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(Chest. 2002;121:2044-2051.)
© 2002 American College of Chest Physicians

A Longitudinal Study of Chest Radiographic Changes of Workers in the Refractory Ceramic Fiber Industry*

James E. Lockey, MD, MS, FCCP; Grace K. LeMasters, PhD; Linda Levin, PhD; Carol Rice, PhD; James Yiin, MPH; Susan Reutman, PhD and Diane Papes, BS

* From the Division of Occupational and Environmental Medicine, Department of Environmental Health, University of Cincinnati College of Medicine, Cincinnati, OH.

Correspondence to: James E. Lockey, MD, MS, FCCP, Division of Occupational and Environmental Medicine, Department of Environmental Health, University of Cincinnati College of Medicine, 3223 Eden Ave, Kettering Building, ML 0056, Cincinnati, OH 45267-0056


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objective: This industry-wide longitudinal study examines chest radiographic changes of workers manufacturing refractory ceramic fibers (RCF).

Design: Chest radiographs were obtained every 3 years and were interpreted using the 1980 International Labour Organization classification for pneumoconiosis. Three exposure metrics were calculated: duration and latency in a production job, and cumulative exposure (fiber-months per cubic centimeter).

Participants: The radiographic survey included 625 current workers at five manufacturing sites and 383 former workers at two of the five sites.

Measurements and results: Pleural changes were seen in 27 workers (2.7%). Of workers with > 20 years of latency from initial production job or 20 years of duration in a production job, 16 workers (8.0%) and 5 workers (8.1%) demonstrated pleural changes, respectively. Results from the cumulative exposure analysis (> 135 fiber-months per cubic centimeter) demonstrated a significant elevated odds ratio (OR) of 6.0 (95% confidence interval [CI], 1.4 to 31.0). The incidence of irregular opacities at profusion categories >= 1/0 was similar to other nonspecified dust-exposed worker populations at 1.0%, and showed a nonsignificant elevated OR in regard to cumulative fiber exposure of 4.7 (95% CI, 0.97 to 23.5).

Conclusions: RCF are significantly associated with pleural changes that were predominantly pleural plaques, but have not resulted in a statistically significant increase in interstitial changes.

Key Words: interstitial fibrosis • man-made mineral fibers • man-made vitreous fibers • pleural plaques


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Refractory ceramic fibers (RCF) are a subgroup of man-made vitreous fibers (MMVF) produced from a mixture of alumina and sand or kaolin. The raw materials are melted and subsequently fiberized through a steam-blowing or spinning process that produces fibers with an average diameter of 1 to 5 µm. RCF can withstand temperatures up to 1,260°C and are primarily used as high-temperature insulating linings for furnaces and kilns. RCF may be sold as bulk fibers, or blanket, or made into felt, boards, specialty shapes, and various types of paper and textile products. Of the total production of MMVF in the United States, which also includes glass fiber and mineral wool, RCF constitute 1 to 2%.1

Because of concern for human respiratory effects associated with RCF exposure, in part, based on previous animal research,2 3 4 5 a US industry-wide study of workers involved with RCF manufacturing was undertaken in 1987. Previous cross-sectional results of workers from five manufacturing facilities demonstrated a significant relationship between RCF production job tasks and predominantly pleural plaques on chest radiography.6 A retrospective cohort study at two plant sites where past RCF exposures were reconstructed demonstrated coherence at a statistically significant level among three exposure metrics, time since first RCF production job, duration of RCF production employment, and cumulative fiber exposure (fiber-months per cubic centimeter) and pleural plaque findings.7 The purpose of this study was to update the results of the longitudinal radiographic findings.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Subjects
A prospective industry-wide health study was initiated in 1987 with current workers at three facilities and with current and former workers at two facilities where RCF and RCF products are produced.6 7 Additional details of subject recruitment and data collection are reported elsewhere.8 9 10 Eligible workers had a minimum of 1 year of employment between 1953 (commencement of RCF production) and December 31, 1996. Female clerical employees who left work prior to study initiation in June 1987 were required to have held a production job for a minimum of 4 h/wk for 1 year to be included in the study. There was a total cohort of 1,331 subjects with at least one chest radiograph and 1 year of employment by December 31, 1996. Workers were asked to sign an informed consent that was approved by the University of Cincinnati Medical Center. At time of study initiation, baseline health and structured occupational history questionnaires were administered in person to each worker, and weight and height were measured to calculate body mass index (BMI [weight divided by height squared]). The occupational history elicited comprehensive information about the subject’s RCF employment and other work experiences, including potential asbestos exposure prior to, during, or subsequent to RCF employment. Workers with < 5 years between hire into the RCF industry and the date of the last chest radiograph were excluded (n = 237), as were employees working in sales who had unknown exposure to RCF (n = 86).

