Evan Michael Shannon, MD, MPH, Assistant Professor of Medicine at UCLA David Geffen School of Medicine
LOS ANGELES — When evaluating the effects of extreme heat events, California veterans with common cardiometabolic diseases have increased risk of mortality, particularly those who reside in lower socioeconomic status neighborhoods or are experiencing homelessness, according to a recent study.
The time-stratified case-crossover study published in JAMA Network Open investigated whether extreme heat events are associated with increased risk of mortality among California veterans with cardiometabolic disease. The study also examined how this risk differs by race and ethnicity, neighborhood health-related social risk factors and history of homelessness.1
Study authors are affiliated with UCLA David Geffen School of Medicine in Los Angeles; Stanford University School of Medicine in Stanford, CA; VA Greater Los Angeles (CA) Healthcare System; and VA Palo Alto (CA) Healthcare System.
Extreme heat events, which are periods in which temperatures are significantly higher than usual, will become progressively more common as global mean temperatures increase, the Intergovernmental Panel on Climate Change reports. Extreme heat is the deadliest in North America, and this risk is intensified among people with underlying cardiometabolic disease, who are less physiologically capable of adapting to the increased cardiac workload. Many classes of medications used to treat cardiometabolic disorders can also increase risk from heat exposure, the study pointed out.
In addition, veterans have risk factors that place them at high risk of adverse outcomes associated with extreme heat events. Veterans who seek VA care are typically older and have higher rates of cardiometabolic disease compared with nonveterans with advanced age and cardiometabolic comorbidity. Research has been limited on the associations between extreme heat events and adverse outcomes for veterans and high-risk veteran subpopulations, the authors explained.
“I noticed that, while heat waves have been linked to death in the general U.S. population, this had not been studied in veterans, who might be at even greater risk due to higher rates of cardiac risk factors and older age compared to the general U.S. population,” Evan Michael Shannon, MD, MPH, assistant professor of medicine at UCLA David Geffen School of Medicine, told U.S. Medicine.
This study focused on California veterans, because extreme heat events are projected to disproportionately increase in the western U.S., and one-third of veterans experiencing homelessness reside in California, according to the investigators.
Click to Enlarge: Map of California With Number of Extreme Heat Days Using National Centers for Environmental Information Data and 95th Percentile Threshold White spaces denote regions without unique zip code. Source: JAMA Network Open
The researchers used administrative data from the VA Corporate Data Warehouse to identify veterans with a California address from Oct. 1, 2015, to Sept. 30, 2021, and a diagnosis of a cardiometabolic condition of hypertension, diabetes, ischemic heart disease, congestive heart failure, chronic kidney disease, stroke or peripheral arterial disease.
Extreme heat events were defined by daily maximum temperatures greater than the 90th, 95th and 97.5th percentiles of 2006 to 2020 historical normal temperatures from National Center for Environmental Information meteorological data. Patient residential addresses were linked with the nearest weather station. The associations of same-day to 4-day extreme heat events with mortality were assessed.
The study’s main outcome was individual all-cause mortality during warm months (April-October). A time-stratified case-crossover design was used to investigate associations between extreme heat events and mortality. Subgroup analyses were performed by race and ethnicity, Area Deprivation Index (ADI) for patient address and homeless status.
Among the 277,876 veterans with a diagnosis of one or more cardiometabolic conditions who resided in California between 2015 and 2021, the authors determined that 13,556 veterans (97.9% men; median age, 78 years old) experienced mortality during the study period. Extreme heat events were significantly associated with increased risk of mortality for same-day extreme heat events at the 95th percentile of historical mean temperatures.
The analysis also found that veterans residing in lower socioeconomic status neighborhoods and veterans experiencing homelessness had increased risk of mortality compared with those living in higher socioeconomic status neighborhoods and housed veterans, although these differences weren’t statistically significant.
In California veterans with common cardiometabolic disease, there was an estimated increased odds of mortality of 10% to 18%, depending on the temperature threshold used to define extreme heat events and the extreme heat event duration, the study reported.
“It is important to note that extreme heat can be deadly for veterans with common cardiometabolic conditions, such as hypertension and diabetes,” said Shannon, who is also affiliated with VA Greater Los Angeles Healthcare System. “These risks may be even greater for lower income and homeless experienced veterans.”
“Physicians may use these findings such that during clinical encounters, especially in usual care settings, they can discuss with patients ways to mitigate risk from extreme heat,” he said. “For example, they may provide guidance on ensuring adequate hydration during heat. Or to know the location of the nearest cooling centers.”
In addition, physicians can use these findings to advise patients about the high risk of adverse events posed by heat waves, including death, Shannon added.
“It will be important for the VA and other integrated health systems to develop comprehensive plans so that patients know how to protect themselves during heat waves, which are expected to increase in frequency and intensity,” he recommended.
The authors suggested this study can inform the development of heat-response plans within and outside the VA that incorporate anticipatory alerts directed at socioeconomically vulnerable subgroups based on forecasted extreme heat event severity and length.
The findings are in accordance with other analyses that have shown increased risk of mortality during extreme heat events, though no previous studies have specifically investigated all-cause mortality among veterans with cardiometabolic disease.
Limitations of the study include having no measure of indoor heat exposure, so patients may have had access to air conditioning that would have reduced exposure to outdoor heat. Also, the definition of homelessness might not have encompassed all veterans experiencing homelessness.
The study included veterans enrolled in homeless programs, including transitional housing programs, and the researchers couldn’t determine shelter status based on the data. Lastly, the findings might not be generalizable to veterans outside California and are likely not fully generalizable to non-VA users because of differences in VA care delivery compared with health care available to the general U.S. population.
- Shannon EM, Chen L, Yuan A, Chary A, Gabrielian S, Eisenman DP, Washington DL. Extreme Heat, Social Factors, and Mortality Among California Veterans With Cardiometabolic Disease. JAMA Netw Open. 2025 Nov 3;8(11):e2545524. doi: 10.1001/jamanetworkopen.2025.45524. PMID: 41288973; PMCID: PMC12648351.
