THE importance of silicosis as an occupational disease has been demonstrated in many industries. Such investigations have shown rock drilling in hard-rock mining and other drilling operations to be hazardous, even including open drilling in quarries, road construction, and building excavation. It is recognized, however, that the silicosis hazard among drill operators, as in other dusty operations, varies widely with the free silica content of the rock and with the degree to which the operation is confined. Because of the extensive rock excavation work in New York State the control of silicosis among drillers and associated workers is regarded as a most important health problem. For this reason considerable effort has been directed toward its solution. An important step in this connection was the development of a code of dust control requirements for rock drilling.' Since it was to apply to all drilling operations,