Sales of e-cigarettes have slowed, in part due to warnings by public health experts that the devices may be dangerous. The New York Times reports a growing number of scientists and policy makers say 40 million American smokers could use the devices to help them stop smoking. “We may well have missed, or are missing, the greatest opportunity in a century,” David B. Abrams, Senior Scientist at the antismoking group Truth Initiative, told the newspaper. “The unintended consequence is more lives are going to be lost.” Some experts warn e-cigarettes can be a stepping stone to smoking regular cigarettes for young people. The full effect of using the devices will not be known for years, they warn. Others note that vaping is much less dangerous than smoking, because e-cigarettes do not contain the deadly tar found in regular cigarettes.