Background to the Use of Terminology in the Roadmap

Because the term asbestos does not denote a single mineral but rather is used to encompass a set of minerals with specific industrial characteristics and commercial value, there have been challenges and controversies in determining both what specific set of minerals and what set of characteristics should be included in a definition of asbestos. The current regulatory definitions used by the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA) define six recognized minerals as varieties of asbestos:² chrysotile (a member of the serpentine group of sheet silicates) and five fibrous forms of the amphibole group of double-chain silicates: riebeckite asbestos (also termed crocidolite), cummingtonite-grunerite asbestos (also commercially termed amosite), anthophyllite asbestos, tremolite asbestos, and actinolite asbestos (29 CFR 1910.1001(b); 29 CFR 1926.1101(b); 30 CFR 56.5001(b)(1); 30 CFR 57.5001(b)(1); 30 CFR 71.702(a)).

The 1990 NIOSH Recommended Exposure Limit (REL) outlines limits for airborne asbestos fibers, a term that encompasses the six minerals defined as asbestos by OSHA and MSHA and particles greater than 5 μm in length and having (1) an aspect ratio of 3:1 or greater and (2) the mineralogical characteristics (i.e., crystal structure, elemental composition) of their nonasbestiform analogs (NIOSH, 1990a,b). Table 1-1 replicates and compares the terminology used by the regulatory agencies, OSHA and MSHA, for asbestos and that suggested by NIOSH in its 1990 recommended exposure limit for airborne asbestos fibers.

TABLE 1-1

Minerals Addressed by OSHA and MSHA Regulations and the NIOSH Recommended Exposure Limit.

The central focus of the Roadmap is on establishing research to assess the potential for asbestos, asbestos analogs, or other mineral particles with specific physical or chemical characteristics to impact human health. The generic term elongated mineral particles is used by NIOSH in the Roadmap to encompass a broad spectrum of mineral particles of a specific size and aspect ratio but the term has no current regulatory or mineralogical connotation. As noted above, the committee prefers the term elongate mineral particles (see Chapter 3 for discussion). Increasing the specificity of the terminology to the extent possible will aid in clarifying the central message of the Roadmap. Within the range of vari ous types of elongate mineral particles, the potential health impact of exposures may vary widely, although this is yet to be fully explored.

Mineral Variability

Approximately 3,000 minerals exist in nature. Mineral identification is based on mineral crystal structure and crystal chemistry. Mineral crystal growth habits³ and the chemical substitutions in the crystal structure, influenced by the variable conditions of growth of a mineral, are additional characteristics that can be used to describe a mineral and distinguish it from another. The mineral growth environment and chemical substitutions have the potential to cause variations in a mineral’s optical properties, surface chemistry, and crystal structure, as well its tendency to break down or decompose. Within a particular mineral deposit or rock, the constituent minerals can be heterogeneous, even at the micrometer scale, varying in composition, crystal structure, and/or habit, for example. The potential diversity of minerals occurring in mineral deposits and rocks creates challenges when identifying those minerals associated with health impacts and when characterizing human exposures, whether at the mine site or during or following processing into manufactured products.

Nature of the Exposures

The nature and extent of human exposures to asbestos in the United States have changed over the past 50 years. In the 1970s, most asbestos mining ceased in the United States, with the final mine closing in 2002 (NTP, 2005; Virta, 2006). This ended more than a hundred years of production (1890 to 2003) of an estimated 3.29 million metric tons of asbestos. Additionally during that time, 29.6 million metric tons of asbestos were imported into the United States for industrial uses, including roofing materials, flooring, friction materials in brakes and clutches, and asbestos-cement pipes (Virta, 2006). Health consequences of exposure to asbestos, particularly its association with the development of asbestosis, lung cancer, and mesothelioma after a long latency period, became known in the 1950s and 1960s and led to regulations in the United States in 1971. Use of asbestos in the United States peaked in 1973 at 803,000 metric tons per year, and by 2003, U.S. consumption was at 4,650 metric tons (Virta, 2006). The European Union banned new uses of asbestos in 2005, and asbestos is fully or partially banned in many other countries.

