Heat Waves, Urban Heat Islands, and Wellbeing and Health: a Mobile Sensing Approach

While the number of heat wave days in France will likely continue to increase over the century, extreme heat is associated with excess mortality and morbidity in urban heat islands. The first objective of H3Sensing is to investigate outdoor environmental, building, dwelling, situational, and behavioral determinants of objectively assessed personal heat stress over daily movements during warm periods. We will explore environmental determinants associated with micro-urban heat islands at an unprecedented level of accuracy; we will assess building and dwelling characteristics associated with in-home heat stress; and we will examine how daily activities (trips, etc.) and practices (how people dress, manage heat at home) contribute to heat stress. The second aim is to investigate how these heat stress determinants and momentary and cumulated heat stress itself are related to physiological indicators of heat stress, sleep, thermal discomfort, and well-being. One hundred eighty (180) participants recruited through internet advertisements will be followed over 4 day periods in March-May and then a second time in June-September 2025. Research assistants will use a detailed assessment sheet (defined from the a priori visit to 10 dwellings) to collect information on building and dwelling characteristics. Participants will carry a smartphone with a GPS receiver and the Eco Emo tracker application permitting to identify their trips and visited places and to collect additional information on these trips (including their access to air conditioned). GPS data will permit to dynamically assess exposures in participants' everyday mobility. To move beyond simplistic assessments of personal heat stress, we will assess it in an ambulatory way considering personal ambient temperature, humidity, wind speed, and radiant temperature combined into the universal thermal climate index. Additionally, a meteorological station will be placed in the bedroom, while sensors measuring solar flux will be installed outdoor on the windows and balcony. Participants will also carry two bracelets for the measurement of physiological parameters (blood pressure, skin temperature, galvanic skin response as a marker of sweating, and heart rate). As part of the VF++ project, core temperature data will be additionally collected in a subsample of participants (e.g., 20 participants). Personal thermal discomfort, well-being, and sleep quality will be self-reported on the smartphone using the Eco Emo tracker application. An a posteriori phone questionnaire will collect detailed information on heat-related practices and health impacts over the observation period. Analyses of short-term effects will use repeated measure models. Investigations of momentary objective heat stress as the outcome will use statistical techniques for complex mixtures to handle the wide set of exposures. Analyses of sleep quality, well-being, and physiological measures as the outcomes will use two complementary designs: fixed-effect analyses of matched observations in the warm and in the cooler periods for the same participants; and participant-level fixed-effect analyses of pooled observations in the warm period. H3Sensing brings together epidemiologists, urbanists, and climate scientists. As research-to-policy translation, experts in urbanism involved in the project will formulate concrete technical recommendations and will identify practical solutions (in a pedagogical document of synthesis) to help urban actors willing to integrate considerations related to urban heat islands in their urbanistic regulations, public policies, and urban planning projects.