Asbestos-associated diseases. Science, public policy, and litigation

Minimal pathologic changes of the lung and asbestos exposure

A group of 199 autopsy subjects was investigated for minimal pathologic pulmonary changes possibly resulting from asbestos exposure. According to the standards proposed by the Pneumoconiosis Committee of the College of American Pathologists, features consistent with asbestosis grade 1 (AG1) include findings of bilateral pleural plaques, high concentrations of asbestos bodies (ABs) in digested lung tissue, and a history of occupational risk. Similar changes without evidence of ABs on histologic section and referred to as small airway lesions (SALs) present a less well-correlated association. In this study, SALs showed significant differences when compared with the features observed in subjects without possible asbestos-related pulmonary fibrotic changes. Minimal bronchioloalveolar fibrotic changes with concomitant presence of ABs can be considered a mild pneumoconiotic lesion (AG1), and SALs may be regarded as an additional indicator of asbestos exposure.

Asbestos-related malignancy

Asbestos-associated malignancies have received significant attention in the lay and medical literature because of the increasing frequency of two asbestos-associated tumors, lung carcinoma and mesothelioma; the wide distribution of asbestos; its status as a prototype environmental carcinogen; and the many recent legal compensation proceedings, for which medical testimony has been required. The understanding of asbestos-associated carcinogenesis has increased through study of animal models, human epidemiology, and, recently, the application of modern molecular biological techniques. However, the detailed mechanisms of carcinogenesis remain unknown. A wide variety of malignancies have been associated with asbestos, although the strongest evidence for a causal association is confined to lung cancer and mesothelioma. Epidemiological studies have provided evidence that both the type of asbestos fiber and the industry in which the exposure occurs may affect the rates of asbestos-associated cancers. It has been shown that asbestos exerts a carcinogenic effect independent of exposure to cigarette smoking that, for lung cancers, is synergistically enhanced by smoking. Other questions remain controversial, such as whether pulmonary fibrosis necessarily precedes asbestos-associated lung cancer and whether some threshold level of exposure to asbestos (including low-dose exposures that may occur in asbestos-associated public buildings) may be safe. Mesothelioma, the most closely asbestos-associated malignancy, has a dismal natural history and has been highly resistant to therapy. However, investigational multi-modality therapy may offer benefit to some patients. A description of the processes through which compensation claims for asbestos-associated malignancies are evaluated illustrates for physicians the legal system's approach to possible injury from toxic substances. The differences between scientific and legal reasoning about the causes of diseases with long latency, especially when they are imputed to toxic exposures are substantial, and may impede the application of toxicological evidence to legal disputes.

Screening of autopsy populations for previous occupational exposure to asbestos

A screening procedure to select, in autopsy populations, subjects having a major likelihood of previous occupational exposure to asbestos is described. To test our necropsy population we searched for pleural plaques (PPs); the optical count of both lung asbestos bodies (ABs) and uncoated mineral fibers (UMFs) at least 10 microns in length was recorded. In the adult autopsy population studied in the Turin area, the predictive value given by a positive test for large-size PPs (mostly bilateral) was about 55%. This level of probability did not rise in relation to the AB counts, whereas an increase to over 75% was observed if more than 10,000 UMFs/g dry lung were present. In subjects without PPs or with small-size plaque lesions (mostly unilateral), predictive values of positive tests were 20 to 30% when ABs and UMFs were found to be below 100 and 10,000/g, respectively, and increased to approximately 40% if the AB count exceeded 500/g and to almost 70% for an UMF count above 100,000/g. In subjects without PPs or with small-size plaque lesions, the probability of being nonexposed was greater than 90% if neither ABs nor UMFs were found. This autopsy screening may be a reliable tool in selecting cases most probably related to occupational exposure.

Fiber size and number in workers exposed to processed chrysotile asbestos, chrysotile miners, and the general population

We analyzed chrysotile and chrysotile-associated amphibole (largely tremolite) asbestos fibers in 21 workers exposed to various types of processed (milled) chrysotile ore, 20 long-term chrysotile miners, and 20 members of the general population (controls). Significantly greater amounts of both chrysotile and tremolite were found in processed-ore workers and miners than in controls. On average, the mean fiber lengths and aspect ratios for the mining and processed-ore-exposed workers were similar and were significantly greater than the values seen in the controls; within the processed-ore group, there was a marked variation in these parameters, and some workers appeared to be exposed to fairly long, thin fibers. It was found empirically that the fiber size data, and to a lesser extent the concentration data, could be used to classify workers accurately into those with processed-ore exposure and controls. We conclude that fiber sizes in the lungs of processed-ore-exposed workers are similar to those of chrysotile miners and are considerably longer than those found in the general population; some processed-ore workers have longer fibers which might be responsible for higher disease incidences in certain working groups; tremolite accompanies chrysotile in a variable proportion of workers exposed to processed chrysotile products and might be important in the genesis of mesothelioma in such workers; and mineralogic analysis will usually detect exposure even when chrysotile has largely disappeared from lung tissue.

