Low-level fiber-induced radiographic changes caused by Libby vermiculite: a 25-year follow-up study

Abstract

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.

Figure 1.

Prevalence of pleural radiographic changes including localized and diffuse pleural thickening by age and cumulative fiber exposure (<1 and ⩾fiber/cc-years). Dark gray bars = fiber/cc-years < 1; light gray bars = fiber/cc-years ⩾ 1; black bars = proportion of diffuse pleural thickening. Numbers above bars = localized/diffuse; numbers below bars = total number of participants in each category.

Figure 2.

Association of pleural radiographic changes with cumulative fiber exposure, according to models including various cofactors. BMI = body mass index; CFE = cumulative fiber exposure in quartiles; hire = date of hire. For each model, CFE quartiles 2, 3, and 4 are represented in sequential order. Odds ratios with 95% confidence intervals are represented on a log scale.