As a rare early-season heat wave sends temperatures soaring across Southern California, new research from UCLA reveals a troubling reality: extreme heat does not threaten all communities equally.
Heat maps developed by the UCLA Center for Healthy Climate Solutions show that emergency room visits during heat events cluster disproportionately in lower-income communities with less tree cover, including South Los Angeles, the San Fernando Valley and East Los Angeles. Formerly redlined neighborhoods — areas once systematically denied investment — are often several degrees hotter than better-rated neighborhoods in the same city.
"Heat maps reveal something temperature data alone can't: which communities are most vulnerable to heat-related health impacts," said Dr. David Eisenman, a physician and professor at the UCLA Fielding School of Public Health and co-director of the center. "Knowing who is hit hardest is essential for planning targeted interventions — especially as heat waves grow more frequent and severe."
The findings carry urgent implications as record high temperatures have already been broken on the first day of the current heat wave, which is expected to continue. Latino and Black communities face elevated risk of heat-related illness, due in part to the physical characteristics of their neighborhoods.
Census tracts with the least access to air-conditioned public spaces such as libraries and shopping malls saw a 38 percent increase in heat-related deaths for every degree Celsius rise in maximum temperature, according to UCLA research. The protective effect of walkable access to cooled spaces held even after researchers adjusted for social vulnerability.
Some neighborhoods in the northeast San Fernando Valley experience twice the rate of excess emergency room visits compared to the Los Angeles County average during heat events.
Yet the research also points to solutions. UCLA researchers found that increasing tree canopy and improving the reflectivity of roofs and pavements could reduce temperatures by up to 2 degrees Celsius during heat events. Under plausible scenarios, such interventions could prevent 25 to 50 percent of heat-related emergency room visits, with even greater benefits in the most vulnerable communities.
Heat remains the leading weather-related cause of death, yet it often escapes public attention.
"Heat kills quietly," Eisenman said. "There is no dramatic footage or visible destruction for the cameras. A person dies alone in a hot apartment, and the death certificate may say cardiac arrest, not heat."
Outdoor workers, older adults and people without air conditioning face the greatest danger. Many cardiac, blood pressure and psychiatric medications impair the body's ability to regulate temperature, putting older adults on these drugs at particular risk without their knowledge.
Eisenman urged residents to reject the assumption that living in Southern California provides natural immunity to heat waves.
"Your body only adapts to heat through repeated exposure to it, and most people on the California coast simply do not get that exposure because the weather is so mild most of the year," he said. "The assumption that we already know how to handle heat is one of the things most likely to get people killed."
For those without air conditioning, experts warn that fans alone can be dangerous when temperatures exceed 95 degrees Fahrenheit. Above that threshold, fans circulate hot air and accelerate fluid loss without providing meaningful cooling. The most effective immediate steps include applying cold water to the neck, armpits and groin, taking cool showers or getting to an air-conditioned space.
The single most important action during a prolonged heat event, Eisenman said, is checking on vulnerable neighbors and relatives — in person.
"Not a text. Go knock on the door and see them for yourself," he said. "People who live alone, particularly older adults, are at elevated risk during heat events because no one notices when they begin to deteriorate."
Analysis of the 1995 Chicago heat wave showed that neighborhoods with stronger social ties had lower death rates even after controlling for socioeconomic factors.
Residents can check their neighborhood's heat risk using CalHeatScore, a statewide tool that ranks extreme-heat health danger by ZIP code and links users to nearby cooling resources. The UCLA Heat Maps, which visualize community-level health burdens from 2008 to 2018, are currently informing Los Angeles County's Heat Action Plan.
Climate projections show more frequent and severe heat events coming to Southern California, with more humid nights that make recovery during sleep increasingly difficult.