Radiographic Data Collection
Chest radiographs at several medical facilities in each local community were evaluated, and a provider was selected. Chest radiographs were masked of all identifying information and were submitted to three experienced certified B-readers and radiologists for independent evaluation. In addition, at each reading, 10 to 25% of radiographs were from individuals from a general population obtained from the same medical facilities, and subsequently mixed with the study radiographs and submitted to the B-reader panel members for evaluation using the 1980 International Labour Office classification of radiographs of pneumoconiosis.11 Chest radiographs were obtained approximately every 3 years. At two plant locations, posteroanterior and right and left anterior oblique radiographs were evaluated throughout the study; at three plants, only posteroanterior radiographs were obtained initially for two or three periods, and then subsequently oblique radiographs were added. The posteroanterior and oblique radiographs were evaluated concurrently as a group of three by each B-reader.

For this analysis, a radiographic reading was defined as positive when the median interpretation of three B-readers was consistent with either pleural and/or interstitial changes. Because of interreader and intrareader variability of chest radiograph interpretations, it was possible for an individual’s initial radiographic readings to change in subsequent radiographs from positive to negative or vice versa. Therefore, our longitudinal definition of a positive chest radiographic reading was determined a priori as follows: once a subject’s chest radiographic readings were reported as positive by two of three readers (the median reading), then that positive radiograph reading plus all subsequent B-readings were used to define the median reading for the final positive or negative determination. Thus, at least one half of the total number of B-readings after and including the first positive radiograph reading must be positive for the subject to be ultimately identified as positive. By initiating the count with the first positive radiograph readings, while allowing for the median reading of the last radiograph to be negative, this case definition was derived to be sensitive and inclusive. Pleural thickening was defined as diffuse thickening of the pleural membrane that included blunting of the costophrenic angle, while blunting of the costophrenic angle only with no other pleural change was excluded. Pleural plaques were defined as pleural thickening with or without calcification along the chest wall, diaphragm, and/or pericardium not otherwise classified as diffuse pleural thickening. Interstitial changes were defined as parenchymal opacities of a round and/or irregular nature.

Exposure Assessment
The occupational history was updated by interview yearly for current workers and every 3 years at time of chest radiography for the former workers, and included job titles, activities, and location and dates of all jobs. Three exposure metrics were calculated for this study: time worked in a production job (duration), time since beginning the first production job (latency), and cumulative exposure (the summed product of duration and exposure intensity in each job). Each of these was calculated to the subject’s last radiograph date on or before December 31, 1996, including those who had radiographic findings develop during follow-up. The definition of a production job required that persons spend at least 10% of their time in the production areas of the facility. Personnel who did not fit this criteria, ie, spending less time in production areas (quality control laboratory workers), accumulated fiber exposure in these other jobs, which contributed to the cumulative exposure metric but not to the duration or latency metric. Hence, the construction of duration and latency focuses on the jobs with relatively higher exposure. Cumulative exposure, however, which accounts for potential exposure in production and nonproduction workers, is generally recognized as the most valid summary exposure measure associated with chronic effects over a working lifetime.12 Other metrics, also, have been suggested,13 14 but these must be selected with an underlying rationale for biological plausibility.

