Climate change and the rapid evolution of the insect have helped drive up malaria deaths and brought dengue and other mosquito-borne viruses to places that never had to worry about them.

By Stephanie Nolen Stephanie Nolen reported this story from five countries in Africa and Latin America, part of a special project looking at the fight against mosquito-borne disease.

Along hundreds of miles of Lake Victoria’s shoreline in Kenya, a squadron of young scientists and an army of volunteers are waging an all-out war on a creature that threatens the health of more people than any other on earth: the mosquito.

They are testing new insecticides and ingenious new ways to deliver them. They are peering in windows at night, watching for the mosquitoes that home in on sleeping people. They are collecting blood — from babies, from moto-taxi drivers, from goat herders and from their goats — to track the parasites the mosquitoes carry.

But Eric Ochomo, the entomologist leading this effort on the front lines of global public health, stood recently in the swampy grass, laptop in hand, and acknowledged a grim reality: “It seems as though the mosquitoes are winning.”

Less than a decade ago, it was the humans who appeared to have gained the clear edge in the fight — more than a century old — against the mosquito. But over the past few years, that progress has not only stalled, it has reversed.

The insecticides used since the 1970s, to spray in houses and on bed nets to protect sleeping children, have become far less effective; mosquitoes have evolved to survive them. After declining to a historic low in 2015, malaria cases and deaths are rising.

Climate change has brought mosquitoes carrying viruses that cause dengue and chikungunya, excruciating and sometimes deadly fevers, to places where they have never been found before. Once a purely tropical disease, dengue is now being transmitted in Florida and France. This past summer, the United States saw its first locally transmitted cases of malaria in 20 years, with nine cases reported, in Texas, Florida and Maryland.

“The situation has become challenging in new ways in places that have historically had these mosquitoes, and also at the same time other places are going to face new threats because of climate and environmental factors,” Dr. Ochomo said.

Scientists around the world are pressing hard for new solutions, including novel technologies that Dr. Ochomo is testing. They have developed some promising approaches, including a new generation of tools that modifies mosquitoes biologically, and genetically, to block disease.

But such efforts have been stymied by cost and regulatory hurdles. The process for getting any of these tools to the places where children fall ill with each new season of rain involves years of testing and regulatory reviews that are painfully slow and badly underfunded.

“It’s just ridiculous how much time we are wasting before we can get into the field and actually start saving lives,” said Bart Knols, a Dutch vector biologist who runs mosquito-borne disease elimination projects across Africa, Asia and the Caribbean.

The biggest killer

Malaria has killed more people than any other disease over the course of human history. Until this century, the battle against the parasite was badly one-sided. Then, between 2000 and 2015, malaria cases dropped by a third worldwide, and mortality decreased by nearly half, because of widespread use of insecticide inside homes, insecticide-coated bed nets and better treatments. Clinical trials showed promise for malaria vaccines that might protect the children who make up the bulk of malaria deaths.

That success lured new investment and talk of wiping the disease out all together.

But malaria deaths, which fell to a historic low of about 575,000 in 2019, rose significantly over the next two years and stood at 620,000 in 2021, the last year for which there is global data.

There were more cases of dengue in Latin America in the first half of this year — more than three million — than in all of 2022. Bangladesh is in the grip of its largest-ever dengue outbreak, with 120,000 cases by the end of August. Cases of and deaths from chikungunya and other mosquito-borne infections have started rising, too, in many regions of the world.

One of the main reasons is that mosquitoes are highly adaptable. As more and more people are protected by nets or sprays at home, mosquitoes have begun to bite more outdoors and in the daytime, instead of indoors and at night, the historic pattern of the malaria vector species in Africa. Because the genetic makeup of mosquitoes evolves quickly in response to changing environmental conditions, they have also developed resistance to the class of insecticides in wide use — while the malaria parasite itself is increasingly resistant to the once highly-effective drugs used to treat it.

And a new mosquito that thrives in urban areas has come from Asia to Africa, where the spread of malaria had always been confined largely to the countryside. That change has made more than 100 million additional people vulnerable to mosquito-borne infections, researchers at the University of Oxford recently estimated.

