To address the dearth of knowledge about person-based and trip-level exposure, we developed the Icarus model. Icarus uses mesoscale traffic model—activity-based model—to analyze the heat exposure of regions of interest at an individual level. The goal with Icarus was to design accurate, granular models of population and temperature behavior for a target region, which could be transformed into a heat exposure model by means of simulation and spatial-temporal joining. By combining and implementing the most robust software and data available, Icarus was able to capture person-based exposure with unparalleled detail. Here we describe the model methodology. We use the metropolitan region of Phoenix, Arizona, USA to carry out a case study using Icarus.

Heat exposure for urban populations has become more prevalent as the temperature and duration of heat waves in cities increase. Occupational exposure to heat is a major concern for personal health, and excessive heat exposure can cause devastating outcomes. While occupational heat exposure studies have traditionally focused on environmental temperature, work intensity, and clothing, little is known about the daily exposure profile of workers, including their daily travel and working patterns. This study developed a novel measure of exposure and reprieve dynamics, the moving average hourly exposure (MAHE) to balance short-duration but high-exposure events and capture the inability to reprieve from exposure events. MAHE was assessed by combining an activity-based travel model (ABM) and the Occupational Requirement Survey to simulate urban workers' total daily heat exposure. The simulation considers daily travel, work schedules, and outdoor working frequency. The simulation was conducted for 1 million workers in Phoenix, Arizona, using Mean Radiant Temperature (MRT). The results show that 53% to 89% of workers in Phoenix's construction, agriculture, transportation, raw material extraction, and entertainment industries will likely experience MAHE over 38°C for at least an hour. These industries also have up to 34% of the laborers exposed to over 7 hours of continuous 38°C and above MAHE exposure. The location of the most intense heat exposure was identified near the downtown and central business districts, significantly different from the home locations of the workers in suburban and rural areas. Formulating the MAHE balances heat risk events with cooling benefits and aids in identifying individuals with prolonged high heat exposure.