To estimate current exposure intensity to RCF at the five manufacturing locations, workers were randomly selected to wear personal air-monitoring samplers during the work shift.10 From 1987 to 1988, the range of time-weighted average exposure estimates were 0.01 to 1.04 fibers per cubic centimeter for the blanket line, 0.03 to 0.61 fibers per cubic centimeter for dry fabrication, 0.01 to 0.27 fibers per cubic centimeter for wet fabrication, 0.01 to 0.47 fibers per cubic centimeter for furnace operations, and 0.02 to 0.62 fibers per cubic centimeter for maintenance. This process was repeated quarterly. Details on the sampling protocol have been published.10 Similar exposure levels for these participating plants also have been reported by others.15 Historical exposure data were available at two plants for calculating pre-1987 in-plant exposures.16 17 Measurements of fiber concentration (fibers per cubic centimeter) in each work location were used to estimate exposures for specified time periods for > 80 job titles.16 Overall, exposures over time have decreased. The maximum exposure estimate was 10 fibers per cubic centimeter in the 1950s for carding in a textile operation; subsequent engineering changes reduced this estimated exposure to < 1 fiber per cubic centimeter. Workers at these two facilities were characterized by their cumulative lifetime RCF exposure (defined as fiber-months per cubic centimeter) by adding the products of job duration in months times job-specific fiber exposure levels across time. Four fiber-months per cubic centimeter categories were derived, using a priori requirements that the mean of each category would be statistically different from that of adjacent categories.

Historically, exposure to asbestos has been the primary risk factor for fiber-associated pleural and parenchymal changes. Therefore, detailed open-ended and closed-ended questions were asked concerning possible workplace and home asbestos exposure. The total time of contact was summed to obtain an asbestos duration of exposure variable. These asbestos exposure questions included jobs in the mining or milling of asbestos; spraying asbestos insulation; remodeling jobs prior to 1980 using filler and grout, taping and spackling, and mortar and plaster; textile manufacturing with asbestos products; jobs using paper and cement products containing asbestos; products used for fire proofing, insulation, or acoustical treatment; new construction work with asbestos products; gutting and demolition of commercial buildings; ship building, including repairing and refitting; manufacturing or repairing of automobiles, involving gaskets, clutch plates, and break lining; and sheet metal work with asbestos insulation. In addition to occupational asbestos exposures, workers were queried on changing their automobile brakes, potential home use of asbestos materials, and about friends or relatives who might have brought asbestos into their residences.

The total duration of exposure also was calculated for other respiratory hazards, possibly associated with interstitial changes. "Other" respiratory hazards included work with beryllium, cobalt, zeolites, attapulgite, raw kaolin, silicon carbide, talc, vermiculite, wollastonite, and moldy hay. Total (nonoverlapping) months among these other exposures both before and after RCF employment was calculated. Also, a detailed cigarette smoking history was used to calculate total pack-years of direct exposure to tobacco smoke. In summary, duration and latency in a RCF production job, asbestos exposure, and exposure to other dusts related to occupational lung disease were available for all workers. Cumulative exposure to RCF was calculated for workers at two locations only.

Chest Radiograph Statistical Analysis
Odds ratios (ORs) of pleural changes adjusted by logistic regression for duration of asbestos exposure and both categorical age (<= 50 years, > 50 years) and age defined continuously (in years at the last radiograph) were used with each metric of RCF exposure. The three RCF exposure metrics were as follows: RCF production duration, > 10 to 20 years and > 20 years, compared to 0 to 10 years across five manufacturing sites; RCF production latency, > 10 to 20 years and > 20 years, compared to 0 to 10 years across five manufacturing sites; cumulative exposure, > 15 to 45 fiber-months per cubic centimeter, > 45 to 135 fiber-months per cubic centimeter, and > 135 fiber-months per cubic centimeter, compared to > 0 to 15 fiber-months per cubic centimeter across two manufacturing sites. Each measure of RCF exposure was calculated from initial exposure to date of last radiograph for all workers to allow consistent periods of observation for all cases and noncases. A potential relationship between BMI and pleural changes18 was evaluated by logistic regression testing for linear trend.

