Health Effects of Inorganic Fibers

It has become fashionable to start discussions of disease related to fibrous inorganic materials by referring to Pliny the Younger (A.D. 61-114), who commented in a letter on the sickness of slaves who worked with asbestos. His observation was forgotten, as evidenced by the fact that during the Middle Ages, Paracelsus (1493–1541) as well as Agricola (1494–1556) wrote extensively on “miner’s disease” without mentioning asbestos. Later, Zenker (1867) coined the word pneumo(no)coniosis to describe the diseases endemic to coal and iron miners. Differential diagnosis of the pulmonary disorders, tuberculosis, silicosis, pneumonia, and other lung disease was attempted thereafter, although the varieties were often confused even by experienced physicians. The industry that provided asbestos to modern society started at about this same time (in the 1870s). The first indication of pulmonary disorder in an asbestos worker came in an autopsy report of fibrosis by Dr. Montague- Murray at Charing Cross Hospital, London, in 1899–1900 (Peters and Peters, 1980). By 1902 asbestos was included in the list of dusts considered injurious by the Lady Inspector of Factories, Adelaide Anderson (Oliver, 1902). Auribault (1906) appears to have been the first to note high mortality in workers in an asbestos mill and weaving establishment, but he attributed their illness to calcium carbonate dust rather than asbestos. Scarpa (1908) believed the pulmonary disease of 30 asbestos workers was caused by tuberculosis, and Fahr (1914), who published the case of a female asbestos worker who died of “pleuro-pneumonia . . . with a large number of crystals in pulmonary tissue of a peculiar nature,” was clearly somewhat mystified at the presence of nonbiological materials. It was Cooke (1924, 1927, 1929) who first defined asbestos as a specific etiologic agent in pulmonary fibrosis. He described extensive fibrosis with thickened pleura and adhesions to the chest wall and pericardium in asbestos workers and noted the presence of abundant mineral matter (“curious bodies”), but also tubercular lesions. The term asbestosis was used in the 1927 publication. Pancoast and Pendergrass (1925) argued that the fibrosis seen in asbestos workers was a result of ad-mixed silica and an expression of “asbestosilicosis,” signifying uncertain etiology of the observed symptoms, a view that survivied into the 1930s (Lynch and Smith, 1935).

Fibrous Minerals and Synthetic Fibers

A mineral is a naturally occurring, crystalline inorganic compound with a specific chemical composition and crystal structure. Minerals are commonly named to honor a person, to indicate the geographic area where the mineral was discovered, or to highlight some distinctive chemical, crystallographic, or physical characteristic of the substance. Each mineral sample has some obvious properties: color, shape, texture, and perhaps odor or taste. However, to determine the precise composition and crystal structure necessary to accurately identify the species, one or several of the following techniques must be employed: optical, x-ray diffraction, transmission electron microscopy and diffraction, and chemical and spectral analyses. The long history of bestowing names on minerals has provided some confusing legacies. Many mineral names end with the suffix “ite,” although not most of the common species; no standard naming practice has ever been adopted. Occasionally different names have been applied to samples of the same mineral that differ only in color or shape, but are identical to each other in chemical composition and crystal structure. These names, usually of the common rock-forming minerals, are often encountered and are therefore accepted as synonyms or as varieties of bona fide mineral species. The Fibrous Minerals list (Appendix 1) includes synonyms. A formal description of a mineral presents all the physical and chemical properties of the species. In particular, distinctive attributes that might facilitate identification are noted, and usually a chemical analysis of the first or “type” specimen on which the name was originally bestowed is included. As an example, the complete description of the mineral brucite (Mg(OH)2), as it appears in Dana’s System of Mineralogy, is presented as Appendix 3. Note the complexity of this chemically simple species and the range of information available. In the section on Habit (meaning shape or morphology) both acicular and fibrous forms are noted. The fibrous variety, which has the same composition as brucite, is commonly encountered (see Fig. 1.1D) and is known by a separate name, “nemalite.” Tables to assist in the systematic determination of a mineral species are usually based on quantitative measurements of optical properties (using either transmitted or reflected light, as appropriate) or on x-ray diffraction data.

What Is an Inorganic Fiber?

Fibers are everywhere around us. They are essential parts of the human body, our hair, for example; the threads in our clothing, natural or synthetic; the insulation in our houses. Natural fibers have been useful to humans for more than ten thousand years. They were mixed with clay before firing to strengthen and reinforce pottery vessels, making them more durable. Textiles that combined the fibers of flax and asbestos were known in ancient times for their seemingly magical resistance to fire and decay. It was industrialization, however, that caused a dramatic increase in the use of natural inorganic or mineral fibers. By the late nineteenth century asbestos had become an important commodity with a variety of commercial applications. It served as insulation to control heat generated by engines and, because of its incombustibility, as a fire retardant in its more recent general use as building insulation. Asbestos fibers are found worldwide in many products: as reinforcement in cement water pipes and the inert and durable mesh material used in filtration processes of chemicals and petroleum, for example. However, asbestos is not the only inorganic fiber in use today. Synthetic inorganic fibers abound. Glass fibers have replaced copper wire in some intercontinental telephone cables. Fiberglas (a trade name) has become the insulation material of choice in construction. Carbon and graphite fiber composites are favored materials for tennis racket frames and golf clubs. Fibrous inorganic materials have become commonplace in our everyday lives. As the use of inorganic fibers increased, there were some indications that fibers might be hazardous to our health. Since the first century A.D. it was suspected that asbestos might be the cause of illness among those who mined and processed the material. Asbestosis, a debilitating and sometimes fatal lung disorder, was documented and described in the nineteenth century. Within the last 25 years, lung cancer and mesothelioma have also been linked to asbestos exposure among construction and textile workers, as well as others exposed to dusts containing asbestos fibers. Although the etiology and specific mechanisms that give rise to these two cancers are not yet understood, concern for the health of exposed workers led the governments of the United States and other countries to specify the maximum allowable concentrations of asbestos in the ambient air of the workplace.