When a number of persons is exposed to a particular pneumoconiosis-producing dust for the same period under apparently similar conditions, some of them develop the disease while others escape. It is possible that the less easily affected group comprises merely those who by chance or through some inborn or acquired local physical difference, e.g. nose-breathing, receive smaller doses of the noxious dust into the lungs. It is also possible that they are a selected group of peculiar constitutional type. The latter view has been accepted by some authorities and used in practice in selecting entrants to the mining industry, on the ground that by excluding certain anthropometric types the incidence of pneumoconiosis, or of pneumoconiosis complicated by tuberculosis, would thereby be reduced, though no adequate statistical data have so far been published to support this hypothesis.