Akl et al. BMC Public Health 2011, 11:244 http://www.biomedcentral.com/1471-2458/11/244 Page 9 of 12 Figures 5 Waterpipe use among adults. Survey (GYTS) in a number of countries (Additional file 4) [54]. Also, consistently with our findings, two recent non-systematic reviews of the topic found that water- pipe tobacco smoking is increasing in prevalence world- wide with 10-20% prevalence in Arab American young adult populations in the United States [55,56]. This study has a number of strengths. First, and to our knowledge, this is the first systematic review of the preva- lence of waterpipe smoking and its associated factors across countries, age groups and genders. Second, we used the Cochrane Collaboration methodology for conducting systematic reviews, i.e. using a very sensitive and compre- hensive search strategy, a duplicate and independent selec- tion process, a duplicate and independent data abstraction process, and a rigorous appraisal of the methodological quality of included studies. Third, we abstracted the data on cigarette and other forms of tobacco smoking, wher- ever applicable, for comparative purposes. The study also has a number of limitations. First, only four studies were conducted at national levels [20,36,45,52]. Second, studies varied by whether they reported the prevalence of waterpipe only smokers or all waterpipe smokers. They similarly varied by the type(s) of measure of waterpipe smoking reported (e.g. current, ever, and regular). However, the studies were consistent in the definitions of these measurement types. Third, only one study used a validated tool to measure expo- sure to waterpipe smoking, a practice that can vary widely in terms of frequency of use, the length of each use, and quantity of tobacco per single use [57]. Third, all of the included studies were cross-sectional in design and did not allow analyses for time trends. The prevalence of waterpipe smoking is alarmingly high, particularly among school students [11,16-19,21,29,46-49,52] and university students [12,22-28,42,50]. This prevalence is consistently compar- able to cigarette smoking among school students [16-21,29,46-48,52] and university students [12,23-28,50]. The high prevalence of waterpipe smoking among the youth could have been exacerbated by a common belief that this form of smoking is less harmful than cigarette smoking [8,58]. School students are parti- cularly vulnerable because of the additional risk of sec- ondhand waterpipe smoking (compared with secondhand cigarette smoking) given the social accept- ability of this form of smoking [23]. Pregnant women represent a similarly vulnerable group in which the pre- valence is concerning because of the evidence of