"Before I knew it, I was gasping for air. It doesn't even take me 10 minutes to get from home to my other job. Just like that," says Franklin, who says an attack feels like drinking water through a pinched straw.

"Your windpipe is that straw and that water is the breaths you can take," Franklin says. "You're trying to bring your air through as much as you can."

Doctors in emergency departments near Baltimore's hotter neighborhoods say they prepare each summer for an increase in heat-related conditions.

"A lot of times the heat played a factor in making a chronic condition acutely worse," says Dr. Amit Chandra, chief of emergency medicine at the University of Maryland Medical Center Midtown Campus. He says that's especially true for cardiovascular conditions.

"The worse your circulatory system is, the worse you are at getting cool," says Chandra, who is also a faculty member at the University of Maryland School of Medicine. A weak or damaged heart might struggle to pump extra blood to the skin so heat can radiate off the body. Even sweating, which also gets rid of heat by evaporation, can put stress on the heart.

Respiratory conditions can become aggravated too, in part, because heat can actually worsen air quality and because conditions like asthma and COPD can be triggered by high heat and humidity.

Chandra says even looking at patients' medical records wouldn't necessarily tell the full story of how heat could be harming their health.

"We wouldn't diagnose them at the end of the day with heat exhaustion or heat stroke necessarily unless their temperature went up," Chandra says. "So there's probably quite a few folks that are affected by the heat, and we're not really tracking or measuring."

Regardless of where they live, people in poverty are more vulnerable to many chronic conditions, including some made worse by heat, like asthma and heart disease.

"Our patients are plagued by poverty, substance abuse and unfortunately some of the patients don't have great access to health care," says Dr. Reginald Brown, chair of emergency medicine at Bon Secours Hospital Baltimore in West Baltimore and a faculty member at the University of Maryland School of Medicine.

"As far as the impact [of heat] to our patients, it's just another thing that complicates their lives," he added.

The urban heat island

Cities in general tend to be hotter than their natural surroundings, thanks to a phenomenon known as the urban heat island.

"If you have less green cover, you will almost always have higher temperatures, and greater exposure to heat," says Brian Stone, director of the urban climate lab at the Georgia Institute of Technology.

Trees provide shade, but they also cool the environment down through the evaporation of water from their leaves — a process similar to how humans sweat to cool down.

"When you pave over an area, particularly if it had green plants, you have interrupted that cycle," Stone says. "Not only have you sealed the surface, you have put a lid on it, so evaporation cannot happen."

Pavement — particularly if it's black — absorbs heat and holds it in. At night, a city of more than 1 million people can be as much as 22 degrees warmer than its surroundings. Even the buildings themselves, Stone says, can create a sort of canyon that traps heat.

Given these elements, Stone says it makes sense that many low-income areas are hotter than richer areas.

"Lower-income parts of the city tend to have less green cover," Stone says. "That's something that we see across a lot of cities."

It's also not unusual, Stone says, for poorer areas to be located near "undesirable land uses" such as nearby highways or industrial areas, which create even more heat.

The pattern of who lives closer to those sources of heat is not just a matter of poor versus rich, it is also often a matter of black and brown versus white. Nationwide, many of the low-income communities NPR found to be hotter — often with fewer trees, more concrete and closer to highways and factories — are also communities of color.

"It wasn't just a coincidence that communities of color end up in some of the most undesirable places," says Miya Yoshitani, executive director of the Asian Pacific Environmental Network, which focuses on environmental justice issues affecting working-class Asian and Pacific Islander immigrant and refugee communities.

Yoshitani explains that policies like redlining — a practice, beginning in the 1930s and banned by the Fair Housing Act in 1968, in which neighborhoods were marked high risk for mortgage lenders in large part based on their racial makeup — forced people of color into less desirable areas. In Baltimore, the city's hottest neighborhoods, many of which are predominantly African American, still line up fairly consistently with the neighborhoods marked "hazardous" on a 1937 map created by the Home Owners' Loan Corp.

"People of color, African American communities, indigenous communities in the beginning and then immigrant communities as they came to the United States were not given a choice about where they could live, where they could raise their families, where they could work," Yoshitani says. "Those choices were made for them and that legacy continues today."

"They can't escape it"

In the majority of the cities NPR mapped, poverty was linked to heat, adding a second layer of risk to an already at-risk population. This is not only because poverty itself is a health hazard, but because poverty is also tied to other factors that can make it harder to get cool.

"People with money, of course, can do that a lot better than people with less money," says Dr. Georges Benjamin, executive director of the American Public Health Association.

He says no one is immune to climate change, but wealthier people can more easily control their exposure to heat by using air conditioning — which on its own contributes to climate change — or even by moving to a cooler part of the city.

