Part of a series examining the effects of climate change here in Massachusetts

CHELSEA, Mass. — As coastlines recede with global warming, so-called heat islands are growing. These are dense urban areas where cement or asphalt cover most of the ground, where multi-story buildings — often brick — bake in the sun, and where there are few trees.

Daily temperatures in these spots can be 20 to 50 degrees hotter than in leafy suburbs. For residents of these islands, health risks rise with the heat.

This map of Greater Boston shows land surface temperatures on July 7, 2015. The redder areas are warmest. The map was created on July 3, 2017. (Courtesy of The Trust For Public Land mapping tool)

A Summertime Spike In Medical Issues

At 11 a.m. one early summer morning, it's a humid 80 degrees inside Fausto Alvarado's third-floor apartment in Chelsea. When it's hot, the 88-year-old from Honduras struggles with every breath.

"I can’t get enough oxygen, and I'm very tired," says Alvarado. "I almost can't breathe."

Alvarado is just back from a week in the hospital and still on antibiotics. He has a lung condition: chronic obstructive pulmonary disease, or COPD. When it's hot, Alvarado says he sometimes feels like he's drowning.

Two ceiling fans whir and the windows in Alvarado's combined kitchen/living room are wide open to release steam and odors. His landlord, Trinity Management Co., supplies an air conditioner, but artificially cold air is hard on his lungs, too.

Fausto Alvarado looks down while his daughter, Cruz Romero, administers an intravenous injection of an antibiotic to treat his lung infection. Alvarado's medical condition does not permit him to have an air-conditioner, which during the summer months makes breathing difficult. (Jesse Costa/WBUR)

Alvarado is alarmed to learn he lives within a heat island, one of the hottest areas in Greater Boston. His daughter, Cruz Romero, worries about the future of her community.

"More and more people are going to get sick more often," she says.

That’s the prediction of doctors who study climate change: more dehydration and kidney failure, more difficulty with emphysema, asthma and other lung conditions, more heart problems and heat stroke.

Some research shows older Americans are adapting to the heat, learning to stay indoors near air conditioners, but Chelsea Deputy Fire Chief John Quatieri isn’t seeing it.

"Usually in the summer months we see a spike in medical calls, whether it’s dehydration or people just passing out," Quatieri says.

Chelsea firefighters also get more fire calls, when fans and air conditioners plugged into extension cords short out or grills spark a blaze. But Quatieri didn’t realize that parts of his hometown are hotter than others and more likely to see heat-related problems.

Click to enlarge: The land surface temperature map for July 7, 2015, zoomed in on Chelsea. The map was created on July 3, 2017. (Courtesy of The Trust For Public Land mapping tool; Jesse Costa/WBUR)

Most of Chelsea is a heat island, meaning temperatures are consistently hotter than average. Quatieri looks at a heat map of Chelsea. Inside the red patches, one of which includes the fire station where he is standing, the surface temperature high will reach 140 degrees on a typical July day.

"This area that’s marked in red, this is where we’d see the spike in calls during the summer," says Quatieri, shaking his head as he remembers the past weekend. "We were very busy, and most of the calls were in this Broadway area right here."

Satellite data shows temperatures in the hottest parts of Chelsea, Everett, Somerville and Boston are 10, 20, sometimes 40 degrees higher than in the tree-lined, spacious neighborhoods of Melrose, Arlington, Newton and Brookline. As global temperatures rise, Chelsea is partnering with the Worcester Polytechnic Institute to determine the extent and magnitude of heat island effects. Some of the challenges are already clear.

Chelsea, with 35,080 residents, is the smallest city in Massachusetts, but it is the second most densely settled (after Somerville). In Chelsea, most residents are low to moderate income. Seventy-two percent of residents rent, and Chelsea senior planner Alex Train says many spend more than 30 percent of their income on housing. The housing stock is older.

Infrastructure That Keeps The Heat

Train looks down Broadway, toward Fausto Alvarado’s subsidized apartment. There’s a line of 80- to 90-year-old buildings made of brick and stone.

"Those materials retain heat," Train says. "So, for example, you’ll have a 90-degree day here in Chelsea and while that evening, it may drop down to the 50s and 60s, those buildings are still retaining the heat it collected during the afternoon."

The Chelsea Square Apartments in an area identified as a heat island in the city. (Jesse Costa/WBUR)

During heat waves, Chelsea opens schools, the senior center and City Hall — buildings that have central air. There’s a back-up micro-grid power plan in the works to make sure these buildings stay cool during brown-outs. Alerting all residents about the dangers of heat and poor air quality is difficult because there are at least 35 different languages spoken in this 1.8-square-mile city.