The multiplying risks, experts say, mean there’s an urgent need for a method to protect people from all mosquitoes — one that will help defend against malaria, but also dengue, yellow fever and whatever pathogen lurks around the corner. (Only female mosquitoes bite; they need the protein in blood to produce eggs.)

But it takes a decade or more to design, develop, test and produce a new technology or intervention. Contrast that with the six-week life spans of mosquitoes, which are constantly evolving to elude the ways we try to kill them.

The bulk of the money for these efforts to date has come from high-income nations and private philanthropists, but funding levels have plateaued. Multiple researchers said it was increasingly difficult to motivate the kind of investment they need for large-scale trials of new methods.

“Sooner or later, funding bodies are going to divert that money to other things,” Dr. Knols said. “They’re going to say, ‘We’re putting it into agriculture, or into schooling.’”

A new problem

In the towns and villages of Busia County in Kenya, the roads begin to fill while the sky is still the streaky purple of dawn, with farmers on the way to their fields, children in freshly pressed uniforms walking to school and moto-taxi drivers reporting to the market.

Dr. Ochomo’s research has found that the mosquito Anopheles funestus is feasting on them: The species, once thought to bite mostly sleepers in their beds at night, now bites outside in the daytime.

Four in 10 people on these red-dirt roads are carrying the malaria parasite, even if they don’t have symptoms, studies by Dr. Ochomo and his colleagues have found. Some outdoor and daytime biting was likely happening all along, but no one was really tracking it because the focus was on the vulnerable sleepers.

Twenty years ago, in the early days of the mass distribution of bed nets, malaria case rates plunged immediately, and there was an optimism that the nets might be enough, said Audrey Lenhart, the chief of entomology at the U.S. Centers for Disease Control and Prevention. Instead, she said, they have helped to create a new problem.

“Think about it: You put bed nets everywhere, then the mosquitoes that bite people inside, they’re going to die out,” Dr. Lenhart explained. “The ones that are going to survive are the ones that are biting people sitting outside, biting livestock, the ones that aren’t in the houses, right? So then those are the ones that are reproducing and keeping the population of mosquitoes there.”

Insecticides that don’t work

Most of the current insecticides in use are pyrethroids, which were developed in the 1970s and derived from the chemical compounds in an ancient mosquito deterrent made by crushing aster flowers. They have been used for everything, including bed nets and for spraying on walls.

With mosquitoes around the world now highly resistant to them, there is an urgent search for something new.

In 2005, the Bill & Melinda Gates Foundation invested $50 million in a project called the Innovative Vector Control Consortium to search for effective insecticide compounds. The consortium asked large agrochemical companies to look in their chemical libraries for molecules that might affect mosquitoes in novel ways and be safe and durable enough.

“We started with four-and-a-half-million compounds, and we’re down to four,” said Nick Hamon, who recently retired as the consortium’s chief executive.

To work as a new insecticide, compounds have to be safe for humans, come in solid form and not be water soluble. And they have to kill mosquitoes in a substantively different way than pyrethroids do, because mosquitoes develop resistance not just to an individual chemical, but to the way the chemical kills them.

Companies must now complete the safety and testing process on the only four compounds that tick all those boxes. It is intensely costly and slow, said Susanne Stutz, the chief chemist at the German chemical company BASF.

“It’s always a race with the mosquito, who is faster: Usually, the mosquito wins because it develops the resistance much faster than new products come out,” she said.

The World Health Organization requires two large randomized clinical trials, carried out in two different geographic and epidemiological settings, showing a significant positive impact on public health in both, before it will recommend use of a new intervention against mosquitoes. The organization says the policy is designed to make sure that countries with limited means are making the best decisions about where to spend their money and to ensure that products are backed by rigorous evidence. Yet the world’s multiplying mosquito problems increasingly require solutions tailored to specific situations: What works to protect children in the African Sahel won’t be what works to protect loggers in forests in Cambodia.

