A method of operating a detection system includes switching the system from a normal mode for sensing smoke to a high sensitivity mode for sensing airborne particles, such that in the high sensitivity mode the detection system is configured to discriminate between particles of diameters less than 2.5 micrometers and 10 micrometers. Transmitting light from one or more light sources of the detection system into a monitored space, and detecting scattered light at one or more light sensing devices of the detection system. The detection of scattered light is indicative of one or more indoor air quality conditions in the monitored space.