What do we know – and need to know - about extreme heat?

Dr. David Eisenman, a physician and professor in the UCLA Fielding School of Public Health's Department of Community Health Sciences, studies extreme heat as a public health threat. He also serves as a professor in residence at the David Geffen School of Medicine at UCLA, in the Division of General Internal Medicine and Health Services Research, as director of the UCLA Center for Public Health and Disasters and co-director of the UCLA Center for Healthy Climate Solutions.

Q: We often hear that it's "hotter in the city." What is the "urban heat island effect," and why does it make certain Los Angeles neighborhoods much more dangerous during a heatwave than others?

Cities are warmer than surrounding areas because urban surfaces such as asphalt, concrete, and roofs absorb solar radiation during the day and emit it back at night. Dense neighborhoods can be several degrees hotter than nearby greener areas, and they stay hotter at night, when the body needs to recover.

The UCLA Heat Maps show that excess emergency department visits during heat events cluster in lower-income communities with less tree canopy and worse scores on the Healthy Places Index, such as South Los Angeles, the San Fernando Valley, and East Los Angeles. Formerly redlined neighborhoods are often hotter than better-rated neighborhoods in the same city. Latino and Black communities are more likely to be at risk of heat-related health problems due partially to these characteristics of their neighborhoods.

The good news is that we know how to change this. We showed that increasing tree canopy and the reflectivity of roofs and pavements can reduce temperatures by up to 2 degrees C during a heat event, and that avoiding 25% to 50% of emergency room visits during heat events is a realistic outcome under plausible scenarios. The benefits are even larger in heat-vulnerable communities like the northeast San Fernando Valley, where some neighborhoods see twice the rate of excess ER visits compared to the county average.

Q: Heat waves don't always make the news like hurricanes or fires, yet heat is the leading weather-related cause of death. Why do we tend to underestimate it, and how many extra deaths do we see on a hot day in Los Angeles?

Heat kills quietly. 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. Heat rarely gets credited as the underlying cause the way a flood or wildfire would. This invisibility is compounded by who dies for instance, in Los Angeles, extreme heat events increase deaths among people experiencing homelessness, a population already hidden from public view.

I should add that in California we may be reaching a point where wildfire smoke overtakes heat as the dominant climate-related cause of death. Rachel Connolly, Miriam Marlier and Michael Jerrett’s team at UCLA found that wildfire smoke contributed to more than 52,000 premature deaths in California from 2008 to 2018 alone. 

The point is that heat and smoke are both under-recognized causes of death, both driven by climate change, and in California, smoke may now be the larger killer. Neither gets the attention it deserves.

Q: If I don't have air conditioning at home, what are the most effective ways to lower my core temperature immediately? Is a fan enough when it's 95+ degrees?

At or above roughly 95 degrees Fahrenheit, a fan alone can make things worse. When ambient air temperature exceeds skin temperature, a fan circulates hot air and accelerates fluid loss without meaningful cooling.

The most effective immediate steps are cold water applied to high-vascular areas - neck, armpits, groin - where blood vessels run close to the surface and heat exchange is efficient. Cool or cold showers can rapidly reduce core temperature. Getting to an air-conditioned space is the most reliable solution, and the research supports this more strongly than is commonly appreciated. Our research found that as maximum temperatures rise, heat-related illness mortality increases more slowly in census tracts with greater walkable access to libraries, shopping malls, and other publicly air-conditioned spaces - the protective effect held even after adjusting for social vulnerability. Census tracts with the least access to cooled spaces saw a 38% increase in heat-related deaths for every degree Celsius rise in maximum temperature. Being able to walk to cool air shapes whether extreme heat kills you, which is why access to public cooling centers is a genuine public health issue and not merely a convenience.

Q: Your research highlights community resilience. What is the single most important thing a person can do for their neighbors or elderly relatives during a multi-day or even weeks-long heat event?

Check on them in person. Not a text. Go knock on the door and see them for yourself.

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.

When you see them, ask whether they have cold water, assess if their home temperature feels comfortable and ask whether they want a ride to a cooling center. This matters in practical terms. Air conditioning ownership in LA is far lower than in inland cities like Phoenix and many times people don’t use their air conditioner because of the added cost. Ask about their medications. Many cardiac, blood pressure, and psychiatric medications - antipsychotics and anticholinergics in particular - directly impair thermoregulation, and older adults on these medications may not recognize how impaired they are.

Q: Does living in a warm climate like southern California make us "immune" to heat waves, or is that a dangerous myth?

It is a dangerous myth. 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. When an unusual heat wave arrives, bodies that have not recently needed to cope with it are suddenly asked to, without the physiological tools to do so. 

Climate projections show more frequent and severe heat events coming to southern California, with more humid nights that make it harder for the body to recover while sleeping. The assumption that we already know how to handle heat is one of the things most likely to get people killed.