Wake-up call from new study on humid heat

This humid heat can elevate body temperatures to potentially fatal levels, scientists warn.
A girl bathes her sibling as heat and humidity make life miserable in Bhubaneswar. Image used for representative purposes only
A girl bathes her sibling as heat and humidity make life miserable in Bhubaneswar. Image used for representative purposes only

CHENNAI: The Southwest monsoon that has just covered the entire nation, bringing relief from the unprecedented heatwaves this year, but it might not have been as effective in regions with high humidity. Blame it on the warm spells and humid heat extremes, which are becoming more widespread during the monsoon season due to global warming.

The season’s relatively higher humidity, when combined with high temperatures during warm spells, or monsoon breaks, can cause extreme heat stress. This humid heat can elevate body temperatures to potentially fatal levels, scientists warn.

A recent paper by a group of researchers at Cornell University said there has been a consistent increase in areas exposed to extreme levels of humid heat during monsoons in India between 1951 and 2020. As of 2020, close to 43 million sq km, where an estimated 67 crore people live, suffer from extreme moist heat during monsoon-break.

In the Indo-Gangetic plains, as many as 3.7 to 4.6 crore people face elevated risk of moist heat, which is majorly driven by irrigation practices, the researchers said.

With the projected increase in warm spells, a widespread increase in moist heat can pose a significant challenge to the health of people, especially those who work outdoors, the researchers said.

“The consistent increase in exposed area (of close to 43 million sq km above 31°C can adversely impact the labour-intensive work during the monsoon break,” the authors wrote in the study published in the journal Earth’s Future.

Humidity is the villain

The relatively higher humidity levels are what make the monsoon breaks feel hotter. Global warming is causing temperatures to increase, not just during summers, but across seasons. And humidity affects the human body's ability to fight off the heat.

On a monsoon day, a temperature of 35°C can cause the human body to heat up faster than under the same temperature on a dry summer day. Sweating helps our bodies regulate temperature. When it's hot, we sweat, and when the sweat evaporates, it takes away some of the heat and helps cool our bodies.

(a) Surface area experiencing over 31°C moist heat for at least six continuous hours from 1951 to 2020
(a) Surface area experiencing over 31°C moist heat for at least six continuous hours from 1951 to 2020

But when the air is humid, it is already saturated with moisture and cannot absorb any more moisture. As such, sweat evaporates very slowly on a hot and humid day, thereby keeping our bodies hotter for a longer period. This can lead to heat exhaustion or heat stroke if precautions aren’t taken.

Wet-Bulb Globe Temperature

The Cornell researchers found that the Indo-Gangetic plains and the eastern coast of India were most vulnerable to moist heat extremes with wet-bulb temperatures reaching 38°C. The Wet-Bulb Globe Temperature (WBT) is used to estimate the heat stress under direct sunlight. It is measured by wrapping a wet cloth over the bulb of a thermometer. The evaporation of water from the cloth cools the thermometer, causing the mercury to dip. But when the air is humid, evaporation is very slow. In such cases, mercury readings remain closer to the dry temperature, which is on the higher side.

The wet bulb method is used to calculate heat stress in humans exposed to direct sunlight. Researchers agree that a wet-bulb temperature of 35°C is the safe limit for humans. Zach Schlader, a physiologist at Indiana University Bloomington told MIT Technology Review that beyond this point, human bodies won’t be able to lose heat sufficiently to maintain their core temperature.

Call to protect outdoor workers

The study showed that people in the Indo-Gangetic plain and eastern coast are experiencing humid heat beyond the safe limits for human survivability.

It highlighted, in particular, the case of agriculture and construction workers, who are exposed to direct sunlight for much of their working hours. In contrast, summer has minimal impact on the area exposed to extreme moist heat.

The researchers called for a revision of outdoor working hours, especially in regions experiencing extreme moist heat during monsoon.

They also highlighted how rising temperatures negatively affect labour productivity. “For instance, a 3°C increase in global warming can reduce labour productivity by 7% and contribute to at least 4% reduction of GDP in India, which leads to inflation in crop prices,” the authors warned.

(B) Population exposed to over 31°C moist heat for at least six continuous hours during 1960 to 2020; and 
(c)Map of the extreme moist heat risk for 1951 to 2020
(B) Population exposed to over 31°C moist heat for at least six continuous hours during 1960 to 2020; and (c)Map of the extreme moist heat risk for 1951 to 2020

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