Winter Races More Dangerous Than Heat

Courtney Bruce and Adelina Sotnikova running in a cross-country race with a large pack

Cold mornings, not heat waves, turned out to be the bigger danger for marathon runners in a massive new study out of Japan.

Quick Take

Researchers tracked 4.53 million marathon runners in Japan from 2011 to 2020 and found 75 cases of sudden cardiac arrest during races.
Every 1-degree Celsius drop in starting temperature raised cardiac arrest risk by about 5 to 6 percent.
The danger grew worse in races held during autumn and winter, not summer.
Separate studies on general cardiac arrest, including one from Germany, also link cold and humidity to higher heart risks outside of running events.

A Massive Study Flips The Script On Race-Day Danger

Most runners worry about overheating during a race. A study published through JACC: Advances looked at nearly every marathon held in Japan over nine years and found the opposite pattern. Cold starting temperatures, not hot ones, were tied to a higher chance of sudden cardiac arrest on the course. Researchers tracked 4.53 million race participants and documented 75 cardiac arrest cases during that span.

The research team measured more than just temperature at each race start. They logged humidity, sunlight exposure, wind speed, rainfall, and air pressure for every event in the dataset. After running the numbers, temperature stood out as the clearest weather factor tied to cardiac arrest risk, with each 1-degree Celsius drop raising the odds by roughly 5 to 6 percent. The pattern showed up most clearly in races held during colder months.

Why Winter Races May Hit Hearts Harder

Cold weather makes blood vessels tighten, which raises blood pressure and forces the heart to work harder. Add in the physical strain of running 26.2 miles, and that combination can stress a heart that already has hidden problems. The American College of Cardiology has separately noted that extreme weather, whether too hot or too cold, predictably increases the number of medical emergencies at endurance races.

That same organization pointed to a heat-stress threshold where medical problems spike once conditions get too warm, showing weather affects runners on both ends of the thermometer. Their review found that adverse event rates during endurance races climb considerably once the heat index crosses a certain point, a reminder that extreme conditions of any kind test the body during long-distance effort.

The Pattern Shows Up Far Beyond The Finish Line

Cardiac arrest tied to cold and humid weather is not unique to marathon runners. A time-series analysis published in the journal Heart found that average temperature, daily temperature swings, and humidity were each independently linked to higher rates of cardiac arrest outside hospitals, across the general population, not just athletes.

A ten-year study from the German Resuscitation Registry backed that up, finding that high-humidity days carried a statistically significant increase in cardiac arrests outside hospitals. Even young athletes are not immune to weather-driven risk. A Japanese registry tracking school-age runners found a U-shaped curve, where cardiac arrest risk climbed at both cold and hot extremes, bottoming out around 68 degrees Fahrenheit. Together, these findings suggest race organizers and runners alike should treat cold-weather marathons with the same caution typically reserved for summer heat warnings, since the body’s cardiovascular strain does not wait for warm weather to show up.