The preliminary death toll from two recent heat domes that struck the United States this July likely understates the full impact of this summer’s extreme temperatures, experts caution. The delay and difficulty in accurately attributing deaths to heat highlight shortcomings in the U.S. approach to assessing fatalities linked to severe weather events.
Most states rely on official death certificates to report heat-related deaths, but these documents can take weeks to be issued and often omit heat as a contributing factor. “These are mass mortality events,” said Kristie Ebi, an epidemiologist at the University of Washington’s Center for Health and the Global Environment. “Without clear numbers, it is challenging for individuals and policymakers to grasp the full danger posed by extreme heat.”
In contrast, many European countries use an excess mortality analysis method, which compares the number of deaths during a heat event to expected baseline levels. This approach was widely employed during the coronavirus pandemic and has been applied to recent heat waves, revealing substantial undercounts in official tallies. European officials attributed more than 10,000 excess deaths to the continent’s record-breaking heat wave in June.
Joan Casey, an environmental epidemiologist at the University of Washington, emphasized that the U.S. possesses the capability to implement similar analyses but must prioritize real-time understanding of heat-related mortality. “The methods are fairly standard, but we need to collectively decide that this is important,” she said.
To estimate fatalities from the two July heat domes, inquiries were made to state health departments across 31 states where extreme heat warnings were issued by the National Weather Service. Responses varied markedly: nine states indicated it could take weeks or months to complete death record analyses, four did not respond, and two states—Ohio and Wyoming—reported not tracking heat-related deaths. Among the reporting states, New Jersey recorded the highest number of heat-related fatalities, estimating 29 deaths, though Ebi noted this does not necessarily indicate worse conditions or preparedness compared to other states. It is possible that unrecognized deaths elsewhere remain uncounted.
The heat waves brought temperatures exceeding 100 degrees Fahrenheit across more than two dozen states, shattering previous all-time highs. Millions faced prolonged exposure to these extreme conditions. Researchers from the World Weather Attribution network concluded that the intense combination of heat and humidity observed in early July would have been virtually impossible without climate change driven by greenhouse gas emissions.
Extreme heat exerts significant physiological stress, increasing heart workload to dissipate heat and triggering heavy sweating to conserve water. Failure to maintain core body temperature can lead to organ failure. According to Garyfallos Konstantinoudis, an epidemiologist at Imperial College London, focusing solely on heat stroke underestimates the scope of heat-related deaths. Individuals with chronic illnesses such as cardiovascular disease or diabetes are especially vulnerable, as heat-induced strain can provoke fatal complications.
Death certificates often list immediate causes such as heart attacks or kidney failure, with environmental factors like heat rarely recorded. This is compounded by the fact that many coroners, who are often elected officials with varying medical expertise, may not identify heat as an underlying contributor. This leads to underreporting and delayed recognition of heat-associated mortality.
Maryland exemplifies a rigorous approach, where at least 19 heat-related deaths in early July were determined through comprehensive investigations, including scene assessments and postmortem examinations, according to state health officials. This process contrasts with more straightforward classifications of deaths from floods or hurricanes. “Heat-related deaths can be more difficult to definitively identify because heat often acts as a physiologic stressor,” noted Department of Health spokesperson David McCallister.
Because many heat-related fatalities go unrecognized, the phenomenon has been dubbed a “silent killer.” Excess mortality analysis serves as a critical tool to estimate deaths missed by traditional methods. While it cannot pinpoint individual cases, it provides more timely data on overall mortality spikes during heat events.
European countries widely use this method to inform public health responses. For instance, Spain’s public health agency publishes daily heat death estimates, and France revised its early June heat wave death toll upward to more than 5,700 within three weeks. These figures reflect not only temperature trends but also challenges in adapting to heat—air conditioning remains uncommon in many parts of Europe, the fastest-warming continent.
Beyond immediate health impacts, excess mortality studies help quantify climate change’s role in escalating heat-related deaths. Konstantinoudis and colleagues modeled temperature and mortality data to estimate that anthropogenic warming contributed to approximately 38 percent of heat wave fatalities in England and Wales, equating to more than 800 deaths.
“This gives us an important argument for climate mitigation,” Konstantinoudis said, highlighting the need for accurate and timely mortality data to drive both public awareness and policy action. Experts argue that improving U.S. monitoring and reporting of heat-related deaths is essential as extreme heat events grow more frequent and severe in a warming world.