People exposed to secondhand smoke in the womb or as children are at an increased risk later in life for an irregular heart rhythm known as atrial fibrillation, a new study suggests.

"We can't say that secondhand smoke definitely caused atrial fibrillation" based on this type of study, said senior author Dr. Gregory Marcus from the University of California-San Francisco. "We do need to confirm these findings."

Atrial fibrillation causes the heart's upper and lower chambers to stop working together, according to the U.S. Heart, Lung and Blood Institute. The condition increases the risk of stroke, and can cause chest pain and heart failure.

Cigarette smoking is linked to the development of atrial fibrillation, but the link with secondhand smoke exposure was unknown, the researchers write in the journal HeartRhythm.

Using data from 4,976 people taking part in an Internet-based study on heart health, the researchers analyzed participants' exposure to secondhand smoke and whether or not they had atrial fibrillation.

Overall, about 12 percent of participants reported having atrial fibrillation. Those with the condition had an average age of about 62, compared to an average age of about 50 among those without atrial fibrillation.

The researchers accounted for factors that may have affected subjects' risks for atrial fibrillation, including age, sex, race, other health conditions and smoking and alcohol use.

They found that people exposed to secondhand smoke in the womb or during childhood were about 40 percent more likely to report atrial fibrillation than those who weren't exposed.

The risk was even higher among people who did not have other risk factors for atrial fibrillation, the researchers write.

Dr. Cuno S.P.M. Uiterwaal of the University Medical Center Utrecht in The Netherlands speculated in an email to Reuters Health that the impact of secondhand smoke may be more obvious in people without other more significant atrial fibrillation risk factors.

"Papers like these aim to draw attention to the possibility that early-life secondhand smoke exposure may not only have short term consequences, such as to the fetus, but also long-term hazards to offspring," said Uiterwaal, who was not involved with the study.

Preventing second-hand smoke exposure in early development and life may be one way to reduce the risk of atrial fibrillation later on, the researchers say.