DALLAS: In a study sure to fuel the anti-smoking lobby, medical researchers announced that regular exposure to second-hand smoke can double the risk of heart disease. The 10-year study, conducted by the Harvard Medical School and Brigham and Women's Hospital, focused on more than 32,000 female nurses who never smoked and had no history of heart disease, stroke or cancer. The findings, published Tuesday in the Dallas-based American Heart Association journal Circulation, showed that those women who reported "regular" exposure to tobacco smoke at home or at work had a 91 percent greater chance of contracting heart disease. Those who reported "occasional" exposure were 58 percent more at risk. Despite the dramatic finding, TIME's Dick Thompson reports that additional studies are needed regarding how human cells actually react to second-hand smoke before they can hope to cinch a nationwide ban on smoking in workplaces and public sites. Thompson also warns that the study is flawed because participants, not researchers, defined what constitutes "regular" versus "occasional" exposure. "The study could prove less significant once it is more closely analyzed," he says.