NEW DELHI — On a sweltering Wednesday in June, a thin woman named Rehmati gripped the doctor’s table with both hands. She could hardly hold herself upright, the pain in her stomach was so intense.

She had traveled for 26 hours in a hot bus to visit her husband, a migrant worker here in the Indian capital. By the time she got here, the temperature was 44 degrees Celsius, and she was in an emergency room.

The doctor, Reena Yadav, put Rehmati, 31, on a drip. She told her she would be fine. Rehmati leaned over and retched.

Extreme heat can kill, as it did by the dozens in Pakistan in May. But as many of South Asia’s cities get even hotter, scientists and economists are warning of a more far-reaching danger: Extreme heat is devastating the health and livelihoods of tens of millions more.

If greenhouse gas emissions continue at their current pace, they say, heat and humidity levels could become unbearable, especially for the poor.

It is already making them poorer and sicker. Like the Kolkata street vendor who squats on his haunches from fatigue and nausea. Like the woman who sells water to tourists in Delhi and passes out from heatstroke. Like the women and men with fever and headaches who fill emergency rooms. Like the outdoor workers who become so weak or so sick that they routinely miss days of work, and their daily wages.

«These cities are going to become unlivable unless urban governments put in systems of dealing with this phenomenon and make people aware,» said Sujata Saunik, who was a senior official in the Indian Ministry of Home Affairs and is now a fellow at the Harvard University School of Public Health. «It’s a major public health challenge.»

A recent study in several of South Asia’s largest cities found that if warming continued, by the end of the century, wet bulb temperatures — a measure of heat and humidity that can indicate the point when the body can no longer cool itself — would be so high that people could not survive for six hours or more.

A heat wave in Ahmedabad, in western India, in May 2010resulted in a 43 percent increase in mortality, compared with the same period in previous years, a study found. Since then, city-funded vans distribute free water in the hottest months. In the eastern coastal city of Bhubaneswar, parks are kept open so outdoor workers can sit in the shade.

Across the region, a recent World Bank report concluded, rising temperatures could diminish the living standards of 800 million people. Worldwide, among the 100 most populous cities where summer highs are expected to reach 35 degrees by 2050, 24 are in India, the Urban Climate Change Research Network said.

Rohit Magotra, deputy director of Integrated Research for Action and Development, is trying to help Delhi respond to the danger. The first step is to quantify its human toll. «Heat goes unreported and underreported. They take it for granted,» he said. «It’s a silent killer.»

On a blistering Wednesday, workers covered their faces with bandannas as they built a freeway extension in Delhi. The sky was hazy with dust. Skin rash, dry mouth, nausea, headaches were everyday ailments, the workers said. Every 10 to 15 days, they had to skip a day of work and lose a day’s pay. By 2030, extreme heat could lead to a $2 trillion loss in productivity, the International Labor Organization estimated.

Joyashree Roy, an economist at Jadavpur University in Kolkata, found that most days in the summer are too hot and humid to be doing heavy physical labor without protection.

Yet, people pedaled bicycle rickshaws, hauled goods on their heads and built towers of glass and steel. Only a few people are protected in air-conditioned homes and offices.

Researchers are seeking solutions. In Ahmedabad, city funds have been used to slather white reflective paint over several thousand tin-roofed shanties, bringing down indoor temperatures. In Hyderabad, a similar effort is being tested.

Rajkiran Bilolikar, who led the cool-roof experiment, said that as a child, he would visit his grandfather in Hyderabad. There were trees all over. He could walk, even in summer.

Now a professor at the Administrative Staff College of India in Hyderabad, he can’t walk much. His city is hotter. There are fewer trees. Air-conditioners have proliferated, but they spew hot air outside.

Mr. Bilolikar had resolved not to use his air-conditioner. But through his open windows, his neighbor’s air-conditioner blew hot air into his apartment. His 3-year-old daughter became so overheated that her skin was hot to touch.

Reluctantly, he shut his windows and turned his air on.

Warning that cities will be unlivable as temperatures rise.

© 2018 The New York Times