The investigators conducted a randomized double-blind crossover trial among 60 healthy college students in Shanghai, China from November to December, 2015. These students lived in dormitories that were randomized into two groups and alternated the use of true or sham air purifiers for 9 consecutive days with a 2-week wash-out interval. All participants and research staffs were blinded to the group assignment. All participants were required to stay in their dormitory with the windows/doors closed for at least 50% of the time on weekdays and for 80% on weekends throughout the 9-day intervention period. All interventions started at 7 a.m. to avoid issues related to diurnal variation. Health endpoints, including blood pressure, fractional exhaled nitric oxide were evaluated and biological samples such as morning urine, fast blood and buccal cells were collected at baseline and immediately after the completion of each intervention period.