3.3 Fiber-Induced Diseases3.2g) RDRP Publications of Special Note Relating to Silica-Induced Disease | 3.3a) Reduce Occupational Hazard Associated with Asbestiform Fibers Contaminating Vermiculite from a Mine in Montana
Most respirable particles deposited in the alveolar region of the lung are cleared by scavenging alveolar macrophages that phagocytose particles and transport them out of the lung, mainly via the mucociliary “escalator” lining the airways. Particles that are fibrous (i.e., length>>diameter) present a special clearance challenge. When inhaled, fibers tend to line up with the air stream in the airways, and their effective aerodynamic diameter much more closely approximates particle diameter than length. Thus, even relatively long fibers can be deposited in the alveolar regions if their diameter is small enough. Faced with the challenge of removing long fibers that exceed their physical capacity, macrophages engage in “frustrated” phagocytosis, spilling digestive enzymes and other cellular contents into the alveolar space and initiating pathophysiologic processes that lead to disease (inflammation, fibrosis, and malignancy). It follows that several major fiber characteristics that determine fiber toxicity are dose, dimension, and durability. Highly durable respirable fibers with relatively long length would be expected to be more toxic than short and less durable respirable fibers, all else being equal.
Fibers of occupational health interest can be classified as mineral or organic, and as natural or synthetic. Asbestos (including the six types that have been used commercially: actinolite, amosite, anthophyllite, chrysotile, crocidolite, tremolite) is perhaps the most notorious natural mineral fiber. Some other natural mineral fibers that closely resemble commercial asbestos are equally hazardous (e.g. some of the amphibole fibers that contaminate vermiculite ore once mined in Libby, Montana). Other natural mineral fibers (i.e., fibrous talc, wollastonite, attapulgite, and mordenite fibers) differ considerably from commercial asbestos and appear to be associated with substantially less toxicity. Synthetic vitreous fibers, including Refractory Ceramic Fibers (RCFs), are increasingly used as asbestos substitutes and can be hazardous, especially if they are highly durable and have the proper dimensional characteristics. Evolving technologies allow commercial production of RCFs with specified dimensions and durability; some are highly durable. Fibrous glass and mineral wool are examples of less durable synthetic vitreous fibers. Modern methods of producing and processing synthetic organic fibers (e.g. nylon, rayon, polyester, etc.) pose increased potential for creating respirable fiber hazards encountered by workers, and durability of these fibers varies depending on the material. Engineering controls methods are available to reduce exposures, and there is a need to recognize the hazard and to adopt appropriate control measures (e.g. for nylon flock-associated respiratory fibers and RCFs).
Estimates of the total number of workers potentially exposed to all types of respirable fibers have not been made. However, summing available estimates for several types of fibers (asbestos, fibrous talc, fibrous glass, wollastonite and attapulgite fibers, and RCFs), nearly four million workers may be at risk (A3-85).77,78 Asbestosis is the prototypical fiber-induced interstitial lung disease. This chronic lung disease is characterized by interstitial inflammation and subsequent scarring, leading to radiographic and functional abnormalities that can be disabling and fatal. Affected occupational groups largely reflect the historical major end user industries: shipbuilding/repair, construction materials, and other friction and insulation applications. But, because asbestos has been used in several thousand products with a myriad of uses, risk has been widespread and, as recently as the 1980s, NIOSH estimated that 1,500,000 workers remained at risk for asbestosis.79 National death data distinguish asbestosis as the only major pneumoconiosis for which mortality is increasing in the U.S., from fewer than 100 deaths in 1968 to nearly 1500 in 2002, due in part to long latency between exposure and disease and perhaps also to improved recognition and diagnosis. However, OSHA and MSHA asbestos sampling data indicate substantial declines in exposure for manufacturing, construction, and mining over the past two decades.80 According to a RAND Corporation study, $54 billion had been paid out in asbestos-illness cases as of 2002 and as many as 2.4 million claims could be filed, costing businesses about $210 billion more.81
Fiber-induced interstitial fibrosis would alone warrant substantial research and preventive effort, but occupational exposures to asbestos and related fibers also cause sometimes debilitating non-malignant disease of the pleura (the membrane of tissue that lines the chest cavity) and highly fatal malignant diseases, including lung cancer and mesothelioma, a cancer of the pleura and other similar tissues. A poorly understood process of transmigration moves fibers to these locations, distant from where they initially deposit in the lungs.
When NIOSH was established, asbestos-related occupational disease was already of major concern. In fact, the very first NIOSH Criteria Document was “Criteria for a Recommended Standard: Occupational Exposure to Asbestos” published in 1972 (and later revised in 1977). This criteria document played an important role in moving OSHA and MSHA to establish more protective occupational PELs for asbestos. In turn, these standards relied on sampling and analysis methods for assessing asbestos (and other fiber) exposures, developed by NIOSH and published as part of NMAM (chapter 12).
Subsequent concerns relating to fiber-induced lung disease have come to the attention of RDRP in a variety of ways. Over the past decade, these include: an increasing use of asbestos substitutes that pose their own occupational health risks (e.g. refractory ceramic fiber and carbon nanotubes); request for technical assistance from OSHA (e.g. asbestos-contaminated vermiculite); requests for HHEs (e.g. respirable nylon fibers in flock plants); and industry efforts to refute potential health hazards associated with cleavage fragments of asbestos minerals.
In addition, RDRP working groups have made recommendations for fiber-related research. In the early 1990s, our scientists reviewed the literature, including the recommendations of a recent National Institute of Environmental Health Sciences (NIEHS) sponsored Fiber Toxicology Research Needs workshop82 and wrote an internal document entitled “Fiber Exposures and Lung Disease Research Strategy”83 .
This strategy provided a framework to guide our fiber-related research, including the following objectives:
More recently, a group of RDRP scientists updated priority areas of research for addressing health risks posed by occupational exposure to fibers. Their final report (“NIOSH Interdivisional Fiber Subcommittee Final Report”A3-86) recommended research, including the following objectives:
A number of fiber-related research activities have been carried out by RDRP. A February 2006 search of NIOSHTIC-2 (a bibliographic database for occupational safety and health literature) documents, grant reports, and other communication products produced and/or supported by NIOSH, returned nearly 1500 items related to fibers.
The remainder of 3.3 highlights examples of several RDRP fiber-related activities. In addition to investigator-initiated research to address these scientific objectives (chapter 3.3d), our investigators have been faced with unplanned opportunities first evidenced by the occurrence of respiratory diseases in unexpected settings. Two examples of such unplanned opportunities are one relating to asbestos-contaminated vermiculite (chapter 3.3a,) and the other to respirable fibers unwittingly generated during production of flock (chapter 3.3b). In addition, an example is provided of a focused effort by our researchers to work towards prevention of an anticipated fiber-related occupational hazard associated with a major asbestos substitute (chapter 3.3c).
77. NIOSH . Occupational Respiratory Diseases. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 86-102.
78. NIOSH . Criteria for a Recommended Standard: Occupational Exposure to Refractory Ceramic Fibers. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2006-123.
79. NIOSH . Occupational Respiratory Diseases. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 86-102.
80. NIOSH . Work-Related Lung Disease Surveillance Report 2002. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003-111.