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D. Loomis1, S. H. Wolf2, J. M. Dement3, D. B. Richardson2. 1University of Nevada; 2University of North Carolina; 3Duke University
ObjectivesTo evaluate mortality among workers exposed to the chrysotile form of asbestos.
MethodsWorkers employed for at least 30 days between 1 January 1950 and 31 December 1973 in any of four plants in North Carolina, USA that produced asbestos textile products were enumerated and vital status was ascertained through 31 December 2003. Standardised mortality ratios (SMRs) comparing death rates in the cohort and the national population were computed using the NIOSH Life Table Analysis System. Poisson regression was used to compute adjusted mortality rate ratios by duration of employment.
ResultsAfter exclusion of workers with missing or invalid data, 5782 individuals were included in the analysis. Follow‐up resulted in 184087 person‐years of observation, with 2585 workers known dead, 2821 alive and 376 unknown. There were 275 deaths from lung cancer. Mortality from all causes and all cancers was higher than expected with SMRs of about 1.4 and lung cancer mortality was significantly elevated (SMR 1.96, 95% CI 1.74 to 2.21). Under a 20‐year lag assumption, the SMR for lung cancer was 2.35 (95% CI 2.04 to 2.70). The SMR for lung cancer increased with time since first exposure (0.92 for 0–10 years, 1.21 for 10–20 years, 1.65 for 20–30 years and 2.37 for 30+ years). Relative to workers employed for <10 years, the rate ratio for lung cancer increased linearly among workers employed for 10–20 years and 20–30 years, and to a lesser degree among those employed for 30 years or more. Elevated SMRs were also observed for pneumoconiosis (SMR 3.37, 95% CI 2.62 to 4.25), mesothelioma (SMR 11.1, 95% CI 3.03 to 28.4) and cancer of the pleura (SMR 12.7, 95% CI 3.44 to 32.3). The SMR for mesothelioma and pleural cancer combined (categories of cause of death that overlap) was 11.8 (95% CI 8.84 to 17.7).
ConclusionLung cancer mortality was significantly greater than expected among these asbestos textile workers exposed to chrysotile and increased with both time since first exposure and exposure duration. The excess risk was largely consistent with that observed among South Carolina asbestos textile workers. There was also evidence of increased risk of mesothelioma.
Key wordschrysotile; mesothelioma; lung cancer
J. C. McDonald1, A. Gibbs2, J. M. Harris1, F. D. Pooley3, G. Berry4. 1National Heart and Lung Institute (Imperial College); 2Llandough Hospital; 3University of Cardiff; 4University of Sydney
ObjectivesA full account of mortality in a cohort of 1154 persons, 97% female, who had been employed in 1940–44 on the assembly of military gas masks using filter pads containing 20% crocidolite has recently been published. Important findings were that of the 65 deaths from mesothelioma (28% of the peritoneum), among 632 from all causes, the first was in 1963 and the last in 1994, whereas a further 5.0 cases would have been expected between 1995 and 2003 (p=0.0005). To test the hypothesis that crocidolite, though biodurable, is slowly removed, data available on mineral fibre concentrations in lung tissue at autopsy have been analysed.
MethodsLung tissue samples were obtained from 50 of the 65 cases of mesothelioma, and 20 from deaths from other causes. Lung burden concentrations of all fibrous minerals of aspect ratio of at least 3:1 were obtained from wet lung tissue and identified using energy dispersive x ray analysis. These were analysed in relation to time since first employment and duration. As crocidolite fibre counts ranged from 1 to 1949, medians rather than means were first used in this analysis. A further analysis after logarithmic transformation of the data gave much the same result.
ResultsThe median concentrations in crocidolite fibres/gram, by decade of death since 1940, fell from 65 to 10 for pleural mesothelioma, from 93 to 15 for all causes and from 455 to 9 for all other diseases, except peritoneal mesothelioma. For the latter (10 cases), the median concentrations were all well above 100 throughout.
ConclusionThe steady decline in concentration with time since exposure, together with a similar trend in mesothelioma mortality, confirms the hypothesis that crocidolite and, by inference, other amphibole fibres are slowly removed from the lung. The consistently higher counts for peritoneal than pleural tumours is noteworthy. The findings overall suggest that mesothelioma mortality is unlikely to be significantly increased after about 40 years from most recent amphibole exposure.
