Montana & Mesothelioma - Results
Description of the participants. Of 12,829 persons screened by telephone, 3,527 did not meet the criteria for eligibility and 66 had unknown eligibility status. Of those eligible, 1,689 either did not schedule an appointment or did not report for medical testing, 231 refused medical testing, 4 had died before they were tested, and 5 were physically unable to come in for testing. The remaining 7,307 current and former residents of Libby and the surrounding area participated in the medical testing program (6,149 in the first wave and 1,158 in the second wave, resulting in a 78.6% participation rate). Among those who participated, 81% stated that they currently lived in Montana, and of those, 80% currently resided in the Libby, Troy, or Eureka areas. Most of the participants outside of Montana came from Idaho, Oregon, and Washington. Because participants in the first and second years of medical testing were similar in demographic characteristics, exposure pathways, concern about neighborhood contamination, and prevalence of outcomes, the data from both years were combined.
Of the 7,307 participants, 6,668 (91.2%) were [greater than or equal to] 18 years of age and therefore were eligible for and received chest radiographs. During the two waves of medical testing, the physician on site determined that 525 radiographs could not be read because of poor quality, and these were later repeated.
Characteristics of the participants are presented in Table 1. The participants were almost evenly divided by sex with 49% male and 51% female. The majority of participants were 18-64 years of age (76.0%). Almost half of the participants were former or current smokers. Roughly 74% of participants had lived in the Libby area for [greater than or equal to] 14 years. Many of the participants were overweight. A BMI of 25-29.9 is considered overweight, and a BMI of [greater than or equal to] 30 is considered obese; 67% of participants had a BMI of [greater than or equal to] 25, with almost 32% of all participants in the obese category.
The 29 exposure pathways used in the analyses and the number of participants reporting each pathway are presented in Table 2. These include occupational, recreational, household, and other potential exposures. Participants may have reported one, several, or none of these exposures. The most common pathways were recreational activities along Rainey Creek Road (4,898, or 67.4%), playing in the baseball fields near the expansion plant (4,772 participants, or 65.5%) and playing in the vermiculite piles (2,442, or 33.7%). The most common occupational exposure was dust exposure at non-WRG jobs (2,396, or 32.8%), and the least common occupational exposure was working in a shipyard or ship construction or repair (129, or 1.8%).
Males were much more likely than females to have reported occupational exposures. Among those who reported having worked at WRG, 341 (92.2%) were male and 29 (7.8%) were female. Males also were more likely than females to have been exposed to asbestos or vermiculite during nonoccupational activities, although differences were less pronounced when compared with the occupational exposures. For instance, among the exposure pathways, males were more likely than females to report dust exposures at work (76.8% vs. 23.2%), working at any job with exposure to asbestos (71.2% vs. 28.8%), vermiculite exposure at other jobs (77.2% vs. 22.8%), frequently playing in vermiculite piles (61.1% vs. 38.9%), or frequently popping vermiculite (54.4% vs. 45.6%). The only notable exception was that females were more likely than males to have been a household contact of a WRG worker (60.0% of household contacts were female).
Pleural and interstitial abnormalities. Table 3 presents the crude prevalence rates of pleural and interstitial abnormalities by exposure pathways for 6,668 participants, [greater than or equal to] 18 years of age, who underwent chest radiography. The pathways presented here are not mutually exclusive; for example, a participant who reported having been a WRG worker, playing on the baseball field, and using vermiculite for gardening would be represented in all three categories. The overall prevalence of pleural abnormalities was considerably greater than the prevalence of interstitial abnormalities. Almost 18% of participants who underwent chest radiography had a pleural abnormality, compared with only 1% of participants having an interstitial abnormality. The exposure pathway with the highest unadjusted rate for pleural abnormalities was being a former WRG worker, with 186 (51.0%) having pleural abnormalities. WRG workers also had the highest rate of interstitial abnormalities (3.8%). Other pathways associated with high rates of pleural abnormalities included history of asbestos exposure in the military (42.9%), working in a shipyard or performing shipyard construction or repair (34.9%), and being a secondary contract worker for WRG (34.8%). The lowest prevalence of pleural abnormalities (14.4%) was seen for the "sometimes" exposure category of playing at the baseball field near the expansion plant.
The rate of pleural abnormalities increased from 5.1% in younger adults 18-44 years of age, to 20.0% for participants 44-65 years of age, to 39.7% for participants [greater than or equal to] 65 years of age. Males had a significantly higher rate of pleural abnormalities (26.6%) compared with female participants (9.1%). The crude OR for pleural abnormalities among males compared with females was 3.61 (95% CI, 3.14-4.15). Current and former smokers (ever smokers) were twice as likely to have findings of pleural abnormalities than those who never smoked (crude OR, 2.18; 95% CI, 1.91-2.49). Participants with a high BMI were more likely to have a finding of pleural abnormalities than were those with a lower BMI. This risk increased with increasing quartiles of BMI. Compared with the first quartile, the crude ORs for pleural abnormalities were 1.80 (95% CI, 1.44-2.24) for the second quartile, 2.80 (95% CI, 2.253-3.49) for the third quartile, and 3.71 (95% CI, 2.99-4.60) for the highest quartile. Increasing length of residence in the Libby area was also associated with increasing risk of pleural findings. Compared with participants residing in the Libby area for 14 years, crude ORs ranged from 0.91 (95% CI, 0.73-1.13) for a residential duration of 14-21 years to 3.62 (95% CI, 3.00-4.36) for residential duration of 34 years.
