Document vVDm7wykqBb8jz2MxrgNd897m

cidence of asbestosis with relation to occupation, concentration of the dust, and length of exposure in the fifty-six persons examined in this study is indicated in Table XI. Obviously the examination of such a relatively small group prevents the formation of definite con clusions as to the influence of these factors. Nor is it possible from our findings to establish the maximum safe concentration of asbes tos dust in the air. However, the results of this investigation show the necessity of a reduction of the dust concentrations in those operations, shown in Part II of this study, where there is continuous exposure to high concentrations. Summary (1). The concentration of dust in asbestos fabricating plants de pends primarily on the grade of asbestos; mill fiber gives rise to a higher concentration than crude fiber. Operations arranged in de creasing concentration of dust are preparing, carding, weaving, spin ning, twisting, winding, and warping. (2) . The averaee particle size of asbestos dust is stated in two diameters, longitudinal and transverse. (3) . Petrographic analyses of dust encountered in asbestos fabri cating plants collected in the workers' breathing zone shows it to contain no free silica. (4) . -' Fourteen, or 2S% of fifty-six workers employed in the tex tile departments of asbestos fabricating plants in Pennsylvania had both clinical and roentgenological evidence of asbestosis. Acknowledgments The Department of Labor and Industry of the Commonwealth of Pennsylvania wishes to thank the management and employes of the plants studied for their cooperation and interest which made the success of this study possible; Dr. H. K. Pancoast and Dr. E/P. Pendergrass,pf.the University of Pennsylvania,^whose interpreta tions of the films have been used; Dr. G. W. Grier of the University of Pittsburgh, for his interpretation, assistance, and advice in the roentgenological study; Dr. J. Evans Scheehle. Secretary of the Pennsylvania Department of Welfare, in extending to the Depart ment of Labor and Industry the facilities of his department; Dr. C. A. Laubach. roentgenologist and cardiologist of the Norristown State Hospital, for doing a large majority of the X-ray work, and his technicians for the routine laboratory examinations; Dr. J. D. Heard and Dr. A. B. Fuller of the University of Pittsburgh, and Dr. C. C. Wolferth of the University of Pennsylvania, for their as sistance and interpretation of the cardiograms. Bibliography 1. Amiante. Hvg. du trav., 1:191-192, 1930. 2. Anderson, H. V., and Clark, G. L.: Application of X-rays in the Classification of Fibrous Silicate Minerals Commonly Termed Asbestos. Ind. and Eng. Chem., 21:924-933, 1929. 28