New data show extreme heat waves were associated with higher all-cause mortality throughout the United States, with increased risks seen among men, the elderly, and non-Hispanic Black individuals.
Around the world, millions of individuals die each year from complications brought on and exacerbated by air and water pollution, while reducing air pollution from a single emitter—the burning of fossil fuel—could save 50,000 lives annually in the United States alone.
But pollution-associated deaths are not the only casualty of a warming planet.
This week, headlines from global news organizations documented the extreme toll of rising temperatures in India and Pakistan, and new research published today in JAMA Network Open detailed to what extent heat is linked with all-cause mortality in the United States.
Findings revealed extreme heat was associated with higher county-level all-cause mortality, with greater increases seen among certain populations including men, older adults, and non-Hispanic Black individuals.
The data represent the effects of extreme heat in the contiguous United States measured between 2008 and 2017, 4 years prior to the hundreds of heat-related deaths seen throughout the Pacific Northwest last July.
“Extreme heat is already associated with a higher mortality rate across the contiguous United States. As extreme heat events increase over the coming decades, this will likely become an even greater issue,” said lead author Sameed Khatana, MD, MPH, in an email to The American Journal of Managed Care® (AJMC®). He continued, “efforts to mitigate the effects of extreme heat on the health of vulnerable populations is crucial to prevent these disparities from widening further.”
Khatana is an assistant professor of medicine at the Perelman School of Medicine, University of Pennsylvania, and physician at the Philadelphia Veterans Affairs Medical Center.
Heat can be deadly in a number of ways. Not only does extreme heat exacerbate wildfires, but it can also lead to power outages, cutting off access to air conditioning or home medical devices, and result in heat stroke or render medications less effective.
As previous research has focused mainly on higher mortality rates in urban areas, due in part to the urban heat island effect, investigators set out to assess any heterogeneity across different populations experiencing extreme heat.
For the cross-sectional study, extreme heat was defined as a maximum heat index greater than or equal to 90°F and in the 99th percentile of the maximum heat index in the baseline period (1979 to 2007), authors explained.
Each year, around 700 deaths in the United States are directly attributed to heat exposure, but the total from any cause related to extreme heat is likely higher, they noted. In the current study, extreme heat days were associated with an average 1373 additional deaths per year, with variations present depending on the definition of extreme heat employed.
Different aspects of built environments, including tree cover and air conditioning use, impact the relationship between human health and heat, while “vulnerable and historically oppressed communities may be disproportionately affected by extreme heat because of differences in the burden of medical comorbidities, access to health care, and living in areas at greater risk of extreme heat exposure.”
To conduct the analysis, researchers assessed heat index data from every county in the contiguous United States for each day between May to September of the years studied. Heat index information combines that of air temperature and relative humidity to indicate the temperature humans perceive. Data from 1979 through 2007 were used as a baseline period with heatwaves defined as 3 or more consecutive extreme heat days.
Mortality data were gleaned from the National Center for Health Statistics; rates were age-adjusted based on 2000 US Census Data.
In 2008, 219,495,240 adults aged 20 years and older resided in the studied counties. Of these just over half were female and nearly 18% were over the age of 65. In all 3108 counties assessed, between 2008 and 2017 there was a median of 89 extreme heat days (IQR 61-122) throughout the summer months.
Adjusted analyses showed each additional extreme heat day a month was linked with 0.07 additional deaths per 100,000 adults (95% CI, 0.03-0.10 death per 100,000 adults; P = .001).
For older vs younger, male vs female, and non-Hispanic Black vs non-Hispanic White adults, increases in mortality rates rose to 0.19, 0.12, and 0.11 deaths per 100,000 individuals, respectively.
Geographically, extreme heat days recorded throughout the study period were concentrated in the southern, midwestern, and western United States, while the association with all-cause mortality was significant in metropolitan counties but not in non-metropolitan counties.
Furthermore, “the association between extreme heat days and all-cause mortality was essentially unchanged when the number of poor air quality days per month or the proportion of land that is forested and land that is developed were included in the primary model,” researchers found.
Elderly individuals and those with medical comorbidities are at a heightened risk of heat-related poor outcomes, as they have impaired thermoregulation. Projections also estimate the United States will suffer an additional 20 to 30 extreme heat days per year by 2050, exacerbating the problem already compounded by a growing elderly population.
Taken together, these factors may lead to sharp increases in health care utilization like increased emergency room visits or hospitalizations. “As extreme heat events increase in frequency, intensity, and duration across the country, these trends may continue to increase and potentially increase the stress on health systems' capacity to take care of patients, particularly from vulnerable populations,” Khatana said.
To prepare for a potential influx of patients during a heat wave, health systems and emergency medical services can build up their capacity during times of the year when heat waves are more common.
“Similar to how a hospital may prepare for a flu season, by working with meteorological and environmental agencies, hospitals may be able to predict when an extreme heat event is likely and begin to prepare for that,” Khatana explained, adding ensuring adequate staffing levels, increasing emergency department capabilities, building cooling centers, and increasing outreach to vulnerable populations could all contribute to this process.
As a greater number of non-Hispanic Black individuals reside in urban areas, this factor may account for the increased toll of extreme heat seen in this population. But additional socioeconomic factors such as lower access to health care could also play a role.
“Like many other public health challenges in the United States, extreme heat has had a disproportionately higher impact on the health of minoritized groups such as non-Hispanic Black individuals,” authors wrote, adding Black households have been shown to produce lower per capita carbon emissions compared with White households, once again highlighting the disproportionate impact of climate change on those who contribute least.
Although the study was not designed to identify causes of increased mortality among subgroups, Khatana explained that men tend to have a greater prevalence of cardiovascular risk factors and disease which have been shown to be linked with heat-related mortality. Men of working age also tend to account for a higher proportion of the agricultural and construction workforce, meaning they come into direct contact with heat more often.
Due to the observational nature of the study, no causal conclusions can be drawn and residual confounding may have been present.
Khatana SAM, Werner RM, and Groeneveld PW. Association of extreme heat with all-cause mortality in the continuous US, 2008-2017. JAMA Netw Open. Published online May 19, 2022. doi:10.1001/jamanetworkopen.2022.12957