Occupational Exposure to Carbon Nanotubes and Nanofibers

common purification technique involves selectiveoxidation of the amorphous carbon and/orcarbon shells at a controlled temperature followedby washing or sonicating the material in an acid(HCL, HNO 3, H 2SO 4) or base (NaOH) to removethe catalyst. As there are many types of purificationprocesses, purified CNT and CNF will exhibitdifferences in the content of trace elements and residualmaterials [Liu et al. 2008; Hou et al. 2008].A growing body of literature indicates a potentialhealth hazard to workers from exposure to varioustypes of carbon nanotubes and nanofibers. A numberof research studies with rodents have shownadverse lung effects at relatively low-mass doses ofCNT (Tables 3–2 and 3–7), including pulmonaryinflammation and rapidly developing, persistent fibrosis.Similar effects have been recently observedwith exposure to CNF (Table 3–6). It is not knownhow universal these adverse effects are, that is,whether they occur in animals exposed to all typesof CNT and CNF, and whether they occur in additionalanimal models. Most importantly, it is notyet known whether similar adverse health effectsoccur in humans following exposure to CNT orCNF, or how airborne CNT in the workplace maycompare in size and structure to the CNT aerosolsgenerated in the animal studies.Because of their small size, structure, and low surfacecharge, CNT and CNF can be difficult to separatein the bulk form and tend to be agglomeratedor to agglomerate quickly when released in the air,which can affect their potential to be inhaled anddeposited in the lungs. The extent to which workersare exposed to CNT and CNF in the form ofagglomerates or as single tubes or structures isunclear because of limited exposure measurementdata, but airborne samples analyzed by electron microscopyhave shown both individual and agglomeratedstructures [Johnson et al. 2010; Methner etal. 2010b; Birch et al. 2011b; Dahm et al. 2011].This Current Intelligence Bulletin (CIB) summarizesthe adverse respiratory health effects that have beenobserved in laboratory animal studies with SWCNT,MWCNT, and CNF. A recommended exposure limit(REL) for CNT and CNF is given to help minimizethe risk of occupational respiratory disease in workersas well as guidance for the measurement andcontrol of exposures to CNT and CNF.2 NIOSH CIB 65 • Carbon Nanotubes and Nanofibers