Three urban environments, office, apartment and restaurant, were selected to investigate the indoor and outdoor air quality as an inter-comparison in which CO2, particulate matter (PM) concentration and particle size ranging were concerned. In this investigation, CO2 level in the apartment (623 ppm) was the highest among the indoor environments and indoor levels were always higher than outdoor levels. The PM10 (333 lg/m3), PM2.5 (213 lg/m3), PM1 (148 lg/m3) concentrations in the office were 10–50 % higher than in the restaurant and apartment, and the three indoor PM10 levels all exceeded the China standard of 150 lg/m3. Particles ranging from 0.3 to 0.4 lm, 0.4 to 0.5 lm and 0.5 to 0.65 lm make largest contribution to particle mass in indoor air, and fine particles number concentrations were much higher than outdoor levels. Outdoor air pollution is mainly affected by heavy traffic, while indoor air pollution has various sources. Particularly, office environment was mainly affected by outdoor sources like soil dust and traffic emission; apartment particles were mainly caused by human activities; restaurant indoor air quality was affected by multiple sources among which cooking-generated fine particles and the human steam are main factors.