Wet-bulb heat explained: Why humidity makes extreme summer heat deadlier
India’s current heatwaves highlight the dangers of wet-bulb heat, where high humidity prevents sweat evaporation, increasing risks of heatstroke, organ strain, and death even at lower temperatures. Experts warn that prolonged exposure to wet-bulb temperatures above 31°C can be life-threatening, particularly for vulnerable groups like the elderly, outdoor workers, and those with pre-existing conditions.
India’s persistent heatwaves have renewed attention on wet-bulb temperature (WBT), a measure combining air temperature and humidity to assess heat stress on the human body. Unlike standard temperature readings, WBT accounts for humidity’s role in hindering sweat evaporation, making conditions far deadlier. Doctors explain that high humidity saturates the air, preventing sweat from cooling the body effectively, leading to rapid core temperature rises and strain on vital organs like the heart, brain, and kidneys. WBT is measured using a wet cloth over a thermometer bulb; high humidity reduces evaporation, causing the thermometer to read closer to actual air temperature, amplifying perceived heat. Dr Sunil Rana of Asian Hospital notes that while traditional thermometers show air warmth, WBT reveals the combined threat of temperature and moisture. Dr Rahul Sharma of CK Birla Hospitals adds that humid conditions disable the body’s natural cooling mechanism, forcing the heart to work harder to circulate blood to the skin, increasing cardiovascular risks. Humidity exacerbates heat dangers by slowing sweat evaporation, which normally cools the body on dry days. Dr Neeraj Gupta of Marengo Asia Hospitals warns this can trigger dehydration, dizziness, heat exhaustion, and heatstroke, especially in prolonged exposure. High humidity also strains the heart as it pumps blood harder to dissipate heat, posing severe risks to the elderly, outdoor laborers, and individuals with chronic illnesses like diabetes or heart disease. Research previously suggested humans could survive WBT up to 35°C under shade with fluids, but newer studies indicate health risks may start as low as 31°C to 32°C during extended exposure. Dr Rana emphasizes that prolonged exposure near 35°C can cause dehydration, heatstroke, organ failure, and even death, even in healthy individuals. Vulnerable groups—including children, pregnant women, and those on certain medications—face heightened peril in humid heatwaves. Coastal and densely populated urban areas in India are particularly at risk due to persistent high humidity. Experts urge public health measures like hydration, shade access, and early warning systems to mitigate wet-bulb heat risks during extreme weather events.
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