Study cities and participants
Selection of the study cities and participants has been described previously in detail.15 In short, the cross-sectional study was conducted in 27 districts of seven northeast cities in China from 2012 to 2013, including six districts in Shenyang, three districts in Anshan, three districts in Benxi, five districts in Dalian, three districts in Dandong, four districts in Fushun and three districts in Liaoyang. In each district, one or two kindergartens, primary schools and middle schools were randomly chosen. In each grade of the selected school, one class was randomly chosen and all the students in the class were enrolled. Totally, we obtained 68,647 participants aged from 0 to 20 years.
This study was carried out in accordance with the World Medical Association Declaration of Helsinki Ethical Principles for Medical Research Involving Human Subjects. The Human Studies Committee of Sun Yat-sen University approved the study protocol and all the parents or guardians of the participants signed the informed consent before starting data collection.