Wet-bulb heat explained: Why humidity makes extreme summer heat deadlier

Wet-bulb heat reflects how difficult it becomes for the body to cool itself during humid weather, making prolonged summer exposure increasingly risky

India’s ongoing heatwaves are once again raising concerns about how much extreme heat the human body can safely endure. While air temperature remains the most closely watched measure, experts say humidity can make conditions far more dangerous by preventing sweat from cooling the body effectively, pointing to a phenomenon known as wet-bulb heat.

 

According to doctors, rising wet-bulb temperatures are a growing public health concern, particularly in coastal cities and densely populated urban areas where humidity levels remain high for long periods.

 

What exactly is wet-bulb temperature?

 

Wet-bulb temperature (WBT) combines air temperature with humidity levels to measure how stressful heat conditions can become for the human body.

 

 

Dr Sunil Rana, Associate Director & Head- Internal Medicine (Unit III), Asian Hospital, explained, “The temperature that is read on a thermometer reflects only how warm the air is, while the wet-bulb temperature, which takes into account both temperature and humidity, tells us the humidity effect.”

 

The term comes from the way it is measured. A wet cloth is placed over the bulb of a thermometer and as the water evaporates, it cools the thermometer and lowers the reading. However, when humidity levels are high and the air is already saturated with moisture, less evaporation takes place. As a result, the wet-bulb temperature remains much closer to the actual air temperature, making conditions feel far hotter and more dangerous for humans.

 

Dr Rahul Sharma, Additional Director - Cardiology at CK Birla Hospitals, added that wet-bulb heat becomes especially dangerous because, “In extremely humid conditions, the sweat is not able to evaporate much, and so the body loses its ability to cool down naturally.”  

 

“The core body temperature increases more quickly as a result, which often means heavy burden on the heart, brain, kidneys, and blood vessels," he added.

 

Why humidity makes heat more dangerous

 

On dry summer days, sweat evaporates quickly from the skin and helps cool the body. However, humid air is already saturated with moisture, which slows down or even prevents evaporation and hinders the body’s natural cooling process.

 

Dr Neeraj Gupta, Clinical Director - Pulmonology, Critical Care & Sleep Medicine at Marengo Asia Hospitals, warned that if body temperature is not reduced through sweating, it can lead to dehydration, fatigue, dizziness, heat exhaustion, and even heatstroke.

 

Doctors also noted that high humidity increases cardiovascular strain because the heart has to pump blood harder towards the skin in an attempt to cool the body. This can become particularly risky for elderly people and those with pre-existing illnesses, says Dr Gupta.

 

When does wet-bulb heat become life-threatening?

 

"Earlier research suggested that humans could generally survive wet-bulb temperatures of around 35°C if they were resting in the shade and had access to fluids. However, newer studies have found that dangerous health effects may begin much earlier, at around 31°C to 32°C, particularly during prolonged exposure even under those conditions," informed Dr Rana.

 

Dr Sharma said prolonged exposure to wet-bulb temperatures close to 35°C can trigger dehydration, heatstroke, heart stress, and organ failure, even in otherwise healthy individuals.

 

Who is most at risk?

 

Experts say some groups are significantly more vulnerable during wet-bulb heat events. These include:

 

 Older adults and young children

 Outdoor workers and labourers

 Pregnant women

 People with heart disease, diabetes, obesity, or kidney disorders

 Individuals taking certain medications

 Residents in densely populated urban areas

 

Doctors said people should pay attention to warning signs such as headache, dizziness, muscle cramps, nausea, fatigue and rapid heartbeat immediately.

 

Excessive sweating or unusually dry skin, breathlessness, confusion and fainting may also indicate serious heat stress, while fever, seizures or unconsciousness can signal severe heat-related illness requiring urgent medical attention.

 

Risk of more wet-bulb heat events

 

According to experts, climate change is increasing both temperatures and humidity levels across many parts of India. Coastal regions and large urban centres are considered especially vulnerable. "This increases strain on healthcare, emergency services, water supply, and urban planning systems, making heat preparedness and awareness higher priorities than ever before," added Dr Rana.

 

Dr Sharma added that public health systems can no longer rely only on temperature warnings because humidity also plays a major role in heat exposure risks. He stressed the importance of cooling spaces, hydration awareness, and better protection for vulnerable groups.

 

How to stay safe during wet-bulb heat

 

Drink water regularly, even before feeling thirsty

Avoid stepping outdoors during peak afternoon heat

Wear loose and light cotton clothing

Reduce strenuous outdoor activities

Use fans, ventilation, or air conditioning whenever possible

Include water-rich foods like watermelon, cucumber, and citrus fruits in the diet

Check in regularly with the elderly and children

 

“Proper hydration and cooling are critical for avoiding heat-related emergencies,” Dr Rana said.

 

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