Public health problems caused by indoor pollution (sick building syndrome) have been a concern for some time. In Indonesia, there are no regulations regarding pollutants that have the potential to become hazardous substances and contribute to just general health problems. PMb PM25, PM10, and TSP can affect health, and, in Indonesia, little research has been done on their indoor concentration levels. This study aims to determine the indoor variables and occupant activities that affect the concentrations of CO, NO2 and the particulates. Thirty seven (37) apartments were 24h sampled, measured and interveiewed during December 2017 to April 2018 using direct interview. Results show that occupants activities and indoor interior configuration affect concentration of indoor pollutants. The number of furniture units significantly contributes to the worsen indoor air quality along with humidity. Opening window (natural ventilation) was found to increase coarse particulates (PM10 and TSP), CO concentration, significantly and on the contrary reduce NO2 concentration. Other variables such as number of fan and exhaust fans gave mixed result as both reduce particulates but on the other hand may increase gas concentrations. Further study is needed to investigate the cause of worse indoor quality during natural ventilation as well as occupants activities.