The city is investing in a longer term cooling plan. With help from the state, Chelsea has planted 2,000 trees since 2013. But again, inside this dense, urban heat island, there are setbacks. Roughly 30 percent of the trees have died.

"Part of that is from gas leaks underground that are killing off the trees, methane gas leaks," says Roseann Bongiovanni, executive director at the nonprofit GreenRoots. "That’s a cost to the city; it’s time and effort."

GreenRoots, working with the city of Chelsea, has built two playgrounds and gardens, places that aim to provide refuge.

"Here you can feel a lot less of that heat," says Bongiovanni in one of those gardens, dwarfed by a flowering bush. "You can hear the birds chirping. You feel the wind. There are lots of opportunities to recreate, to be calm, to be in some shaded areas. We’re trying to replicate that throughout the entire city."

But in Chelsea, as in many cities, the heat menace emerges in surprising places, like school playgrounds.

"These days, the good news is we don’t have asphalt on the ground, but we’ve replaced that with rubberized surfaces," says Dr. Aaron Bernstein, a pediatrician at Boston Children's Hospital. He looks at the ground beneath slides and a jungle gym at a school within one of Chelsea's heat islands. "In this case, it’s pitch-black, which will expose those kids to more heat than if they were standing on this concrete, which is a lighter color."

How much more heat on this partly sunny, mid-70-degree day? A handheld temperature gun shows concrete at the entrance to the playground is 82 degrees. The black rubberized surface is 96 degrees.

"That's crazy, right?" Bernstein says. He worries about how kids with asthma would fare on this overheated playground.

All the kids who live within heat islands may be at risk for more stress at home. In the emerging world of climate science, research shows heat interferes with sleep, increases aggression, and contributes to some mental health problems.

Bernstein, who is the program director for climate, energy and health at Harvard’s Chan School of Public Health, has advice for anyone in or outside a heat island who takes medicine that causes a patient to retain water and not sweat, or, on the other hand, to pee a lot. Ask your doctor, he says, if you need to do anything different when it's hot to make sure you don't get dehydrated and that your body can cool itself.

"Because we do see higher rates of hospitalization in heat waves," Bernstein says. "It's not clear if it's the disease itself or these medications, but some of these medications do make us get dehydrated, and they can impair our ability to sweat."

Patients in the Northeast appear to be more vulnerable to the effects of rising temperatures. But high temperatures are usually not the only reason for this.

"The risk is higher when we see a more drastic change in temperature," says Francesca Dominici, co-director of Harvard's Data Science Initiative. "Our body tends to be more susceptible and at a higher risk for disease when you have days jumping, as an example, from 40 to 80 degrees," which happens more often in the Northeast than in other parts of the country.

Residents Seek Doctors' Notes To Stay Cool

As temperatures rise, especially inside heat islands, many doctors encourage patients to stay close to or have access to air-conditioners. But are air-conditioners a medical necessity? That question has launched a battle in Chelsea this summer. Here’s the deal:

The city’s housing authority is telling residents they must remove the AC in any room with just one window, often a bedroom, because it blocks an escape route. The authority says this is a building code requirement. Residents are flooding their doctors with requests for letters, hoping to prove they need to keep air-conditioners in their bedrooms.

"The clinic has been barraged with a whole bunch of people requesting this very same letter," Dr. Lisa Carr, a primary care physician at the MGH Chelsea HealthCare Center.

Carr says there are no guidelines about air-conditioners for medical use. So doctors at this clinic created a policy. They’ve agreed to write letters that say air conditioning is needed for children who use a daily asthma medication. For adults, doctors are left to decide: Would the patient have fewer migraines or less depression? Would that rash go away with air conditioning?

"It’s really tough to try to sort these things out," Carr says. "I’m sure there’s lots of people that would benefit from having air conditioning in the really hot summers here."

Prescribing air-conditioners might make sense as health care payments shift and doctors are encouraged to spend money upfront to help keep patients healthy and out of the hospital. But more use of air-conditioners will also compound the problem of rising energy use and climate change.

Beyond Chelsea

The maps below show daytime and nighttime heat islands as measured by changes in land surface temperature across Boston. The dots show concentrations of populations vulnerable to heat. Click to enlarge the images. And use this website to find local places to keep cool.

(Courtesy the city's Climate Ready Boston report)

The graph below shows projected annual heat-related deaths in Boston.

(Courtesy the city's Climate Ready Boston report)

Correction: An earlier version of this report, including in captions on two maps, said surface temperature readings were for a day this week. The readings were from July 7, 2015, and show temperatures on a typical July day. We regret the errors.