Understanding the behavior of indoor air quality (IAQ) parameters under different operating conditions is essential for effective control of ventilation systems to maintain a healthy and comfortable indoor environment, especially in sensitive spaces such as hospitals and schools. This study investigates the operating conditions of ventilation and indoor air quality by analysing the correlations of indoor and outdoor air quality parameters. The study examines the ability of ventilation systems to maintain adequate indoor air quality by measuring carbon dioxide and particulate matter (PM2.5 and PM10). Air was monitored in a university seminar room and a hospital patient room, both located in Trondheim, Norway. The sensors used for monitoring were placed around the rooms and in the inlet and outlet ducts. The rooms were in normal use during the measurements. The measurements were taken over a period of several weeks. The results provided information on the air quality and ventilation of the measured premises. The results also show the different profile of air properties in different occupied spaces. The diurnal patterns of CO2 and PM2.5 provided information on the typical behavior and ventilation performance of the spaces. The impact of weather and outdoor air quality on indoor air quality was also investigated. There was a clear correlation between indoor and outdoor particulate concentrations. In summary, examining correlations of different IAQ parameters can give additional insights into ventilation performance and how it affects indoor air quality.