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In an evaluation of carbon nanotubes (CNTs) for the IARC Monograph 111, the Mechanisms Subgroup was tasked with assessing the strength of evidence on the potential carcinogenicity of CNTs in humans. The mechanistic evidence was considered to be not strong enough to alter the evaluations based on the animal data. In this paper, we provide an extended, in-depth examination of the in vivo and in vitro experimental studies according to current hypotheses on the carcinogenicity of inhaled particles and fibers. We cite additional studies of CNTs that were not available at the time of the IARC meeting in October 2014, and extend our evaluation to include carbon nanofibers (CNFs). Finally, we identify key data gaps and suggest research needs to reduce uncertainty. The focus of this review is on the cancer risk to workers exposed to airborne CNT or CNF during the production and use of these materials. The findings of this review, in general, affirm those of the original evaluation on the inadequate or limited evidence of carcinogenicity for most types of CNTs and CNFs at this time, and possible carcinogenicity of one type of CNT (MWCNT-7). The key evidence gaps to be filled by research include: investigation of possible associations between in vitro and early-stage in vivo events that may be predictive of lung cancer or mesothelioma, and systematic analysis of dose–response relationships across materials, including evaluation of the influence of physico-chemical properties and experimental factors on the observation of nonmalignant and malignant endpoints.

Exposure to low-toxicity dusts, which have previously been viewed as 'nuisance dusts', can cause chronic obstructive pulmonary disease or other nonmalignant respiratory disease. In Britain, the 'de facto' airborne exposure limits for these dusts have remained unchanged for30 years; currently, they are 10mg m(-3) for inhalable dust and 4mg m(-3) for respirable dust. During this time, exposures in industry have decreased and although in the past, many occupational dust exposures may have exceeded these limits, today this is less likely. However, there is good evidence from epidemiology and toxicology studies that current dust exposures may still present a risk to workers and that for some of those who are affected, there are devastating health consequences. Numerous researchers and others have drawn attention to the necessity to control dust exposures to levels lower than are currently accepted in Britain. It is proposed that until regulators agree on the safe occupational exposure limits for low-toxicity dusts, health and safety professionals should consider 1mg m(-3) of respirable dusts as a more appropriate guideline than the value of 4mg m(-3) currently used in Britain.