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HEALTH CONSULTATION

LIBBY ASBESTOS SITE
LIBBY, LINCOLN COUNTY, MONTANA


DISCUSSION

Health Outcomes
The increase in deaths due to asbestosis during the 20-year period examined in this review was statistically significant. Excess asbestosis-related mortality strongly indicates that historical exposure to asbestos in Libby was much greater than in other areas of Montana or the United States.

One of the questions raised by this review was why deaths due to lung cancer were not as elevated as those related to asbestosis. Current knowledge surrounding asbestos exposure and the association with lung cancer and asbestosis tells us that these diseases are two distinct pathologic processes sharing the same dose-related causative agent [24]. However, controversy remains as to whether asbestosis is actually a risk factor for lung cancer [25]. Several factors could explain the elevated asbestosis mortality and lack of elevated lung cancer mortality in the Libby population, such as diagnostic biases, temporal variations in the two disease processes, differences in the dose-response relationship between the two outcomes, and differences in lifestyle choices among the decedents, particularly smoking.

There are no accurate state or national mesothelioma mortality rates to make comparisons with the Libby population. Therefore, it is difficult to evaluate precisely the degree to which mortality due to mesothelioma is significantly elevated in Libby. Statistics on mesothelioma mortality are not routinely published because of problems in the diagnosis and classification of this rare disease.

Limitations
Although Libby has been a fairly stable community over the last 30 years, there have inevitably been "in migration" and "out migration" of individuals who may have been exposed through the mining and milling of asbestos-contaminated vermiculite in the area. Population mobility could affect the results of this analysis, particularly if individual mobility was associated with exposure or disease status. Residents could have moved to larger cities like Spokane, Washington, for medical treatment or they could have relocated after the mine and mill closed.

This analysis is limited by the lack of lifestyle information, particularly decedent's smoking history. If smoking was markedly different in the Libby population than in the comparison populations, the analysis could easily be confounded. Amandus et al. assessed the smoking habits of Libby workers and found that the proportion of current or former smokers was 15% higher than the national average [16]. However, smoking estimates for Lincoln County from 1990 to 1997 were comparable to state rates according to data from the CDC's Behavioral Risk Factors Surveillance System [26].

The usefulness of death certificate data is limited for asbestos-related deaths. Diagnoses of nonmalignant respiratory diseases are less likely to agree with autopsy data compared with other diagnoses [27]. Numerous reports have documented discrepancies between categorization of cause of death through clinical findings when compared with autopsy diagnoses [28]. Even when autopsies are performed, this information is rarely used to supplement or update the underlying cause of death on death certificates [27]. This is particularly true with respect to malignant mesothelioma, a sentinel outcome for asbestos exposure. In a prospective follow-up of workers in an asbestos products factory, Newhouse et al. reported that mesothelioma was misdiagnosed on 40% of death certificates [29]. Accurate reporting of the diagnosis of mesothelioma on death certificates is even worse for the general public, ranging from 20% to 29% [30,31]. In an analysis of the correlation between death certificate information and mesothelioma incidence as documented in a state-based cancer registry, only 12% of the mesothelioma deaths were correctly identified using the ICD-9 codes for pleural and peritoneal malignancies [32]. The Libby study population had a low autopsy rate (under 5%) that could contribute to nondifferential misclassification of underlying causes of death.

Diagnostic bias could also erroneously contribute to elevated asbestosis mortality statistics in Libby. The diagnosis of asbestosis is typically made through the combination of a physical examination and a thorough occupational history. If the patient has a restrictive respiratory disorder and has a strong occupational history of potential asbestos exposure, the physician is likely to diagnose asbestosis. However, for the 11 cases of asbestosis documented in this mortality review, eight different physicians diagnosed asbestosis. Three of these physicians did not practice in Libby or in Lincoln County. Therefore, the broad number of physicians diagnosing asbestosis in the Libby study population diminishes the potential that diagnostic bias is solely responsible for the increase in asbestosis mortality.

A greater effort was made to ascertain the residential history for decedents listing the Libby Care Center as their last known address. This attempt to gather accurate residential history for a select group within the study population may have introduced an information bias. However, ATSDR thought it important to pursue gathering this data since residential location was an integral part of the mortality review.

ATSDR Response
This mortality review is only one component of ATSDR's overall effort in assessing the adverse health impacts of asbestos exposures in the Libby community. Other ongoing activities include medical screening, health education, and additional epidemiologic analyses. An extensive, population-based screening program for respiratory effects is currently being conducted in the Libby area. Healthcare provider and community health education activities are also being implemented in Libby. ATSDR, in cooperation with a regional pulmonologist, is developing a case-series of persons already diagnosed with asbestos-related diseases to assess significant occupational and environmental exposure pathways. These diverse activities should assist in characterizing the extent of asbestos exposure in Libby while helping individuals in the community understand their health status as it relates to potential asbestos exposure.

The Pew Environmental Health Commission recently published recommendations regarding the need for surveillance of many environmental-related exposures and adverse health outcomes [33]. Their report addressed the current problems with the nation's environmental health infrastructure while emphasizing the need for proactive surveillance of environmental exposures and related health effects. Chronic respiratory disease, an underlying cause of elevated mortality in the Libby study population, is included in the list of adverse health outcomes recommended for inclusion in this national tracking system. It may have been possible to recognize the elevation in adverse health effects exhibited in Libby if a nationwide tracking system like this had been in place over the last few decades. However, without such surveillance, it is difficult for federal or state public health agencies to identify increases in morbidity and mortality associated with environmental exposures in small, rural communities like Libby.



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