India Isn’t Ready for a Deadly Combination of Heat and Humidity

R Lakshmanan has been making steel frames in the southern Indian city of Chennai for 20 years. His job involves standing for long hours outdoors at construction sites, pounding screws with careful precision onto steel rods. Each day he makes nearly 600 frames, which end up becoming the skeleton of a home. Often he works 12-hour shifts, beginning at 6 am. He always feels fortunate when he gets to work under a shady tree.

But this year, that protection hasn’t been enough. Ever since temperatures in March hit a sizzling 38 degrees Celsius—4 degrees above normal for Chennai—the conditions have been stifling. The metal frames Lakshmanan works with have been too hot to touch, the steel burning his fingertips and leaving behind painful sores. He has seen construction workers, especially women, collapse around him, and has had to take breaks during the workday to cope with fits of dizziness and nausea. “On some days, there’s so much heat, it feels like you’re living in a fireball,” he says.

When faced with these conditions, our bodies call upon a well-known mechanism to keep us from overheating: sweating. As perspiration evaporates from the skin, it cools the body’s temperature. But if the air is not only hot but also already filled with moisture, less sweat can evaporate, and this safety feature fails. In India, high temperatures and humidity are increasingly combining to pose a deadly threat—one the country isn’t prepared for.

This danger to human life is measured using “wet-bulb temperature”—the lowest temperature that air can be cooled to via evaporation. It’s determined by wrapping the bulb of a thermometer in a wet cloth and seeing what temperature is recorded. Essentially the bulb is you—or me, or Lakshmanan—the wet cloth is our sweating skin, and the temperature recorded is the coolest we can hope to get by sweating.

When heat and humidity combine to push wet-bulb temperatures past 32 degrees Celsius, physical exertion becomes dangerous. Consistent exposure to high wet-bulb temperatures—35 degrees Celsius and above—can be fatal. At this point the sweating mechanism shuts down, leading to death in six hours. On May 1, 2022, the wet-bulb temperature in Lakshmanan’s home city of Chennai hit 31 degrees Celsius. The same day, the district of Ernakulam in the Indian state of Kerala recorded a wet-bulb temperature of 34.6 degrees Celsius—a record high for the area.

“Without the mechanism to rid the body of that excessive heat, there are many physiological changes that happen in quick succession,” says Vidhya Venugopal, a researcher in public health at the Sri Ramachandra Institute of Higher Education and Research in Chennai.

Raise your internal temperature by 3 to 4 degrees Celsius, and you’ll start to struggle. “As the body tries hard to restore your core temperature, all other processes slowly grind to a halt,” Venugopal says. Blood vessels dilate and circulation slows, particularly to the extremities. Not enough blood will flow to the brain, affecting its functioning. You lose alertness, become drowsy, and don’t feel thirst anymore. Soon organs shut down, one by one. “When the brain stops giving messages to the heart, the pulse slows and the person goes into a coma,” she says.

“Humidity aggravates the killing power of heat,” says Ambarish Dutta, professor of epidemiology at the Indian Institute of Public Health in Bhubaneswar. “It can trigger catastrophic events like heart attacks and strokes, aggravate secondary conditions like diabetes, change the regulatory capacity of the kidneys, affect the endocrine system by triggering stress hormones. In short, it’s a silent killer.”

World Weather Attribution, an international collaboration that analyzes extreme weather events, estimates that India and Pakistan’s recent heat wave has led to at least 90 deaths across both countries. During India’s 2015 heat wave, wet-bulb temperatures in the southern state of Andhra Pradesh rose to 32 degrees Celsius. That year, the heat killed over 2,500 people.

Such events are going to become increasingly common as climate change warms the world. What magnifies the problem is that as temperatures rise, so does the absolute humidity in the atmosphere, says Jane Baldwin, assistant professor in the Department of Earth System Science at the University of California, Irvine. Thanks to what’s known as the Clausius–Clapeyron relationship of thermodynamics, “for every 1-degree increase in temperature, you see a 7 percent increase in humidity,” she explains. It means that for countries like India, climate change has a compounding impact. The effect is strongest over the world’s oceans, and particularly the Indian Ocean, whose rapid warming is a big trigger of South Asia’s high wet-bulb temperatures.

In response, the Indian government has had an expanding heat action plan in place since 2013. Its biggest feature is an early warning system for forecasting heat waves in cities across the country, with alerts aired by the media. But beyond this, safeguards for people vary significantly across the country. Some states have systems for closing workplaces and schools when temperatures are high, some for distributing rehydration salts and offering more drinking water to the public. Others have plans to increase tree cover in cities and to fit buildings with reflective roofs that keep their interiors cooler. The latter, though, are long-term actions that have yet to be properly implemented.

