Heat is a leading weather-related cause of death worldwide and heat waves are increasing globally in terms of frequency, duration, and intensity. Global heat-related deaths could quadruple by midcentury. As with many environmental hazards, numerous factors impact how heat might affect any given person and there are significant gaps in our understanding related to indoor heat and its effect on health. Despite growing interest in establishing standards and guidelines, there is currently no clear consensus on a safe maximum upper limit for indoor temperature. There is conclusive evidence of links between high outdoor temperatures and human health yet research on this correlation does not typically explicitly consider indoor heat exposure. Considerably more research has been completed on healthy, active individuals than for more heat-susceptible populations and the impacts of moderate heat stress on the health of large populations are not well understood.

We conducted a literature review on the impact of indoor thermal conditions on health, recognizing that air temperature alone cannot describe thermal exposure. We introduce the concept of a standardized maximum safe indoor temperature, defined for still air conditions, 50% relative humidity and mean radiant temperature equal to air temperature. Equivalent temperatures with respect to the thermal load on the body can then be calculated for various air velocities, humidities or mean radiant temperatures using the standard effective temperature (SET) model. For U.S. policymakers, we propose adopting a standardized maximum safe indoor temperature of 28 °C. We recognize that the adoption of standardized maximum safe indoor temperatures may vary around the world, but the framework we propose to adjust the standardized upper limit for humidity, air motion, and radiant temperature could be used globally. We also identify important knowledge gaps to guide future research on the relationships between heat and health that could support informed cost-benefit analyses. @en