ORs also were calculated for interstitial changes scored as profusion categories >= 1/0 (the primary outcome) adjusted for pack-years and age (years). Separate analyses adjusted for pack-years and asbestos exposure as well as pack-years and exposure to other substances were tried. These did not provide as good a fit to the data, based on the Akaike information criterion that compares the goodness of fit of models with different explanatory variables.19 One worker with kaolinosis was excluded from the interstitial analyses. For the interstitial analyses, age is a predictor and, therefore, was modeled as a continuous variable rather than categorically, since the minimum and median ages of profusion categories >= 1/0 cases were 57 years and 71 years, respectively. Analyses were then performed with radiographs scored for profusion categories >= 0/1.

Because of the known association of asbestos with pleural plaques, one investigator (J.E.L.) reviewed all workers with clinically relevant asbestos exposure as defined by job tasks and industry, as well as type, duration, and intensity of exposure. After this review, seven workers were excluded (including one plaque case) for both the pleural change and interstitial analysis. Since the findings were similar, these are not shown.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Radiography
As described in Table 1 , there were 1,008 workers from five manufacturing locations; most were white (81%), male (85%), and had an average age of 46.6 years at time of the last chest radiograph. Approximately 20% of workers were in nonproduction jobs, approximately 36% had > 10 years of duration in RCF production jobs, and the average latency period between first production job and last chest radiograph was 16.4 years. On average, the workers had 2.7 separate radiographic evaluations over the study investigation.


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  		Table 1.. Characteristics of Study Subjects (n = 1,008, All Plants) for the Radiographic Analysis*

 
There were 27 workers (2.7%) with pleural changes, of which 19 were bilateral plaques (70%) and 3 were unilateral plaques (11%). No pleural changes were identified in workers hired after study initiation in 1987. The distributions of pleural cases by duration of time employed and time since first hired (latency) for both RCF production and nonproduction workers until the date of first chest radiographic change are shown in Table 2 for comparison purposes with other published studies. The proportion of study workers with pleural changes with increasing duration, latency, and cumulative exposure categories demonstrated an exposure-response trend (Table 3 ). In the 0- to 10-year production duration category, there were 1.6% pleural changes, compared to 4.0% and 8.1% in the > 10- to 20-year exposure, and > 20-year exposure categories, respectively. Multiple logistic regression analyses adjusted for age and potential asbestos exposure showed an increasing trend of adjusted ORs of 2.5 and 3.2 for workers in the > 10- to 20-year RCF production duration and > 20-year duration, respectively, compared to the baseline group of 0- to 10-year duration. A statistically significant OR of 7.4 was found for workers with > 20 years since hire in a production job compared to the baseline group. BMI was also evaluated and found not to be related to either pleural changes (n = 27) or pleural plaques (n = 22).


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  		Table 2.. Duration and Latency of RCF Employment for Workers With Pleural Changes*

 

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  		Table 3.. Relation Between Duration, Latency, and Cumulative Fiber Exposure and Pleural Changes Adjusted for Asbestos Exposure and Age*

