Capturing heat illness in vulnerable populations through the lens of older adults: a scoping review protocol of health administrative data.

introductionHeatwaves are among the fastest-growing climate-related threats to human health, increasing in frequency, intensity and duration with climate change. Older adults are disproportionately affected, reflecting intersecting physiological, social and economic vulnerabilities. Beyond mortality, heatwaves drive substantial but often under-recognised morbidity, including emergency visits and hospitalisations for cardiovascular, respiratory, renal and metabolic conditions. Health administrative data provide valuable opportunities to improve understanding of these phenomena and for the quantification of their impacts. However, comparability is limited by heterogeneity in case definitions, that is, the criteria used to determine which health events are counted as cases: some studies only clinically diagnosed heat illness, while others also capture outcomes plausibly triggered or exacerbated by heat. It is further constrained by differences in International Classification of Diseases (ICD) versions, national adaptations, coding practices and adoption timelines across countries. No synthesis has yet mapped these heterogeneous approaches for older adults, despite their over-representation among those most affected by heat-health risks. This heterogeneity limits the ability to capture the true burden of disease and to inform adaptation planning. METHODS AND ANALYSIS: We will conduct a scoping review to map how heat-related diagnoses among older adults are identified and measured in health administrative data. The review will follow the initial methodological framework of Arksey and O'Malley refined by Levac ETHICS AND DISSEMINATION: Ethical approval is not required for this review as it involves secondary analysis of published and publicly available data; for more information, contact University of Montreal's Research Integrity department at