A model human head was exposed in a wind tunnel to coal dust with a maximum particle size of about 100 μm. The wind speed varied between 1 and 8 m s −1 . A pump giving a sinusoidal flow pattern simulated inhalation through nose or mouth, tidal volume and breathing frequency being adjusted to physiological values. The total airborne dust concentration was determined by isokinetic probes. The sampling efficiency of the model head under the various conditions in the wind tunnel was determined by comparing the size distribution of the total airborne dust with that of the dust collected on the filter in the nose or mouth. On the basis of these data and those of OGDEN and BIRKETT (1977, 1978), a general definition of inhalable dust is possible. An example of such a definition for conditions at a coal face is given.