Children who grow up breathing their parents’ secondhand smoke are more likely to develop a heart rhythm disorder known as atrial fibrillation than the kids of nonsmokers, a recent study suggests. Based on families in the decades-long Framingham study, researchers found that half of the children of smokers included in the analysis were exposed to at least a half pack, or 10 cigarettes, a day – and for every pack-a-day increase in smoke exposure, kids’ risk of developing atrial fibrillation in adulthood climbed by 18%. A small part of the risk for adult atrial fibrillation was associated with the tendency of smokers’ children to become smokers themselves as adults, the study team notes in the Journal of the American College of Cardiology. “Smoking may have unintended and unanticipated downstream harms that have not previously been considered as part of the actual total negative consequences of smoking,” said Dr. Gregory Marcus, senior author of the study and an atrial fibrillation researcher at the University of California, San Francisco. “These findings demonstrate how parental behaviors and exposures can meaningfully influence their children in ways that may not become apparent until late in adulthood,” Marcus said by email. For the study, researchers examined data on 2,816 children and at least one of their parents. Overall, 82% of the kids were exposed to secondhand smoke by a parent at some point during childhood. A total of 404 kids, or 14.3%, developed atrial fibrillation after researchers followed half of the children for at least four decades. Exposure to secondhand smoke from parents was also associated with a 34% higher risk that children would become smokers, researchers report in the Journal of the American College of Cardiology. And, becoming a smoker explained about 17% of the increased atrial fibrillation risk tied to parental smoking. In atrial fibrillation, electrical impulses in the upper chambers of the heart are chaotic, causing that part of the heart muscle to quiver rather than contracting normally. As a result, blood doesn’t move as well to the heart’s lower chambers. This can lead to the formation of clots that can travel through the arteries to the brain and cause a stroke. The study wasn’t designed to prove whether or how secondhand smoke exposure during childhood might cause atrial fibrillation. “Early exposure to tobacco smoke may result in damage to the pulmonary veins, the heart, or the interface between the two, which may increase the propensity to atrial fibrillation,” Marcus said. One limitation of the study is that researchers lacked smoking data for a large proportion of parents, which may have resulted from limited awareness of the health risks of smoking when some of the families started being followed in the 1950s, researchers note. It’s also unclear how much exposure to smoke kids had in the womb, which might also impact atrial fibrillation risk. Even so, the results should offer parents who smoke yet another reason to quit, Marcus said. “Smoking unfortunately remains common, especially among young individuals of child-bearing age,” Marcus added. “Complete smoking avoidance or cessation is the most healthy strategy for parents that want their kids to experience long-term health.”