Marathons can be risky for hearts, but not necessarily those of the runners. It takes longer for nearby residents to get to a hospital for emergency heart care on the day of a race and they’re less likely to survive, a U.S. study finds.

Any event that draws a crowd and causes traffic detours — parades, ball games, concerts, fairs — may cause similar problems, researchers warn.

It’s more than inconvenience: For every 100 people suffering a heart attack or cardiac arrest, three to four more died within a month if they had sought care on a marathon day versus another time, the study found.

It was published Wednesday by the New England Journal of Medicine, just before Boston’s annual 26.2-mile (42-kilometer) race, set for Monday.

The publication timing was by chance, but “hopefully it will raise some attention around the issue,” says the study leader, Dr. Anupam Jena of Harvard Medical School.

The study included marathons in Boston, Chicago, Honolulu, Houston, Los Angeles, Minneapolis, New York City, Orlando, Philadelphia, Seattle, and Washington between 2002 and 2012. It did not include 2013, when bombs at the Boston Marathon might have caused unusual delays.

Jena conceived the study after his wife entered a race last year and he couldn’t get through traffic to watch her run.

“She made the offhand remark, ‘Gee, I wonder what happens to people who need to get to the hospital during one of these large races,'” he says.

Researchers used records on Medicare patients, figuring they were likely to be area residents, not race participants. They looked at heart attacks or cardiac arrest, when the heart suddenly stops beating. They compared death rates for patients hospitalized on the day of the race versus five weeks before or after it, or in surrounding zip codes less affected by closed roads.

The rate of death within 30 days was 28 percent for those stricken on a marathon day versus 25 percent for the others, even though about the same number of people sought care each time.

Average ambulance times were more than four minutes longer on race days. Patients who came by car probably had delays, too, but researchers had no information on that.

“That’s not the time to drive yourself,” Jena says. If you think you might be having a heart attack, “you should call 911, particularly when it’s a marathon day or large public event.”

Dr. Howard Mell, a doctor in Rockford, Illinois, and a spokesman for the American College of Emergency Physicians, says the study shows that planners must think more broadly about medical needs for crowded events.

“If you’re going to have a big street fair, you don’t just need to plan for the people at the fair,” he says.

For the public, the message is “don’t avoid care” just because you dread “going out in that mess,” Mell says. Putting off seeking help just makes matters worse.