Heat Exposure during Outdoor Activities in the United States Varies Significantly by City, Demography, and Activity

Environmental heat is a growing public health concern in cities. Urbanization and global climate change threaten to exacerbate heat as an already significant environmental cause of human morbidity and mortality. Despite increasing risk, very little is known regarding determinants of outdoor urban heat exposure. Assessment of heat exposure at the individual level can be difficult due to the need of high-resolution time-activity data, and as a result, focus is often on place based studies, or person-based studies with small samples.

To determine how exposure varies by city, demography, and activity, and to provide additional evidence for building community and national-scale resilience to extreme heat, we assess US outdoor urban heat exposure across a large sample of urban inhabitants. We estimate outdoor urban heat exposure by pairing individual-level data from the American Time Use Survey (2004–2015) with corresponding meteorological data for 50 of the largest metropolitan statistical areas in the US. We also assess the intersection of activity intensity and heat exposure by pairing metabolic intensities (metabolic equivalent of task) with individual-level time-use data. This allows us to quantify heat exposure as ‘exposure intensity’ by combining measures of activity duration, activity intensity, and environmental heat. We model an empirical relationship between demographic indicators and daily heat exposure intensity with controls for spatiotemporal factors. We find higher outdoor heat exposure among the elderly and low-income individuals, and lower outdoor heat exposure in females, young adults, and those identifying as Black race. Traveling, lawn and garden care, and recreation are the most common outdoor activities to contribute to heat exposure. We also find individuals in cities with the most extreme temperatures do not necessarily have the highest intensity of outdoor heat exposure. The findings reveal large contrasts in outdoor heat exposure between different cities, demographic groups, and activities. Resolving the interplay between exposure, sensitivity, adaptive capacity, and behavior as determinants of heat-health risk will require advances in observational and modeling tools, especially at the individual scale.