This storyoriginally appeared on High Country News and is part of the Climate Desk collaboration.

If heat is the enemy, Marcela Herrera thought she was ready for battle last summer at her familys north Los Angeles apartment.

Old air conditioner units chugged away on windows in three rooms. Extension cords snaked into box fans on the floor, positioned along a hallway to push cooler air towards warmer spots. Bamboo shades, bent blinds and curtains beat back the sun.

But none of that prevented her eldest son, Edwin Daz, from getting a nosebleed each time a heat wave crested over the familys dense working-class neighborhood. And as outdoor temperatures climbed into the 90s, the 17-year-old suffered painful, debilitating migraines. The family doctor recommended that he try to stay cooler for the sake of his health.

Western communities, including Los Angeles, are aware that urban heat is a serious and growing threat to public health, and the warming climate only increases the problem. Its not as visible as other catastrophes, but the implications can be far reaching, says Elizabeth Rhoades, who works on climate issues in Los Angeles Countys Department of Public Health.

Predictions are for longer, more frequent, and more severe heat events throughout the Southwest, especially in Los Angeles and Phoenix. Studies in the last decade suggest that heat especially impacts very old and very young city dwellers, poor neighborhoods, and those without central air conditioning: people like Edwin Daz and Marcela Herrera. But researchers are still learning about how people are affected by excessive heat in the places where they spend most of their timeinside their homes. Few policies exist to protect the most vulnerable, and doctors say the conditions are poorly tracked.

Heat is sneaky. It worsens pre-existing conditions, such as heart and lung disease, kidney problems, diabetes and asthma, more often than it kills directly. People end up going to the hospital because heat affects their health, makes their asthma worse or something worse, says David Eisenman, a professor of medicine and public health at UCLA. But its not technically coded as that in the records. Its coded as worsening asthma. So we really undercount the number of cases where heat is a factor.

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And urban heat is layered. Los Angeles is as much as 6 degrees hotter than surrounding areas because of whats called the heat island effect. Sprawl defines not just heat islands but what some call an archipelago of high temperatures across modern urban areas. Geography, wind patterns, tree cover and concrete all work to create hotspots where temperatures are higher and air pollution is worse. In fact, climate models suggest that Herreras San Fernando Valley neighborhood, far from ocean breezes, will warm 10 to 20 percent faster than the rest of Los Angeles.

Theres been this assumption that we can all cool off somehow. And in some ways that might have been true 100 years ago, Eisenman says. We dont have access to the natural cooling environment like we did before.

The landscapes cooling elements disappeared long before Edwin Daz and his mother arrived in the valley. Their Pacoima neighborhood derives its name from the Native Tongva word for a place of running water. (These days, the now concrete-locked Pacoima Wash, a flood-control channel, is often dry.) After World War II, the neighborhood boomed when developers marketed boxy homes to African-Americans shut out of other parts of the valley by racial covenants.

Today, Pacoima is overwhelmingly Latino. And its single-family homes have produced a complex urban density, says Max Podemski, planning director for the community advocacy group Pacoima Beautiful. Lawns have given way to paved-over yards. Second-dwelling units, divisions within ranch homes, and modified garages can house several families together.

Thats just totally ubiquitous here, Podemski says. And these converted dwellings, uncounted and unpermitted, may or may not have insulation or air conditioners or windows to catch a breeze: The city just doesnt have data about it.

To understand more about how heat moves through Pacoima housing, last summer I built small electronic sensors to record dozens of heat and humidity measurements an hour, during parts of August, September and October: the hottest months in Los Angeles. One sensor went in Edwins bedroom.

In early afternoon, that sensor recorded temperatures equal to those recorded outside, at the weather station at Van Nuys Airport. Evening temperatures in Edwins room were up to 9 degrees higher than outside.

Those results tell a similar story to what a group of researchers, community activists and scientists found in about 30 homes equipped with similar sensors in New Yorks Harlem last year. Buildings have a memory for heat, says Adam Glenn, the founder of AdaptNY and a member of the community climate change observation project, ISeeChange. In New York, old stone buildings hold onto thermal radiation, especially on higher floors, late into the night. So the danger to people continues even when the heat wave is over.

But the ways buildings respond to climate vary. In Herreras apartment, a lack of insulation, common in older California houses, may be the key factor. In the evening, she says, We can feel the warmth in the walls.

The blanket of heat smothering LA hasnt escaped City Halls notice. Mayor Eric Garcetti has set an ambitious goal to lower the citys overall temperature 3 degrees in 20 years. LAs Office of Sustainability is studying where and how to deploy landscape-level cooling strategies, such as planting trees and developing cooler pavements. But it will take years to even know whether the goal is achievable.

In the meantime, renters like Herreras family battle excessive heat mostly alone. According to the Census Bureaus National Housing Survey, half as many rental properties in Los Angeles have central air as do owner-occupied units. Coping costs money. In summer, Herreras power bill can be as high as $200 a month.

As temperatures rise in the Southwest, so do the stakes for city dwellers. In Phoenix, the Maricopa County Health Department has closely tracked heat-related death for more than a decade, producing an exhaustive report each year breaking down cases by age, ethnicity, economic background and other risk factors.

Arizona State University researchers are working with Maricopa and Los Angeles counties to better understand how heat causes sickness and death, and how to counteract it.

Many of us believe that no one should die prematurely because of heat, and there are significant public costs associated with heat just in the health-care sector alone, says David Hondula, an ASU climatologist who studies heat impacts. Heat-associated deaths are climbing in Phoenix, but the reasons remain unclear. If we cant even answer that question, figuring out the best strategy to keep Phoenicians safe, or residents of Los Angeles safe, in a future that is expected to be warmer than it is today, would seem almost impossible, Hondula says.

With summer coming, the Daz-Herrera family has made some changes, insulating the ceiling of Edwins room and adding more air conditioners.

Paying for this has meant skimping elsewhere: fewer outings, no new clothes. Herrera worries that tight finances will force them to turn the air conditioners off. Still, all the changes weve made are helping us, she says. Its better to invest a bit more because health comes first.

Molly Peterson has been covering the environment with a focus on water and climate change since 2002. Formerly with NPR and Southern California Public Radio, she now writes for all kinds of public media outlets, and contributes to Laws and Nature, which tracks environmental policy. Shes based in Los Angeles.

This story was made possible with support from the Center for Health Journalism at The University of Southern California, while iSeeChange contributed heat sensor data.

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