The most common cause of death among firefighters at work is not burns or smoke inhalation, but heart attacks, and a new study reports that those attacks are 10 to 100 times as likely to occur when firefighters are suppressing a fire as when performing nonemergency duties. Cardiovascular events account for 45 percent of deaths of firefighters on duty.

While all risks increased with age, there was no significant increase in risk during emergency medical responses or nonfire emergencies.

The authors acknowledge that the data on time spent in various duties are estimates, and that there is wide variation among fire departments. They also point out that their information on cause of death depended on brief narratives that often lacked autopsy information.

Part of the problem, said Dr. Stefanos N. Kales, the medical director for employee health at the Cambridge Health Alliance and the study’s lead author, is that firefighting involves extremely heavy exertion and then long stretches of sedentary activity. The job itself does not keep firefighters in shape.

“We hope that this would reinforce efforts in the fire community to promote fitness training and wellness activities,” Dr. Kales said. “We’re also hoping it’s going to make physicians who care for firefighters more cognizant of the risks, and more aggressive with treating firefighters’ other risk factors.”