One additional proposal is the idea of the “cooling room”—an air-conditioned space in a hospital where people with heat exhaustion can recover. But there are no significant coordinated nationwide efforts yet to help vulnerable, low-income workers access these. It’s also rare for employers to provide any kind of health insurance specifically for heat stress or to institute emergency care in case of heat stroke, especially in the unorganized labor sector, which contains over 700 million workers in India.

In the face of this, academics like Venugopal are researching what else can be done. Working with about 120 companies, she’s focusing on how industries can reduce heat exposure for their workers. Recommendations include engineering solutions—such as cooling workspaces with air conditioners—while in industries that involve additional heat exposure, such as glass welding or steel-making, she suggests additional tools be used, like heat shields, special uniforms, or even vests piped with cooled air or fitted with ice packs. While these may sound outlandish, they are “standard industrial hygiene practices,” Venugopal says. “But they need to be adopted more widely.”

Where technical solutions can’t be used, there are still administrative controls that could help, she adds—like frequent breaks for people working under heat wave conditions. The body has another protective mechanism—exhaustion—that tells you to rest when overheating, but workers may not always have the opportunity to stop working, she says. In such cases, Venugopal recommends that two people do a job on rotation if it requires heavy exertion or being outdoors.

But the challenge lies in getting buy-in across India’s many cottage and small-scale industries. Until the government adopts stringent mandates requiring employers to protect their staff from extreme heat, individual businesses can choose whether to adopt or ignore suggestions coming from advisers like Venugopal.

It’s also not just workers who need to take care when wet-bulb temperatures are high. Rising humidity and heat lift nighttime temperatures as well, which affects everyone. “When the humidity rises, the temperature doesn’t drop quite so fast at night,” says Steven Sherwood, a professor at the University of New South Wales Climate Change Research Center in Australia. When the sun goes down, infrared radiation disperses some of the heat that has built up on Earth during the day. “When humidity is higher, there is greater cloud cover, which acts like a blanket preventing that escape of heat,” he says.

At night the body should recover from the daytime assault of heat, but because nights are getting hotter, that recovery is hampered, says Dutta. Whenever people talk about the effects of heat, they usually refer to its direct effects—such as heat exhaustion and stroke, which can be fatal or debilitating—but these are only the tip of the iceberg, he says. “If heat stays high in the night, it affects the body’s homeostasis, its ability to regulate and maintain its internal body temperature.” Upset this and your cellular and metabolic activities become disrupted, which can be a driver of disease, and can even be fatal itself. This is a big concern, given that only an estimated 8 percent of Indian households have access to air conditioning.

Analysis by World Weather Attribution suggests that climate change has made deadly weather events in South Asia 30 times more likely than they used to be. In the pre-industrial age, extreme heat waves would crop up once every 3,000 years. Now the probability is once every 100 years. Across India, on average nine heat waves were recorded every year from 1980 to 1999. The average between 2000 and 2019 is almost triple that, at 23.

South Asia is also not the only area at risk. Potentially fatal mixtures of heat and humidity have been increasing around the globe. Coastal cities on the Persian Gulf seem particularly susceptible to very high wet-bulb temperatures in the future, says Luke Harrington, senior research fellow in climate science at the New Zealand Climate Change Research Institute. According to data from NASA, other countries will experience more critical wet-bulb temperatures in the future too, including the United States. States such as Arkansas, Missouri, and even Iowa are at risk. And while some places may have more resources to handle the issue, people outside of India might not be so adapted to cope.

We can’t be certain that dangerous temperature thresholds are going to be breached more frequently around the world—but it is likely, says Sherwood. “At 3 to 4 degrees Celsius of global average warming, a lot of places will experience more fatal wet-bulb temperatures, which will lead to more deaths,” he says, referring to the amount of warming the world is likely to see this century if climate action isn’t taken. If this is the future that’s ahead of us, then how heat increases in India—and is handled—could be a lesson for what the rest of the world should expect.

For Lakshmanan, the heat remains an immediate problem. He can’t afford to let it affect his concentration, he says. If he’s off the mark by even half an inch while crafting his metal frames, all of the material he uses is wasted, and he’s responsible. “There are machines to do my job now, so I need to be alert regardless of the conditions I work in,” he says. “But this summer especially, it’s been really hard. It has tested the limits of our endurance.”

India Isn’t Ready for a Deadly Combination of Heat and Humidity

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