Workers with Libby amphibole exposure: retrospective identification and progression of radiographic changes

To assess how early pleural and/or parenchymal abnormalities consistent with asbestos exposure could be ascertained and to identify factors associated with progression.

Informed consent was obtained under an institutional review board-approved protocol. Multiple sequential chest radiographs obtained between 1955 and 2004 in 84 workers exposed to amphiboles associated with vermiculite in the town of Libby, Montana, were studied. A panel of three NIOSH B readers reviewed each worker's longitudinal chest radiograph series in reverse chronologic order and achieved a consensus reading for each radiograph. Measures of exposure were compared between workers with and those without progression of parenchymal and pleural abnormalities.

Because of the way the study was designed, all subjects had pleural (n = 84) and/or parenchymal (n = 26) abnormalities on the most recent chest radiograph. Compared with other investigations that used different methods, this investigation revealed shorter latency periods (defined as the interval between date of hire and date of earliest radiographic detection) for circumscribed pleural plaque (median latency, 8.6 years) and pleural calcification (median latency, 17.5 years). Pleural abnormalities progressed in 64 workers, while parenchymal abnormalities progressed in 14. No significant differences were found with regard to measures of exposure between workers with and those without progression.

The latency period for the development of pleural plaques may be shorter than previously reported. Early plaques are subtle and may not be detectable except at retrospective review.