Climate Science and Policy

Extreme heat is the leading cause of climate related mortality in the United States with marked socio-economic inequities. Yet, the vast majority of the scientific literature focuses on outdoor heat while people spend a considerable amount of time indoors. This study quantified the indoor-outdoor temperature differential (ΔT, defined as indoor minus outdoor temperature in °C) across 21 San Diego County census tracts using co-located PurpleAir sensors during the May-October 2023 warm season and examined its relationship with sociodemographic vulnerability, heat mitigation factors and sensor placement equity. Indoor temperatures exceeded outdoor temperatures in 90% of measured tracts, with a mean ΔT of +4.07 °C. On days meeting the San Diego County heat emergency threshold of 33.94 °C, the mean indoor-outdoor temperature difference decreased to +1.08 °C. Analysis of indoor temperature and sociodemographic variables found that no variable significantly predicted indoor temperature. However, outdoor temperature was significantly associated with poverty, renter-occupancy, non-white residents, indicating that heat disparities are reflective of geographic sorting into hotter inland areas. Green Space coverage using EVI data was strongly negatively correlated with cumulative environmental burden. Over 94.2% of San Diego County census tracts lacked an indoor sensor. Sensor placement was highly concentrated in coastal, higher income and lower burdened census tracts. These findings indicate the need for policy change, such as residential indoor temperature maximum standards, expanded indoor sensor coverage in high-burden communities, green space investment and air conditioning affordability programs.