Beginning in the 1920s and continuing into the mid 1970s, asbestos insulation and asbestos containing building materials were commonly used in the construction industry. Asbestos can take the form of paper or hard block insulation on hot water piping and heating ducts, or moulded pipe fitting insulation and joint compounds on furnaces. It can also be found in wall and attic insulation, floor tiles, sheet flooring, ceiling tiles and acoustical (stucco) finishes. Outside, asbestos can be found in shingles or siding materials and in asbestos cement pipes. A known carcinogen, asbestos is a potential health hazard if it is damaged or disturbed during renovation or demolition activities, or if damaged and deteriorating asbestos containing materials are present in a building. Exposure can become a health concern when high concentrations of asbestos fibers are inhaled over a long period of time, creating fibrous changes and scarring in lung tissue.