JUUL and similar pod-based e-cigarettes have been popular with teenagers and young adults since they came on the market in 2015, but little has been known about their health effects. A new systematic review led by researchers from Harvard T.H. Chan School of Public Health found that while the products may contain lower levels of harmful ingredients than conventional cigarettes, there is no evidence that even these lower levels are safe for youth. The study also found that the devices' efficient delivery of nicotine fosters greater dependence than other types of e-cigarettes.