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Low-Level Fiber-induced Radiographic Changes Caused by Libby Vermiculite

A 25-Year Follow-up Study

¹ Departments of Environmental Health and Internal Medicine, ² Department of Rehabilitation Sciences, and ³ Department of Environmental Health, University of Cincinnati College of Medicine, Cincinnati, Ohio; ⁴ Department of Radiology and Diagnostic Imaging, University of Cincinnati Hospital, Cincinnati, Ohio; and ⁵ Agency for Toxic Substances and Disease Registry, Centers for Disease Control and Prevention, Atlanta, Georgia

Correspondence and requests for reprints should be addressed to James Lockey, M.D., M.S., Departments of Environmental Health and Internal Medicine (Pulmonary Division), University of Cincinnati College of Medicine, 3223 Eden Avenue, ML 0056, Cincinnati, OH 45267. E-mail: james.lockey{at}uc.edu

Rationale: From 1921 to 1990, vermiculite ore from Libby, Montana, was shipped worldwide for commercial and residential use. A 1980 study of a manufacturing facility using Libby vermiculite was the first to demonstrate a small but significant prevalence of pleural chest radiographic changes associated with amphibole fibers contained in the ore.

Objectives: This follow-up study of the original cohort evaluated the extent of radiographic changes and cumulative fiber exposure (CFE) 25 years after cessation of exposure.

Methods: From the original cohort of 513 workers, 431 (84%) were living and available for participation and exposure reconstruction. Of these, 280 (65%) completed both chest radiographs and interviews. Primary outcomes were pleural and/or interstitial changes.

Measurements and Main Results: Pleural and interstitial changes were demonstrated in 80 (28.7%) and 8 (2.9%) participants, respectively. Of those participants with low lifetime CFE of less than 2.21 fiber/cc-years, 42 (20%) had pleural changes. A significant (P < 0.001) exposure–response relationship of pleural changes with CFE was demonstrated, ranging from 7.1 to 54.3% from the lowest to highest exposure quartile. Removal of individuals with commercial asbestos exposure did not alter this trend.

Conclusions: This study indicates that exposure within an industrial process to Libby vermiculite ore is associated with pleural thickening at low lifetime CFE levels. The propensity of the Libby amphibole fibers to dramatically increase the prevalence of pleural changes 25 years after cessation of exposure at low CFE levels is a concern in view of the wide national distribution of this ore for commercial and residential use.

Key Words: vermiculite • pleural disease • amphiboles • fibrosis • mineral fiber

AT A GLANCE COMMENTARY Scientific Knowledge on the Subject It has been previously demonstrated amphibole fibers can cause pleural thickening and interstitial fibrosis related to latency from initial exposure and duration of exposure. What This Study Adds to the Field Pleural and interstitial changes can occur at low lifetime cumulative amphibole fiber exposure levels in a dose–response manner.

 

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