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INTRODUCTION

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Silicosis is a fibrotic lung disease produced by the inhalation of dust-containing free crystalline silicon dioxide (SiO2). Free silica and silicates represent a large part of the earth's crust. Silicon and oxygen are the two most important elements in the crust; about 27.7% of its composition is silicon, and 46.6% is oxygen. Free silica, the most widespread naturally occurring substance known to have a fibrogenic effect on the lungs, occurs in crystalline and amorphous forms. The crystalline forms that are fibrogenic are quartz, tridymite, and cristobalite; cryptocrystalline forms (consisting of minute crystals) are flint, chert, opal, and chalcedony. There are numerous forms of amorphous silica.

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At high temperatures (800–1000°C), quartz, the most common crystalline form of free silica, is converted into tridymite, and at even higher temperatures (1100–1400°C) it is transformed into cristobalite. Flint, chert, opal, chalcedony, and amorphous forms of free silica, including kaolin and diatomaceous earth, are also transformed into tridymite and cristobalite at these temperatures. This effect of high temperatures is of importance, since both tridymite and cristobalite are more potent than quartz in producing pulmonary fibrosis.

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History
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Silicosis undoubtedly originated in antiquity with the mining and processing of metals and building stone. Agricola, in his book De Re Metallica (1556), was probably the first to recognize the adverse effects of inhaled dust. The first monograph on miners' diseases, Von der Bergsucht by Paracelsus in 1567, included a classic description of “miners' phthisis.” Van Diemerbroeck described how the lungs of stonecutters dying of “asthma” cut like masses of sand (Anatomi Corporis Humani, 1672). Bernardino Ramazzini included a description of diseases of stonemasons and miners in De Morbis Artificium Diatriba (1700). In England, the disease (phthisis) was described in flint knappers, needle pointers, knife grinders, fork sharpeners, and cutters of sandstone. John Scott Haldane (1923) described the cellular storage and retention of dust, including the long-term retention of silica, and recommended better ventilation of mines and factories. The distinction between tuberculosis and silicosis followed Koch's discovery of the tubercle bacillus in 1882. The earliest description of silicosis in the United States, in the nineteenth century, was of employees of a cutlery plant; the disease was then detected among miners. Tunnel work generated numerous cases of silicosis. The tunnel at Gauley Bridge in West Virginia, where many workers contracted both acute and chronic silicosis in the 1930s, attracted much public attention. This resulted in the initiation of dust suppression and respiratory protection methods, improved industrial hygiene, and the introduction of laws for compensation of silicosis victims.

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Although the magnitude of the silicosis risk was gradually reduced in tunnel drilling and mining operations, significant silica exposure continued to occur in other industrial operations, such as foundries, the manufacture and use of silica flour, the production of detergent soaps with a high content of free silica, and sandblasting.

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Work Exposures

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