ABSTRACTMore than one million firefighters in the United States provide critical emergency medical services incommunities they serve and have been on the front lines of healthcare delivery including throughout theCOVID-19 pandemic. As a result of exposure to occupational stressors a high proportion of firefightersexperience considerable stress-related illness burden including chronic pain psychological distress (i.e.anxiety and depression) and posttraumatic stress disorder. Although interventions have been developed toaddress this high need of reducing effects of occupational stress exposure in order to improve firefighter well-being (e.g. cognitive-behavioral therapy resilience training) not all of these modalities appeal to allfirefighters nor are they easily implemented without direct in-person contact. One modality that has shownpromise to reduce distress in various populations is meditation including meditation delivered by smartphoneapps. To the best of our knowledge no smartphone-based meditation interventions designed to cultivate bothmindfulness and feelings of social connection to others have been tested with firefighters. This study willtherefore test the efficacy of a 10-day smartphone-based meditation app intervention among N=192 careerfirefighters. The app was developed by Health Minds Innovations (HMI Madison WI) and is designed toenhance both mindfulness (awareness) and social connection to others in order to reduce anxiety. Our grouprecently piloted tested this app and found that firefighters exhibited reduced anxiety (a key component ofpsychological distress) and burnout as well as improved function of the stress hormone cortisol from beforeto after use of the app. Although these encouraging results suggest a low-cost scalable smartphone-basedmeditation app may be effective to improve firefighter well-being our pilot study lacked an attention controlcomparison needed to establish intervention efficacy. Therefore in collaboration with 3 metropolitan firedepartments in the United States we will test the efficacy of the HMI meditation app to reduce psychologicaldistress compared to a rigorous active attention control (i.e. a Health Education app based on our priorwork)(Aim 1). We will also determine whether the effect of the HMI meditation app of psychologicaldistress is mediated by mindfulness and perceived social connection (Aim 2). The proposed research willprovide important evidence of efficacy about a smartphone-based meditation app intervention with likelyhigh impact that cultivates both mindfulness and social connection in order to reduce psychologicaldistress in frontline workers in the fire service. This research supports NIOSH's Strategic Goal 7 / ActivityGoal 7.14.1 (to develop interventions that integrate protection from work-related health hazards withpromotion of prevention to advance worker well-being) and NORA Objective 6 (promote healthy workdesign and well-being).