Q. Scientific session: Loneliness as a public health challenge: new measures, trends, and protective factors

Abstract Rationale and Objectives Loneliness has emerged as a global public health concern with broad implications for mental and physical health across populations. Yet existing tools and frameworks tend to individualize what is often a structurally embedded and socially patterned experience. This scientific session aims to: 1. Reframe loneliness as both individual and collective phenomenon across the lifespan; 2. Introduce and validate new metrics that move beyond standard loneliness scales; 3. Share national-level trend data and forecasts on loneliness in Finland to 2040; 4. Explore adolescent loneliness in relation to mental health and substance use, highlighting social exclusion and protective factors in family and school contexts. Added Value This session synthesizes three complementary lines of research-scale development, population-level modeling, and adolescent mental health-to build a multi-dimensional understanding of loneliness. It will bring theoretical clarity and methodological innovation to a field dominated by reductive metrics. Key contributions include: • A novel scale capturing collective loneliness at multiple social levels; • Empirical evidence of sharply rising loneliness across age groups in Finland, with long-term projections; • Identification of differential patterns of adolescent loneliness and the moderating roles of institutional support. These insights are timely for policymakers and researchers responding to WHO's recommendations on strengthening social connection as a pillar of public health. Coherence Between Presentations Each presentation addresses loneliness at a different scale and life stage, offering layered and non-redundant insights: • Beattie et al. introduce and validate the Collective Loneliness Scale, theorizing this neglected type of loneliness as illbeing arising from alienation from social groups (groups, community, society); • Parikka et al. provide a population-level analysis showing past and future loneliness trends by gender and age-groups up to 2040; • Tunkkari et al. focus on adolescents, differentiating between social, emotional, and ostracism-based loneliness, and examining protective buffers in familial and institutional contexts. Together, the presentations trace loneliness as a developmental and structural issue, offering converging evidence and distinct lenses. Workshop Format This session follows the Scientific session format. We propose: • Three presentations of ∼12 minutes each; • A short integrative commentary (1 speaker, 6 minutes) to draw out thematic connections; • A moderated Q • All presenters have agreed to structure their contributions to maximize thematic clarity and audience engagement. Key messages • Loneliness is increasing across age groups; projections to 2040 highlight urgent need for action. • New measures and tailored interventions are needed to address both interpersonal and collective loneliness.