Rapid review: health and maximum indoor temperature thresholds in high income countries

There are currently no formal guidelines on maximum indoor temperature thresholds in household, community or workplace settings in the UK, despite the built environment being a major determinant of heat-related mortality and morbidity. This rapid review considered at what temperature threshold(s) hot indoor environments begin to harm human health in high income countries, with a view to informing potential future recommendations and UK policy on this topic. We looked for articles measuring human health and high indoor temperatures in high income countries. Searches of Medline, Embase, Web of Science and Scopus identified 5,642 articles published between 2017–2024, of which 12 articles were included. Countries, populations, settings, and methods of exposure measurement differed within the final set of studies. Health outcomes were varied and included measures such as risk or odds of death, indicators of morbidity, ambulance calls, and self-reported heat related illness, among others. Thresholds at which adverse health outcomes were observed spanned a wide range of temperatures, from 17 °C to 31 °C. While there was a correlation between high indoor temperatures and health outcomes, the body of evidence was not coherent or consistent. There remains a dearth of evidence on safe maximum temperatures in indoor settings. Future research should study directly measured or well-modelled indoor temperature with acute health outcomes for groups at highest risk of heat related morbidity and mortality, to inform adaptation policy in the context of a rapidly warming climate. Until there is extensive scientific data to support a maximum indoor temperature threshold, 26 °C may be the most suitable threshold for a maximum indoor temperature threshold for at-risk groups in keeping with the existing guidance.