The behaviour of plumes from tall chimneys is now fairly well understood. To achieve this understanding it has been necessary to develop high speed recording devices for sulphur dioxide concentration and plume height and to use recorders of atmospheric turbulence and temperature profile at heights up to some hundreds of metres. Instruments for pollution control, in particular the control of dust, have a different requirement. They must be simple; and in addition must measure meaningful functions of the basic variables rather than a single variable. A dust gauge has been developed which measures in the field a function of dust concentration, dust velocity and particle size, and is also dependent on wind direction. The function is considered to represent the 'nuisance value' of the dust emission. A dust monitor has been developed which measures, before emission, the same function of the dust variables, and can be used for emission control. It supplements the smoke meter type of instrument which measures a different function of dust concentration and particle size. The zero stability of both instrument types has been studied and improved. The averaging time of a measurement is an important conception both in the investigation and the control of pollution. In monitoring instruments it must be related to the 'integrating time' of each type of complaint. This is the time required for a pollutant to produce its full effect from an arbitrary starting time.