INTRODUCTION High rates of smoking among adolescents remain a public health concern. This study investigates smoking behavior and use of smokeless tobacco among Danish high-school students and assesses how smoking and use of smokeless tobacco cluster in schools and school classes. We estimate the trend in cigarette smoking from 1997 to 2014. METHODS We used data on 70 243 students, from 3 214 school classes in 119 high schools, who participated in the Danish National Youth Study from 2014. We had information on 87% of all Danish high schools and 85% of eligible students. We also used data from 1997 on 26 644 high-school students from a similar data set to assess the chronological trend in smoking. We calculated prevalences and intraclass correlation coefficients (ICCs) to estimate between-school and between-school class clustering in smoking and use of smokeless tobacco. RESULTS In all, 14% of boys and 11% of girls were daily smokers. A large fraction of the variation in smoking and the use of smokeless tobacco was attributable to the school and school-class level (ICC of 0.19, 0.12, 0.16 and 0.27, for daily smoking, waterpipe smoking, use of electronic cigarettes and snuff/chewing tobacco, respectively). Daily smoking decreased from 15% in 1997 to 12% in 2014, while more students were occasional smokers in 2014 than in 1997 (30% vs 18%). CONCLUSIONS The prevalence of smoking was high among Danish high-school students and had changed little since 1997. The school and class environment accounted for a large part of the variation in smoking behavior.
INTRODUCTIONDespite the solid evidence of smoking's addictive properties and the comprehensive associated harms, high rates of smoking among adolescents remain a global public health concern. In 2015, 18 countries in western and central Europe had a smoking prevalence among girls, aged between 15 and 19 years, higher than 15%, and only three countries worldwide had significant decreases in adolescent smoking in the period from 2005 to 2015 1 . During the last decade, alternative smoke and smokeless products have become available and are being increasingly used; most popular are electronic cigarettes (e-cigarettes) and waterpipe, and also smokeless products such as snuff and chewing tobacco [2][3][4][5] . While there is little doubt that these products in themselves are less dangerous than cigarettes 6,7 , they are not harmless and there is concern that their use is associated with an increased risk of smoking initiation 3,[8][9][10] . High school constitutes a time of transition characterized by physical, psychological and social changes during which environment and opinions of peers become increasingly important to young people while parents' influence declines [11][12][13] . The school is an important setting that influences young people as they spend a large part of their day interacting with the school environment and other students. Research