"On the other hand, the folks with less money, they're going to be in their one home. And they're going to have to deal with the conditions in their one home," Benjamin says. "If they're going to be in an area where it's real hot, they're going to have to find other ways to adapt, but they can't escape it."

The urban poor, already often in hotter environments and already at higher risk for health problems, will have a harder time escaping climate change.

"It is the most significant public health problem that we have. It's going to be here for a long time. And it's getting worse," Benjamin says.

"This is ours"

There are strategies to cool down a city: investing in public transit, designing roofs that reflect sunlight, and planting more trees, among others.

In Baltimore, the city is working to combat urban heat. The government has installed cool roofs, turned vacant lots into community green space and strategically planted and maintained trees in low-income neighborhoods, among other initiatives. But the city's own arborist told the Howard Center that Baltimore is not on track to meet its goal of increasing the tree canopy to 40% by 2037.

"We're doing a lot with a little," says Anne Draddy, the city's sustainability coordinator.

The neighborhood where Shakira Franklin lives has increased its tree canopy over time. But by 2015, it still was among the city's lowest. Franklin says she's not optimistic the city will be able to cool down her neighborhood anytime soon.

"The city has a lot of responsibility. And I think that we would be close to the bottom of the list to be honest," she says.

There's a grassy vacant lot near her apartment where Franklin often takes a break from her job as a landscaping crew supervisor at Bon Secours Community Works, a nearby community organization owned by Bon Secours Health System.

It's one of the few places in the neighborhood with a lot of shade — mainly from a large tree Franklin calls the mother shade. She helped come up with the idea to build a free splash park in the lot for residents to cool down in the heat. Now Bon Secours is taking on the project.

"This was me taking my stand," Franklin says. "I didn't sit around and wait for everybody to say, 'Well, who's going to redo the park?' "

Daniel Greenspan, an architectural fellow working on the project, says they're about halfway to their current fundraising goal — with plans to buy the lot from the city.

On a hot Saturday this summer, Bon Secours and the neighborhood's community association threw a party in the lot. Children ran through streams of water from a pop-up fountain, while adults discussed and voted on potential designs for the new park.

"Our kids, they deserve it," says Franklin, who has two children. "I just feel like it's a long time coming to just have something to say that we built this here for us. This is ours."

Franklin says the park would become a refuge for people who can't escape the heat — and it will indeed be a place to cool off in this neighborhood. But worldwide, heat waves are getting hotter and more frequent, and the past five years have been the hottest ever recorded.

To determine the link between heat and income in U.S. cities, NPR used NASA/U.S. Geological Survey satellite imagery and U.S. Census American Community Survey data. An open-source computer program developed by NPR downloaded median household income data for census tracts in the 100 most populated American cities, as well as geographic boundaries for census tracts. NPR combined these data with TIGER/Line shapefiles of the cities.

The software also downloaded thermal imagery for each city from NASA's Landsat 8 satellite, looking for days since 2011 in June, July and August when there was less than 4% cloud cover. NPR reviewed each of the satellite images and removed images that contained clouds or other obscuring features over the city of interest. In cases when there were multiple clear images of a city, we used the thermal reading that showed a greater contrast between the warm and cool parts of the area of interest. In cases where there were no acceptable images, we manually searched for additional satellite images and found acceptable images from Landsat 8 for every city except for Hialeah and Miami, Fla., and Honolulu, which are frequently covered by clouds.

For each city, NPR aligned the satellite surface temperature data with the census tracts. For each census tract, the software trimmed the geography to only what is contained within the city of interest's boundaries, then removed any lakes, rivers, ocean, etc. It calculated a median temperature reading for each census tract. When all of the tracts in a city were completed, it calculated a correlation coefficient (R) of the tracts to find the relationship between income and heat.

The satellite data measure temperature at a surface, such as the ground or a rooftop. We used this measurement rather than ambient temperature, which measures the air about 2 meters above the ground. Measuring air is a more accurate measure of how people experience heat, but satellite data are more widely available than air temperature data. Using that data allowed us to provide a more complete snapshot of temperature trends across many cities.

View NPR's full data and code on Github.

To determine the relationship between heat and poverty in Baltimore, the Howard Center for Investigative Journalism used block-by-block temperature data showing variations in Baltimore's urban heat island captured by researchers at Portland State University and the Science Museum of Virginia on Aug. 29, 2018. For each "community statistical area" in Baltimore, the Howard Center computed a median temperature and joined it to a U.S. Census American Community Survey data set with the poverty rate for each area and then calculated the correlation coefficient (R).

Copyright NPR 2021.