Objective To understand the knowledge,attitude and behavior on health hazards of haze and secondhand smoke exposure among middle school students in Beijing,and provide us with guidance to carry out effective health education for middle school students.Methods A total of 1 294 students were selected randomly using stratified cluster sampling method;1 085 valid questionnaires were recovered.Among them,739 were junior high school students and 346 were senior high school students.The questionnaire included questions on middle school students' knowledge on haze and secondhand smoke exposure,their attitudes towards the judgement of severity of haze and secondhand smoke exposure and their utilization of protective measures when they are exposed to haze or secondhand smoke.Pearson's chi-squared test was performed to assess differences in knowledge,attitude and behavior between junior middle school students and senior high school students,boys and girls.Results Middle-school students' awareness rate on "the haze and exposure to secondhand smoke may cause respiratory disease" was the highest,which was 93.55％ and 95.76％ respectively.The awareness rate of the fact that haze,secondhand smoke exposure can cause cardiovascular disease was 38.99％ and 55.85％ respectively;while the rate of their awareness on the haze and exposure to secondhand smoke can lead to cancer was the lowest,which was 54.10％ and 69.95％ respectively.The differences was significant in the awareness rate of the fact that respiratory disease,cardio-cerebrovascular disease and cancer can be caused by the haze (x2=732.539,P＜0.05),likewise the differences were significant in the awareness rate of the above three kinds of diseases can be caused by secondhand smoke exposure (x2=460.291,P＜0.05).In the comparison between the severity of haze and smoking hazards,35.94％ of students believe that haze hazards is heavier than tobacco;48.02％ of the students think that the dangers of smoking is greater than haze;16.04％ of the students cannot determine the extent of damage of the haze and smoke.The proportion of students who think smoking was more harmful than haze was higher in junior high school group than in senior high school group.The two groups had a significant difference in the understanding of the severity comparison between haze and smoking (x2=13.014,P＜0.05).The differences between boys and girls in the view of the severity comparison between haze and smoking are also statistically significant,more boys believed smoking was more harmful than haze (x2=9.650,P＜0.05).In the aspect of protection from haze and secondhand smoke exposure,86.36％ of the students would like to protect themselves from haze,while 55.94％ of the students would take measures to prevent the secondhand smoke exposure initiatively,the junior high school students performed better than the senior high school students in this two aspects(x2=17.125,P＜0.05;x2=45.785,P＜0.05).Conclusion While middle school students have a certain understanding of the impact of haze and secondhand smoke exposure on health hazards,their understanding is limited.They have not paid enough attention to the harm of secondhand smoke exposure and their awareness of active refusal to secondhand smoke exposure needs to be improved.Schools should strengthen the health education for students and educate them not only to stay away from haze but also should be far away from secondhand smoke exposure.