 
Cumulative fiber exposure was calculated for 637 workers; 21 of the 637 workers (3.3%) had pleural changes. The average (± SD) exposure was 0.8 ± 0.7 fibers per cubic centimeter for pleural change cases vs 0.5 ± 1.1 fibers per cubic centimeter for the 616 noncases. Results of the analysis showed statistically significant ORs of 5.6 and 6.0 for workers in the exposure categories of > 45 to 135 fiber-months per cubic centimeter and > 135 fiber-months per cubic centimeter, respectively, compared to the group with > 0 to 15 fibers-months per cubic centimeter (Table 3) . Hence, all three exposure metrics (duration, latency, and cumulative fiber) demonstrated increasing ORs with increasing exposure. The effects of continuous-duration asbestos exposure and categorical age also were statistically significant using each exposure metric. The difference between the adjusted and unadjusted estimates for each exposure metric demonstrated that the adjustment removed substantial confounding, especially at the highest exposure levels. However, because the dichotomization of age may leave residual confounding, we reanalyzed the three exposure metrics with age as a continuous variable and asbestos as a continuous or as a categorical variable. ORs and significance levels were similar for each exposure metric.

For the analyses of interstitial changes, one worker with kaolinosis was excluded. There were 10 workers (1.0%) remaining with interstitial changes profusion categories >= 1/0. Because of the small number of cases, each of the three exposure metrics (duration, latency, and cumulative exposure) were reduced to dichotomous. As shown in Table 4 , there were three interstitial cases (0.5%) in the 0- to 10-year RCF production duration category and seven cases (1.9%) in the > 10-year group. Multiple logistic regression analyses demonstrated a fourfold increase in risk that was not statistically significant (OR, 4.1; 95% confidence interval [CI], 0.9 to 26.6). Pack-years (OR, 1.4; 95% CI, 1.2 to 1.8), and age (OR, 6.5; 95% CI, 2.8 to 19.1) for 10-year intervals were significant. The cumulative fiber-months per cubic centimeter exposure metric, as well as covariates pack-years and age, showed similar findings (Table 4) . Because of few cases, the covariates measuring duration of asbestos exposure and duration of exposure to other respiratory substances replaced age, and were each analyzed separately with pack-years in the model. Neither were significant, and they were not included in the final model. For the analysis of interstitial profusion categories >= 0/1, the ORs were lower compared to >= 1/0 (Table 4) . Pack-years and age again were significant for each exposure metric.


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  		Table 4.. Interstitial Analysis Evaluating Duration, Latency, and Cumulative RCF Fiber-Months Per Cubic Centimeter Adjusted for Pack-Years and Age*

 
The quality of the chest radiograph and the radiograph readings was evaluated. Over the study investigation, there were 2,707 sets of radiograph obtained. Only 14 of these radiographs (0.5%) were scored as unreadable in regard to interstitial changes, none were plaques, and 93.5% were scored as radiograph quality 1 or 2 (good and acceptable, respectively). Further, there was no significant difference in the radiograph quality for cases and noncases. Intrareader reliability and interreader reliability for the B-readers median radiograph interpretation were also evaluated. The intrareader {kappa} was 0.67 while the interreader {kappa} was 0.59 for the readings of the group of three radiographs, indicating overall good agreement.20 21