Dr. Ochomo is the principal investigator on two large randomized clinical trials of mosquito interventions. In one $33 million project, researchers are testing the effectiveness of spatial repellents — squares of plastic film that can be hung on the walls inside homes and that dispense low doses of a chemical that confuses mosquitoes and prevents them from biting — in both dengue and malaria risk areas.

S.C. Johnson & Son Inc., the Wisconsin-based company that developed the spatial repellent being tried in Kenya, has donated millions of dollars in products for testing. Such largess is unusual — and not a sustainable pathway for vector control research, said John Grieco, a professor of biological sciences at the University of Notre Dame who coordinates the multicountry spatial repellent trial, which is also running in countries including Mali and Sri Lanka.

The spatial repellents and most other new tools are commodities: items that have to be bought, then bought again six months or a year later. The protection they offer is temporary, as is the funding that allows for their purchase.

The Gates Foundation, the major backer of most of the clinical trials of the commodities, has also had to cover most of the cost of BASF’s testing of new chemicals for use on bed nets, because there is not sufficient profit incentive for a private firm to do it, Dr. Stutz said.

“How do you keep the companies that know how to innovate in this space?” Dr. Hamon asked. The Innovative Vector Control Consortium lost one of its key industry partners in 2017.

“They just said, ‘We can make more money somewhere else,’” Dr. Hamon said.

Some experts believe the emergence of dengue fever, and now malaria, in middle- and high-income countries could generate new funding because it creates a wealthier market that may spur new corporate investment.

Skeptics in the entomology world look at the race for new commodities, and suggest it ignores a lesson from history: They say that only the same strategies that high-income nations used more than a century ago will once again give humans the edge over mosquitoes. In Europe and the America in the early 1900s, and in Brazil in the 1930s, it was large-scale environmental management and changes in housing that made a difference, said Silas Majambere, a Burundian vector ecologist who has worked all over Africa, Europe and Asia.

That means draining breeding sites, spraying larvicides (which are biological toxins and don’t cause resistance) on water that can’t be drained and moving homes out of swampy areas. To protect against urban mosquitoes, people need screened windows and solid walls and roofs with closed eaves: better houses.

“If we had spent the last 40 years doing these things, with the same budget, where would we be today?” Dr. Majambere said.

Hope and challenges

While malaria cases are far lower than they were 20 years ago in the Busia area, the stalled progress means the disease continues to erode family health, income and futures.

“When there is a malaria case in the house, it suspends life,” said Mary Oketeti, a farmer who lives about an hour’s drive outside the town of Busia. She gets malaria three times a year, and her 12-year-old daughter twice as often as that. The family then has to spend what is needed for treatment.

“If there’s a chicken in the house you sell it,” she said.

A chicken might be worth 600 Kenyan shillings, or $5; a trip to the medical clinic, with transport, a diagnostic test and drugs for malaria, will cost at least that much. Repeated bouts of malaria keep children out of school and adults from working; they wipe out savings. Ms. Oketeti said she must stay home from the fields she farms to care for a sick family member for a few days every month.

Dr. Ochomo and his team recently received data from the midpoint of the clinical trial of spatial repellents. Malaria cases were significantly lower in families that had them compared to those that had devices that used placebo repellents. If that trend holds, the next challenge will be convincing the W.H.O. to endorse the use of spatial repellents, then the Kenyan government to buy them.

It won’t be hard to convince people in Busia to use them, though, he said.

“People already know that nets are not enough, they need something more, and they’re happy to see us,” he said. “They say, ‘Finally, someone is coming to try to help with this.’”

Extreme weather has decimated crops and incomes in northern India. To soothe their anxiety, agricultural workers are turning to tobacco and alcohol.

Sanket Jain Sep 27, 2023 2:00 AM

Kamal Sonavane knew she’d pass out if she chewed smokeless tobacco one more time. It was a scorching April afternoon in the middle of another of India’s brutal heat waves, and with no job to go to, the farmworker had already chewed tobacco five times that day. “Even an addicted person avoids doing this in extreme heat because there’s a risk of fainting,” she says.

