Hidden Vulnerability and Heat-Triggered Decompensation: Rethinking Urban Overheating Beyond Clinical Risk

Urban overheating is increasingly recognised as a major public health challenge under climate change, yet existing frameworks remain heavily focused on acute medical risk and clinically vulnerable populations. This paper introduces \textit{hidden vulnerability} to describe a broader population who remain socially and economically active under ordinary conditions but experience progressive destabilisation during sustained indoor heat exposure. Drawing on a sensor-enhanced longitudinal housing survey in Southwark, London during the summer of 2023, combining indoor environmental monitoring with weekly health and wellbeing questionnaires, the study examines how overheating affects everyday urban life. The findings reveal that impacts extended well beyond thermal discomfort. Participants described cascading disruptions across physical, functional, and psychological domains, sleep loss, exhaustion, impaired self-care, reduced concentration, emotional strain, and increased caring burdens, that interacted and reinforced one another, progressively eroding the capacity to maintain ordinary routines. Critically, these processes were observed not only among participants with chronic conditions but also among those without formally recognised health vulnerabilities, suggesting that the threshold for destabilisation is lower, and more widely distributed, than existing frameworks assume. The paper argues that hidden vulnerability is an urban condition, produced by the intersection of housing quality, social infrastructure, and accumulated disadvantage. The response calls for a conceptual framework for redefining heat vulnerability beyond demographic and clinical categories, and an empirical foundation for a social cure approach to urban heat governance.