Common problems in asbestos-related pulmonary diseases

Exposure to asbestos is an important public health hazard in all industrialized societies. Although the pulmonary manifestations of prolonged asbestos fiber exposure are well known, there is considerable disagreement over specific clinical features and employment policies. Three of these controversies are the following: a) Can lung cancer be ascribed to asbestos exposure in subjects who lack evidence of asbestosis? b) Is there any clinical significance to benign asbestos pleural changes? c) Is it advisable to change jobs when evidence of asbestosis or benign pleural changes develop? The purpose of this presentation is to help develop a suitable public policy for Israel. Such a policy would have clinical importance as well as economic, legal, and social consequences for the individual and society as a whole.

Asbestos burden and the pathology of lung cancer

To determine whether we could distinguish asbestos-related lung cancers from unrelated ones, we typed and quantified by electron-optical methods the asbestos fibers in the lungs of 75 men with lung cancer. All but eight men had some history of asbestos exposure. On the basis of combined amosite and crocidolite (AC) concentrations, we divided the subjects into three groups (AC fibers per gram of dry lung): low (less than 10(5)); intermediate (10(5) to 10(6)); and high (greater than 10(6)). Age, smoking history, latent period, and type and location of tumors were similar in all three groups. Of 62 evaluated subjects, zero of 14 in the low group, seven of 29 in the intermediate group, and five of 19 in the high group had asbestosis. Epidemiologic studies suggest that persons exposed to concentrations of asbestos that can cause asbestosis are at increased risk for lung cancer. Thus, the subjects in our intermediate and high concentration groups may have been at increased risk for cancer, even when they did not have asbestosis. Because large burdens of asbestos do not always cause pulmonary fibrosis, asbestosis may be a poor marker of fiber-related lung cancer.

Does asbestos exposure cause gastrointestinal cancer?

The relationship between asbestos exposure and gastrointestinal malignancies is unlike the well-established correlation between occupational asbestos exposure and the subsequent development of pleuropulmonary neoplasms and mesotheliomas. Cohort studies on occupationally exposed workers suggest an association between asbestos and gastrointestinal cancer, but evaluation of dose-response, tissue analysis, animal experiment, and cell culture data yield inconsistent conclusions. No simplistic cause-effect relationship can be ascribed to asbestos at the present time, and the answer to the question, "Does asbestos exposure cause cancer?" must await the results of additional studies.

Chronic airflow limitation: its relationship to work in dusty occupations

The classic diseases of dusty occupations may be on the decline, but this is not the case for chronic nonmalignant lung disease characterized by airflow limitation. This group of diseases, almost certainly multifactorial in etiology, occurs in those engaged in dusty occupations as well as in those who are not. Among the environmental factors concerned, cigarette smoking is clearly one of the most important, but occupational exposures are increasingly implicated. It is also clear that not all with similar exposures are affected, pointing to the importance of host or personal factors. Evidence is now accumulating in support of what has been called the Dutch hypothesis. This explanation of the natural history of chronic airflow limitation suggests that an "asthmatic tendency" is a necessary factor whether the putative exposure is to cigarettes or to other airborne pollutants. Further research should therefore be directed towards clarifying the relationships of acute and chronic airway dysfunction in response to airborne pollutants of all types.

The role of analytical techniques in the diagnosis of asbestos-associated disease

There is increasing concern over the adverse health effects resulting from asbestos exposure. The mineralogy of asbestos and the pathology of asbestos-associated diseases is briefly reviewed. Techniques for tissue sampling, histopathological diagnostic criteria and the role of light microscopy (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X-ray diffractometry (XRD)--in the identification and quantitation of asbestos bodies and fibers in the lung tissue samples--are discussed. The value of a systematic quantitative approach is emphasized in order to differentiate the dose relationships and disease patterns.