Abstract: A dual monitor comprises a piezoelectric sensor in the form of a quartz crystal and an electrode system thereon. An exposure device in the form of a dust discriminator is provided to expose the piezoelectric sensor at least periodically to a stream of a gas which may contain dust or other organic or inorganic particulate material whose presence in or absence from the gas is to be monitored. The gas of said stream consists primarily of air. An electronic monitoring device is provided coupled to the electrode system of said piezoelectric sensor to monitor variations in the resonant frequency of said piezoelectric sensor induced by the aggregation of dust or other particulate material on said piezoelectric sensor.