RESPIRABLE >10 m FIBROUS AND PLATY PHYLLOSILICATES

Abstract

A total of 24 tissue sections from the lungs of 8 vineyard workers with silicate pneumoconiosis were searched for >10 m fibrous and platy particulates, identified earlier as primarily mica phyllosilicates. The purpose of the study was to augment the very limited data available on the range of physical fiber lengths and the light microscopic characteristics of >10 m phyllosilicates that reach human respiratory tissues. Brightly birefringent crystals having an estimated length longer than 10 m were measured using a calibrated reticule eyepiece. Of 148 particulates confirmed as >10 m in length, 137 were fibrous (aspect ratio >3) and 11 were platy forms, with the largest plate measuring 37 m x 28 m. Mean length of the fibers was 19 m, with several as long as 35 m. Mean width was 2.6 m and mean aspect ratio was 8.9. Most of the >10 m fibrous particles were rodlike or fibroplaty, the latter with irregular contours and nonuniform diameters. The majority of the >10 m fibers, and all but one of the 7 fibers 35 m in length, were apparently recent deposits since they were found in respiratory tissues showing macrophage response but little or no fibrosis. Dissolution and short axis fragmentation of >10 m fibers was frequent. Long axis fragmentation (splintering) of >10 m platy particulates was also frequent and appeared to be a mechanism for in vivo generation of >10 m fibers. While the findings pertain to vineyard workers exposed to exceptionally high levels of phyllosilicates, the atmosphere of a rural area was involved. Moreover, community atmospheres may at times contain high levels of phyllosilicates due to industrial and vehicular emissions, construction and agricultural activities, and natural events such as dust storms. Personal and local exposures to commercial products containing silicates are additional considerations. Pertinently, everyone's lung contains on average billions of particulates and millions of fibers that are largely phyllosilicates. The 5 m fibers warrant special attention in view of their recognized fibrogenic potential, the uncertainty about their collection as part of PM-10 monitoring, and the poorly defined pathogenesis of most fibrosing lung diseases. Moreover, there are data indicating that PM-10 monitoring does not include all >10 m fibrous and platy particulates that reach respiratory tissues, whether particulate measurement is by physical or aerodynamically equivalent diameter (AED). It would seem prudent to collect particulates of all sizes that are present in the ambient air, to measure them according to physical as well as AED size, and to evaluate their chemical, physical, and pathobiologic properties.