Abstract: A control system in an air monitor is programmed to selectively direct air from more than one source over a sensor and various algorithms are then used to provide feedback regarding the difference between the particulate readings at each of the sources. In one embodiment, a data curve for a reading from one air source during a first time period is overlaid with a line which is linearly extrapolated beyond the end of that time period and the data curve for a reading from a second air source during an immediately subsequent second time period is overlaid with a line which is linearly extrapolated beyond the start of that time period. The difference in the contaminant level between the extrapolated lines at the end of the first time period and the beginning of the second time period approximates the difference in contaminant content between the two air sources.