Demonstrating the protective effect of a 70-year-old occupational exposure limit against pneumoconiosis caused by mica
By Kristian Fried, Ph.D., Dr. rer. nat., DABT, ERT, Senior Consultant
Workers involved in crushing, milling, screening, and bagging of mica scrap are at increased risk to develop pneumoconiosis, a progressive material overloading of the lung that can lead to fibrosis and, in the later stages, to dyspnea. Pneumoconiosis is only seen after 10–20 years of respiratory mica exposure, and it can have a latency period of up to 40 years—today’s cases date back to exposures during the second half of the 20th century. An occupational lifetime exposure level of 3 mg/m3 respirable mica dust has been considered to present no risk of pneumoconiosis since 1951 when the American Conference of Governmental Industrial Hygienists (ACGIH) established a 20 million particles per cubic foot (mppcf) (3.5 mg/m3 respirable particles) exposure limit. As a result, numbers of unspecified and other pneumoconioses in the United States have steadily declined since the early 1970s. Data from the National Institute for Occupational Safety and Health documents a 91% decrease between 1972 and 2014 (i.e., the peak of documented cases and the latest reported data) for combined cases of aluminosis, berylliosis, stannosis, siderosis, and fibrosis from production and use of bauxite, graphite fibers, wollastonite, cadmium, Portland cement, emery, kaolin, antimony, and mica. Ample evidence indicates that the 70-year-old occupational lifetime exposure level of 3 mg/m3 respirable mica dust is protective of workers’ health.