Table 1.

Description of the mISkin app’s main features, including behavior change techniques and rational for inclusion.

Feature	Description	Behavior change techniques[27]	Rational for inclusion (evidence-based and theory-based)
Skin sensitivity assessment with feedback	A set of 5 questions about skin reaction to the sun, based on previous literature (eg, [33,34]. After completion, participants receive feedback about their specific skin type and their reaction to the sun (eg, “You have skin type III. Sometimes burns, usually tans”).	Provide information on consequences of behavior to the individual.	Understanding their personal risk of sunburn will help people shape outcome expectations, which in turn will impact goal setting. Evidence: A systematic review [21] outlines the importance of understanding the consequences of excessive sun-exposure. Theory: Social Cognitive Theory postulates that people tend to form outcome expectancies about the results of given actions [35]. In line with these outcome expectancies, people will engage in actions that are likely to produce positive outcomes and dismiss those that result in negative consequences [35].
NHSa Choices “How to apply sunscreen” Videob	The video provides information how to properly apply sunscreen, stating specific information about quantity, frequency, SPF, how to apply it before leaving the house, where to apply it, and guidance on sunscreen costs. The video also demonstrates how to apply sunscreen properly by showing a model doing it. The importance of other methods of sun-protection is also discussed in the video (ie, covering up and seeking shade). Special attention is devoted to children and the need for additional information about sun-protection. The risk of sunburn and skin cancer is also highlighted in the video. A snapshot from the NHS Choices video “How to be Sun Smart” was also included to foster social comparison on sun-protection habits.	Provide information on consequences of behavior in general, provide information on where and when to perform the behavior, provide instructions on how to perform the behavior, and demonstrate the behavior.	The video tackles all important instructions regarding sunscreen app, providing a complete display of the “how to do it” technique. The video also provides information about other methods of sun-protection and the consequences of excessive sun-exposure. Evidence: systematic review [21]. Theory: In the Social Cognitive Theory, instructions on how to engage in a specific behavior are essential to translate a goal into action, which will in turn foster self-efficacy and subsequent further behavior change [36].
UVc photographs	The app submenu “How to be SunSmart” also includes ultraviolet photographs of the face (male and female). Before displaying the pictures, a brief description is provided.	Provide information on consequences of behavior in general; fear appeals.	The inclusion of these types of photographs helps highlight the harmful effects of exposure to ultraviolet rays on people’s appearance and, subsequently, promotes sun-protection habits. Evidence: Various systematic reviews [21,25,26]. The desire to have a tan is a central motive for sun exposure, as most people believe that a tan will improve personal appearance (eg, [37-39]. Research also shows that people find others more attractive when they have a tan [21,37,38]. Thus, interventions that highlight the negative effects of exposure to ultraviolet rays on one’s appearance might lead to significant behavior change (eg, [21,37]. Theory: Social Cognitive Theory hypothesizes that people will engage in actions that are likely to produce positive outcomes based on outcome expectancies [35].
“Sun safety quiz”	This component engages holidaymakers in the “Sun Safety Quiz” by answering true or false to questions on general principles of sun-protection practices, information on positive consequences of sun-protection, tanning, vitamin D and UV Index. This is a gamification component, in which participants receive performance-based rewards (ie, positive feedback and final score message). Feedback provided also highlights others’ use of sun-protection to facilitate social comparison.	Provide feedback on performance, provide information on consequences of behavior in general, provide information about others’ approval, provide normative information about others’ behavior, and facilitate social comparison.	A gamification feature was included in the quiz with feedback about performance and the provision of relevant information to facilitate social comparison. Evidence: Even though no conclusive evidence was unveiled by the completed systematic review [21], other systematic reviews have shown that self-regulatory strategies [31,40] and gamification [21,41,42] can be effective in changing other health behaviors. Theory: According to the Control Theory [43], feedback on performance provides external feedback on achievements and can lead to behavioral change. The Social Cognitive Theory hypothesizes that referential performance is induced by a process of social self-judgment, where social comparison is central. The provision of opportunities for social comparison is therefore an important strategy to influence referential performances and promote behavior change [35].
“Sun Alert service”	An algorithm was designed to define the main rules for interaction between the app and participants (Figure 3). This interaction is especially important to establish rules for delivering prompts for sun-protection. These prompts will occur between 10 AM and 4 PM and will depend on participant location (indoors or outdoors information based on smartphone GPS). Participants will receive approximately 2 prompts per day. In these prompts, a forecast of the levels of ultraviolet radiation will also be provided.	Prompt practice	Several studies showed that forgetfulness is a key barrier for sun-protection [44]. We believe that prompting will help individuals remember about sun-protection methods at least at two points in the day: (1) the start of the day, just before temperature starts increasing (ie, 10 AM), and (2) at midday when sun-protection (eg, seeking shade) is most needed. Evidence: Systematic reviews [15,21,31] and a previous trial on sunscreen use [19]. Theory: The Social Cognitive Theory envisages prompting as a key strategy for behavior change. Prompting enables individuals to experience mastery which promotes self-efficacy [45].
Diary record: ecological momentary assessment	Real-time data capture through the smartphone app is also used for assessment of sun-protection practices. This assessment will occur randomly between 11 AM and 3 PM, if the individual is outside (as detected by the GPS on the smartphone). Sun-protection practices will be represented by the use of symbols or pictures (please see Figure 5) and participants will only need to touch the screen to record the use of sun-protection.	Prompt self-monitoring	Self-report is prone to inaccuracies and biases in the reporting of behavior [46]. Smartphones can be an effective and feasible alternative to self-report for sun-protection assessment, especially because these devices can collect behavioral events in natural settings and produce time- and date- stamp events [47]. Evidence: Previous systematic reviews have shown the efficacy of this strategy in changing behavior [31,32,47,48]. Theory: Self-monitoring is a key strategy for behavior change for both the Control theory [43] and the Social Cognitive Theory [36]. Monitoring present behavior can lead to comparisons between actual behavior and standards and, subsequently, adjustments in performance in order to reach behavioral standards.

^aNHS: National Health Service.

^bPermission was granted by NHS Choices to be used in the mISkin application.

^cUV: Ultraviolet.