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To the Editor:
The recent publication by Larson, Williamson, and Antao “Follow-Up of Libby, Montana Screening Cohort: a 17-Year Mortality Study”1 is appreciated by all who recognize (1) the frequency and unique clinical features of Libby amphibole (LA) diseases,2–5 (2) their prevalence in locations far removed from Libby,6 and (3) the risks from exposure to similar amphiboles in surface rocks in different parts of the world.7,8 The 8043 screenees examined by Larson et al in Lincoln County and elsewhere in the United States represent slightly more than 10% of the estimated 70,000 living in the area for various periods of time during the 17 years. More than half (54%) of those screened were under 50 years of age and therefore less likely to evince asbestos-related disease (ARD).
Cause of death was from death certificates obtained from the National Death Index. It is well recognized that death certificates are acceptable for clearly identified causes of death-like malignancies (with the exception of mesothelioma) but not for chronic diseases like asbestosis especially in the presence of multiple comorbidities. The authors quote Selikoff and Seidman9,10 to this effect but do not elucidate on the much more accurate alternative used in the publications of these eminent authorities on ARD: best evidence based on informed review of the clinical records, radiographs, and pathology. Death certificates identified less than half the deaths caused by asbestosis (47% or 201 of 427 in long-term workers in an occupation at obvious risk, insulators. Larson et al cite potential diagnostic bias in favor of ARDs in their study: “death certificate certifiers in the Libby community may have been overly aware of ARD.” Having practiced in Libby, been familiar with the diagnoses rendered by the other physicians in the area and evaluated hundreds of patients with ARDs, the second signatory to this letter can state quite emphatically that the opposite is more likely to be true: Medical records and death certificates of many physicians in the community did not note ARD when it was clearly evident clinically and radiographically.
Additionally, many persons in the Libby area moved elsewhere, especially when the mine began to shut down, a process that was completed in 1990. A recent publication reported asbestos-related pleural thickening in almost half (48%) of 198 persons who left Libby on graduation from high school.11 Surely, deaths among these expatriates would not be certified by a practitioner “overly aware of ARD” who is practicing in Arizona or California. Another failing of death certificates in the Libby area itself is that many are completed by nonphysicians: Coroners and even undertakers.
Limiting radiographic identification of ARD to the chest x-ray film even in multiple projections and even if read by one or more B readers will fail to identify many patients with asbestos-related pleural thickening and/or irregular opacities. Signatory 1 compared B reading of x-ray films in two projections with low-dose-CT scans in 2760 atomic weapons workers potentially exposed to asbestos.12 CT showed pleural thickening in 271 (9.8%), only 54 of whom (20%) showed on x-ray film; interstitial opacities were seen in 76 CTs (2.8% of those examined), of whom only 10 (13%) were identified on x-ray film. The striking disparity between CT and x-ray film identification of ARD would be even greater for the very thin parietal pleural thickening characterized as “lamellar”, which is typical of LA exposure.
We must conclude from these considerations that the frequency of nonmalignant asbestos-related mortality in the Libby population reported by Larson et al is greatly underestimated.
A mortality study of LA exposure looking at all routes of exposure (occupational, household, community) and utilizing both death certificates and best evidence and CT scan as well as x-ray film would be appreciated by all who have concerned their professional lives with ARD.
1. Larson TC, Williamson L, Antao VC. Follow-up of the Libby, Montana screening cohort: a 17-year mortality study. J Occup Environ Med
2. Whitehouse AC. Asbestos-related pleural disease due to tremolite associated with progressive loss of lung function: serial observations in 123 miners, family members, and residents of Libby, Montana. Am J Ind Med
3. Black B, Szeinuk J, Whitehouse AC, et al. Rapid progression of pleural disease due to exposure to Libby amphibole: “not your grandfather's asbestos related disease.”. Am J Ind Med
4. Miller A. “Not your grandfather's pleural disease”: rapid progression, ventilatory impairment, and chronic pleuritic pain from Libby vermiculite/amphibole. Commentary. Am J Ind Med
5. Miller A, Szeinuk J, Noonan CW, et al. Libby amphibole disease: pulmonary function and CT abnormalities in vermiculite miners. J Occup Environ Med
6. Agency for Toxic Substances and Disease Registry (ATSDR). Summary Report: Exposure to asbestos-containing vermiculite from Libby, Montana, at 28 processing sites in the United States. Atlanta, GA:Department of Health and Human Services; 2008.
7. Wolfe C, Buck B, Miller A, et al. Exposure to naturally occurring mineral fibers due to off-road vehicle use: a review. Int J Hyg Environ Health
8. Baumann F, Buck BJ, Metcalf RV, McLaurin BT, Merkler D, Carbone M. The presence of asbestos in the natural environment is likely related to mesothelioma in young individuals and women from Southern Nevada. J Thorac Oncol
9. Selikoff IJ. Use of death certificates in epidemiological studies, including occupational hazards: discordance with clinical and autopsy findings. Am J Ind Med
10. Selikoff IJ, Seidman H. Use of death certificates in epidemiological studies, including occupational hazards: variations in discordance of different asbestos-associated diseases on best evidence ascertainment. Am J Ind Med
11. Szeinuk J, Noonan CW, Henschke CI, et al. Pulmonary abnormalities as a result of exposure to Libby amphibole during childhood and adolescence—The Pre-Adult Latency Study (PALS). Am J Ind Med
12. Miller A, Widman SA, Miller JA, Manowitz A, Markowitz SB. Comparison of X-ray films and low-dose computed tomographic scans: demonstration of asbestos-related changes in 2760 nuclear weapons workers screened for lung cancer. J Occup Environ Med