A woman sits on a city park bench while wiping her brow in the midday heat.

Extreme Heat Hazards

Increasingly frequent extreme heat days are putting more people’s health at risk.

By Jackie Powder

Less than a month into Summer 2024, extreme heat has blanketed the U.S., previewing a season of heat advisories and air quality alerts. The high temperatures come with a long list of health risks, especially for people over 65 and children, as well as those with chronic conditions.

Heat-related deaths in the U.S. have increased from approximately 1,602 in 2021 to 2,302 in 2023, according to the U.S. Department of Health and Human Services. And extreme heat can also have societal impacts, contributing to rising violence, declines in worker productivity, and difficulty concentrating in school, says Jaime Madrigano, ScD, MPH, an associate professor in Environmental Health and Engineering.

In a Q&A, Madrigano, who is leading a project exploring heat vulnerability and environmental equity in New Orleans neighborhoods, discusses the increase in extreme heat events, the effect on unprotected communities, and why heat is known as the silent killer.

Is there a standard definition of extreme heat?

Typically, extreme heat is defined locally, based on historical norms of a given area. Sometimes communities put mitigation measures into effect when a threshold is reached for a day or two. But we’re learning from several studies that the temperature thresholds that some communities use to activate alerts and advisories aren’t actually aligned with when health impacts start to occur. For example, one study found that in some regions of the U.S., excess hospitalizations occurred at lower heat index thresholds than those which are used to trigger heat alerts. They may be using a threshold based on a larger area or they may not be accounting for all the vulnerabilities in the community. So it is important for public health scientists and practitioners to do heat and health studies in a given location.

Is extreme heat becoming normal?

We are seeing more frequent and successive extreme heat days, and the season over which we tend to see them is lengthening, depending on where you are in the country. Those trends are going up as climate change continues, and we’re expected to see another record breaking summer. We’re also starting to see extreme heat events more often in locations that have historically not experienced these high temperatures, such as New England, and that means that you have a greater number of people at risk for exposure.

Heat is and has been for a long time the number one weather-related killer in the United States.

When we talk about extreme heat the focus is generally on outdoor temperatures, but what are the concerns about heat risks indoors?

The indoor environment is often something that as scientists we have less access to measure in studies. For people who are at risk, we know that air conditioning is the most effective adaptation measure in terms of keeping them protected and safe. Sometimes in non-air conditioned homes, the temperature can rise higher than the levels outside. Typically, [in extreme heat events], when we see people most at risk either dying in their home or becoming ill, they don’t have, or are not using, air conditioning.

In New Orleans last summer, we sampled a particularly low-income population to measure temperatures in homes and almost all [households] had some form of air conditioning—it’s the South—but many were also facing high utility bills. If someone has only one window unit and they’re trying to keep multiple rooms cool or are running it intermittently to keep utility bills down, it may not be possible to keep the house cool enough.

There are also areas of the country that have had extreme heat events that typically don’t have a high penetration of air conditioning, so there’s a greater at-risk population. That was the case in the Pacific Northwest in 2021.

How does equity come into the picture in the context of extreme heat?

A lot of work has looked at the current urban landscape where places were redlined compared to areas that weren’t redlined. In study after study, all across the country, those redlined areas have far fewer trees and they tend to be hotter. They don’t have the environmental amenities for cooling provided by vegetation and green spaces. There’s more impervious surfaces—black top, concrete, and asphalt—that tend to trap heat, and generally, more pollution. And many studies that have looked at indicators of socioeconomics, or race and class, have shown that communities within cities that have higher proportions of low-income populations or communities of color tend to be areas that have been disinvested in for long periods of time, and they tend to be hotter.

Some cities are starting to think about ordinances, similar to those that require landlords to run heat during the winter, to protect people from heat. And some have already put into effect thresholds for indoor temperatures in terms of heat in the summer. A cooling ordinance in Phoenix, Arizona, requires a minimum temperature for cooling and ventilation in rental units; and Chicago adopted an ordinance that requires certain residential buildings to cool common areas.

The term “Heat Katrina” has popped up in the media. Could it happen?

Heat is and has been for a long time the number one weather-related killer in the United States. It’s known as this silent killer because it’s not like a Katrina. We’re concerned about major storms and hurricanes and the public health impacts they can have, but those are very obvious weather events. You can see the physical displacement. You can see the physical exposures, whereas you don’t see that with heat. We see thousands of heat-related illnesses and deaths, but oftentimes they aren’t counted as heat hospitalizations or heat deaths because of the way we code things, so we know the numbers are often much higher than what might be reported, even after a particularly high-temperature heat wave.

I don’t know that we’re going to see a Heat Katrina, but we’ve been seeing, in an ongoing way, high numbers of adverse health consequences associated with extreme heat and we expect that to continue to rise.