Firefighters face a far greater risk of dying of heart problems while battling a blaze than was thought, suggests a large U.S. study that offers more evidence of their need to stay in shape.

The risk of a heart-related death while putting out a fire was up to 100 times higher than the risk during downtime even though fighting fires accounts for only a small percentage of these workers’ time, Harvard researchers found.

About 100 firefighters die in the line of duty each year and previous research has shown that nearly half of the deaths are due to heart disease. The vast majority – about 70 percent – of the nation’s roughly 1 million firefighters are volunteers.

Experts say diet and exercise should be priorities at the firehouse.

“You may not be able to prevent all these deaths, but to the degree you can prevent some deaths by paying attention to underlying risk factors and better fitness programs, that’s the goal,” said Dr. Linda Rosenstock, dean of the UCLA School of Public Health, who was not involved in the study.

The study, published in Thursday’s New England Journal of Medicine, doesn’t address whether firefighters have an overall higher risk of dying from heart disease than the general population.

Rosenstock said fire departments could do more to improve health by requiring annual physicals and fitness tests. Departments also should have wellness and fitness programs to reduce heart disease risk factors such as obesity and high blood pressure, she said.

Firefighting is a physically demanding job that involves heavy lifting of equipment and exposure to toxic chemicals. Recruits are generally healthy and physically fit, but their health can decline over time because many firehouses don’t require regular exercise or yearly medical exams. Also, the health requirements are usually less stringent for volunteers, who tend to continue firefighting as they age.

In the Harvard study, researchers examined a federal registry of 1,144 on-duty firefighter deaths between 1994 and 2004. Excluded were the 343 firefighters who perished in the Sept. 11, 2001 terrorist attacks.

Nearly 40 percent – or 449 – of the on-duty deaths during that period were due to heart disease. Thirty-two percent of the heart-related deaths occurred while fighting blazes: 13 percent responding to an alarm; 17 percent returning from a call and 13 percent during physical training.

The researchers also calculated the odds of dying from a heart attack by taking into account the estimated amount of time spent performing different duties.

They found the risk of death from heart disease was highest during active firefighting – up to 100 times greater than the risk of dying during administrative work – though firefighting made up no more than 5 percent of a firefighters’ time. Increased risk of death was also found for other emergency duties such as responding to a call and returning from the scene of a fire.