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Pneumoconiosis is the general term for lung disease caused by inhalation and deposition of mineral dust, with asbestosis more specifically being pneumoconiosis caused by asbestos inhalation. (See Etiology.)
The word asbestos is derived from Greek and means inextinguishable. The term refers to a group of naturally occurring, heat-resistant fibrous silicates, the fibers of which are long and thin (length-to-diameter ratio >3) and either curved or straight. The curved fibers make up serpentine asbestos (chrysotile is the prime example), and the straight fibers make up amphibole asbestos. (See Etiology and Workup.)
Several different types of amphiboles (ie, amosite, anthophyllite, tremolite, actinolite, crocidolite) have been recognized. Chrysotile is by far the most common type of asbestos fiber produced in the world and accounts for virtually all asbestos used commercially in the United States.
The production and use of asbestos increased greatly between 1877 and 1967. In the 1930s and 1940s, scientists recognized a causal link between asbestos exposure and asbestosis. In the 1950s and 1960s, researchers established asbestos as a predisposing factor for bronchogenic carcinoma and malignant mesothelioma. (See Epidemiology and Prognosis.)
Inform patients of the work-related causes of asbestosis (see Medical Care). For patient education information, see Bronchoscopy.
The incidence of asbestosis varies with the cumulative dose of inhaled fibers; the greater the cumulative dose, the higher the incidence of asbestosis. Experts estimate a 1% risk of developing asbestosis after a cumulative dose of 10 fiber-year/m3.
All types of asbestos fibers are fibrogenic to the lungs. Amphiboles, particularly crocidolite fibers, are markedly more carcinogenic to the pleura. Fibers with diameters smaller than 3 micrometers are fibrogenic because they penetrate cell membranes. Long fibers (ie, >5 micrometers) are incompletely phagocytosed and stay in the lungs, setting up cycles of cellular events and the release of cytokines.
The initial inflammation occurs in the alveolar bifurcations and is characterized by the influx of alveolar macrophages. Asbestos-activated macrophages produce a variety of growth factors, including fibronectin, platelet-derived growth factor, insulinlike growth factor, and fibroblast growth factor, which interact to induce fibroblast proliferation.
Oxygen free radicals (eg, superoxide anion, hydrogen peroxide, hydroxy radicals) that are released by the macrophages damage proteins and lipid membranes and sustain the inflammatory process. A plasminogen activator, which is also released by macrophages, further damages the interstitium of the lung by degrading matrix glycoproteins.
Individuals probably differ in their susceptibility to asbestosis based on respiratory clearance and other unidentified host factors. People who smoke have an increased rate of asbestosis progression, likely due to impaired mucociliary clearance of asbestos fibers.
As there is some continuing uncertainty about the mode(s) of action of asbestos in the genesis of diseases, an expert group proposed cooperative action by diverse scientific disciplines to address such issues as terminology, mineralogy, test materials, and experimental models.
Exposure to amphibole asbestos fibers is linked to the production of autoantibodies. Moreover, a study indicated that asbestos-related abnormalities occur more often in individuals who test positive for antinuclear antibodies (ANAs) than they do in persons who test negative for them. The study was conducted on the population of Libby, Montana, where mining, transportation, and processing of asbestos-contaminated vermiculite caused increased risk of asbestos-related pleural and lung diseases.
Serum samples showed that the majority of persons sampled were positive for ANAs. It was also found that the risk of developing pleural or interstitial abnormalities was more than 3 times greater in the ANA-positive individuals than it was in persons who were ANA negative.[4, 5]
Sources of asbestos exposure
The risk of asbestos exposure in the United States occurs mainly through the processing, manufacturing, and end-use of asbestos.
