What is a "safe" indoor, warm season, temperature?

In high income nations, individuals spend ~90% of their time indoors. However, there is limited evidence directly linking indoor heat exposure to health outcomes. Our study investigated the indoor environments of people receiving emergency medical care in New York City, NY (NYC) and Atlanta, GA U.S. from May to September 2016. The first objective was to determine the relative influence of outdoor conditions as well as patient characteristics and neighborhood sociodemographics on indoor temperature and specific humidity. The second objective was to determine if extreme heat cases experience hotter and more humid indoor conditions as compared to controls.

Paramedics carried portable sensors into buildings where patients received care to passively monitor indoor temperature and humidity. For the first objective, Generalized Linear Models (Gaussian family, identity link) associated outdoor conditions and sociodemographics against indoor conditions (NYC, N = 3959; Atlanta, N = 2014). Consistent with previous studies, indoor specific humidity was strongly related while temperature was modestly related to outdoor conditions. In NYC, the heatwaves modified indoor/outdoor temperature associations. On heatwave days, some households intensified their use of air conditioning. For the second objective, the study compared indoor heat exposure of extreme heat cases versus controls. In NYC, extreme heat related illness cases were more likely to experience higher heat exposure (indoor temperature > 28 °C) compared to controls. In contrast, indoor heat exposures were similar between cardiovascular cases and controls.