Heterogeneity in Individually Experienced Temperatures (IETs): New insights into heat exposure from Boston and Phoenix

Urban environmental health hazards, including exposure to extreme heat, have become increasingly important to understand in light of ongoing climate change and urbanization. In cities, neighborhoods are often considered a homogenous and appropriate unit with which to assess heat risk.

This paper presents results from two studies that examine the variability of individually experienced temperatures (IETs) within urban neighborhoods. In July 2013, 23 research participants were recruited from Boston's South End neighborhood and equipped with Thermochron iButtons that measured the air temperatures surrounding individuals as they went about their daily lives. Participants also filled out daily surveys and participated in exit interviews. In September 2014, the same procedure was repeated in four Phoenix neighborhoods.

The Boston study found that IETs were heterogeneous within the neighborhood and that outdoor temperatures overestimate mean experienced temperatures during a heat wave. The Phoenix study investigated how heterogeneity varied between neighborhoods that differed in key elements such as racial diversity, income, age, and processes like gentrification.

Findings from daily surveys and exit interviews in Boston suggest that behavior (schedules, preferences, lifestyles) and access to cooling resources (air-conditioned homes and business, parks, and pools) were variable among participants. Further, Individual attributes such as gender, race, socioeconomic status, age, and neighborhood tenure, also proved important in predicting heat exposure. The Boston study indicated that individual heat exposure can best be understood when the intersection of individual attributes, behavior, and access to cooling resources are framed by the neighborhood social and biophysical context including how individuals and groups used public resources such as parks and private resources such as coffee shops and corner stores. Together, these parameters created intra-group variation, which was a partial driver of heterogeneity within the neighborhood.

The Phoenix study provided an expanded understanding of how these dimensions interact, especially in a city that experiences significantly higher temperatures than Boston for a longer seasonal durational. Different neighborhoods experience different outdoor ambient temperatures, and data collected in this study build a more complete picture of how intra-city differences in outdoor temperatures manifest themselves into the IETs of urban residents.

Individual differences are an overlooked determinant of heat exposure and should be better integrated into multi-scalar analyses. The IET approach is an innovative tool to collect information about how individuals actually experience temperatures within cities. Together, the Boston and Phoenix IET studies combine individual, neighborhood, and city differences in heat exposure such that we can better design innovative heat-health intervention and mitigation strategies.