Until the 1980s in the US, asbestos was a popular material in industrial, commercial and construction applications. Asbestos typically found its way into homes and workplaces during construction or renovation, when asbestos-containing products such as floor tiles or roofing felt were installed. These products remained popular for many decades because they were cheap, made from easily-acquired materials and minerals, and had beneficial fire-retardant properties. Products such as asbestos roofing felt are a particular hazard for the workers who installed these dangerous materials. Asbestos roofing felt was typically installed on flat roofs, such as those on large commercial buildings, institutions and schools. The felt consisted of large sheets of asbestos material painted over with asphalt. As long as these asbestos materials are sealed beneath a layer of asphalt, they post no particular danger to those inhabiting the building and surrounding area. However, workers who installed these large sheets of asbestos were likely exposed to dangerous, friable asbestos fibers when manipulating and installing the sheets. Years later, many of these roofers have developed serious illnesses as a direct result of their exposure to the friable asbestos. Johns-Manville Corporation was founded in 1858 as the H. W. Johns Manufacturing Company of New York, N. Y. and was based on the principle uses of asbestos as fire resistant roofing material. In 1886 the inception of the Manville Covering Company of Milwaukee, Wisconsin was founded on the principle uses of asbestos as a heat insulating material. In 1901 H. W. Johns Manufacturing Company and Manville Pipe Covering Company merged to form H. W. Johns-Manville Company of New York, N. Y. The company was reincorporated as Johns-Manville Corporation in 1926. Between 1900 and the mid 1980s, asbestos was used in over 3,000 different products. During the 20th century, more than 30 million tons of asbestos were used in industrial facilities, homes, schools, shipyards, steel mills, power plants and commercial buildings in the United States. Asbestos is term used to describe six naturally occurring minerals with similar properties. In the simplest of terms, asbestos is a rock mined from the ground. Asbestos has several key physical properties, including its durability and resistance to heat and combustion. It is also fibrous in nature, which allows it to be spun and woven into cloth. Most chemicals do not affects asbestos, and it is also does not conduct electricity well. Asbestos is an extremely fibrous mineral and mining, milling, processing, or use of asbestos and its products create many small fibers. Because of their thin shape and small size, the asbestos fibers easily pass through the body's natural defenses designed to trap debris within the respiratory systems before reaching the lungs. Once inside the lungs, the asbestos fibers slice into the sensitive tissue causing irritation and scarring. Because asbestos fibers are so durable, the body is not able to break them down to remove them. Once in place, the fibers continue to generate scar tissue, progressively damaging the lungs; extensive scarring can lead to the development of asbestosis. Asbestos may also cause the development of mesothelioma or lung cancer.