Manufacturers commonly use asbestos in the following products:
Products containing asbestos cement - Pipes, shingles, clapboards, sheets
Vinyl-asbestos floor tiles
Asbestos paper in filtering and insulating products
Material in brake linings and clutch facings
Textile products - Yarn, felt, tape, cord, rope
Spray products used for acoustic, thermal, and fireproofing purposes
Occupations associated with asbestosis include the following:
The risk of uncontrolled removal of sprayed-on asbestos was highlighted, in a study of 2 workers, by the presence and persistence of asbestos fibers and bodies in the bronchoalveolar lavage (BAL) fluid of these workers even after several months.[6, 7]
Another study, published in 2012, provided a detailed assessment of the health hazards of exposure to asbestos-containing drywall accessory products.
Occurrence in the United States
Asbestos consumption (per capita) peaked in 1951, declined gradually until 1971, and fell rapidly thereafter. No reliable information exists regarding the number of people presently at risk of asbestos exposure in the United States and in other countries. Since the early 1940s, as many as 10 million workers in the United States may have been exposed to asbestos. In 1972, reports estimated that 250,000 persons were at risk. By the 1980s, the number of active asbestos miners and millers had fallen to a few hundred.
Strict regulation (eg, prohibition of asbestos sprays in buildings, controls on the level of asbestos fibers in the air) has drastically reduced the risk of asbestosis development. However, persons who have been previously exposed to asbestos continue to be at risk for asbestosis and other asbestos-related diseases.
Bans on asbestos use are in place in several countries, including Australia, Japan, South Africa, and the nations of the European Union; use is restricted in the United States and Canada. However, trends in developing countries and countries that are emerging as economic powers indicate an increasing problem with asbestos-related diseases. These shifts in the global epidemiology have prompted a call for a global ban on asbestos.[11, 12, 13]
The following complications can result from asbestos exposure:
Right-sided heart failure
Progressive respiratory insufficiency
Progressive respiratory insufficiency
See the list below:
The risk factors for developing this complication are as follows:
Cumulative amount of asbestos inhaled
Degree of dyspnea
Combined pulmonary and pleural involvement
Honeycombing visible on radiographs
High number of neutrophils, eosinophils, and fibronectin in bronchoalveolar lavage (BAL) fluid
A higher risk of lung carcinoma has been found in patients with asbestosis. Patients with asbestosis are also at risk for malignant mesothelioma and carcinomas of the upper respiratory tract, esophagus, biliary system, and kidney.
People who smoke are likely to develop chronic bronchitis and obstructive airway disease and are prone to respiratory tract infections. Moreover, people who smoke are at high risk for the development of bronchogenic carcinoma because asbestos and tobacco smoke are synergistic in carcinogenicity. Individuals who both smoke and are exposed to asbestos are several times more susceptible to the development of lung carcinoma than are individuals who have neither exposure.
Some studies show that asbestos exposure alone, without a smoking history, increases the risk of lung carcinoma 6-fold. Asbestos exposure also increases the risk of developing malignant mesothelioma and cancers of upper respiratory tract, esophagus, kidney, and biliary system.
In addition, a meta-analysis of several studies of women who were occupationally exposed to asbestos found sufficient evidence for a causal association between asbestos exposure and ovarian cancer.
Asbestosis may coexist with other asbestos-related diseases, including calcified and noncalcified pleural plaques, pleural thickening, benign exudative pleural effusion, rounded atelectasis, and malignant mesothelioma of the pleura.
Ameille et al found no causal relationship between airway obstruction and asbestos exposure. Their study evaluated lung function in persons (n=3660) with previous occupational exposure to asbestos. No significant correlation was shown between pulmonary function parameters and cumulative asbestos exposure.
Study of mortality trends in the United States has shown that, while deaths from other pneumoconioses are declining, deaths from asbestosis are increasing. Further, the death rate is not expected to decrease for several years. One model predicts 29,667 deaths from 2005 to 2027.
Estimated annual years of potential life lost before age 65 years attributable to asbestosis totaled 7267 in the years 2001-2005 and represented a significant increase from 1968-1972.
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