Yet Sonavane repeated the familiar ritual: adding the slaked lime to the tobacco leaves, then putting the mixture in her mouth. “I would have anyway collapsed, either because of the heat waves or the mounting stress,” she says, sitting in her two-room brick house in Bhadole in the Indian state of Maharashtra. Anxious about money, her lack of work, and the extreme heat, she turned to the tobacco once again.

Climate change is making farming in Maharashtra harder. This in turn impacts day laborers, who are hired when agricultural help is needed. “Every few months, farmers report losses caused by heat waves or floods,” says community health worker Shubhangi Patil, who serves the Kolhapur district where Sonavane lives. When crops fail, earnings become more precarious, and farm laborers “resort to substance use to forget their problems,” says Patil. It’s a prevailing issue across the region, Patil says.

It’s also a phenomenon that isn’t limited to India—or to countries with predominantly low- and middle-income wages. Research from other regions has found groups responding to the pressures of climate change by increasing their consumption of alcohol and other substances, with potentially deleterious effects on their health.

A landless farmworker in her mid-60s, Sonavane has been toiling in the fields of Kolhapur for over 25 years. A decade earlier, she says, she didn’t chew smokeless tobacco. “I despised it,” she says. “Today I can’t stay even a few hours without it.”

The weather, she says, started to get bad in western Maharashtra in 2019. “This region has seen two floods, unbearable heat, incessant rainfall, hail storms, and a drought,” all in the past three years, Sonavane says. Farmers have faced tremendous losses: 36 million hectares of sugarcane, onions, rice, and other crops lost over the past five years, according to Maharashtra’s department of agriculture. Farm workers are currently finding it difficult to get even eight days of work a month because crop damage is so common, Sonavane says.

With no resources for dealing with the stress of being out of work, Sonavane stumbled across the solution of soothing her anxiety with smokeless tobacco, which costs just 10 rupees ($0.12) a packet. Like cigarettes and vapes, chewing tobacco contains nicotine, a central nervous system stimulant. Users say it elevates their mood; improves concentration; and relieves anger, tension, and stress. “They desensitize grief, sadness, and negativity for a while,” says Kolhapur-based clinical psychologist Shalmali Ranmale Kakade, referring to tobacco and other commonly abused substances, such as alcohol.

But nicotine is also highly addictive, and in heavy tobacco users, those positive effects may simply be the consequences of staving off withdrawal. Repeatedly chewing tobacco also causes many types of cancer—including those of the mouth, oesophagus, stomach, and bladder.

Sonavane has become so accustomed to using tobacco that she now has fixed times in the day for taking it. But if she’s particularly stressed, she’ll chew it more frequently. “After trying it out a few times, people start associating liquor, tobacco, or any form of abuse as relief, which eventually becomes a loop,” says Ranmale Kakade. Health worker Patil says she’s witnessed people using substances to manage climate stress in at least 200 villages across her district.

The climate crisis has multiple ways of pushing people toward using mood-altering substances. Research published in the journal Perspectives on Psychological Science notes that people may worry about climate change itself, or feel anxious about the destabilizing effect it can have economically and socially. Changes to the weather—in particular, more days of extreme heat—can also impact people’s physical and mental health, driving them to self-medicate with substances that they can access. A shifting climate also has the power to alter people’s established behavior patterns, which might allow the formation of new, unhealthy habits.

And India is not the only country where this is taking place. In 2019 and 2020, Australia experienced severe bushfires that burned 19 million hectares of land and over 3,000 homes, killing more than 56,000 livestock animals. A paper published earlier this year, which assessed the mental health of 746 Australians aged 16-25, found “greater presentations of depression, anxiety, stress, adjustment disorder symptoms, substance abuse, and climate change distress and concern,” even among participants who weren’t directly exposed to the fires.

Another paper published this April in the Journal of Environment Management found a relationship between heat waves, declining mental health, and alcohol consumption in Chinese people aged 50 and above. Looking at a sample of China’s population, it concluded that extreme heat led to more frequent drinking, with people reducing positive behaviors, such as exercising. But the total extent of climate-related substance abuse worldwide is currently unknown, limited by the relatively small amount of research that’s been done on the topic.