   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Pleural Changes
The vast majority of cases of pleural plaques in the general population are related to previous occupational and/or environmental exposure to asbestos, and occurs most commonly > 15 to 20 years after initial exposure. Castellan and colleagues22 demonstrated a 0.21% rate of pleural abnormalities in blue-collar workers with minimal history of exposure to respiratory hazards. A population in Sweden studied by Hillerdal23 demonstrated a background prevalence of bilateral pleural plaques of 0.3% for men > 40 years old who denied any exposure to asbestos. The prevalence of pleural plaques in general populations can vary from 0.0 to 6.8%, depending on the definition used to record the presence of a plaque on a chest radiograph, the potential for previous occupational exposure to asbestos, and the age, gender, and location (country as well as urban vs rural) of the survey group.24 The results of the radiographic analysis in this study demonstrated increasing ORs between the occurrence of pleural changes and duration of employment, as well as a significant relationship with the > 20-year latency from initial employment category in a RCF production job. The overall prevalence of pleural changes was 2.7% (27 changes among 1,008 eligible workers). Of the 27 cases of pleural changes, 22 cases (82%) were plaques; of these, 86% were bilateral. Five of 62 workers (8.1%) with > 20 years of employment in RCF production job tasks had pleural changes (OR, 3.2; 95% CI, 0.9 to 9.7), as did 16 of 201 workers (8.0%) with > 20 years since the first RCF production job (OR, 7.4; 95% CI, 2.3 to 32.6). Six of 61 workers (9.8%) with cumulative RCF exposure > 135 fiber-months per cubic centimeter had pleural changes (OR, 6.0; 95% CI, 1.4 to 31.0). When pleural plaques alone (n = 22) were evaluated, the results across all three exposure metrics for the higher exposure categories were significant, including the metric of > 20 years of RCF production exposure (OR, 4.7; 95% CI, 1.3 to 16.3). CIs are based on the profile likelihood function, which is believed to be more accurate than those based on asymptotic normality of parameter estimates, especially for small sample sizes.25

In a previous report, a nested case-control study was undertaken evaluating asbestos exposure using an extensive interview that obtained information on work tasks and practices. RCF remained a significant risk factor for plaques.7 Because age may act as a surrogate for unrecognized asbestos exposure, and also may be associated linearly with exposure (ie, workers with longer RCF duration and latency will also be older), the primary analysis was with age modeled as a dichotomous variable (<= 50 years, > 50 years). Age was also examined as a continuous variable, and findings remained essentially unchanged with respect to each RCF exposure metric. Age was significant when modeled categorically or continuously for all exposure metrics. Duration of asbestos exposure was significant when age was modeled categorically for all exposure metrics. When age was modeled continuously, the effect of asbestos exposure no longer remained significant in the cumulative exposure metric.

Interstitial Findings
In the initial report from the 1987 survey, there were no irregular opacities for profusion categories >= 1/0.6 In the follow-up study of two plant sites where historical fiber exposure estimates were reconstructed, parenchymal changes of 1.1% were demonstrated among former workers, 0.7% among production workers, and 0.5% among the combined group of current (production and nonproduction) and former workers.7 The prevalence of irregular opacities in this update is 1.0% (10 irregular opacities in 1,007 current and former employees). Seven of the 10 cases had changes at profusion categories >= 1/1, and three cases were at 1/0. Adjustment for pack-years of cigarette smoking was included in the analysis because of reported association between smoking and low-profusion-level irregular opacities at the lung bases.22 In addition, age was included in the model because of the likelihood of a higher prevalence of interstitial changes with increasing age from nonoccupational causes. After adjustment for pack-years and age, both of which were significant, there was a trend, but not at a statistically significant level, between interstitial changes (profusion categories >= 1/0), and duration of employment within RCF production job tasks, and cumulative RCF exposure. Seven of 363 workers (1.9%) with > 10 years in RCF production job tasks demonstrated interstitial changes (OR, 4.1; 95% CI, 0.9 to 26.6). Five of 61 workers (8.2%) with cumulative exposure > 135 fiber-months per cubic centimeter demonstrated interstitial changes (OR, 4.7; 95% CI, 0.97 to 23.5). When the analysis was repeated excluding three cases with last chest radiographic readings negative for interstitial changes, the results were similar: > 10 years of RCF production (OR, 5.5; 95% CI, 0.9 to 72.4), and cumulative RCF exposure > 135 fiber-months per cubic centimeter (OR, 2.4; 95% CI, 0.3 to 13.9). When five subjects with profusion category 0/1 were included in the analysis (total of 15 cases), the OR for an effect related to RCF production duration and cumulative exposure both decreased in comparison to profusion categories >= 1/0. This decrease is likely the result of including chest radiographs interpreted at profusion category 0/1, which represent very subtle and therefore highly variable radiographic interpretation.

