Document J3XjovBrErEJoqOErknnYrGvZ

ten Ti$k / trucking ickers, loaders, jte sacks ith sacks that : to 'escape) rtructlon, ship- nnlng, twistg, vveaving, ating, mould oiling, 9 loggers, painters, i^^orkers. uipment / other ction sites, such *s, electricians 'or all o< these ppers. steam joiners, shipers, masons, aulkers bs on refits, dry pairs operations tee fitters and :ans, plumbers, i linings, apaWmen, body echanics I , . several forms osure. The oce family mem:he recognition he source here ASIIE5TOS-RE1.ATEO LUNG DISEASES 193 is presumed to be the dust brought home in the worker's overalls (10, 31), Likewi.se, the associa tion between residence near a mine, mil), or factory and the occurrence of mesothelioma brought to light the importance of neighborhood exposures (27). Such neighborhood exposure is also presumed to account for occurrence of pleu ral plaques and/or calcification in residents of the mining area of Finland (32). Pleural plaques have also been described in agricultural populations (33-35); in some instances, they have been attributed to the working of soil that contains asbestos fibers, e.g., in Bulgaria (33, 34). The surprisingly high prevalence of asbestos bodies (as evidence of exposure) in routine au topsies indicates an environmental exposure (or the residents of most of the larger cities of the world; however, the amounts in the general atmosphere are small (36), and these autopsy find ings should probably be regarded more as an index of exposure rather than of disease poten tial. The Advisory Committee report that fol lowed the Lyon Conference (7) concluded that "there is at present no evidence of lung damage by asbestos to the general public," and "the amount of asbestos in the Jungs of members of the general public is very small compared to those occupationally exposed." Finally, attention lias recently been directed toward the widespread occurrence of asbestos fibers in certain natural water sources (37-40). An event that brought this to the notice of the general public was the discharge of mine tail ings containing fiber into Lake Superior, a source of drinking water to many cities in the center of the North American continent (38). It is now also recognized that fibers also occur in many natural waters, particularly in mining re gions; however, the Advisory Committee report emanating from the Lyon Conference (7) judged there to be no evidence at present of "an increased cancer risk'resulting from asbestos fibers present in water, beverages or food or in the fluids used for the administration of drugs." The question must, however, remain under close scrutiny. The Fate and Biologic Effects of Inhaled Asbestos Particles Deposition in the Lung Whether or not inhaled asbestos fibers will be deposited in the lung depends on the aerody namic behavior of the particles, the dimensions of the respiratory tract they enter, and the pat tern of breathing that carries the particles. The aerodynamic behavior of particles is a function mainly of diameter, but also of size, shape, and density. The varying characteristics of commer cially used asbestos fibers make it obvious that the environment to which asbestos workers are exposed will contain particles having great variation in sice and composition. These include fibers (so-called if their length is at least 3 times their diameter), which may be long (as long as 200 /*m) or short. Ill addition, a working en vironment is likely to include a range of smaller particles and/or fibers released from the break down and disruption of the primary fiber, as well as those due to any other nonasbestos par ticles added by the mining or industrial pro cess. Inhaled asbestos particles follow the moving airstreams with each inspiration, and, once they make contact with any part of the surface of the airways or airspaces, are not resuspended in the expiratory airstTeams (41, 42). Deposition of the larger inhaled particles (more than 5 pm in di ameter) occurs mainly in the nose (assuming nose breathing) and major airways, owing to inertial impaction and sedimentation. Because of Brownian movement, deposition of the small er particles (less than 1 pm in diameter) occurs mainly in the more peripheral airways and air spaces. Tin's deposition profile is summarized in figure 3. Deposition patterns can be profoundly modi fied by breathing patterns; nose breathing causes a high retention rate, even of smrdl fibers, within the nose (41, 42). However, under work ing conditions, including heat and exertional stress, most workers resort to mouth breathing. Deeper, slower respirations favor a more even distribution of inspired air, and, thus, a more even distribution of inhaled particles. Lung volume also influences distribution and, possi bly, retention patterns, particularly if breathing occurs at less than the normal functional resid ual capacity (FRC) in the range of airway clo sure (43, p. 98). Likewise, there is some evidence that the state of the airways in smokers is such (hat inhaled particles will penetrate less deeply into the bronchial tree and thus tend to be de posited more centrally than in nonsmokers (44). Pulmonary Clearance Clearance of particles deposited on the mu cous blanket is brisk, with half-times of minutes