around sunscreens and commensurate non‐scientific questioning of sunscreen safety and utility, often amplified through social media, this manifesto has been written to reflect accurately and concisely the status of our understanding based on current scientific knowledge. We argue that sunscreen products are not a threat but essential tools in furthering human health globally.

Background: Ultraviolet radiation (UVR) is used as treatment for psoriasis and other skin diseases, but UVR can induce DNA mutations that may lead to skin cancer development. While skin cancers have been documented in patients treated with phototherapy, a limited number of epidemiological studies have examined the incidence of skin cancer in people receiving narrowband ultraviolet B (NB-UVB) treatment. Information on the mutagenicity of NB-UVB would help inform about the potential skin cancer risks of this treatment. Objectives: To determine the mutation burden in human skin resulting from a NB-UVB treatment course and to use this data to estimate the total number of NB-UVB exposures whereupon skin cancer surveillance should begin. Methods: Biopsies of normal skin were obtained before and after a course of NB-UVB from 16 patients with psoriasis. Epidermal DNA was sequenced using nanorate sequencing (NanoSeq) to determine the mutational signatures and mutation burden from NB-UVB. Results: The NB-UVB treatment course increased the number of mutations in skin. Median increase in mutation burden was 0.55 substitutions per Mb in infrequently sun-exposed (buttock; n = 14) skin and 0.89 substitutions per Mb in frequently sun-exposed (forearm; n = 10) skin (P < 0.001). Change in mutation burden due to NB-UVB ranged from 1.16- to 10.50-fold in buttock skin and from 0.93- to 2.33-fold in forearm skin. This increase was mainly attributable to UVR exposure-linked mutational signatures, SBS7a and SBS7b, with some evidence that mutational burden was related to the genetic background of the individual. Modelling the change in mutation burden from NB-UVB relative to minimal erythema dose (MED) in comparison with average mutation burden in keratinocyte skin cancers allowed estimation of total lifetime exposures at which patients are likely to require skin cancer surveillance; for patients with a MED equal to 2 standard erythemal doses (SEDs), skin cancer surveillance should be offered at 422, 165 and 58 NB-UVB exposures for those receiving low, typical and high levels of sun exposure, respectively. Conclusions: A treatment course of NB-UVB causes UVR-induced mutations in the healthy skin of patients with psoriasis. Relating mutation burden to MED and sun behaviour habits allows estimation of when to begin skin cancer surveillance according to total lifetime NB-UVB exposures.

Objective: To propose a strategy for using sunscreens that optimizes the balance between skin health and adverse effects to humans and the environment.Methods: A model was developed to explore the relationship between sunscreen usage and personal sun exposure throughout the year in populations resident at different latitudes.Results: There is little biological justification in terms of skin health for applying sunscreen over the 4–6 winter months at latitudes of 45° N and higher, whereas year‐round sunscreen is advised at latitudes of 30° N and lower. Avoiding sunscreen application at times when it is biologically unnecessary results in an annual reduction in sunscreen use of 25%. Furthermore, using products containing UV filters over the winter months at more northerly latitudes could lead to a higher number of people with vitamin D deficiency. The single largest use of sunscreen is on a sun‐seeking holiday and encouragement to make more use of clothing and shade would appreciably reduce the sunscreen burden to the environment.Conclusion: We need to use sunscreens wisely so that we gain the health benefit from their use while at the same time limiting possible harm to ourselves and the environment.

Beach holidays in areas of strong sunlight are popular and sunscreen is often the primary means of photoprotection favored by many people. The object of this study was to estimate how effective sunscreen is in preventing sunburn under high ultraviolet (UV) levels. We used a computational model to determine how the quantity, frequency, substantivity, and labeled SPF of applied sunscreens impact on the predicted erythemal response in unacclimatized skin over the course of a 7‐day holiday in a high‐solar environment. Our results indicated that sunscreen on its own may be insufficient to prevent sunburn in white skin on a sun‐seeking holiday that combines prolonged exposure with high UV levels. Nevertheless, sunscreens have a valuable role to play on a beach holiday especially if an SPF30 or higher is chosen, the product is applied liberally and uniformly, application is started early into the sun exposure period and repeated at 2‐hourly intervals throughout the day, and the product binds well to the skin. The main limitation of our investigation is that it is not an observational study but rather a computational model and while all models are wrong, some, including this one, are useful.

Thirty years ago, it wasn’t uncommon to read in the dermatological literature statements like: ‘UVB [ultraviolet B irradiation] intensity declines from noontime apex, but UVA intensity remains relatively constant throughout the day’¹ and ‘It is useful to remember that the level of UVA reaching the Earth’s surface is very similar in both summer and winter’.²

Background Evidence on validation of surrogates applied to evaluate the personal exposure levels of solar ultraviolet radiation (UVR) in epidemiological studies is scarce. Objectives To determine and compare the validity of three approaches, including (i) ambient UVR levels, (ii) time spent outdoors, and (iii) a modelling approach integrating the aforementioned parameters, to estimate personal UVR exposure over a period of six months among indoor and outdoor workers and in different seasons (summer/winter). Methods This validation study was part of the EU ICEPURE project and was performed between July 2010 and January 2011 in a convenience sample of indoor and outdoor workers in Catalunya - Spain. We developed linear regression models to quantify the variation in the objectively measured personal UVR exposure that could be explained, separately, by the ambient UVR, time spent outdoors, and modelled UVR levels. Results Our 39 participants – mostly male and with a median age of 35 years- presented a median daily objectively measured UVR of 0.37 standard erythemal doses (SEDs). The UVR dose was statistically significantly higher in summer and for outdoor workers. The modelled personal UVR exposure and self-reported time spent outdoors could reasonably predict the variation in the objectively measured personal UVR levels (R²= (0.75, 0.79)), whereas ambient UVR was a poor predictor (R²=0.21). No notable differences were found between seasons or occupation. Conclusions Time outdoors and our modelling approach were reliable predictors and of value to be applied in epidemiological studies of the health effects of current exposure to UVR.

The development of a computational model is described that allows time‐varying erythema and acclimatization to be determined following repeated exposure to solar ultraviolet (UV) radiation. Good agreement was observed between experimental data and model predictions. Results are presented that illustrate how daily, adventitious sun exposure over the course of a year can result in occasional erythema generally in the late spring/early summer, while at the same time showing how the photoadaptive response of the skin changes throughout the year at different latitudes and for different sun‐reactive skin types. The model is predicated on three physiological responses; the erythemal response as a function of increasing UV dose, the time courses of erythema and adaptation in response to a single UV exposure. Mathematical models complement observational data and the outputs from the model may help in planning future experimental studies, as well as providing insights into mechanistic responses.