The prevalence of 1.0% for interstitial changes in the RCF population is similar to other nonspecified dust-exposed worker populations. The prevalence of irregular opacities at profusion category >= 1/0 was 1.02% in a blue-collar worker population with > 5 years of exposure to dust or other respiratory hazards.22 For combined round and irregular opacities, the prevalence was 1.9%; for a nonexposed population, the prevalence was 0.21%. Though the findings in regard to interstitial changes in this study were not statistically significant, our study power was low. We estimated the statistical power for testing the effect of duration RCF production employment shown in Table 4 (OR, 4.1; p = 0.09) to be approximately 39%.26 Smoking has been found in some studies to be associated with low-profusion-level irregular opacities in presumably normal individuals.22 It may be possible that smoking and fiber exposures together increase the likelihood of finding irregular opacities. Smoking, recorded as pack-years in this study, was a significant predictor of irregular opacities.

A study of US workers from five fiberglass and two mineral wool manufacturing plants did not demonstrate an overall increase in small opacities when compared to a non-MMVF-exposed comparison group.27 Twenty-three workers (1.6%) with identified opacities were at profusion level 1/0 or 1/1, and occurred with exposure to fibers of respirable size. The primary type of opacities was irregular, and there was an association with various exposure indexes at profusion level 1/0 but not 1/1 at the plant identified as manufacturing ordinary and fine-diameter fibers (average fiber diameters > 3 µm and 1 to 3 µm, respectively). A study of production workers with exposure >= 15 years to rotary-spun fiberglass for insulating appliances demonstrated a prevalence of irregular opacities (>= 1/0 to 2/1) of 3.5%, and when including profusion category of >= 0/1 of 7.7%; pleural plaques or thickening was noted in 5.6%.28 A question had been raised, however, about airborne asbestos fibers at the plant site studied in regard to a potential confounding factor.29

An initial survey of seven European RCF manufacturing facilities in 1987 demonstrated pleural changes in 15 workers (2.8% of 543 available chest radiographs).30 Three of these 15 workers had costophrenic angle obliteration only, and of the 12 workers with unilateral or bilateral pleural changes, 8 workers had other possible explanations other than RCF exposure. Seventy-seven of 543 radiographs (14.2%) were interpreted with changes of profusion category >= 0/1, and 38 radiographs (7%) at profusion categories >= 1/0. A significant association between small opacities was noted with production plant, smoking, and age. A similar association was also demonstrated with years since first employment, years of employment, prior asbestos exposure, and current nonrespirable fiber exposure levels. There was, however, no association with cumulative indexes of exposure, and the 15 cases (2.8%) of predominantly irregular opacities at profusion categories >= 1/0 were not related to respirable fiber exposure indexes. Overall, the authors concluded that RCF exposure was unlikely the predominant cause of the radiographic findings. In further analysis, cumulative exposure ranged from 0 to 22.94 fiber-years per cubic centimeter, with a mean of 3.84. When the radiograph interpretations were further analyzed in relationship to cumulative exposure, there was no association between the prevalence of small opacities and exposure.31

