However, the design of with-in person comparison in the current study enabled us to avoid the potential confounding factors caused by inappropriate selection of controls. Inclusion of
smoking individual in the study allows us to investigate the influence of air pollution on both smokers and non-smokers, and investigate the potential modifying effects of smoking on the biological responses to dramatic changes in air pollution. The design with a bi-directional intervention strengthens the scientific rigor and provides internal validity of our results. Our finding successfully filled in the gap in understanding related to the short- term effects of particulate matter air pollution on healthy adults.