NEW YORK - That long soak in a hot tub to warm up during the winter could hold some unexpected dangers, a Japanese study has found.

Taking a hot bath on a cold day may spell trouble for the heart, said a team led by Chika Nishiyama, at Kyoto Prefectural University of Medicine School of Nursing, with the rate of cardiac arrests during bathing rising ten-fold from summer to winter.

The findings could be especially significant in Japan, where bathing is a key part of life both at home and as a form of recreation, with people flocking to the country's numerous hot springs for long, relaxing soaks.

"In Japan, most people take a deep, hot bath, since traditional Japanese homes are not well-insulated as in the west and central heating is quite uncommon," Nishiyama and colleagues wrote in "Resuscitation."

The team based their study on data from nearly 11,000 cardiac arrests in the western prefecture of Osaka between 2005 and 2007.

Prior to the cardiac arrest, 22 percent of people had been sleeping, 9 percent had been bathing, 3 percent had been working, and 0.5 percent had been exercising. The rest had been doing "nonspecific" or unknown activities.

When looking at cardiac arrest rates, at the top of the list was 54 arrests per 10 million people per hour of bathing, followed by 10 per 10 million people per hour of exercising.  

For bathers, the risk was tied to outside temperatures, with more cardiac arrests on colder days.

While it's still unclear how to explain the link, getting into a hot tub on a frigid day causes a rapid blood pressure drop, which stressed the heart.

"Preventive approaches such as warming a bathroom and hallway or refraining from taking a deep, hot bath could be important for high risk people," Nishiyama and colleagues wrote.