Air quality sensors are incorporated into an aircraft ventilation system in order to detect the nature and concentration of possible contaminants in the air flow passages. Data from the sensors may also be used to monitor air quality trends, determine corrective measures, and provide maintenance alerts. The sensors are provided at strategic locations throughout the ventilation system, such as before and/or after an air compressor and before and/or after a recirculation system, in order to identify where the problem is occurring.