Understanding the wet-bulb effect: How heat and humidity affect us

Wet-bulb temperature is used to depict the temperature at which our bodies will be unable to cool themselves down by sweating

As most parts of India are experiencing heatwave conditions, it has reignited the discourse about how one can protect themselves from rising temperatures and how much high-temperature a person can stand. 

According to media reports, 13 people died of sunstroke in Maharashtra recently, but these headlines do not tell the whole story as they only talk about one factor which is high temperature. 

When it comes to the impact of high temperatures on humans, we generally ignore humidity, which also plays a huge role in how we actually experience heat.

That's where the role of wet-bulb temperature (WBT) comes into play. WBT is used to depict the temperature at which our bodies will be unable to cool themselves down by sweating. This measure is a combination of temperature and humidity. 

The term WBT comes from how it is measured. One can measure it by wrapping a wet cloth over the bulb of a thermometer and the evaporation of water from the cloth will cool the thermometer down. This lower temperature is the WBT, which can never go above the dry temperature. 

If the humidity in the surrounding air is high, the WBT will be closer to the dry temperature. In humid conditions, it becomes harder for people to cool down through sweating as water evaporates slowly in humid conditions.

As long as the wet-bulb temperature is well below the skin temperature, the body can release heat to your surroundings through sweating. But as the wet-bulb temperature approaches our core temperature, we lose the ability to cool down, hence the discomfort.

According to a paper published in the peer-reviewed journal ScienceDirect, even heat-acclimatised people cannot carry out normal outdoor activities in wet-bulb temperatures above 32°C, which is the equivalent of a heat index of 55 degrees. 

At wet-bulb temperatures above 35 degrees, even healthy people with unlimited water, shade, and no physical activity will die of heatstroke after a few hours of exposure. This temperature can be reached at an actual temperature of 45°C with a relative humidity of 50 per cent or at about 39°C with 75 per cent humidity, said the National Institute of Health, US.

At such extremely high wet-bulb temperatures, there is so much moisture in the air that sweating becomes ineffective at removing the body’s excess heat. After about six hours or more, it can lead to organ failure and death in the absence of access to artificial cooling. 

In order to protect ourselves from sunstroke, it is advisable to avoid prolonged exposure to the sun and to have plenty of water, even if not thirsty, to avoid dehydration.