Defining Asbestos Particulates for Monitoring Purposes

Abstract

The specific health hazards associated with exposure to asbestos are defined in terms of definitions of the asbestiform minerals—definitions that distinguish between acicular fragments and true fibers and that relate to the respirability of airborne particulates. The respiratory hazards include asbestosis, mesothelioma, and cancer. Estimates of cancer incidence—ranging from those of Joseph Califano, U.S. Secretary of Health, Education, and Welfare, to those of Doll and Peto—are evaluated critically. Bronchogenic cancer incidence is reviewed and discussed. The role of synergism with cigarette smoking in disease is discussed. The incidence of mesothelioma in relation to human epidemiology, the numerous other substances that cause mesothelioma in animals, the inactivity of anthophyllite asbestos, an epidemic in Turkey, and experience in the mines of Quebec Province, Canada, are examined. Cancers of the gastrointestinal tract associated with the ingestion of asbestos from drinking water or from lung clearance mechanisms are examined critically. The dearth of reliable human epidemiology and extensive animal studies are considered. Dose-response relationships associated with asbestosis are presented, and the threshold exposure limit determined for chrysotile miners is mentioned. One cohort of some 250 men occupationally exposed to moderately controlled airborne chrysotile concentrations for 25 years is evaluated. Dose-response relationships associated with cancer are discussed. The single-fiber hypothesis is criticized. Background incidence of asbestosis in persons without occupational exposure is discussed, and intrinsic lung defense mechanisms are touched upon. Hazardous exposure conditions for asbestos are defined. The epidemiology of the asbestos miners of Quebec Province is summarized, and recent experience is considered. The British criterion of acceptable risk, based on a 1% probability of developing the earliest clinical signs of asbestos-related disease, is discussed in light of society's willingness to accept reasonable risks and in light of the impracticality of zero risk. The effects of short intense exposure and their overwhelming effects on lung clearance mechanisms are discussed. The geographic distribution of asbestos-bearing geological formations is mentioned and hazards associated therewith are touched on. The hazardous entities, that is, those airborne particulates associated with asbestosis, are defined in terms of their dimensions in relation to respirability, deposition in the lungs, lung clearance mechanisms, tissue cell dimensions, and cytotoxicity. The origin of the 3:1 aspect ratio for fiber definition is delved into. Some animal studies on the effects of fiber dimensions are mentioned. The dimensions of the hazardous entities involved in cancer are presented. The dimensions of asbestos found as living tissue burden in humans are compared with the size distribution of airborne atmospheric particulates. Based upon all the foregoing considerations, definitions are developed of airborne fibrous particulates to be considered for monitoring purposes. The various definitions of asbestiform particles are mentioned. The respective limitations of electronic and optical microscopy are discussed. Definitions leading to efficient, reproducible hazard assessment are proposed. Finally, acceptable, realistic occupational exposures to asbestos are proposed, and demands for zero exposure are set against the perspective of the total quality of life.