The percentage of high school smokers who smoke at least 11 cigarettes a day is on the decline, a new study by the Centers for Disease Control and Prevention finds. ‘Light’ smoking—defined as between one and five cigarettes a day—is increasing among U.S. high school students. The CDC researchers note that while the decline in heavy smoking among teens is encouraging, even light smoking can have detrimental health effects.

Reuters reports that overall, 19.5 percent of high school students call themselves smokers. The percentage of teen smokers who smoked at least 11 cigarettes per day dropped from 18 percent in 1991 to 7.8 percent in 2009. During that same period, teen smokers who smoke between one and five cigarettes daily increased from 67.2 percent to 79.4 percent, the researchers report in the American Journal of Preventive Medicine. The study used data from an annual survey of between 14,000 and 16,000 students.

Study co-author Dr. Terry Pechachek told Reuters, “With fewer cigarettes, the price effect, smoke-free policies and a change in the broad public awareness of risk, the heaviest patterns of use are becoming very rare.”  He said he is concerned about the increase in light smokers. “It’s still a very risky behavior. We want to get across to people that although this is a positive trend, it’s very unacceptable to have so many children exposing themselves to something so addictive. The greatest danger is minimizing the risk.”