Once a bad climate event has passed, people’s behaviors don’t necessarily revert to how they were before. “Exposure to climate-related stressors—such as storms, floods, wildfires, and droughts—can lead to physical and psychological distress that often persists for months after the event,” says Francis Vergunst, an associate professor of psychosocial difficulties at the University of Oslo and a coauthor of the Perspectives paper.

Vergunst notes that in most cases, people’s new habits won’t tend to meet the diagnostic criteria for a substance-use disorder. However, the substance-use behaviors will still be harmful to the individual, their family, or the community.

Balaso Thorwat, who is in his mid-70s, lives in Khochi, a village in Kolhapur. It has flooded frequently in recent years. After losing vegetables worth $350 to flooding in 2019 and 40,000 kilograms of sugarcane worth over $1,400 in 2021, Thorwat was extremely stressed. “I was unable to sleep because of the mounting losses,” he says.

Thorwat went to the nearby public doctor, who offered him only the most basic advice: “Don’t stress.” With no one to talk to about his problems, it was easier for him to turn to country liquor—a type of locally made alcohol sold informally, often made from fermented sugarcane molasses, which is then distilled. “Whenever I feel stressed, I drink liquor. At least I can sleep peacefully after that,” he says. When I asked him how often he gets stressed, he replied: “Daily.” When alcohol didn’t help, he started chewing smokeless tobacco as well.

“Even teenagers are now seen resorting to substance abuse,” says Shubhangi Kamble, a health care worker from Arjunwad, another flood-affected village in Kolhapur, noting that in her observations, the issue has become much more prominent in the past three years. After speaking to over a hundred people from drought- and flood-affected villages, Kamble found a pattern: “There’s a lot of stress because people aren’t able to understand how the climate will keep changing.”

During the first nine months of 2022, India faced climate disasters on nine out of 10 days. And research released in April this year found that more than 90 percent of India’s population—over 1 billion people—are now vulnerable to the adverse effects of heat waves. But while the impact of climate disasters is large and growing, the country’s capacity to help those driven toward addiction is insufficient. A 2019 government report acknowledged that the “reach of the national programs for the treatment of substance-use disorders is grossly inadequate.”

India also doesn’t have enough mental health professionals. For the 833 million people who live in rural areas, the country has just 1,224 subdistrict hospitals and 764 district hospitals. The latest World Health Organization statistics show that per million people, India has only three psychiatrists working in the mental health sector, and fewer than one psychologist. When Sonavane was feeling the acute stress of the heat wave back in April, she says she wasn’t able to get any help or counseling. This was the same for many other farmworkers affected by the heat wave, say the health workers WIRED spoke to.

To be treated and prevented, climate-related substance abuse needs to be better recognized, and for that, clear evidence of the existence and size of the problem is required. In India and elsewhere, there’s a lack of data to establish a direct link between climate change and substance abuse—but this could be fixed. “Examine hospital admissions data for substance use-related problems before, during, and after a severe weather event,” suggests Vergunst. “Another option is to use survey data to track substance-use patterns alongside climate-related stressors such as heat waves or floods.”

Until these efforts are made, climate-related substance abuse will continue as an underreported and undertreated health problem. Sonavane and Thorwat know the health risks of heavy use of substances like alcohol and tobacco. But the stress they are dealing with outweighs these concerns. “I’ve spent my entire life working in the soil,” says Sonavane. “The stress will only go away after I die finding work in this soil.”

Tens of millions of Americans are struggling to cope with soaring temperatures. OSHA is developing new workplace safety standards, but they are not yet complete.

By Noah Weiland Reporting from Terminal C of the Dallas Fort Worth International Airport. Oct. 5, 2023, 3:00 a.m. ET

Anthony Soto, a 22-year-old baggage claim employee at the Dallas Fort Worth International Airport, crumpled to the floor near gate C15 after a seizure last October that he attributed to hot indoor conditions and strenuous lifting. In record-setting heat in Texas this past summer, Mr. Soto, who has epilepsy, had four more seizures that left him speechless, his body unresponsive, he said.

