When scorching heat descends on New York City in the summer, Harlem resident Evelyn Jenkins-Smith finds breathing difficult when she goes outside.

“My chest feels heavy,” said the retiree, who suffered a minor stroke two years ago. So the 73-year-old widow cloisters herself in her sweltering apartment, which lacks air conditioning.

Jenkins-Smith is among the growing number of city dwellers whose health is at risk from increasingly hotter summers – a threat that has prompted cities nationwide to look for innovative ways to keep their most vulnerable residents safe and cool.

Rife with asphalt and concrete that absorb and radiate heat, many U.S. cities amount to giant heat traps, scientists say.

The phenomenon is known as the urban heat island effect. It can add as much as 5 degrees Fahrenheit (3 degrees Celsius) to daytime temperatures in cities, according to the U.S. Environmental Protection Agency.

The problem is even worse at night when city temperatures can be as much as a whopping 22 degrees Fahrenheit (12 degrees Celsius) warmer than green rural areas, where heat is more effectively released back into the atmosphere, the agency said.

Countrywide, heat is the number one cause of death from extreme weather events – including threats such as floods and hurricanes, according to the National Weather Service. Its data show that high temperatures killed an average of 131 people a year in the United States between 1987 and 2016.

When the heat index – a measure that combines temperature and humidity – climbs above 95 degrees Fahrenheit (35 degrees Celsius), it becomes harder to sweat off heat, leaving older adults, in particular, at risk of suffering heat stroke.

High temperatures also can aggravate pre-existing health issues, such as heart conditions, according to the U.S. Centers for Disease Control and Prevention.

Future risks look even greater unless there is quick action to curb climate change, scientists say.

In New York City, an estimated 3,300 people could die each year, beginning in 2080, from intense heat due to climate change, according to a study published last year in the journal Environmental Health Perspectives.

“We underestimate the threat of extreme heat because it’s an unseen threat,” said Larissa Larsen, a University of Michigan urban planning professor who has studied the effects of extreme heat on health for 15 years.

CITIES STEP UP

But as climate change has sent temperatures skyrocketing, urban planners and health officials have scurried to find solutions, said Kurt Shickman, who heads the Global Cool Cities Alliance. The non-profit in Washington D.C. helps dozens of cities devise urban-cooling policies.

From Louisville to Chicago, and Los Angeles to New York, mayors and city councilors are pouring millions of dollars into plans to fight intensifying urban heat, Shickman said.

Flat rooftops are being painted white to reflect light and cool homes. Armies of seasonal workers plant trees, which cool the air due to their ability to evaporate water. And dark pavement is being replaced with materials that cool faster.

In addition, many cities have designated public cooling centers – such as air-conditioned libraries and community centers – for use when the blistering heat becomes a particular health danger.

“Cities are really taking the lead,” Shickman said.

But in their efforts to work out where they should paint roofs, plant trees and replace pavement, cities have uncovered an unpleasant truth: Heat doesn’t strike evenly, and it’s the poorest neighborhoods that are most at risk.

In New York City, for instance, municipal authorities have produced a map that combines the thermal and socio-economic characteristics of neighborhoods to zero in on areas where residents are most likely to die during heatwaves.

They turn out to be some of the metropolis’ poorest areas: the South Bronx, Central Brooklyn, and Jenkins-Smith’s Harlem.

Such mapping confirms the intertwining of poverty and urban heat risk that social-justice advocates and researchers have suspected for two decades.

“Heat affects everyone but there are some people it impacts with harder consequences,” said Brooke Havlik, a spokeswoman for WE ACT for Environmental Justice, a Harlem-based advocacy group.

CASH EQUALS COOL

Sharon Harlan, a professor of health sciences and sociology at Northeastern University in Boston, says she and colleagues have discovered a simple reality: Access to cash can equate with the ability to cool down.

In Phoenix, Arizona’s broiling capital where the July average daytime temperature is 104 degrees Fahrenheit (40 degrees Celsius), “people can buy cooler temperatures”, she said.

Analyzing data from Phoenix’s metropolitan area, Harlan and her colleagues found that for every $10,000 increment in average household income in a neighborhood, the area’s outside temperature was lower by half a degree.

Wealthier neighborhoods with verdant yards and trees had cooler air, they found, while low-income neighborhoods, with few trees, accumulated daytime heat.

But it is indoors – in homes that can be 20 degrees Fahrenheit (11 degrees Celsius) hotter than outside and where most heat-related deaths occur – that being poor takes the biggest toll.

That’s because “low-income households are hesitant to use air conditioning because of the utility costs,” said the University of Michigan’s Larsen.

In New York City, city authorities have tried to address the problem by helping low-income people with medical conditions buy subsidized air-conditioning units.

But such efforts are greatly underfunded, according to the mayor’s office, and the poor remain without air conditioning in much larger numbers than the rich, according to 2014 city data.

In Brownsville, one of New York’s poorest neighborhoods, just 70 percent of households had air conditioning compared to 99 percent in South Shore, one its wealthiest neighborhoods.

Jenkins-Smith, who lives on a tight budget, says she is no fan of air conditioning, which she finds gives her a sore throat.

But neither would she be able to easily afford it since her husband’s death last year left her with a single income.

“It would be a tight squeeze” to afford the bills, she admitted.

COOLING – AND HEATING

Even when it’s affordable, air conditioning shouldn’t be looked at as a clear solution to threats from hotter temperatures, as it comes with problems of its own, warned Forrest Meggers of Princeton University in New Jersey.

Meggers, a professor of architecture, has researched how the use of air conditioning adds to outdoor heat levels in cities.

Heat emitted by cooling systems and cars can account for about a fifth of the heat island effect, he said.

“All machines have to work harder when it gets hotter out, and when they work harder they emit more heat,” said Meggers in a telephone interview.

Using thermal cameras he developed to snap photos of buildings, Meggers captured images of window air-conditioning units shooting plumes of hot air back outside.

“If you’re in a Manhattan business-district building, there’s a huge air-conditioning system exhausting heat up (near) the roof,” he said.

“But as soon as you get to the Bronx or Harlem, you’re going to just see a window air conditioner stuck under an awning, spewing out heat onto the sidewalk,” he said.

“So your neighborhood environment is being negatively impacted because the only affordable air conditioners are small, inefficient units.”

NEW TECHNOLOGY

In the long run, new technology will play a pivotal role in helping solve this problem, Meggers said.

The scientist is among scores toiling in laboratories to engineer new cooling materials. Those include paint or roof shingles that better reflect the sun and rolls of thin glass polymer that both reflect the sun and help objects they are applied to shed heat.

Using such materials, “we can manipulate the way people perceive temperature without depending on air conditioning,” Meggers told the Thomson Reuters Foundation.

Back in Harlem, Jenkins-Smith says that, for now, she’ll stick to her habit of drinking plenty of water and staying put indoors during heatwaves, without turning to air conditioning.

“I don’t need any extra expense,” she said. “I and heat get along.”