A, Proportion of EIR reaching the skin with no protection (SPF = 1) and transmitted through the sunscreen to the skin for SPFs 15, 30, and 60. The SPF is defined as the ratio of the EIR dose that induces the first perceptible erythema on sunscreen-protected skin to the EIR dose that induces the same erythema on unprotected skin. % EIR transmitted is directly related to SPF (% EIR transmitted = 1/SPF): As SPF doubles, the number of photons (ie, % EIR transmitted) is halved. This process is further illustrated in a video at http://www.youtube.com/watch?v=8cc8qRr7oMQ. B and C, Effect of information format on perceived increase in sunscreen effectiveness. Symbols and error bars indicate Bayesian posterior medians and 95% credible intervals; the gray dots (horizontally jittered) represent individual experts’ judgments averaged across sunscreen pairs.
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Herzog SM, Lim HW, Williams MS, de Maddalena ID, Osterwalder U, Surber C. Sun Protection Factor Communication of Sunscreen Effectiveness: A Web-Based Study of Perception of Effectiveness by Dermatologists. JAMA Dermatol. 2017;153(3):348–350. doi:10.1001/jamadermatol.2016.4924
The sun protection factor (SPF) is commonly used to convey a sunscreen’s effectiveness in protecting against UV radiation that causes sunburn (ie, erythema-inducing radiation [EIR]).1 Importantly, the EIR burden depends on the proportion of EIR actually transmitted through the sunscreen to the skin (% EIR transmitted) and not on the proportion of EIR absorbed by the sunscreen (% EIR absorbed). Doubling SPF from, say, 30 to 60 halves % EIR transmitted from 3.3% to 1.7%, thus doubling protection2 (Figure, A). Unfortunately, however, media and health professionals often incorrectly state that SPFs beyond 30 offer only minor improvements in sun protection, arguing that the increase in % EIR absorbed by the sunscreen is less pronounced than the corresponding increase in SPF values3,4 (eg, 96.7% < 98.3% vs 30 ≪ 60). However, only changes in % EIR transmitted directly relate to changes in SPF; changes in % EIR absorbed do not. In this study, we evaluated whether dermatology experts are able to adequately assess improvements in sunscreen effectiveness based on the following information formats: SPF vs % EIR absorbed vs % EIR transmitted.
A total of 261 dermatology experts from Germany (n = 145), the United States (n = 68), Switzerland (n = 35), and Australia (n = 13) participated in a randomized web experiment. The Table presents participants’ demographic characteristics. The study was approved by the institutional review board of the Max Planck Institute for Human Development. Written informed consent was obtained at the beginning of the online study.
Participants assessed the increase in protection for 10 pairs of sunscreens with different SPFs (ie, all 10 pairwise combinations of SPF values 10, 15, 20, 30, and 50). For each sunscreen pair, effectiveness was presented in 1 of 3 information formats: SPF, % EIR absorbed, or % EIR transmitted. Participants assessed the relative increase in effectiveness in 2 ways: (1) in terms of the time protected when using the more effective sunscreen as opposed to the less effective sunscreen; and (2) on a rating scale from 1 (“same”) to 7 (“much higher effectiveness”). Participants were randomly assigned to one of the information formats and task orders (ie, time vs rating task first).
For each pair, we calculated the ratio between the actual effectiveness ratio (ie, the ratio of SPF values) and the perceived effectiveness ratio; ratios greater than 1 imply underestimation. Both tasks were analyzed using Bayesian linear mixed-effects models.5 We estimated fixed effects for information format, task order, and their interaction, as well as random effects for participants and sunscreen pairs. We report posterior medians and 95% Bayesian credible intervals (CIs).
The completion rate (ie, the proportion of consenting participants who completed the study) was satisfactory (ie, 284 out of 482 = 59%) and similar across conditions and populations. Twenty-three participants were excluded because they provided nonsensical answers in the time task. Sample sizes differed across conditions because assignment to conditions was random (SPF, n = 110; % EIR absorbed, n = 85; % EIR transmitted, n = 66).
Results are summarized in the Figure, B and C. In the time task, the increase in sun protection was, on average, underestimated by a factor of roughly 2 in the % EIR absorbed condition (ratio, 2.1; 95% CI, 1.7-2.4), moderately underestimated in the % EIR transmitted condition (ratio, 1.5; 95% CI, 1.2-1.8), and only slightly underestimated in the SPF condition (ratio, 1.3; 95% CI, 0.92-1.6). Underestimation was consistently higher for % EIR absorbed than for SPF (difference between ratios, 0.79; 95% CI, 0.67-0.91).
Likewise, the increase in effectiveness was, on average, rated to be lower in the % EIR absorbed condition (2.8; 95% CI, 2.2-3.5) than in the % EIR transmitted (3.8; 95% CI, 3.1-4.4) or SPF (3.9; 95% CI, 3.3-4.6) conditions. Ratings were, on average, consistently lower for % EIR absorbed than for SPF (1.1; 95% CI, 0.72-1.5).
The comparisons reported held with simultaneous statistical control for language region (English vs German), age, sex, and board certification (yes vs no).
For both professionals and lay people, SPF is key to evaluating the effectiveness of sunscreens because SPF, being the ratio of the 2 clinical end points erythema doses protected/unprotected, correlates with clinical effectiveness.6 Unfortunately, SPF ratings are poorly understood, and inaccurate explanations emphasizing the proportion of EIR absorbed have created widespread confusion and uncertainty, as our web experiment confirmed: When sunscreen effectiveness was presented as % EIR absorbed, dermatology experts underestimated the increase in protection provided by a stronger sunscreen. Assessments based on % EIR transmitted and, in particular, SPF, were more accurate. Therefore, for effective communication of sunscreen effectiveness among health professionals, patients, and consumers, we recommend that SPF be used exclusively and that presentations based on the proportion of EIR absorbed or transmitted be avoided.
Corresponding Author: Stefan M. Herzog, PhD, Center for Adaptive Rationality (ARC), Max Planck Institute for Human Development, Lentzeallee 94, 14195 Berlin, Germany (email@example.com).
Accepted for Publication: October 19, 2016.
Published Online: February 1, 2017. doi:10.1001/jamadermatol.2016.4924
Author Contributions: Dr Herzog had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Herzog, Lim, de Maddalena, Osterwalder, Surber.
Acquisition, analysis, or interpretation of data: Herzog, Lim, Williams, de Maddalena, Surber.
Drafting of the manuscript: Herzog, Lim, Surber.
Critical revision of the manuscript for important intellectual content: All authors.
Statistical analysis: Herzog.
Obtained funding: Osterwalder.
Administrative, technical, or material support: Herzog, Lim, Williams, Surber.
Supervision: Herzog, Lim, de Maddalena.
Conflict of Interest Disclosures: Dr Lim has served as an investigator for Estée Lauder, Ferndale, and Allergan. Dr Surber has served as a consultant for Actelion Pharmaceutical, Galderma, Leo Pharma, and Novartis. No other disclosures are reported.
Funding/Support: This study was supported in part by funding from BASF PCN GmbH, Monheim, Germany.
Role of the Funder/Sponsor: BASF PCN GmbH had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
Additional Contributions: We thank Susannah Goss, MA, Center for Adaptive Rationality (ARC), Max Planck Institute for Human Development, for editing and commenting on the manuscript and Jürg Hafner, MD, Department of Dermatology, University Hospital Zürich, Zürich, Switzerland, for help with recruiting Swiss participants. They received no compensation beyond salary for their contrbutions.