Silica and Silica Compounds

The uses of silica and the potential health hazards for workers or others exposed to dust particles date back thousands of years and are documented. Hippocrates and Pliny both mentioned silica's ability to cause disease and Pliny even described miners who used forms of respiratory protection. The first study of silicosis, in the time of the industrial revolution, was that of Johnstone in 1796 who noted the high mortality of needlepointers at Redditch, England. By 1918, English workers received compensation for disability as a result of silicosis. Silicosis is a pneumoconiosis, but the terms must not be used synonymously. Silicosis, of all the pneumoconioses, has probably claimed the largest number of victims, either alone or in combination with tuberculosis, a condition frequently associated with silicosis. Betts gave the first description of acute silicosis in the United States. In 1932, the American Public Health Association (APHA) developed the definition for the fibrotic lung disease silicosis as A disease due to breathing air containing silica (SiO2), characterized anatomically by generalized fibrotic changes and the development of miliary nodulations in both lungs, and clinically by shortness of breath, decreased chest expansion, lessened capacity for work, absence of fever, increased susceptibility to tuberculosis (some or all of which symptoms may be present) and by characteristic X-ray findings.”. In 1917, Dr. Alice Hamilton described the life of stonecutters in the Barre, Vermont area of the United States. Later the United States Public Health Service studied these workers and issued a report giving preventive measures to combat silicotuberculosis and silicosis resulting from the inhalation of silica-containing dusts for Barre workers and also for other exposed workers. Silica is a natural mineral composed of silicon dioxide, which occurs in either the crystalline or amorphous form. Silica makes up 21% of the earth's crust and is the most common of all chemical compounds. Pure silicon dioxide crystals are found naturally in three polymorphic forms: quartz, the most common; tridymite; and cristobalite. Each of the three is important to human health and make up the crystalline form of silica. Both tridymite and cristobalite appear more fibrogenic than quartz. Silicon dioxide is an acidic oxide, which is practically insoluble in water, but can be attacked by hydrogen fluoride. The amorphous form of silicon dioxide, also called vitreous silica, does not pose a significant threat to human health because it has not been associated with pneumoconiosis. In the few reports that have claimed an association between amorphous silica and disease, the truly amorphous nature of the material has been in doubt. Therefore, for the purposes of this discussion, the review and comments relate to quartz (the most common silicate), which is sometimes called free silica. Human exposures to silica were encountered from the first time man dug into the ground because silica deposits are found in every land mass and stratum from every era and period of geological time. The use of silica in the production of glass probably dates back thousands of years. It is difficult to get accurate figures on the use and production of silica because it is such a universal material and has a multitude of uses. The uses of silica are quite varied and so is the production of silica. Processing operations depend upon the nature of the deposit as well as the desired end product and include crushing, secondary milling to refine particle size, and other methods to further refine the particles. World production was estimated by Davis and Tepordei at 182 million tons in 1983, Asia was the largest producer, followed by Europe, South America, North America, and Africa. This production pattern has been relatively stable for the last 10–15 years. Case reports and surveys by the U.S. Bureau of Mines and the Public Health Service documented the occurrence of pulmonary disease in various worker groups exposed to silica. Public attention was galvanized by the Gauley Bridge (WV) outbreak of acute silicosis among tunnelers of nearby pure quartz. This incident provided an impetus for industrial health reform in the United States and led in 1937 to dust control standards and to the Walsh–Healy legislation. Silicosis rates among granite workers in Vermont decreased dramatically during subsequent decades. From that time to the present, standards have been under continued reevaluation, and decremental changes in permissible exposures to “free” silica dust have reduced but have not eliminated silicosis as a health hazard. Keywords: Silica; Silicon; Nonmining sector; Female workers; Male workers; Exposure assessment; Toxic effects; Cancer; Lung tumor; Amorphous silica; Quartz; Standards; Mica; Pumice; Mining; Portland cement; Standards; non-U.S.