Trees Reduce Daytime Extreme Urban Heat Risk for People Experiencing Unsheltered Homelessness.

Extreme heat poses growing health risks in U.S. cities, particularly for people experiencing unsheltered homelessness (PEUH) who lack consistent access to indoor cooling. This study assessed personal heat exposure among PEUH in Salt Lake City, Utah, one of the fastest-warming U.S. cities. Eight PEUH carried GPS-enabled smartphones and wearable temperature sensors for six days during a period of extreme summer heat. Nearly 10,000 geolocated heat index observations were intersected with a tree canopy polygon layer to quantify the role of urban tree cover in mitigating heat risk among PEUH. PEUH routinely experienced risky heat conditions, with prolonged exposure to heat above moderate (≥ 32.8 °C) and high-risk (≥ 39.4 °C) heat index thresholds. Nights offered little thermal relief; participants spent an average of 69% of nighttime hours in "hot night" conditions (≥ 20 °C). Multilevel modeling revealed that time spent near tree canopies was significantly associated with lower daytime heat index, with 10-15 min under trees linked to a 1.75 °C reduction in heat index during afternoon hours (12-6 PM). Results suggest that trees are a critical resource during periods of extreme heat, but that proximity to trees cannot fully mitigate exposure to hazardous conditions. Improving access to water and indoor cooling, and including PEUH in climate-health planning are additional, critical short-term steps toward thermal equity. While housing remains the most effective heat protection against heat stress, immediate interventions can prevent harm. Trees, often viewed as ecological amenities, are essential public health infrastructure, especially for those with no shelter from rising urban heat.