Exposure to Amorphous Silica Fibers and Other Particulate Matter During Rice Farming Operations

Abstract

Occupational exposure to biogenic amorphous silica fibers was found during all phases of rice farming. Exposure during field preparation was the highest, followed by harvest and then rice stubble burning. The highest personal exposure was 1.9 fibers/cc for fibers >5 μm in length in the respirable dust fraction. The highest level seen in area samples was 9.9 fibers/cc for fibers >5 μm in length in the respirable dust fraction. The median fiber length was 2.8 μm, with a range from 0.5 to 20 μm. (Fibers <d0.5 μm were not counted.) Ninety percent of fibers were <9 μm in length. The median fiber width was 0.9 μm, with a range from 0.2 to 7 μm. Ninety percent of fibers were <2.5 μm in width. Samples for airborne amorphous silica fibers were collected with personal sampling pumps and polycarbonate filters. Samples were analyzed using X-ray fluorescence to identify composition and electron diffraction to determine the crystalline state of fibers. Fiber counting methodology was adapted from standard asbestos analytical procedures. The biogenic amorphous silica fibers were of complex morphology, often having no parallel sides. Although the fibers did not have needlelike or hairlike shapes, ends of some fibers were sharply pointed.