His blue button-down shirt was streaked with sweat on a recent sweltering day as the temperature again neared 105 degrees. Working in such heat “makes us feel unwanted, unhelpful and unworthy,” he said. “The only thing that matters is how long it takes to scan bags.”

Scientists say the record heat this summer was fueled by climate change and that heat waves are likely to grow more intense. But there are few safeguards for tens of millions of workers increasingly exposed to rising temperatures on the job.

The Biden administration is taking steps to create new rules for employers, with two key steps expected in the coming months. A handful of states have put in place standards for work in extreme heat, including California, which requires employers to allow outdoor workers to rest in the shade in temperatures above 80 degrees.

But in other states, workers like Mr. Soto, who makes $15 an hour, continue to suffer as extreme heat spans the summer months and the early fall. Dallas endured a record number of September days with triple-digit temperatures.

“The worst-performing states are just not going to do it on their own,” said Dr. Rosemary Sokas, an occupational health expert at Georgetown University who co-wrote a recent article in The New England Journal of Medicine on the dangers now faced by workers in absence of a federal regulations.

Prodded in 2021 by President Biden, the Occupational Safety and Health Administration is drafting guidelines for indoor and outdoor work in heat, which could allow the federal government to fine employers that violate its recommendations.

But OSHA is still plodding through a labyrinthine rule-making process. The agency is required to go through nearly 50 steps, most of which are mandated by executive orders or by congressional legislation.

By the end of October, officials expect to complete a consultation with small businesses that would be affected by the standards. Business groups have opposed the possible rule, saying it could be onerous and expensive. By early next year, the agency could lay out a timeline for a rule proposal.

“That’s really a major milestone, because that’s the spot where the agency formally alerts the public that we are proposing a rule,” Andrew Levinson, OSHA’s director of standards, said in an interview.

Mr. Levinson said that the agency was planning to publish indoor and outdoor standards together, since workers “may be shuffling between outdoor work environments and then going into a warehouse, or into some other equipment processing area.” He added that OSHA had to consider different varieties of hot weather, like dry and moist, and how they affect the body.

The agency’s current guidance for employers, with little enforcement muscle, may offer clues to its formal heat standard. Among the guidelines, experts say, could be acclimatization — the practice of gradually easing workers into schedules that expose them to extreme heat. Many workers who have died from heat-related causes succumbed as they began a job.

The agency could also require employers to offer workers access to breaks, shade and cold water. In a statement to The Times, Mr. Soto’s employer, Prospect Airport Services, said that he had been stationed in a cooler work area and that it had offered additional breaks to employees working in a baggage-handling space where the air-conditioning had been unreliable.

Federal lawmakers introduced legislation over the summer that would require OSHA to publish an emergency rule within a year after the bill passes, a measure seen as unlikely to pass because of opposition in the Republican-controlled House.

One of its chief backers, Representative Greg Casar, Democrat of Texas, held a “thirst strike” over the summer to urge the fast-tracking of an OSHA rule. “It’s critical a rule is laid out over the next year,” he said, adding, “If we want to make it permanent, we need to pass legislation.”

David Michaels, an epidemiologist at George Washington University who led OSHA during the Obama administration, said that the agency’s current timeline suggested that new standards might not come by next year. Whenever it arrives, the rule “would be a game changer,” he said, adding: “There’s no question. And it will save lives.”

Extreme heat especially afflicts low-wage earners like Mr. Soto. In higher temperatures, workers in poor counties lose more of their pay, researchers have found. And low-income Americans disproportionately suffer from chronic health conditions that make them more vulnerable to heat-related injuries.

People with epilepsy are more prone to seizures in extreme heat. so Mr. Soto received permission from his supervisors to work in cooler baggage claim areas. The daily medication he takes has steadied him.

Yet he is still anxious as he navigates the sun-drenched and unreliably air-conditioned airport five days a week, including the long walk to a staff room for lunch that he said eats up much of his break time. The airport’s heat, he said, “feels like you’re in the gym, in the sauna.”

“You fully start sweating. I start looking at my hands and I think, How am I already sweating? I haven’t done anything,” Mr. Soto added. “My uniform, you can literally see the sweat on your back and stomach.”

