Thermal indices for assessing the impact of outdoor thermal environments on human health: a systematic review of epidemiological studies.

The global rise in temperatures contributes to the increase of climate-sensitive diseases. Despite mitigation efforts, temperatures are projected to keep rising, highlighting the need for integrated methods to assess the impact of thermal environments on human health. This study summarizes the existing evidence on the statistical relationships (associations) between thermal indices and health outcomes. Medline, Scopus, and Web of Science were systematically searched until December, 2023 for studies examining the association between thermal indices and health outcomes in outdoor environments (protocol registration: PROSPERO CRD42023412470). The quality of the included studies was assessed using the United States National Institutes of Health Quality Assessment Tool. The search identified 5038 records, with 310 meeting eligibility criteria and examining 1143 associations. These associations represented 51 countries, primarily in North America (n = 448, 39.2%) and Europe (n = 399, 34.9%). Temperate climates (n = 597, 52.2%) were the most frequently examined. Seventeen indices were identified, with Apparent Temperature being the most common (141 publications, 634 associations). Frequently used indices included also Heat Index, Universal Thermal Climate Index, and Physiologically Equivalent Temperature. About half of the associations focused on mortality and half on morbidity. The most frequently examined associations were for diseases of the circulatory system (n = 304, 26.6%), all-cause morbidity/mortality (n = 288, 25.2%), and diseases of the respiratory system (n = 151, 13.2%). Among associations examining heat-related outcomes (n = 882), 57.8% (n = 510) suggested an increased risk of adverse health outcomes as indices increased, while for cold-related outcomes (n = 367), 44.1% (n = 162) suggested an increased risk as indices decreased. This systematic review reveals significant associations between thermal indices and health outcomes, indicating that thermal indices could be valuable tools for public health planning. However, the diversity in methodologies across studies highlights the need for standardization in methodology and reporting, including the reporting of non-significant findings.