Indoor temperatures above 26 °C impose disproportionate cardiovascular strain on older adults

Climate change is driving more frequent and severe periods of indoor overheating, posing substantial health risks for older adults who spend ~90% of their time indoors. While laboratory studies have characterized cardiovascular responses under controlled conditions, limited evidence exists on the real-world physiological consequences of sustained indoor heat exposure. Therefore, we quantified cardiovascular strain associated with in-home heat and humidity exposures across an entire summer in a subtropical climate. Forty-seven community-dwelling adults aged ≥ 65 years (32 women; median age 72 [IQR: 68–77]) were continuously monitored during summer (December–March) in southeast Queensland, Australia. Heart rate and physical activity were recorded via wrist-worn wearables, while ambient temperature and humidity were measured in three rooms of each participant’s home. Linear mixed-effects and piecewise regression models assessed associations between ambient conditions and heart rate above and below the World Health Organization (WHO) indoor occupancy threshold (26 °C), accounting for sex, activity, and lagged exposures (1–3 h, previous day). Real-time increases in indoor temperature, but not absolute humidity, were significantly associated with higher heart rate (p < 0.001). Above the WHO threshold, heart rate rose by 1.3 beats·min−1 per °C, compared with 0.7 beats·min−1 per °C below this level (p < 0.001). Effects persisted for 1–3 hours post-exposure, indicating immediate and lingering cardiovascular strain. This study provides the first ecologically valid evidence that indoor overheating imposes sustained cardiovascular stress on older adults, with a clear inflection at 26 °C. Findings reinforce WHO guidelines and highlight the need for policy measures to protect heat-vulnerable populations.