Cold Snaps Line Up With Cardiac Arrest Spikes, New Study Finds

On the days when cardiac arrests outside hospitals ran highest, counts climbed nearly a third above a typical day, an extra 19 or so nationwide, and those days shared a common signature: cold, still air. That’s the central finding from a five-year study that tracked every out-of-hospital cardiac arrest recorded by Hungary’s national ambulance service, the kind of sudden, deadly event where a person’s heart stops and paramedics race against the clock to save them.

Researchers compared more than 114,000 cardiac arrest cases against daily weather data collected from late 2018 through the end of 2023, publishing their results in the journal Public Health. Higher cardiac arrest counts were consistently associated with colder temperatures and lower wind speeds, and for every 1 degree Celsius the temperature dropped, daily cardiac arrests rose by 1.4%.

This is not a story about one bad flu season or a single storm. It’s a pattern that repeated across five years, and it held up even when researchers checked their work a second, completely different way, using a separate machine-learning model to test which weather factors were most useful for predicting daily cardiac arrest counts. That second check pointed to the same factors, which is why researchers say weather forecasts could someday give emergency services an early warning, enough lead time to prepare before a surge hits.

How Scientists Tracked Cardiac Arrest Cases in Hungary

Hungary turned out to be a nearly ideal place to study this question. Its ambulance service is centralized, meaning every cardiac arrest case in the country gets logged in one system, with no gaps caused by patients using different hospital networks or private providers. That gave researchers a full national picture instead of a patchwork drawn from a few cities.

After setting aside part of 2020 to avoid mixing in the disruption of the early COVID-19 pandemic, when ambulance protocols and public behavior changed dramatically, scientists were left with 1,584 days of data covering 114,830 cardiac arrest cases. On an average day, close to 61 people suffered a cardiac arrest outside a hospital. Those affected were typically older adults, around 70 on average, and more often men than women.

Weather information came from Hungary’s national weather service and included temperature, wind speed, air pressure, humidity, and pollution levels, all measured as daily national averages. Preliminary checks found strong agreement among weather stations for daily temperature, suggesting the national averages reasonably represented conditions across a country that is not especially small.

Crunching the Numbers on Weather and Heart Emergencies

To connect weather patterns to heart emergencies, the team used a statistical method built for counting daily events, adjusted to account for the fact that some days randomly see more ups and downs in the data than others. They also tested effects across a four-day window, since the body’s stress response to cold or bad air doesn’t always show up the same day something changes outside. Sometimes it takes a day or two, or even three, for the strain to catch up with someone’s heart.

Cold proved the dominant force. Even modest daily drops in temperature nudged the numbers upward. Calmer air mattered too. Wind actually appeared protective, likely because moving air helps clear out pollution and improves air quality overall, and the study found a real drop in cardiac arrests on breezier days.

Winter told the same story from a different angle. Daily cardiac arrest counts averaged close to 66 cases during winter compared with about 56 during summer, a gap of almost 18% that held up across multiple statistical checks.

Confirming the Pattern With Artificial Intelligence

To make sure the effect wasn’t just noise in the numbers, researchers picked out the days with unusually high cardiac arrest counts across the five years, flagging cases that spiked well beyond normal fluctuation. Those flagged days lined up consistently with colder-than-average temperatures and calmer air, matching what the broader statistics had already shown.

Then they ran a second, independent test using machine learning, training a model on the weather data to see which factors were most useful for predicting cardiac arrest surges. It landed on the same two factors: temperature and wind speed. When researchers also fed the model information about how many cardiac arrests had already happened in the days before, its predictions improved considerably, suggesting that pairing recent case history with weather forecasts could sharpen any future warning system.

What Rising Cardiac Arrest Risk Means for Emergency Response

Weather forecasts already give people days of notice before a cold front arrives. That same notice could double as a warning for hospitals and ambulance crews about a likely rise in cardiac emergencies. Because those weather associations can take up to three days to surface, emergency services have a real window to prepare staffing and equipment before a crisis hits, rather than scrambling after it already has.

None of this means weather causes cardiac arrest on its own. Individual risk still depends on personal factors this study could not measure, from age to existing heart disease. What this research adds is a broader pattern across a whole population: when the temperature drops and the air goes still, more hearts stop, and it happens predictably enough that health systems could plan around it instead of being caught off guard.

Disclaimer: This article summarizes findings from a peer-reviewed study and is intended for general informational purposes only. It is not medical advice. The research is observational, meaning it identifies associations between weather and cardiac arrest rates across a population and does not prove that weather directly causes cardiac arrest. Individual risk depends on personal health factors this study did not measure. Anyone with concerns about heart health should consult a qualified healthcare professional, and anyone witnessing a suspected cardiac arrest should call emergency services immediately.

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