The present study aimed to develop and pilot-test feasibility of new laboratory test procedures to evaluate secondary performance attributes of sunscreens suitable for professional outdoor work, which so far have not been taken into consideration when testing sunscreen efficacy. In addition, it should be determined to what extent ten selected sunscreen products of producers with a relevant market share comply with these requirements and producers’ claims.
The usage of sunscreen is an important part of an overall strategy in the prevention of occupationally induced skin cancer, although technical and organizational sun-protective measures, as well as clothing and headgear, should be given priority [17,18,19]. However, studies have shown that, in practice, technical and organizational measures are often highly influenced by the general conditions in the workplace and, therefore are rarely implemented in most cases [23, 26, 40]. The use of personal protective equipment (e.g., long-sleeved clothes, headgear) is also often inadequate [23, 24]. Sunscreens should – as a secondary measure – therefore just add to other protective measures and should especially be applied to areas of the body that cannot be protected with clothing (e.g., forehead, ears, neck, and hands). It needs to be emphasized, however, that AK, cSCC and BCC in outdoor workers predominantly occur in exactly these body areas [10,11,12, 41]. Thus, these body areas are of pivotal importance for adequate photo-protection and skin cancer prevention at workplaces. Furthermore, this highlights the need of sunscreens that meet the specific requirements for outdoor work.
To ensure effective protection against UVR, current guidelines recommend a sunscreen with a high SPF and sufficient UVA photo-protection [17,18,19]. In our study, we confirmed the claimed SPF of the products, in some cases with SPF clearly higher than 50 (see Fig. 1). Nevertheless, it has to be mentioned that these results are not entirely transferable to the application in practice since we used a sunscreen thickness layer of 2 mg/cm2. Under real-life conditions, however, the applied amount is often significantly lower, which leads to a significant decrease in protective performance [27, 42,43,44]. On the one hand, this underlines the necessity of recommending a high SPF; on the other hand, it emphasizes the need to train OW in correct usage, ideally with targeted teaching strategies [18, 43]. Furthermore, we demonstrated the bio-stability of the SPF after physical activity – although to a varying degree. Overall, the bio-stability of the very water-resistant products (P1 - P5) was on average more stable than that of the water-resistant products (P6 - P10), even though for one product, the label “very water-resistant” could not be confirmed. At this point, however, it should be taken into account that the evaluation of water-resistance was carried out with 6 participants and not with 10, as required by the Colipa Guideline [35]. This might limit the validity of our results. These results are similar to the findings of the investigation by Bodekaer et al. [45], who examined the effect of physical activity, heat and bathing – in other words, a simulated day at the beach – on the stability of the SPF in an organic and an inorganic sunscreen. For the practical application, these results indicate that the re-application of sunscreen products every 2 hours in sufficient quantities – with the aim of maintaining the SPF – is of particular importance. According to a recent randomized-controlled trial by Matta et al. [46], which received considerable media attention, the repeated application of sunscreens on 75% of the body area with 2 mg/cm2 led to systemic resorption and increased overall plasma concentration of active ingredients (e.g., between 3.3 ng/mL and 7.1 ng/mL for avobenzone). Although the transferability of these findings to the application of sunscreen in practice is a matter for discussion [47,48,49], the authors encourage use due to the evident protective effects of sunscreens [46]. Nevertheless, this once again underlines the importance of first-line sun-protective measures (e.g., brimmed hats, wearing long-sleeved clothes) to reduce blood transmission by vastly limiting application areas.
The aim of this pilot study was to develop new methods for assessing secondary PA that best mimic working conditions of outdoor workplaces (‘workplace simulation’). Therefore, both a wooden and a metal bar were used to evaluate subjective skin feeling and grip (see PA 4). These bars served to simulate specific parts of working materials or tools – e.g., a wooden hammer shaft vs. a metal scaffold tube. This PA is – among others – essential in the construction industry since secure grip is crucial from an occupational safety point of view. Closely related to this PA – and therefore tested together – is the absorption time of a sunscreen product (PA 3), which can influence safety aspects, but is also a key factor for user acceptance (e.g., through galenic properties). While the aim of our pilot study was to best represent real-life working conditions, it is questionable to what extent the laboratory tests are equal to an assessment by OW in a workplace context. Bauer et al. [27], for example, carried out a randomized-controlled, cross-over trial in the workplace and evaluated the overall acceptance of the daily use of two sunscreens. The examination of our laboratory results by OW in practice, therefore, seem feasible and might be an important step for validation and quality assurance.