How heat injures the body

Dangerous heat waves are affecting more of the country, including states with typically milder climates.

The costs to the economy are vast: In 2021, more than 2.5 billion hours of labor in the U.S. agriculture, construction, manufacturing and service sectors were lost to heat exposure, according to data compiled by The Lancet, the London-based medical publication. Productivity dips heavily in hot weather.

Few states offer more vivid examples of these new perils than Texas. More than 40 people have died in Texas from heat-related causes since 2011, including a lineman and letter carrier over the summer.

The risks to workers were apparent on a series of sweltering late summer days at DFW, where temperatures neared 110 degrees.

Over 650,000 Americans worked in commercial airports as of 2022, according to federal data compiled by the Service Employees International Union. Many have jobs that involve full or partial heat exposure, including wheelchair escorts, shuttle drivers and airplane cleaners that can ask for loitering in hot areas without adequate air-conditioning.

Workers on the tarmac, such as baggage handlers, typically face the highest temperatures and most dangerous conditions. While some industries and employers have allowed workers to clock in early in the morning or late at night to avoid the worst of a day’s heat, flight schedules are fixed. Most airport workers cannot choose the time or place for their work.

Travun Watts, a contractor who makes $14 an hour cleaning American Airlines planes at the airport between 2 p.m. and 10 p.m., fainted one afternoon in August as he waited in a jet bridge in scorching weather.

Sitting in a baggage claim area on a recent afternoon before his shift, Mr. Watts, who has diabetes, recalled waking up at a Dallas hospital, uncertain about what had landed him there. “I felt like I was in a loop, incoherent,” he recalled.

To assess the limits of work in extreme heat, scientists point to what is known as the wet-bulb temperature — a measurement of both temperature and humidity. Above 95 degrees, sweat cannot evaporate and the body cannot cool. Hours outdoors can be fatal.

“When you have hot conditions, there’s increased demand on the heart to pump more blood to the largest organ in our body, which is our skin,” said Dr. Jonathan Patz, a scientist at the University of Wisconsin-Madison who has studied the environmental health effects of climate change.

Extreme heat can wreak havoc on the body’s major organs. The heart and the kidneys can become deprived of blood and oxygen, leading to kidney failure. If the brain becomes overheated and oxygen-deprived, it can halt the signals to the body to cool itself, preventing sweat.

Mr. Watts spent more than three days in the hospital, he said. A nurse still visits him at home once a week to check on him. His job had been unrelenting even after he returned, he added, often involving cleaning as many as 14 planes per shift.

“Instead of giving me five to 10 minutes to set my insulin meter, they’d rush me, make me run from one plane to next, even when I told them it’s detrimental to my health,” he said.

Airports are particularly risky settings for work, with concrete structures and tarmac that easily retain heat, Dr. Patz noted.

Extreme heat can reduce the safety of indoor spaces by reducing airflow and raising the temperature of air-conditioned spaces. Terminal C, where Mr. Watts works, is older than others at the airport, with crowded walkways, unreliable air-conditioning and drinking fountains with lukewarm water.

At 5:30 p.m. on a recent day, as the temperature hovered around 100 degrees, baggage employees rested their heads and arms on the ramps that funneled bags out of flights in Terminal A.

“Any strenuous activity like throwing luggage on a conveyor belt takes a lot more out of you,” said Dr. Frank LoVecchio, an emergency physician who treated airport workers over the summer at the Valleywise Health Medical Center in Phoenix.

“I’ve seen people super red. They look like they just jumped in a pool,” said Zach Bodine, who makes around $15 an hour helping passengers in wheelchairs at the Phoenix Sky Harbor International Airport. He recalled co-workers “throwing up in the bathroom nonstop.”

Mr. Soto, the Dallas baggage claim worker, said that he had considered quitting, a move that could protect his health. But he recalled being a boy who was awe-struck watching planes land at DFW with his father — a feeling that led to his dream of becoming a pilot.

Mr. Soto sometimes rides the airport’s outdoor tram system just to glimpse aircraft. “Everyone wishes they could fly,” he said.