Hot Days Trigger a Sharp, Short-Lived Rise in Stroke Hospital Admissions, Study Finds

On a sweltering afternoon in Nanning, a subtropical city in southern China, emergency departments may see something more dangerous than heat exhaustion. A new time-series study published in the journal Air Quality, Atmosphere & Health reports that when daily mean temperatures climb well above the local norm, hospital admissions for acute ischemic stroke rise sharply—and the effect is concentrated within just a day or two of the heat exposure. The findings add to a growing body of evidence that ambient temperature is not merely background weather but an active, short-term trigger of one of the world’s leading causes of death and disability.

The research team, led by Xiaoxiao Song of the Second Affiliated Hospital of Guangxi University of Chinese Medicine together with colleagues from the Nanning Hospital of Traditional Chinese Medicine and Guangxi Medical University, analyzed 2,382 hospital admissions for acute ischemic stroke recorded between July 1, 2017, and June 30, 2020. The records came from a single tertiary hospital in Nanning, a city whose humid subtropical climate makes it a useful natural laboratory for studying how heat and cold shape cerebrovascular risk. On an average day, the hospital admitted just over two stroke patients, a modest daily count that nevertheless accumulates into a dataset rich enough to detect subtle weather-related patterns.

Methodologically, the study leans on two statistical workhorses of environmental epidemiology. The first is the quasi-Poisson generalized linear model, which handles daily count data such as hospital admissions and accommodates the overdispersion—variance exceeding the mean—that is typical of such series. The second is the distributed lag nonlinear model, or DLNM, a framework that allows researchers to estimate simultaneously how an exposure like temperature affects risk in a nonlinear way and how that effect is spread across time. Rather than asking only whether a hot day produces more strokes on that same day, the DLNM can trace the risk across a window of lag days, here spanning zero to seven days after exposure, and can compute cumulative effects over any sub-window within that range.

The team anchored its comparisons to the median daily mean temperature of 23.30 degrees Celsius, treating this as the reference point against which hotter and colder days were judged. When the mean temperature rose to 29.60 degrees Celsius—a level well within Nanning’s summer routine—the risk of an ischemic stroke admission increased substantially. The relative risk reached 1.347, with a 95 percent confidence interval of 1.058 to 1.714, when cumulative effects over lag days zero to one were considered, and climbed to 1.396 (95 percent CI: 1.085 to 1.797) over lag days zero to two. In practical terms, on such hot days the hospital could expect roughly 35 to 40 percent more ischemic stroke admissions than on a typical day at the median temperature.

Just as striking as the size of the effect is its timing. The association between high temperature and stroke admissions attenuated as the lag window lengthened, fading over longer cumulative periods. This pattern suggests that heat acts as a near-immediate trigger rather than a slow-burning risk factor: the physiological damage it inflicts on vulnerable patients appears to translate into arterial blockages within hours to a couple of days. That short latency has real operational implications, because it means emergency services and stroke units can anticipate surges in demand almost in real time as heat waves roll through a city, rather than bracing for a delayed wave of cases.

The biological plausibility of a rapid heat effect is well supported by prior research. Heat stress promotes dehydration, which hemoconcentrates the blood and increases viscosity, tilting the hemostatic balance toward clot formation. Sweating-driven fluid loss also reduces plasma volume, and studies of heat-stressed humans have documented measurable changes in coagulation responses. Heat further strains the cardiovascular system by increasing cardiac output and cutaneous blood flow to shed excess warmth, while aging blood vessels lose some of their thermoregulatory reflex capacity. Endothelial function, the ability of blood vessel linings to dilate and maintain smooth flow, is itself temperature-sensitive. Inflammatory and coagulation markers rise in hot conditions, and blood pressure—normally lower in warm weather—can fluctuate in ways that destabilize existing atherosclerotic plaques. Any of these pathways could, in a patient with narrowed cerebral arteries, tip the balance toward an occlusive event within a single hot day.

Cold told a different and less conclusive story. When the mean temperature dropped to 10.50 degrees Celsius, the same-day risk estimate was lower than for heat, and the cumulative associations over longer lag windows were inconsistent. The authors are careful here: they note that the low-temperature findings require cautious interpretation. This asymmetry is not unusual in subtropical settings, where winters are mild and cold extremes are relatively rare, limiting the statistical power to detect cold effects. It also contrasts with studies from temperate and northern Chinese cities, such as Beijing and Guangzhou, where distributed lag analyses have often found robust cold-related increases in stroke admissions with longer lag times. The divergence underscores a central theme in climate-health research: temperature effects are regionally heterogeneous, shaped by local climate norms, housing, air conditioning prevalence, and the physiological adaptation of the population.

Recognizing how easily time-series findings can be artifacts of modeling choices, the researchers ran an extensive battery of sensitivity analyses. They varied how the long-term trend and seasonality were adjusted in the models, added air pollutant concentrations as covariates to rule out confounding by poor air quality, changed the maximum lag period, excluded the year 2020—a year distorted by the COVID-19 pandemic’s disruption of hospital care—and adjusted for the Spring Festival window, during which hospital utilization patterns in China shift dramatically. The main findings held up across these checks, lending confidence that the heat-stroke link is not a statistical mirage. The robustness of the hot-temperature effect, contrasted with the fragility of the cold-temperature signal, reinforces the study’s central conclusion.

The stakes of this line of research are rising with the thermometer. Ischemic stroke imposes an enormous global burden, and analyses of the Global Burden of Disease data show it remains a leading cause of death and long-term disability worldwide, with China bearing a particularly heavy share. The World Stroke Organization has issued a scientific statement on stroke and climate change, warning that warming temperatures will translate into additional cerebrovascular events. Meanwhile, studies using hourly heat exposure data have begun to show that even short bursts of high temperature can precipitate ischemic stroke, and occupational health research documents widespread heat stress in working populations. Against that backdrop, a study pinpointing a one-to-two-day window of elevated risk gives public health authorities something actionable: heat-health warning systems can be tuned not just to warn the general population but to alert hospitals, ambulance dispatch, and thrombolysis-capable stroke centers to prepare for demand spikes within 48 hours of extreme heat.

The study’s limitations are those inherent to its design. It draws on admissions from a single tertiary hospital in one city over three years, so the results may not generalize to regions with different climates or to populations with different demographics and healthcare access. Hospital admissions capture only patients who reach care, and prehospital delay—known to be common in stroke—could interact with weather in ways the data cannot reveal. The authors also emphasize that the cold-temperature association, being less consistent, should not be overinterpreted. Still, the core message is clear and increasingly hard to ignore: in a warming world, the days immediately following a heat spike are precisely when vulnerable brains are most at risk, and health systems that plan for that window may save not just comfort but lives.

Subject of Research: The short-term association between ambient mean temperature and hospital admissions for acute ischemic stroke in a subtropical Chinese city.

Article Title: Association between mean temperature and hospital admissions for acute ischemic stroke: a time-series study

Article References: Association between mean temperature and hospital admissions for acute ischemic stroke: a time-series study. (n.d.). https://doi.org/10.1007/s11869-026-02102-5

Image Credits: AI Generated

DOI: 10.1007/s11869-026-02102-5

Keywords: acute ischemic stroke, ambient temperature, heat, hospital admissions, distributed lag nonlinear model, time-series analysis, Nanning, China, climate change, cerebrovascular disease, public health, epidemiology