Changes in occupational exposures have generally followed the trajectory of the use and regulation of asbestos. Mining, milling, and manufacturing exposures in the United States have declined, leading to reductions in long-term occupational exposures, although some exposures continue with imported asbestos-containing products. Limited information is available on the number of workers still exposed to asbestos. OSHA (2008) estimates that 1.3 million U.S. workers in construction and general industry face asbestos exposure on the job. However, exposures generally occur during remediation and abatement efforts, through demolition of buildings with asbestos-containing materials, and during excavation in areas where asbestos occurs naturally; thus, exposures tend to be shorter term and more intermittent. The Roadmap acknowledges the limitations in current understanding of exposures and recommends research on the nature and levels of occupational exposures to asbestos and other elongate mineral particles and on the number of workers exposed. Further, in the United States, although generally below the levels of occupational exposures in mines, nonoccupational exposures can occur in homes, schools, public buildings, and other locations with unabated asbestos insulation or from other exposures. Environmental exposures are possible for residents or workers near asbestos-containing waste sites and active or inactive mines; the most recent well known issues occurred in Libby, Montana, where mined vermiculite was contaminated with asbestos. Internationally, asbestos-containing products (particularly asbestos-cement products) continue to be produced and used in some countries; in 2003, the leading consumers of asbestos were Brazil, China, India, Iran, Kazakhstan, Russia, Thailand, and Ukraine (Virta, 2006).

Because the latency period between asbestos exposure and onset of disease may be several decades, the public health impact of the disease lags behind exposure reductions. For example, the number of asbestosis deaths in the United States reached a plateau in the late 1990s, in part because of the long latency period and the fact that individuals survive many years after disease onset (Moolgavkar et al., 2009). The annual number of asbestosis deaths is anticipated to decrease substantially due to exposure reductions (NIOSH, 2009).

Exposure Assessment

Further complicating the issues regarding asbestos are the challenges in exposure assessment that make it difficult to quantify current exposures and to understand how the levels of exposure were determined in prior studies. Some worker populations that have been studied in epidemiological investigations may have been exposed to complex mixtures of mineral particles, where the level of exposure may be difficult to ascertain in retrospective studies, data on cigarette smoking may not be available, and exposure assessments may have not used direct measurements of exposure but rather surrogates such as job description.

Measures of the extent to which workers or residents are exposed to hazardous airborne particles are conducted primarily by collecting air samples and then counting those that occur as single crystals or in bundles in a representative section of the air sample filter. Cleavage fragments, when identifiable, may or may not be counted depending on the analytic methodology. Detailed methodologies for identifying and counting asbestos fibers have been developed by NIOSH, ASTM International, the International Organization for Standardization, and other organizations using phase contrast microscopy, polarized light microscopy, scanning electron microscopy, or transmission electron microscopy. Electron microscopy methods allow higher spatial resolution and can provide further definition to the nature of the crystalline habit, previously characterized through polarized light microscopy and/or the mineral’s chemical composition. Challenges in assessing exposure include variations in the particle distribution on sample filters, ensuring consistency in inter- and intralaboratory counting techniques, varying methods for preparation of samples, the assumption that small samples represent actual exposures, and subjectivity in assessing what particles are countable in a given analytical method (see Chapter 3).

Broader Context of Research on Exposure to Airborne Particulates

In addition to the research addressed in the NIOSH Roadmap, it is important to recognize the wide breadth of ongoing research on exposures to and health effects from airborne particulates. For example, occupational inhalation exposures to crystalline silica dust are well documented to contribute to silicosis, and coal dust exposure of miners is closely associated with pneumoconiosis. Research on synthetic vitreous fibers is also relevant. Because of the relevance of related research on airborne particulates, it is important for the Roadmap to draw from these other experiences and to coordinate research using an interdisciplinary approach.

Chapter 2 examines recent efforts to develop research roadmaps and discusses the key elements and larger context for framing a broader research strategy.