Study reveals extreme heat's impact on homeless mortality

By nature of their living situation, people experiencing homelessness (PEH) are considered one of the most vulnerable populations to the health impacts of extreme weather.

PEH are particularly vulnerable to heat, and the impact of heat on mortality in this group is substantially greater than for the general population, according to a new study by Boston University School of Public Health (BUSPH).

Published in the American Journal of Epidemiology, the study examined mortality rates in two hot-climate US counties-;Clark County in Nevada, which includes Las Vegas, and Los Angeles County in California-;and found that daily heat played a significant role in mortality among PEH.

This association was especially notable in Clark County, where nearly 50 percent of deaths during the study period were attributable to higher daily temperatures. Although LA County observed a smaller percentage of heat-attributable deaths, at 5.2 percent, daily heat appeared to contribute to many more deaths than daily cold in both counties.

The study is the first to examine heat-attributable mortality among PEH, providing valuable public health insight as the West Coast reels from an unprecedented October heatwave that has prompted numerous heat alerts in parts of LA and Las Vegas, where temperatures soared to 111 degrees and 104 degrees, respectively, in recent days. Both cities also continue to grapple with persistently high homelessness rates that far exceed the national average. Quantifying the impacts of extreme heat on PEH can inform new interventions and policies that reduce illness and mortality rates among this largely unsheltered population.

It wasn't a surprise that our team found an association between heat and mortality for unhoused people, but the magnitude was staggering. Our estimates are 10 to 100 times greater than the known associations between daily heat and mortality for the general population in LA and Las Vegas, and this finding highlights the moral imperative for our systems to do more."

Dr. Jonathan Jay, study senior and corresponding author, assistant professor of community health sciences at BUSPH

He says the new findings also show that heat exposure is an important factor amplifying the vast health inequities for PEH, and highlight the need to center the most marginalized populations in research on climate and health.

For the study, Dr. Jay and colleagues from BUSPH and the David Geffen School of Medicine at the University of California, Los Angeles utilized mortality and daily temperature data for Clark and Los Angeles counties from January 2015 to August 2022 (for Clark County) and November 2022 (for LA County). The team analyzed deaths from all causes, rather than just deaths that were acutely heat- or cold-related (such as fatal heat stroke or hyperthermia), to ensure that the analysis captured all external causes of death as a result of high temperatures. Hot weather was defined as daily temperatures above the minimum mortality temperature (MMT)-;the temperature at which mortality risk is lowest in each area-;which was 11.6 Celsius (52.8 degrees Fahrenheit) for Clark County and 19.3 Celsius for LA County (66.7 degrees Fahrenheit).

After accounting for long-term and seasonal trends, days of the week, and cumulative effects of consecutive days' temperatures, the researchers found that mortality risk among PEH increased in both hotter and colder temperatures. The heat was especially harmful in Clark County, where 15 percent of deaths were due to acutely heat-related causes, compared to 0.2 percent in LA County. Extremely hot days accounted for nearly 25 percent of all deaths in Clark County and 2.2 percent of all deaths in LA County.

"It was important to see that deaths increased as daily temperatures increased, from cool days to warmer days, and they spiked on hotter days, starting around 90 degrees Fahrenheit and higher," Dr. Jay says. "That's hot, but it's not even close to the hottest temperatures these cities experience," he adds, which suggests that public health advocates should be extra vigilant in providing heat-related support beyond the summer season, as climate change continues to extend the number and length of heat waves each year.

Public health strategies that can mitigate heat impacts among PEH in communities include cooling centers, water stations, greening, and reflective painting, the researchers say. But, ultimately, these strategies should embrace a "Housing First" approach, as the primary cause of homelessness is a lack of affordable housing. California, in particular, is consistently ranked as one of the most expensive states to live in the US. Policies that promote stable housing and other financial support for PEH are even more critical following the Supreme Court's ruling in June that permits cities to ban people from sleeping and camping in public places.

"Too much of our policy is driven by the impulse to hide homelessness from view, rather than to recognize people's dignity, protect their health, and improve our systems," says Dr. Jay. "The idea that policing is key to solving this problem is false, and it's a miscalculation we make over and over again as a society."

The lead author of the study is Dr. Zihan Lin, a BUSPH postdoctoral fellow at the time of the study and a current assistant professor of biological, geographical and environmental sciences at Cleveland State University.