Fans reverse from cooling to heating at 32-35°C for adults over 65 at low humidity, with wet clothing extending safe use by 3-5°C

Havenith's team combined biophysical heat-balance equations with human subject trials at controlled temperatures and humidities. They measured skin wettedness, core temperature rise, and convective heat gain across 20-45°C and 15-60% relative humidity, directly testing fan-on versus fan-off conditions. Sample size was 24 adults stratified by age (young vs over-65), revealing a 3°C earlier reversal point for older participants due to delayed and reduced sweating.

This threshold mismatch explains why UK guidance at 35°C under-protects elderly users during dry heatwaves while WHO's 40°C figure overestimates safety in arid conditions. Urban heat-island data from 2022 European events show indoor humidity often drops below 25%, pushing the practical limit lower still for apartment-dwelling seniors. Wet-clothing or misting interventions cut dehydration risk by slowing insensible losses while boosting evaporation.

One immediate rule: when outdoor temperature exceeds 35°C, mist exposed skin and clothing every 15 minutes while running a fan on low; this extends safe fan use until 38°C even at moderate humidity and directly reduces heat-strain reports in the next heatwave season.

Next steps require city-level deployment of misting kits in high-rise elderly housing plus real-time humidity-linked alerts; without them, excess mortality among over-70s will track dry-bulb peaks rather than wet-bulb.