The National Institute for Occupational Safety and Health (NIOSH) has reviewed data characterizing occupational exposure to airborne refractory ceramic fibers (RCFs) and information about potential health effects obtained from experimental and epidemiologic studies. From this review, NIOSH determined that occupational exposure to RCFs is associated with adverse respiratory effects as well as skin and eye irritation and may pose a carcinogenic risk based on the results of chronic animal inhalation studies. In chronic animal inhalation studies, exposure to RCFs produced an increased incidence of mesotheliomas in hamsters [McConnell et al. 1995] and lung cancer in rats [Mast et al. 1995a,b]. The potential role of nonfibrous particulates generated during inhalation exposures in the animal studies complicates the issue of determining the exact mechanisms and doses associated with the toxicity of RCFs in producing carcinogenic effects [Mast et al. 2000]. The induction of mesotheliomas and sarcomas in rats and hamsters following intrapleural and intraperitoneal implantation of RCFs provided additional evidence for the carcinogenic potential of RCFs [Wagner et al. 1973; Davis et al. 1984; Smith et al. 1987; Pott et al. 1987]. Lung tumors have also been observed in rats exposed to RCFs by intratracheal instillation [Manville Corporation 1991]. In contrast to the carcinogenic effects of RCFs observed in experimental animal studies, epidemiologic studies have found no association between occupational exposure to airborne RCFs and an excess rate of pulmonary fibrosis or lung cancer. However, studies of worker populations with occupational exposure to airborne RCFs have shown an association between exposure and the formation of pleural plaques, increased prevalence of respiratory symptoms and conditions (dyspnea, wheezing, chronic cough), decreases in pulmonary function, and skin, eye, and upper respiratory tract irritation [Lemasters et al. 1994, 1998; Lockey et al. 1996]. Increased decrements in pulmonary function among workers exposed to RCFs who are current or former cigarette smokers indicate an apparent synergistic effect between smoking and RCF exposure [Lemasters et al. 1998]. This finding is consistent with studies of other dust exposed populations. The implementation of improved engineering controls and work practices in RCF manufacturing processes and end uses have led to dramatic declines in airborne fiber exposure concentrations [Rice et al. 1996, 1997; Maxim et al. 2000a], which in turn have lowered the risk of symptoms and health effects for exposed workers. In 2002, the Refractory Ceramic Fibers Coalition (RCFC) established the Product Stewardship Program (PSP), which was endorsed by the Occupational Safety and Health Administration (OSHA). Contained in the PSP were recommendations for an RCF exposure guideline of 0.5 fiber per cubic centimeter (f/cm3) of air as a time-weighted average (TWA) based on the contention that exposures at this concentration could be achieved in most industries that manufactured or used RCFs. At this time, the available health data do not provide sufficient evidence for deriving a precise health based occupational exposure limit to protect against lung cancer. However, given what is known from the animal and epidemiological data, NIOSH supports the intent of the PSP and concurs that a recommended exposure limit (REL) of 0.5 f/cm3 as a TWA for up to a 10-hr work shift during a 40-hr workweek will lower the risk for developing lung cancer. Keeping exposures below the REL should reduce the risk of lung cancer to estimates between 0.073/1,000 and 1.2/1,000 (based on extrapolations of risk models from Moolgavkar et al.  and Yu and Oberdörster ). Keeping worker exposures below the REL will also reduce the risk of irritation of the eyes and upper respiratory system. The risk for mesothelioma at 0.5 f/cm3 is not known but cannot be discounted. Evidence from epidemiologic studies showed that higher exposures in the past resulted in pleural plaques in workers, indicating that RCFs do reach pleural tissue. Both implantation studies in rats and inhalation studies in hamsters show that RCFs can cause mesothelioma. Because of limitations in the hamster data, the risk of mesothelioma cannot be quantified. However, the fact that no mesothelioma has been found in workers and that pleural plaques appear to be less likely in workers with lower exposures suggests a lower risk for mesothelioma at the REL. Because residual risks of cancer (lung cancer and pleural mesothelioma) and irritation may still exist at the REL, NIOSH further recommends that all reasonable efforts be made to work toward reducing exposures to less than 0.2 f/cm3. At this concentration, the risks of lung cancer are estimated to be between 0.03/1,000 and 0.47/1,000 (based on extrapolations of risk models from Moolgavkar et al.  and Yu and Oberdörster ). Maintaining airborne RCF concentrations below the REL requires a comprehensive safety and health program that includes provisions for the monitoring of worker exposures, installation and routine maintenance of engineering controls, and the training of workers in good work practices. Industry-led efforts have likewise promoted these actions by establishing the PSP. NIOSH believes that maintaining exposures below the REL is achievable at most manufacturing operations and many user applications, and that the incorporation of an action level (AL) of 0.25 f/cm3 in the exposure monitoring strategy will help employers determine when workplace exposure concentrations are approaching the REL. The AL concept has been an integral element of occupational standards recommended in NIOSH criteria documents and in comprehensive standards promulgated by OSHA and the Mine Safety and Health Administration (MSHA). NIOSH also recommends that employers implement additional measures under a comprehensive safety and health program, including hazard communication, respiratory protection programs, smoking cessation, and medical monitoring. These elements, in combination with efforts to maintain airborne RCF concentrations below the REL, will further protect the health of workers.
Ceramic-materials; Ceramics; Ceramics-industry; Respirable-dust; Fibrous-dusts; Fibrogenesis; Fibrogenicity; Respiratory-system-disorders; Pulmonary-system-disorders; Lung-fibrosis; Lung-disorders; Lung-cancer; Cancer; Airborne-particles; Particulates; Particulate-dust; Standards; Skin-irritants; Eye-irritants