SILICOSIS. Pneumoconiosis is a pulmonary condition re sulting from inhaled dust. Most forms are manifested merely by pigmentation with no effect on function. Two kinds of dust, free silica and asbestos, produce definite pulmonary disease. A few others may provoke changes recognizable in Rontgenograms, but proof of their etiologic significance is lacking.
Silicosis is an industrial disease caused by inhaling large amounts of free silica over a period of years. In normal lungs it is mani fested by numerous small, uniformly distributed, discrete, fibrous nodules, often causing no symptoms. In infected lungs the fibrosis is massive and is accompanied by compensatory emphysema pro* ducing dyspnoea. It specifically predisposes to tuberculosis of which a large but variable proportion of its victims die. Silica is not a mechanical irritant. Its action is physico-chemical and can be modified by the presence of other minerals. Experiments demonstrate that inhalation of silica is retarded when other sub stances are mixed with it in the air and that minerals like gypsum, iron and aluminium retard its action inside the body by coating the surface of silica grains.
Asbestosis is the other generally recognized pneumoconiosis producing symptoms and disability. It is characterized by fine, diffuse fibrosis originating about the terminal bronchioles and spreading into the lung. Such reaction reduces pulmonary elasti city and obliterates many blood capillaries with resultant dyspnoea and sometimes cardiac changes in advanced cases. Associated in fections may heal, accentuating the pre-existing dust fibrosis. Experimental evidence strongly suggests a mechanical rather than a chemical irritation by this fibrous mineral.