The missing exposure-outcome link: “equivalent” outdoor ambient temperatures for assigning heat-health effects to individual-level temperature exposures in three U.S. cities

Abstract

BACKGROUND AND AIM: Heat-health epidemiologic studies often lack information on key determinants of exposure, including occupation, housing, and air conditioning, at the individual or parcel (home) level. Therefore, effect estimates specific to people in subgroups defined by various combinations of these determinants are often not available. We derived subgroup estimates from modeled individually-experienced temperatures (IETs) during 10 days around a past city-specific extreme heat event for three U.S. cities of varying climates: Atlanta, Georgia (hot-humid), Detroit, Michigan (temperate), and Phoenix, Arizona (hot-dry). METHODS: IETs were previously estimated for 2.5 million people in the three cities using modeled data: parcel-linked population microdata, housing-specific indoor temperatures, temporospatially-varying outdoor ambient temperatures, and time activity patterns. To each of the IETs we linked the daily outdoor ambient temperatures as measured at the regional airport—the temperature exposure often used in heat-health studies. We then fit a mixed-effects regression model to predict this often-used temperature exposure, ‘equivalent’ outdoor airport temperature, based on IET and housing type, demographics, air conditioning, and person-specific random effects. Using the equivalent temperatures, we assigned risk ratios (RRs, for increases in outdoor airport temperature) from existing literature on mortality, emergency department visits, and preterm births to each person-day and estimated attributable burdens (using age/race/sex) health event counts. RESULTS:The equivalent temperatures, RRs, and attributable burdens, by design, differed between population subgroups due to variability in indoor and outdoor temperature exposure. For example, heat-associated deaths were 150% higher among Blacks vs. Whites in Atlanta and 40% higher in Detroit. Emergency department visits were 19%, 13%, and 25% higher in Atlanta, Detroit, and Phoenix, respectively, among those without vs. with air conditioning. CONCLUSIONS:When IETs are known, these person-specific equivalent temperatures can be used to estimate subgroup and spatially-varying heat-health burdens for resource allocation for future heat waves and environmental and energy justice policies. KEYWORDS: climate change, indoor temperature exposure, urban heat island, climate vulnerability, extreme weather preparedness