Regarding limitations of our piloting study, the lack of minimum requirements for the PA test procedures should be mentioned. Therefore, a final overall appraisal of the tested products – except for an ordinal ranking, which is a straightforward approach but would be of limited informative value - is only possible to a limited extent. The definition of minimum standards, − i.e., which scores should be achieved in the given tests – is, however, a complex process of consensus building that must be carried out involving different normative institutions. Future studies – once this ongoing process is completed – can consider the minimum requirements and in this fashion provide a ranking of the most suitable sunscreens for professional outdoor work. As this has not yet been included in our pilot study, a weighting of the PA should in addition be considered to prevent selected PA having a too strong influence on an overall estimation. In future studies, the weighting of our suggested PA could also be industry-specific, since their relevance may be varying according to the work environment. Furthermore, prospectively, an interactive ‘online tool’ that allows to weight PA in order to match them to a specific industry could be useful (e.g., very high water resistance for bath attendants, dust and dirt absorption as well as non-slip grip for the construction industry or agriculture). This would facilitate the selection of sunscreens, for both, employers, and employees, and enhance acceptance by the social partners. In addition, it could be useful to establish a certified label (e.g., ‘suitable for the professional sector’). Social accident insurances, ministries of labour or bodies responsible for safety and health at work, could establish such an industry standard.
Another limitation of the present study is the experimental setting, which may only be transferable to the daily practice of the broad spectrum of practical outdoor workplace settings to a limited extent. In particular, the use of 2 mg/cm2 sunscreen does not match the applied amount of sunscreen in practice [42, 43]. The measured SPF, as well as the bio-stability, is therefore probably lower under practical conditions. On the other hand, the amount of sunscreen used leads to rather conservative estimates, especially regarding the secondary PAs (e.g., absorption time and subjective skin feeling). Under real-life working conditions, paradoxically, it might thus be expected that acceptance will be somewhat higher than in the laboratory tests.
Although the experimental setting of our pilot study implies a high degree of standardization, studies that involve human subjects are always prone to a certain variability. This becomes apparent, for example, in the experimental conditions of PA 5 (‘compatibility with textiles’). Since the subjects were allowed to leave the research institute during the investigation, it cannot be completely ensured that they always followed the instructions of the study personnel. One further aspect that limits the transferability of our results is that the assessments of the PA are based on external observers or the statements of the study participants rather than on objective (instrumental) measurements. Even though the assessments were carried out by at least two trained external graders, an objectification through instrumental measurement procedures would be of advantage and could contribute to the reproducibility of our results. Especially for PA 4 (evaluation of grip) or PA 7 (whitening effect), instrumental measurements (e.g., usage of Chromameter assessments or color measurement of image analysis for whitening effect; friction measurements for the evaluation of grip) might be possible. The development and validation of additional objective assessment methods is currently under further consideration; however, subjective assessments cannot be dispensed for PA 2 (eye irritation) and PA 4 (subjective skin feeling).
A strength of the present pilot study is that the development of the PA – except for PA 7 (whitening effect) – was derived from the literature. The assessment of the whitening effect was the only one that was subsequently added during the investigations. In our pilot study, the whitening effect was assessed by two trained observers based on a photo of a subject’s back. Since the extent to which white residue is detected on the skin may be influenced by the skin color of the subject to whose back the products were applied, the results must be interpreted with caution. Nevertheless, it should be noted that the results of the two graders in our study are highly consistent. In future studies, the extent of the whitening effect should be determined with more test subjects in order to be able to exclude confounding influences, e.g., skin color. In addition, a reference product could also be used to facilitate the assessment. Moreover, by using a Chromameter device or color image analysis, an objective measurement method could be of advantage.
The selection of participants for this pilot study focused on eligible subjects who were also able to perform sensory grading with a satisfying quality. In our pilot study, we have a sex imbalance, as most of the participants were women. Future studies should therefore include a larger proportion of men in the test panel to increase representability in terms of the intended target population, although – regarding the very heterogeneous group of OW – representability strongly depends on the respective (industrial) sector.
Overall, the test methods are feasible and seem to be suitable for testing secondary PAs, although the degree of discriminability of the single test methods varied. For example, there were barely any differences in the raw data for PA 2, PA 5, and PA 6. Yet while only carefully selected products according to their producers’ claims were tested, it is all the more remarkable that half of the products did not meet their producers’ claims.
The strength of this study lies in its unique approach and design as until to date no standard test method has been developed to assess other PA than the three mandatory efficacy statements. To our knowledge, this study has identified for the first time a specific test-battery for objectifying the suitability of appropriate sunscreen formulations and thus provided a significant impetus for the current scientific and political focus on improvement of occupational health in highly UVR-exposed OW [50,51,52]. Our study aims to open up a discussion on new testing methods focusing on outdoor workers’ expectations and the intended scope of application that complement the so far mandatory labels and assist in making informed, evidence-based decisions when choosing an appropriate sunscreen product. Furthermore, for manufacturers there now rises a significant opportunity to fulfill this demand by manufacturing quality, efficacious and safe sunscreen formulations specifically targeted for outdoor workers.