A follow-up study of the European RCF manufacturing facilities in 1996 consisted of current workers at six manufacturing facilities and those (from the same plants) who participated in the first survey in 1987 and subsequently left (leavers).32 33 A total of 760 workers provided chest radiographs, representing 88% of the current workers and 36% of leavers. Radiographs from workers from other industries with no exposure to dust or fibers were randomized and also included. Pleural changes were noted in 11% of the radiographs from the study group, with pleural plaques identified in 5%. Of the 40 workers with pleural plaques, 13 workers had unilateral pleural plaques, 24 workers had bilateral pleural plaques with 3 discordant, and 31 workers (78%) reported previous exposure to asbestos. Of those having >= 20 years of latency, 14.4% of workers (n = 22) had plaques compared to 3.0% of workers (n = 18) with < 20 years of latency. In nonmutually exclusive groups, 45% of total workers reported occupational exposure to asbestos outside the RCF industry and 20% reported asbestos exposure within the participating plant sites. For nonoccupational asbestos-exposed workers, there was a statistically significant association between RCF latency and both pleural changes and pleural plaques, with age excluded from the model and pleural changes only with age included in the model. There were 15 workers (2.0%) with small opacities profusion categories >= 1/1 and 59 workers (7.8%) with categories >= 1/0. All but eight of the cases with opacities were irregular in shape. There was no association between small opacities profusion category >= 1/0 and cumulative exposure to respirable fibers or dust. There was a suggestion of an association between profusion categories >= 0/1 (51% of total cohort) and cumulative respirable fiber exposure, but the pattern of occurring in the early (pre-1971) and late (1992 on) exposure periods and not the midexposure period (1970s and 1980s) was not supportive of a cause-effect relationship. In addition, the distribution of small opacities was similar in the non-dust-exposed comparison group. Overall, the authors believed that, although it was difficult to separate the effects of asbestos and RCF exposure in workers unexposed to asbestos occupationally, there was some evidence of a relationship between RCF latency for both pleural changes and pleural plaques but not with duration or intensity of exposure to RCF. The relationship between RCF exposure and small opacities was at best ambiguous, however.

In conclusion, the results of this longitudinal study confirmed our previous cross-sectional study results, demonstrating a statistically significant association between RCF exposure and pleural plaques. There was an increase in interstitial changes but not at a statistically significant level. Overall, the prevalence of pleural plaques has remained constant, rather than increasing, which is likely attributed to the lowering of the RCF exposure levels at the manufacturing facilities in the mid-1980s. In our previous analyses, a nested case-control study was undertaken to more thoroughly evaluate asbestos exposure.7 Further, as an additional approach to adjust for possible asbestos exposure, age was included as a surrogate for unrecognized past asbestos exposure. RCF, however, remained a significant predictor of pleural changes including pleural plaques. In regard to the interstitial changes, the overall prevalence at profusion category >= 1/0 in this study are similar to other nonspecified dust-exposed worker populations. There was a trend with duration of exposure in production jobs and cumulative fiber exposure, but this finding was not statistically significant. The power of the study was low, however, to determine whether or not an actual significant effect exists. The RCF cohort will continue to be followed up in the future to further examine pulmonary morbidity.


   Acknowledgements
 
The authors thank the participants in this study, and the participating radiologists: Ralph Shipley, MD; Harold Spitz, MD; and Jerome Wiot, MD. They also thank the Refractory Ceramic Fiber Coalition: Paul Epstein, MD; Edward Horvath, Jr., MD, MPH; Mark Utell, MD; and Alec Walker, MD, for their review and insights. The authors are grateful to their staff and to their administrative assistant, Connie Thrasher.


   Footnotes
 
Abbreviations: BMI = body mass index; CI = confidence interval; MMVF = man-made vitreous fibers; OR = odds ratio; RCF = refractory ceramic fibers

This study was supported by the Refractory Ceramic Fiber Coalition and National Institute of Environmental Health Sciences grant No. 5-P30-ES06096–09.

Received for publication March 1, 2001. Accepted for publication January 30, 2002.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. . Thermal Insulation Manufacturers Association. (1990) Health and safety aspects of man-made vitreous fibers: part IV. Thermal Insulation Manufacturers Association Stamford, CT.
  2. Bunn, WB, Bender, JR, Hesterberg, TW, et al (1993) Recent studies of man-made vitreous fibers. J Occup Med 35,101-113[CrossRef][ISI][Medline]
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R. C. BROWN, B. BELLMANN, H. MUHLE, J. M. G. DAVIS, and L. D. MAXIM
Survey of the Biological Effects of Refractory Ceramic Fibres: Overload and Its Possible Consequences
Ann. Hyg., June 1, 2005; 49(4): 295 - 307.
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