Secondary exposure occurs when asbestos fibers are released into the air and are inhaled by persons not directly working with asbestos. This secondary exposure can be as extensive and as harmful as direct exposure to asbestos workers themselves. An asbestosis prevalence of 11% in wives, 8% in sons, and 2% in daughters was reported in families of asbestos-exposed shipyard workers. Mesothelioma and other asbestos-related diseases have developed in members of workers' families whose only contact with asbestos was dust from an exposed worker's clothing. Mesothelioma and other asbestos related diseases also have developed in persons who as children lived within one-half mile of an asbestos factory. In the workplace persons not directly working with asbestos or asbestos products also have had secondary exposure to asbestos. An example is the four to five million shipyard workers who were exposed when insulation workers applied asbestos to ships' pipes and hulls. People in contact with work clothes of asbestos workers or with asbestos containing household products have developed pleural abnormalities. Low exposures from work-related, household, and natural sources can induce pleural plaques. For diffuse, pleural thickening, higher exposure levels might be required.