Last summer, as Portland, Oregon, began to sizzle under a brutal, triple-digit heat wave, Vivek Shandas installed a window air conditioner in the bedroom of his home. It's the first such appliance he's owned since moving two decades ago to the City of Roses, a place known for its relatively temperate climate.

"My spouse ran out and got one, like, a week before" the daytime temperatures soared past 100 degrees, says Shandas, a climate adaptation professor at Portland State University. "I installed it, and we ran it all night. But even then, the temperature didn't get below 85, and my son and our two dogs all huddled into the bedroom."

Though it wasn't as effective as he'd hoped, Shandas, who studies "heat islands" – urban areas that are warmer than surrounding outlying areas or even nearby neighborhoods – says he's lucky: He can afford to make his bedroom a little less uncomfortable when the temperatures rise. Many of the people in the neighborhoods he studies aren't as fortunate.

"We have the means and the resources to go buy a bedroom portable window unit," he says. "I have privilege. I have transportation. But other people don't have that coping capacity." That deficiency is increasingly likely to worsen their overall health and even put some lives at risk.

Experts expect the number of heat-related fatalities – the leading weather-related cause of death in the U.S. – will probably rise as extreme temperatures become more common. Thermometer readings in some urban areas can be around 20 degrees hotter than surrounding areas during a heat wave, but Shandas notes temperature differences between areas with green spaces and those without can be significantly higher even within the same city. That increases the likelihood that living amid a heat island with little relief will aggravate residents' chronic diseases and send them to the emergency room.

"As the planet heats up, you see [people with] a disproportionate risk of having heat-related illness and death in those communities," says Dr. Georges Benjamin, executive director of the American Public Health Association. Because the heat stresses bodily systems, he says, life on a heat island "puts [people] at a higher risk of having not only the heat problem, but a complication from their underlying diseases."

During the Pacific Northwest heat wave, for example, the temperature reached a record 116 degrees Fahrenheit in Portland, and the death count for the weather event in Washington and Oregon alone stood at around 200 in August, though it may be much higher. Shandas, meanwhile, says thermometers he set up in some of Portland's heat islands hit 124 degrees, while thermometers he put in other, leafier areas reached 99 degrees – a 25-degree differential.

And because heat islands trap the sun's heat, temperatures don't fall very much after sunset, Shandas says. Without relief, the danger of heat-related or heat-triggered issues can increase.

In his experience, "most of the people who die from high heat die at night," Shandas says. "Our body's ability to thermoregulate changes when we sleep, and if the ambient temperature goes above our internal body temperature – 98.6 degrees Fahrenheit, or 37 degrees Celsius – then our body attempts to release the heat."

In heat-island homes that trap heat but don't have air conditioning, the body struggles to cool itself, which can trigger health complications, Shandas says: "At night, the combination of the physical environment, lack of air circulation indoors and sensitivity of specific populations can increase the likelihood of heat exhaustion."

By definition, heat islands are typically found where asphalt streets, tarred roofs and other hard, dark and impervious surfaces are plentiful but trees, air circulation and open green space are scarce. That composition causes greater absorption, retention and generation of heat by buildings and pavements, and often runs parallel to heat-causing human activities, such as vehicular traffic.

"When you've got the sun beating on a building all day, that building heats up, but then also holds on to that heat into the night," says Greg Kats, founder and CEO of the Smart Surfaces Coalition. The organization promotes the use of green or reflective roofs, porous pavements and other materials and strategies that mitigate the effects of climate change; Benjamin, the APHA executive director, sits on its steering committee.

"People are overheating, and this isn't just outdoors, this is in their homes," Kats says. "What we're seeing is that these problems disproportionately affect low-income communities. Not only are those neighborhoods hotter, but also the residents of those neighborhoods are less likely to have the means to respond to the heat."

Shandas and fellow researchers have found this dynamic appears anything but happenstance: Rooted in the 1930s, redlining by mortgage lenders and the federal government – through which African Americans and others were barred from accessing home loans – helped cement segregation and inequality in American cities. That created templates in which more affluent, majority-white neighborhoods were seen as "good" while minority or ethnic neighborhoods were considered risky for investment.

Over the decades, redlining resulted in disinvestment in "bad" neighborhoods. That, in turn, led to more pavement and parking lots, less tree canopy and a near assurance that these communities would swelter during heat waves.

The intertwined health effects of climate change, redlining and heat islands can be pronounced, Shandas says. He recalls that "back in the day," he led a research project mapping low-income, underserved communities in which the health of residents was particularly vulnerable.

"We worked with the public health agencies to figure out which communities had higher rates of asthma and respiratory illness, heart disease," Shandas says. "And then we added, on top of that, air conditioning access data and transportation access data" from official government sources.

When the "heat dome" descended on Portland, Shandas says, he suspected the vulnerable-health maps, likely heat islands and areas that have a history of underdevelopment due to redlining would overlap, "and that's exactly what we saw play out in this."

"While this is not a causal relationship, the correlations unmistakably point to the amplification of heat waves in areas where past policies have marginalized some community members," he says.

Although redlining has been outlawed, Benjamin says, it perpetuated attitudes that still exist today: "It creates a part of the community where, either through conscious or unconscious neglect, cities don't want to invest."

"So you don't fix the parks, you pave over areas that used to be playgrounds, you don't fix the community up," he says. As a result, he says, dark open spaces absorb heat, and residents -- many living in buildings with inefficient or nonexistent cooling systems – suffer.

Shandas says for him, the link between redlining and heat islands was confirmed when he took thermal-imaging satellite maps and overlaid them on area redlining maps held in an online database at the University of Richmond Digital Scholarship Lab.

"We studied 108 across the country and just looked at surface temperature," he says. "We're finding over and over and over that the areas that were historically redlined were hotter than the areas that weren't."

The health risks posed by heat islands, global warming and socioeconomic inequality, however, also have resulted in innovative solutions to help residents cope – and to potentially save lives.

They include free air conditioners for low-income residents in the Boston area, development of an ultra-white paint that reflects more than 95% of sunlight and a Philadelphia toolkit for creating neighborhood-level heat-relief programs – part of an effort that pivoted to providing more direct cooling relief due to COVID-19. In Maryland, there's a tree-planting effort with the goal of bringing shade to underserved communities.

A comprehensive approach by the Smart Surfaces Coalition advocates for a range of solutions. They include making sweeping changes to urban infrastructure, like replacing regular asphalt with more porous surfaces and using "green roofs" on new buildings, as well as upgrading or modifying existing buildings to better reflect or use solar energy rather than allow buildings to absorb it.

"We're looking at reflective roofs, we're looking at green roofs, we're looking at photovoltaic, solar roofs, and making sure that as new roofs are built for new buildings, we're using it as an opportunity to upgrade that roof," Kats says. "I think one of the cheapest and easiest ways to do that is with a cool, reflective roof. It can be as simple as just painting the roof white."

A recent Smart Surfaces Coalition study focused on Baltimore indicates that implementing strategies that could help diminish heat islands not only could save lives by lowering peak summer temperatures downtown by 4 degrees, but could also exceed costs of implementation by more than $10 billion. Financial benefits would come through avenues such as lower energy costs; such strategies also would lead to job creation and help avoid losses in tourism revenue.

Although the Portland heat dome is considered a rare event, Shandas says it's another harbinger of climate change. Unless humanity changes course, he says, the future will become the present, and more people are likely to live on heat islands – and suffer because of it.