Systematic measurements were made of the dimensions of fibres found in the air in mines and mills for crocidolite, amosite and chrysotile. Samples were collected on membrane filters and examined in light, transmission electron and scanning electron microscopes. The proportion of short fibres was shown to decrease with extent of processing, especially for amosite. No airborne fibres had a diameter greater than 3 micrometers, and all would therefore be capable of reaching the pulmonary alveoli. It was shown that for the same airborne mss of the different varieties of asbestos, fewer amosite fibres would be present than crocidolite or chrysotile fibres. Most of the fibres to which workers are exposed are short and thin. It was also shown that the choice of analytical method and the limits of fibre dimension that can be observed are critical. Thus, fibres greater than 5 micrometers which are visible in the light microscope represent only a small proportion of the total fibre count, and transmission electron microscopic methods can lead to under-reporting of long fibres.