R Lakshmanan has been manufacturing steel frames in Chennai, southern India, for 20 years. His job consists of standing outdoors on construction sites for hours, driving screws onto steel bars with careful precision. Every day he makes almost 600 frames that eventually become the skeleton of a house. He often works 12-hour shifts, starting at 6 a.m. He always feels comfortable when he is allowed to work under a shady tree.
But this year, that protection wasn’t enough. Since the temperatures in March were a crackling 38° Celsius –4° above normal for Chennai– The conditions were oppressive. The metal frames Lakshmanan works with were too hot to touch, the steel burning his fingertips and leaving painful sores. He has seen construction workers, particularly women, collapse around him and has had to take breaks during the workday to cope with bouts of dizziness and nausea. “Some days it’s so hot it’s like living in a fireball,” he says.
Under these conditions, our body resorts to a well-known mechanism to keep us from overheating: sweating. As sweat evaporates from the skin, it cools the body temperature. If the air is not only hot, but also already filled with moisture, less sweat can evaporate and this safety function fails. In India, high temperatures and humidity are increasingly combining to create a deadly threat that the country is unprepared for.
This threat to human life is measured by the “wet bulb temperature” – the lowest temperature to which air can be cooled through evaporation. It is determined by wrapping the bulb of a thermometer in a damp cloth and seeing what temperature is recorded. Basically you – or me or Lakshmanan – are the lightbulb – the wet cloth is our perspiring skin and the temperature recorded is the coolest we can reach by sweating.
When heat and humidity combine to raise wet-bulb temperatures to over 90 degrees Fahrenheit, physical exertion becomes dangerous. Constant exposure to high wet-bulb temperatures—35°C and above—can be deadly. At this point The sweating mechanism shuts down, leading to death within six hours. On May 1, 2022, the wet-bulb temperature is in Lakshmanan’s hometown of Chennai reached 31° Celsius. On the same day, a wet-bulb temperature of 34.6 degrees Celsius was measured in the Ernakulam district of the Indian state of Kerala – a record high for the region.
“Without the mechanism to rid the body of this excessive heat, many physiological changes occur in quick succession,” says Vidhya Venugopal, a public health researcher at the Sri Ramachandra Institute of Higher Education and Research in Chennai.
Raise your internal temperature by 3° to 4° Celsius and you’ll start struggling. “As the body tries to restore your core temperature, all other processes slowly grind to a halt,” Venugopal says. Blood vessels dilate and blood flow slows down, especially in the extremities. Not enough blood flows to the brain, affecting its function. They lose attention, become sleepy and no longer feel thirsty. Soon the organs fell silent, one after the other. “When the brain stops sending messages to the heart, the pulse slows down and the person goes into a coma,” she says.
“Moisture amplifies the lethal effects 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 diseases like diabetes, alter the kidneys’ ability to regulate, 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 the recent heatwave in India and Pakistan has resulted in at least 90 deaths in both countries. During the 2015 heat wave in India, 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 becoming more frequent as climate change warms the world. Compounding the problem is that as temperatures rise, so does the absolute humidity in the atmosphere, says Jane Baldwin, an 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° increase in temperature, you see a 7 percent increase in humidity,” she explains. This means that climate change is amplifying for countries like India. The effect is strongest over the world’s oceans and particularly over the Indian Ocean, whose rapid warming is a major trigger for the high wet-bulb temperatures in South Asia.
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