We’re told the truck will be here any minute. On what will turn out to be one of the first truly scorching days of the year, it’s still pleasant enough a little after nine in the morning. A steady breeze drifts though the northwest San Antonio neighborhood where we stand in the shade of a live oak. Douglas Melnick, the city’s chief sustainability officer, points a pistol-shaped infrared thermometer at the surface of the nearby street and pulls the trigger. The sunny asphalt already registers 86 degrees, while the shaded sidewalk clocks in at a relatively cool 71.

Minutes later, a specialized tanker truck arrives, and its crew of about a dozen soon sets about uncoiling hoses and loosening valves. One member of the team unleashes a steady stream of gray sealant from a spray gun along the edge of the road, and two others level out the thin puddle with squeegees, kicking up a smell reminiscent of wet paint. They work their way first along one curb, then the opposite, before holstering the spray gun and activating ten sealant dispensers fixed to the rear of the truck. Driving slowly up six neighborhood blocks—a little less than a half mile—with the squeegees following closely behind, they fill in the center of the street.

This test site is among ten in San Antonio (one in each city council district) where crews have applied one of five slightly different sealant products. Throughout the summer, researchers from the University of Texas at San Antonio will monitor the effects of the sealant, which is designed to reflect the sun’s heat. The goal is a reduction of the surface temperature of the pavement, and, theoretically, a cooler city.

Heat gets trapped in parking lots, roofs, and sidewalks in urban environments, which also often lack plant life to provide shade and reduce temperatures through evaporative cooling. Paved roads are among the leading drivers of higher temperatures in cities compared with their rural surroundings—a phenomenon known as the urban heat island effect.

Cooling down our urban environments isn’t simply a matter of comfort—it could save lives. In September 2021 more than two hundred medical and health journals joined together to describe climate change as “the greatest threat to global public health,” citing, among other hazards, a slew of research showing that potentially fatal extreme heat is a growing menace. Indeed, heat waves are the leading cause of weather-related deaths in the United States. Last summer (the second-hottest on record) Texas recorded 279 heat-related deaths, a two-decade high that experts say is almost certainly an undercount. And this year, a late-June heat wave was responsible for at least thirteen deaths in the state.

Cooler pavement is by no means a panacea, and research shows that in some conditions, it can actually make us hotter. Even when used effectively, it can seem like applying a bandage to a bullet wound. Experts agree about the need to better understand the effects of heat-mitigation measures, such as planting trees, installing shade structures, and improving access to public cooling centers. But are any of these measures enough in the face of global climate change?

This discussion is about much more than tragic, sudden deaths resulting from heatstroke. Routine exposure to extreme heat takes a toll on the human body. Dr. Elizabeth Matsui, director of the Center for Health & Environment at the University of Texas Dell Medical School, explains that once we’ve exhausted our body’s natural cooling mechanisms, we quickly become susceptible to “a spectrum of direct effects from heat.”

Higher temperatures can exacerbate preexisting conditions, particularly those related to heart function—the journal Nature noted just last week that even moderate heat strains the heart—as well as the respiratory system and kidneys. Pregnant women are especially susceptible to heat illness, and Matsui notes that it also puts them at further risk of high blood pressure, as well as premature labor, which can lead to severe birth defects. Heat is indirectly a driver of injuries as well—having an adverse effect on mental health and an association with an increase in violent crime. Many of these effects fall disproportionately on the most vulnerable, including children, older adults, and those earning low incomes.

Experts also point to the many ways in which heat is an impediment to healthy lifestyles for all of us. High temperatures can disincentivize exercise, for instance, even as physical activity has been shown to reduce the risk of conditions such as cancer, cardiovascular disease, and stroke. “The main health interest I have in heat as a climate change impact is actually not at the far end of the spectrum of heat-related injury and deaths,” says Kevin Lanza, a professor at UTHealth Houston School of Public Health. His focus is on “chronic disease prevention by creating environments that promote safe physical activities as our weather is warming.”

So creating cooler environments, as city leaders are striving to do in San Antonio, may improve health outcomes for everyone. When Lanza was in graduate school, in 2017, he worked on a study aiming to document the urban heat island effect in Dallas. The researchers found that with proper heat mitigation measures, some sections of the city could be as much as fifteen degrees cooler on hot summer days. They also determined that these strategies could reduce heat-related deaths by more than 20 percent. Among other heat-reduction strategies, the study recommended installing thousands of acres of cool pavement.

Neil Debbage leads the team of UTSA students and faculty who are monitoring the newly installed cool pavement in San Antonio. They will routinely take a series of temperature measurements at the installation sites. That includes what’s known as the wet-bulb globe temperature, which takes into account factors such as humidity, wind speed, and direct sunlight. They’ll also measure the net radiation budget, which considers numerous factors related to solar radiation.

