Health Agencies Warn Fans Can Stop Cooling and Become Harmful at Mid‑30s Temperatures

Overview

• Public‑health guidance cited by multiple outlets this week says electric fans cool only up to a threshold and may be ineffective or harmful when ambient air reaches roughly 35 °C to 40 °C, with the UKHSA commonly using 35 °C and the WHO up to 40 °C as reference points.
• The reason is physical: when room air approaches or exceeds core body temperature (about 35–37 °C) the body can no longer shed heat by evaporating sweat, so a fan can blow hot air onto skin and speed heat gain and dehydration.
• Humidity and age change that threshold because high humidity limits sweat evaporation and older people sweat less, so the temperature at which a fan becomes counterproductive can be lower for vulnerable adults.
• Practical steps advised in recent coverage include checking indoor temperature and humidity, moving to a cooler location if thresholds are exceeded, using a fan to expel warm air by aiming it outward at a window at night, and protecting windows with shading or reflective film to reduce indoor heat.
• The wider effect of the guidance is likely to push more people toward cooled public spaces or air conditioning during the ongoing heatwave, increase demand for clear, simple home checks, and raise urgency around protecting older people from dehydration and heat illness.