The study’s scale and methodology set it apart from earlier work. Rather than treating air conditioning as a simple yes-or-no variable, the researchers constructed county-level estimates of annual average household AC usage and then examined how the relationship between daily temperature and mortality shifts across that usage gradient. County-specific temperature–mortality associations were first estimated for each county in the contiguous United States, and those estimates were then pooled through a meta-regression framework keyed to the counties’ average AC usage. This two-stage design allowed the team to move beyond correlation and characterize precisely how the shape of the temperature–mortality curve changes as cooling becomes more common in a population.

The technical results are striking in their internal consistency. When the researchers compared counties at the 10th percentile of household AC usage with counties at the median, three things happened simultaneously. First, the minimum mortality temperature—the outdoor temperature at which deaths are lowest—shifted downward, meaning that the human body’s apparent comfort zone extended toward cooler values. Second, the range of temperatures above that minimum mortality point for which mortality showed no statistically significant increase widened considerably, effectively flattening the most dangerous portion of the heat–death curve. Third, and most directly, the mortality odds ratio at the 95th percentile of temperature fell from 1.022, with a 95 percent confidence interval of 1.011 to 1.033, to 1.008, with a confidence interval of 0.999 to 1.017. In practical terms, an extremely hot day that would have raised mortality risk by roughly two percent in a low-AC county raised it by well under one percent where air conditioning use reached the median.

Perhaps the most policy-relevant discovery in the analysis, however, is what happened beyond the median. When household AC usage climbed further above that midpoint, the mortality odds ratio did not continue to fall. Instead, the benefit plateaued, indicating that the protective effect of residential cooling saturates once usage reaches roughly half of households in a county. Above that threshold, additional adoption delivers diminishing returns for population health. This plateau has profound implications for how governments think about adaptation investments: the priority is not maximizing AC penetration everywhere, but lifting the lowest-usage communities—often the hottest, poorest, and most vulnerable—toward the median, where each additional air-conditioned household buys the largest mortality reduction.

The epidemiological logic behind these findings is grounded in physiology. Extreme heat stresses the cardiovascular and respiratory systems, thickens blood, impairs thermoregulation, and disproportionately kills elderly people, people with chronic disease, and those without access to cooled environments. Indoor cooling interrupts this cascade by lowering core body temperature and reducing the physiological strain of hot nights, which are increasingly recognized as a key driver of heat deaths. Earlier research, including a landmark study of the twentieth-century decline in the US temperature–mortality relationship published in the Journal of Political Economy, had pointed to air conditioning as the leading explanation for Americans’ growing resilience to heat. The new study quantifies that resilience county by county, on modern data, and with the statistical machinery needed to separate AC’s effect from other influences such as demographics, healthcare access, and long-term acclimatization.

To build their AC usage estimates, the team relied on a fine-scale dataset of multidimensional household well-being developed by co-authors Narasimha D. Rao and Karthik Akkiraju and collaborators, published in Scientific Data in 2024, which fuses multiple household surveys to produce spatially detailed portraits of American living conditions. Mortality data came from the National Center for Health Statistics Research Data Center of the US Centers for Disease Control and Prevention, while daily gridded weather data were drawn from the PRISM climate database at Oregon State University and population exposures from the LandScan Global one-kilometer population grid. The researchers also conducted an extensive battery of sensitivity analyses—varying temperature time windows, spline specifications, adjustment for fine particulate matter pollution, and restricting the analysis to summer months—finding that the core results held across all of them.

The maps of averted mortality that emerge from the analysis tell a story of deep geographic inequity. The largest avoided death burdens concentrate where the dual conditions of high heat exposure and widespread residential cooling coincide, while counties in the Southeast, Southwest, and parts of the Midwest show the biggest absolute benefits. Conversely, regions where heat risk is rising but AC adoption lags—often because of poverty, aging housing stock, or energy insecurity—stand out as areas of unmet need. The study’s authors note that in 2020, 27 percent of US households reported difficulty meeting their energy needs, according to the US Energy Information Administration, a reminder that the cooling that saves lives is itself unevenly affordable. Energy burden, in this framing, is a direct mortality risk factor.

Yet the paper is careful not to present air conditioning as an unqualified good. The authors explicitly underscore the need for strategies that balance the survival benefits of AC against the harms of overuse, which include surging electricity demand on the hottest days, greenhouse gas emissions from fossil-fueled generation, waste heat vented into urban streets, and the refrigerant emissions that potentiate further warming. The plateau finding sharpens this calculus: because health benefits saturate near median usage, the marginal emissions cost of pushing adoption far beyond that level yields little additional protective return. The rational adaptation portfolio, therefore, pairs targeted expansion of cooling access for vulnerable and low-usage populations with efficiency standards, grid decarbonization, passive cooling design, urban shade and reflective surfaces, and community interventions such as cooling centers, whose public health effectiveness has been reviewed in the European Journal of Public Health.

The stakes of getting this balance right will only grow. The Lancet Countdown’s 2024 report documented record-breaking climate-related health threats, and studies of population aging project that temperature-related mortality will rise substantially at higher levels of global warming even under optimistic scenarios. The research team’s companion work, published in JAMA Network Open in 2025, estimated the heat and cold mortality burden in the US from 2000 to 2020, providing the baseline against which the new averted-death figures are measured. Together, these findings reframe household air conditioning from a comfort appliance into critical health infrastructure—whose reach, affordability, and carbon footprint will help determine how many people the coming decades of heat will claim. As climate change pushes temperatures past thresholds the human body cannot tolerate, the authors conclude, ensuring equitable access to safe indoor cooling while managing its energy costs stands as one of the defining adaptation challenges of the century.

Subject of Research: The effect of household air conditioning usage on heat-related mortality across the contiguous United States

Article Title: Impact of household air conditioning usage on heat-related mortality in the USA

Article References: Chu, L., Akkiraju, K., Rao, N. D., Dubrow, R., & Chen, K. (2026). Impact of household air conditioning usage on heat-related mortality in the USA. Nature Health. https://doi.org/10.1038/s44360-026-00184-0

Image Credits: AI Generated

DOI: 10.1038/s44360-026-00184-0

Keywords: air conditioning, heat-related mortality, climate adaptation, public health, temperature–mortality association, heat waves, energy insecurity, meta-regression, epidemiology, United States, climate change, mortality burden