Occupational exposure to carbon nanotubes and nanofibers
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Occupational exposure to carbon nanotubes and nanofibers

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Occupational exposure to carbon nanotubes and nanofibers


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    Carbon nanotubes (CNTs) and nanofibers (CNFs) are some of the most promising materials to result from nanotechnology. The introduction of these materials and products using them into commerce has increased greatly in the last decade. The development of CNT-based applications in a wide range of products is expected to provide great societal benefit and it is important that they be developed responsibly to achieve that benefit. Worker safety and health is a cornerstone of responsible development of an emergent technology because workers are the first people in society to be exposed to the products of the technology and the workplace is the first opportunity to develop and implement responsible practices. In this Current Intelligence Bulletin, NIOSH continues its long-standing history of using the best available scientific information to assess potential hazards and risks and to provide guidance for protecting workers. Since it is early in the development of these materials and their applications, there is limited information on which to make protective recommendations. To date, NIOSH is not aware of any reports of adverse health effects in workers using or producing CNT or CNF. However, there are studies of animals exposed to CNT and CNF that are informative in predicting potential human health effects consistent with ways in which scientists traditionally have used such data in recommending risk management strategies. NIOSH systematically reviewed 54 laboratory animal studies, many of which indicated that CNT/CNF could cause adverse pulmonary effects including inflammation (44/54), granulomas (27/54), and pulmonary fibrosis. NIOSH considers these animal study findings to be relevant to human health risks because similar lung effects have been observed in workers exposed to respirable particulates of other materials in dusty jobs. There are well established correlations between results of animal studies and adverse effects in workers exposed to particulates and other air contaminants. Moreover, in animal studies where CNTs were compared with other known fibrogenic materials (e.g., silica, asbestos, ultrafine carbon black), the CNTs were of similar or greater potency, and the effects, including fibrosis, developed soon after exposure and persisted. These are significant findings that warrant protective action. NIOSH conducted a quantitative assessment of risk using the animal studies with sufficient dose-response data, which included two subchronic (90-day) inhalation studies and five additional studies conducted by other routes or durations. The estimated risk of developing early-stage (slight or mild) lung effects over a working lifetime if exposed to CNT at the analytical limit of quantification (NIOSH Method 5040) of 1 μg/m3 (8-hr time-weighted average [TWA] as respirable elemental carbon) is approximately 0.5% to 16% (upper confidence limit estimates). In addition, the working lifetime equivalent estimates of the animal no observed adverse effect level (NOAEL) of CNT or CNF were also near 1 μg/m3 (8-hr TWA). Therefore, NIOSH recommends that exposures to CNT and CNF be kept below the recommended exposure limit (REL) of 1 μg/m3 of respirable elemental carbon as an 8-hr TWA. Because there may be other sources of elemental carbon in the workplace that could interfere in the determination of CNT and CNF exposures, other analytical techniques such as transmission electron microscopy are described that could assist in characterizing exposures. Studies have shown that airborne background (environmental and in non-process areas in the workplace) concentrations to elemental carbon are typically less than 1 μg/m3 and that an elevated exposure to elemental carbon in the workplace is a reasonable indicator of CNT or CNF exposure. Studies have also shown in some manufacturing operations that exposures can be controlled below the REL when engineering controls are used [Dahm et al. 2011]. However, NIOSH has not assessed the extent to which exposures can be controlled during the life cycle of CNT/CNF product use, but since airborne CNT/CNF behave as classical aerosols, the control of worker exposures appears feasible with standard exposure control techniques (e.g., source enclosure, local-exhaust ventilation). Previously in a 2010 draft of this CIB for public comment, NIOSH indicated that the risks could occur with exposures less than 1 μg/m3 but that the analytic limit of quantification was 7 μg/m3. Based on subsequent improvements in sampling and analytic methods, NIOSH is now recommending an exposure limit at the current analytical limit of quantification of 1 μg/m3.

    NIOSHTIC No 20042517

  • Content Notes:
    April 2013.

    This Current Intelligence Bulletin (CIB) was developed by the scientists and staff of the National Institute for Occupational Safety and Health (NIOSH) who participate in the NIOSH Nanotechnology Research Center (NTRC). The document manager was Ralph Zumwalde.

    Also available via the World Wide Web as an Arobat .pdf file (3.41 MB, 184 p.).

    Includes bibliographical references (p. 73-89).




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