Despite the projections made by the researchers in Dallas, studies in Los Angeles and Phoenix have yielded results that cast doubt on the efficacy of cool pavement. Even though in both cities surface temperatures of treated pavement were reduced by more than ten degrees during the hottest parts of the day, there was only a minor or even a negative impact on the air temperature. In L.A., the mean radiant temperature—the most accurate gauge of human comfort used in the study—was actually several degrees higher above the treated surfaces during the hottest parts of the day. That meant a pedestrian walking on the cool pavement would actually feel hotter than if they were walking on standard asphalt.

“In the middle of the day, when the sunlight is most intense, you get what’s called the radiated heat penalty,” explains Kelly Turner, a professor at UCLA and one of the study’s authors. The sun’s heat gets reflected off the sealant-covered pavement, keeping the pavement cool but increasing the mean radiant temperature. “In theory, if you cover the whole city of San Antonio, for instance, with cool paints and cool roofs, you would have such a large decrease in air temperature that it would offset that radiative heat penalty, but we just don’t know yet how much area would need to be treated.”

After watching the crew install the cool pavement at the neighborhood site, I drove northeast to another area that had been coated with sealant a few days before. It consisted of several residential blocks running alongside an elementary school campus. Walking along the street, I felt hotter standing on the treated pavement than on the nearby asphalt, and the sealant cast a glare. Yet when I reached down to touch the pavement, it was noticeably cooler than the untreated road.

Melnick and Debbage have reviewed the studies from other cities and understand the potential drawbacks of cool pavements. “We’re trying to see if that trade-off of slight discomfort for a part of the day is worth the bigger benefits of cooling across a broader scale,” Debbage explains. Even if it’s hotter above the treated roads during the warmest part of the day, there may still be significant benefits in the evening, when the urban heat island effect can be especially brutal. Buildings and pavement radiate heat well into the night, which can get trapped by other structures, keeping the city temperature warmer. “So, at scale, if you had the cool pavements, then basically they don’t have that energy to give off anyways, because it’s been reflected all day long,” Debbage adds.

Melnick repeatedly emphasized the need for multiple approaches to citywide heat mitigation, in some areas using, for example, a combination of tree plantings, reflective roofs, and architectural canopies. Cool pavement alone may not be a good option in pedestrian-heavy corridors.

Research has shown that what Turner terms the community’s “heat burden”—hotter neighborhoods, fewer cooling options, less shade—falls disproportionately on lower-income residents. In Texas, many of last year’s 279 heat deaths involved residents experiencing homelessness or migrants crossing the border, but there were also many who lived in less precarious situations. For those waiting outside for public transit, working an outdoor manual labor job, or simply unable to afford an air conditioner in a section of the city that’s fifteen degrees warmer than a tree-lined affluent neighborhood, extreme heat can easily translate to poor health outcomes. “If you want to reduce the heat burden,” Turner says, “you need to think about sun-blocking interventions versus sun-reflecting interventions.”

A nonprofit that’s trying to bring one of those sun-blocking interventions to more of the state is the Texas Trees Foundation. Established in 1982, it has planted thousands of trees across North Texas. The benefits, beyond the obvious shade and evaporative cooling, can be difficult to quantify, but a 2014 study by researchers at the U.S. Forest Service found that by removing pollution from the air, trees in the U.S. saved more than 850 lives and provided $6.8 billion worth of health benefits.

Texas Trees also commissioned the Dallas urban heat island study that Lanza worked on, among the earliest such research in the country. That laid the groundwork for an ambitious new plan to refashion the Dallas Medical District—a maze of concrete, pavement, office buildings, and hospital complexes that was rated one of the hottest parts of the city—into an urban oasis. The plan includes considerable changes to Harry Hines Boulevard, a major thoroughfare, and installing a ten-acre park on what had been an elevated roadway.

The project’s leaders are well aware that they’re devoting these resources to improving an area that’s home to three of the state’s top hospitals: Children’s Medical Center Dallas, Parkland Memorial Hospital, and UT Southwestern Medical Center. They hope to refashion the district as the beacon of good health that it should be—an architectural testament to all that scientific and medical research tells us about how to ensure healthy lifestyles. Still in its early design phase, the project is expected to be completed in stages over the next decade. “The unique thing that we’re doing in the district is the design team is going to be using real data in real time to make defined decisions,” says Rose Jones, a medical anthropologist selected by Texas Trees to help guide the design. “Our hope with that is it’s going to lay the foundation for where we all need to be moving, which is: it needs to be empirical.”

As did almost all of the experts I spoke with, Jones emphasized the dearth of research about the interactions between heat, the design of cityscapes, and human health. “Heat planning is in sort of a nascent phase,” says UCLA’s Turner. “The science and the interventions, and what we know will work, lag behind the severity of the problem.” The projects in Dallas and San Antonio will gather just such information, which could prove vital in helping to address the challenges imposed by climate change.