Lung-Retained Dose Following Occupational Exposure to Silica

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Abstract
Forty-two men with cancer at varying sites were rated prospectively from interviews for lifetime exposures to crystalline free silica. Information was obtained on concentration, frequency, and reliability of exposure information, as well as smoking, duration of exposure, and clearance time. Fifteen men had silica exposure; all were smokers. Other subjects served as smoking (N = 16) and nonsmoking (N = 9) controls without silica exposure. Paraffin blocks of lung tissue were subsequently obtained, deparaffinized, chemically digested, ashed, and mounted on copper mesh grids. Grids were examined electron microscopically at 10,000× and nonfibrous particles were counted if their diameter exceeded 1.0 μm but was less than 7.0 μm. Fibers were defined as structures longer than 3 μm with an aspect ratio > 3:1 and were counted separately. Mineralogical identification was performed using energy-dispersive X-ray spectrometry. Nonfibrous silica, feldspar, micas and clays, other silicates, and total silica/silicates were found in excess in the silica-exposed subjects. Similarly, nonfibrous silicates containing iron were in excess in the lungs of those having silica exposure. However, there was no evidence of an excess of pure siliceous or silicate fibers other than asbestos. Lung nonfibrous silicate particle content corresponded best with frequency of exposure to silica, and smoking (in pack years) had a significant negative effect on total lung silica/silicate content when combined in a model with frequency of exposure. Concentration, reliability, and duration of exposure each showed a relationship to lung silica/silicate content, but no smoking effect was evident. We conclude that (1) measurements of nonfibrous silica and silicates in lung reflect occupational exposure to crystalline free silica; (2) nonfibrous mica and clays are particularly well correlated with exposure to pure silica; and (3) the effects of smoking and of cessation interval require further study. Case, B.W.; Dufresne, A.; Richardson, L.; Siemiatycki, J.; Takahashi, K.: Lung-Retained Dose Following Occupational Exposure to Silica. Appl. Occup. Environ. Hyg. 10(12):1031–1036; 1995.