A randomized crossover trial will be conducted in 90 primary school children in 3 classrooms in Shanghai, China. The effects of fresh air ventilation/filtration system, fresh air ventilation system with no filtration and natual ventilation will be compared on the effects of reducing indoor PM2.5 and chilhood health. Biological samples of children are going to be collected to investigate the associations between indoor air pollution and biomarkes of certain health effects.

A randomized crossover trial among 90 primary students in 3 classrooms in Shanghai, China will be performed. Three arms are going to be 1) fresh air filtration system, 2) fresh air system with no filtration and 3) natural ventilation. Each arm is going to be in use for five continuous weekdays followed by one week of wash-out. After one-week of wash out, a cross over change between 3 arms is going to be taked. Then the 2nd week of intervention is performed followed by the 2nd wash-out week. The same cross-over change between 3 arms is going to be repeated again until each classroom will be applied by all three arms of ventilation. A total of 5 weeks is required for the project. Children's biological samples including saliva, urine and skin samples will be collected as well as exhaled NO. A questionnaire on children's perception on air quality will be assessed as well.

Inclusion Criteria:

• All invited primary school children in Grade 5 in an artibrarily selected school

Exclusion Criteria:

• Current airway symptoms (cough, airway infection, asthma et al.)
• Acute infectious diseases