This article originally appeared on the National Nanomanufacturing Network's InterNano website. It is licensed under Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported.
In late November, the National Institute for Occupational Safety and Health (NIOSH) published a Current Intelligence Bulletin entitled "Occupational Exposure to Carbon Nanotubes and Nanofibers." The document is not an official "agency determination or policy," and was released solely by NIOSH for peer-review and comment. NIOSH's carbon nanotube recommended exposure limit (CNT REL) is set at 7 μg/m 3 for these preliminary purposes.
The old saying that "a picture is worth a thousand words" is once again proved by the document's cover image which is a remarkable "field emission micrograph of a multi-walled carbon nanotube…penetrating the pleura of the lung." The finely detailed image of a CNT penetrating a lung membrane should cause any reader to stop and closely consider the document's recommendations.
The document begins by noting that while there are no scientific reports of "adverse health effects in workers producing or using carbon nanotubes…or carbon nanofibers," NIOSH is concerned because some studies have shown that the potential for worker exposure exists. Additionally, some in vivo studies have shown adverse reactions to carbon nanotubes in rodents -- including pulmonary inflammation and fibrosis. Further, some in vitro and in vivo studies have led researchers to theorize that inhalation of certain carbon nanotubes in large doses may potentially cause asbestos-type exposure effects.
NIOSH explains that the scientific basis for its CNT REL is an extrapolation from subchronic in vivo toxicity studies in rodents. "The REL is based on the available subchronic and short-term animal dose-response data of early-stage fibrotic and inflammatory lung response to CNT exposure. Benchmark dose (BMD) estimates from the animal data (and the 95% lower confidence limit estimates of the BMD) have been extrapolated to humans by accounting for species differences in alveolar lung surface area. Working lifetime exposure concentration have been calculated based on estimates of either the deposited or retained alveolar lung dose of CNT assuming an 8-hour time-weighted average (TWA) work shift exposure during a 40-hour work week, 50 weeks per year, for 45 years."
However, because a risk of adverse lung effects may occur even below the 7 μg/m 3 CNT REL, NIOSH further recommends reducing airborne levels of nanoscale carbon to as low as possible. Additionally, NIOSH recognizes that the REL is based on a mass dose metric which might not be appropriate for nanoscale materials because "a mass-based sampling method may not be sufficiently sensitive to detect all [carbon nanotubes] and [carbon nanofiber] structures in the air at low mass concentrations." Accordingly, NIOSH recommends additional research to determine the most appropriate dose metric.
The CNT REL is not provided in isolation, NIOSH also recommends that employers and workers follow the detailed workplace safety measures outlined in its comprehensive guidance document, "Approaches to Safe Nanotechnology: Managing the Health and Safety Concerns Associated with Engineered Nanomaterials." To this end, NIOSH provides summary information outlining the basic measures explained in its "Approaches" document.
Another notable feature of the document is its position regarding worker medical screening and surveillance. NIOSH concludes "that workers occupationally exposed to [carbon nanotubes] and [carbon nanofiber] may be at risk of adverse respiratory effects. These workers may benefit from inclusion in a medical screening program recommended as a prudent means to help protect their health." This last quasi-recommendation alters NIOSH's prior position that "[t]he current body of evidence about the possible health risks of occupational exposure to engineered nanoparticles is quite small. Insufficient scientific and medical evidence now exists to recommend the specific medical screening of workers potentially exposed to engineered nanoparticles."
The document closes by listing 15 scientific research needs to "assist NIOSH in evaluating the occupational safety and health concerns of working with [carbon nanotubes] and [carbon nanofiber]."
Finally, for those keeping track, in 2005, NIOSH proposed a REL for ultrafine TiO2 (essentially nanoscale TiO2) of .1 μg/m 3. Bayer Material Sciences previously set a worker exposure limit for its multi-walled carbon nanotube products (BayTubes) at .05 mg/m 3 in 2009. Similarly, NIOSH's REL for carbon black is 3.5 mg/m 3, and OSHA's permissible exposure limit for respirable synthetic graphite is 5 mg/m 3.
National Institute for Occupational Safety and Health, Current Intelligence Bulletin, "Occupational Exposure to Carbon Nanotubes and Nanofibers," p. 3, available at http://www.cdc.gov/niosh/docket/review/docket161A/pdfs/carbonNanotubeCIB_PublicReviewOfDraft.pdf (last visited Dec. 9, 2010).
National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Department of Health and Human Services, Approaches to Safe Nanotechnology: Managing the Health and Safety Concerns Associated with Engineered Nanomaterials (2009) available at http://www.cdc.gov/niosh/docs/2009-125/pdfs/2009-125.pdf.