Table 4 presents the final unconditional logistic regression model in which all exposure pathways, as well as other risk factors and interaction terms, were assessed for their contribution to the risk of pleural abnormalities. The model shows that the following factors were associated with pleural abnormalities: having been a WRG worker, having been a household contact of a WRG worker, having been exposed to asbestos in the military, having played in vermiculite piles, being male, being older, having lived in the Libby area for a longer period of time, having smoked cigarettes, and having a high BMI.
We found significant interaction terms between age and being a former WRG worker and between sex and being a household contact of a WRG worker. Although the odds of finding a pleural abnormality is higher for former WRG workers than for nonworkers, the magnitude of the OR decreases with increasing age. Table 5 shows that estimated odds of finding a pleural abnormality ranges from 4.18 for a 30-year-old former WRG worker compared with a nonworker, to 1.14 for a 90-year-old former worker compared with a non-WRG worker, after adjusting for all other variables in the model. The model also shows that the estimated odds of finding a pleural abnormality is almost five times greater for males than for females (among nonhousehold contacts) after adjusting for other variables in the model (OR, 4.84; 95% CI, 3.83-6.11). The estimated odds of finding a pleural abnormality is 3.62 (95% CI, 2.70-4.83) times greater for females who were household contacts of former WRG workers when compared with females who were not. The corresponding increased odds of pleural abnormalities among male household contacts is 1.71 (95% CI, 1.32-2.22).
As age increases, the odds of finding a pleural abnormality increase, although the rate of that increase slows. For example, among non-WRG workers, the estimated odds of finding a pleural abnormality for a 40-year-old was three times greater than for a 30-year-old (OR, 3.04; 95% CI, 2.69-3.43). However, the odds reduce to 2.02 (95% CI, 1.87-2.18) when comparing a 60-year-old with a 50-year-old. This effect is slightly attenuated for age contrasts involving former WRG workers (OR = 2.45 for a 40-year-old vs. a 30-year-old; OR = 1.63 for a 60-year-old vs. a 50-year-old). Among the recreational exposure pathways, those who played in the vermiculite piles frequently had twice the risk of pleural abnormalities compared with those who never played in the piles (OR, 2.02; 95% CI, 1.59-2.57).
The rate of interstitial abnormalities increased from 0.04% in younger adults 18-44 years of age, to 0.5% for participants 44-65 years of age, to 3.2% for participants [greater than or equal to] 65 years of age. Males had a higher rate of interstitial abnormalities (1.2%) compared with female participants (0.4%). The crude OR for interstitial abnormalities among males compared with females was 2.68 (95% CI, 1.47-4.86). Current and former smokers (ever smokers) were twice as likely to have findings of interstitial abnormalities than were those who never smoked (crude OR, 2.18; 95% CI, 1.91-2.49). BMI was not associated with interstitial abnormalities. Participants residing in the Libby area for > 34 years had a 4-fold increased risk of interstitial abnormalities (crude OR, 4.38; 95% CI, 1.82-10.54).
We assessed the independent contributions of the exposure pathways and covariates to the risk of interstitial abnormalities using multivariate logistic regression. The strongest predictors of interstitial abnormalities were having been a former WRG worker (OR, 2.71; 95% CI, 1.26-5.87) and being male (OR, 2.48; 95% CI, 1.08-5.68). Increasing age, having worked on a ship or done ship repair, and having a history of pneumonia were also significantly associated with interstitial abnormalities.
The models above demonstrate the relative importance of various exposure pathways and covariates in predicting pleural and interstitial abnormalities. However, the majority of participants reported multiple pathways rather than a single exposure pathway. Only 2% of the participants reported "no" to all exposure pathways, whereas 48.1% of the participants reported six or more exposure pathways. The prevalence rates for pleural and interstitial abnormalities among participants with multiple exposures compared with those with no apparent exposures is displayed in Figure 1. An exposure-response relationship is apparent between the number of exposure pathways and the prevalence of pleural abnormalities. Among those who reported "no" to all exposure pathways, 6.7% had pleural abnormalities, compared with an almost 35% prevalence among those reporting [greater than or equal to] 12 pathways. An exposure-response relationship is not seen for interstitial abnormalities, which had a prevalence of about 1% regardless of the number of pathways reported. This trend in increasing pleural abnormalities with increasing number of pathways could not be solely attributed to former WRG workers. This relationship was apparent, although slightly attenuated, even after we removed the workers from the analysis (30.1% prevalence of pleural abnormalities among those reporting [greater than or equal to] 12 pathways). Table 6 presents the results from an unconditional logistic regression model of this relationship controlling for age, sex, BMI, smoking, and residential duration. There was a statistically significant trend of increased pleural abnormalities with increasing number of pathways, with ORs ranging from 1.40 for one pathway to 3.75 for [greater than or equal to] 12 pathways.