Key wordscrocidolite; lung burden; biodurability
JCMcDonaldJMHarrisGBerry Sixty years on: the price of assembling military gas masks in 1940. Occup Environ Med200663 pp 852-855
J. M. Dement1, E. Kuempel2, R. Zumwalde2, R. Smith2, L. Stayner3, D. Loomis4. 1Duke University Medical Center; 2National Institute for Occupational Safety and Health, Cincinnati, OH; 3School of Public Health, University of Illinois, Chicago, IL; 4School of Public Health, University of Nevada, Reno, NV
ObjectivesTo develop a method for estimating fibre size‐specific exposures to airborne asbestos dust for use in epidemiological investigations of exposure–response relationships.
MethodsArchived membrane filter samples collected at a Charleston, SC, USA asbestos textile plant during 1964–1968 were analysed by transmission electron microscopy (TEM) to determine the bivariate diameter/length distribution of airborne fibres by plant operation. The protocol used for these analyses was based on the direct transfer method published by the International Standards Organization (ISO), modified to enhance fibre size determinations, especially for long fibres. Procedures to adjust standard PCM fibre concentration measures using the TEM data in a job exposure matrix (JEM) were developed in order to estimate fibre size‐specific exposures. Bootstrapping was used to estimate variability in the bivariate fibre size distributions and PCM to TEM conversion factors by plant department.
ResultsA total of 84 airborne dust samples were used to measure diameter and length for over 18000 fibres or fibre bundles. Consistent with prior studies, a small proportion of airborne fibres were longer than >5 μm in length and varied considerably by plant operation (range 6.9% (95% CI 3.9 to 9.8) in finishing to 20.8% (95% CI 18.7 to 22.8) in universal winding). The proportion of fibres <0.25 μm in diameter but with lengths >5 μm, and not generally detectable by PCM, was found to range from 4.5% (95% CI 6.7% to 12.8%) in finishing to 11.4% (95% CI 10.1% to 12.5%) in mule spinning. The bivariate diameter/length distribution of airborne fibres and PCM to TEM conversion factors demonstrated a relatively high degree of variability by plant operation.
ConclusionThese data provide new information concerning the airborne fibre characteristics for a textile operation used for previous exposure–response analyses. The TEM data demonstrate that the vast majority of airborne fibres inhaled by these workers were shorter than 5 μm in length and <0.25 μm in diameter, and therefore not included in current PCM fibre counts. A fibre size‐specific JEM was developed for use in fibre size‐specific risk assessments and is being used for risk assessments for the Charleston cohort. Similar methods are being used in a study of four North Carolina, USA asbestos textile plants using predominately chrysotile.
Key wordsasbestos; TEM; PCM
A. H. Harding1, A. J. Darnton2, D. M. McElvenny2, D. Morgan1. 1Health & Safety Laboratory; 2Health & Safety Executive
ObjectivesThe Asbestos Survey is a national survey of asbestos workers in Great Britain. It was established following the introduction of the 1969 UK Asbestos Regulations, to monitor long‐term health of asbestos workers. The aim of this analysis was to report on the updated mortality of the cohort.
MethodsSurvey recruitment began in 1971. Under the 1969 regulations, workers initially attended voluntary medical examinations at 2‐yearly intervals. The 1984 Asbestos Licensing Regulations and the 1987 Control of Asbestos at Work Regulations subsequently required all workers covered by these regulations to attend statutory medical examinations. These workers were also eligible for inclusion in the cohort. At the time of their examination, workers completed a questionnaire with details of smoking habit, occupational history and duration of exposure to asbestos. Survey participants were flagged at the National Health Service Central Register(s) for death registrations. Standardised mortality ratios (SMRs) for deaths occurring before the end of 2005 were calculated using mortality rates for Great Britain.
ResultsAltogether 93622 men with 1676186 person‐years of follow‐up and 4495 women with 103394 person‐years of follow‐up, and 15498 deaths were included in the analysis. At the first medical examination, the mean age was 35 years (SD 12.5 years) and 57% were current smokers. Median exposure length was 5.8 years. The SMRs were 141 (95% CI 138 to 143) for all causes of death, all malignant neoplasms (MN) 163 (159 to 167), MN of the larynx 148 (109 to 195), MN of the trachea, bronchus and lung 187 (179 to 196), MN of the pleura 968 (817 to 1139), MN of the peritoneum 3730 (2979 to 4612), MN of the stomach 166 (149 to 186), mesothelioma (ICD‐10, deaths post 2001) 513 (435 to 601), and asbestosis 5594 (4634 to 6694). SMRs tended to be higher for those first exposed before 1960, those exposed at an early age, for insulation workers and those in the stripping/removal industry.
ConclusionMortality among asbestos workers was significantly higher than in the British population. The Asbestos Survey covers a period of increased regulation in the asbestos industry, and is invaluable for monitoring the effect of changes in the working environment on the long‐term health of asbestos workers.
Key wordsasbestos; cohort; mortality