Reviewing the Evironmental and Human Health Knowledge Base of Carbon Nanotubes

Article excerpt

The worldwide funding devoted to nanotechnology research and development by governments, industry, and venture capitalists was estimated to be around US$9.6 billion in 2005 (Lux Research Inc. 2006). A large portion of this spending is still being allocated to the development of nanoparticulate materials because of their many novel physical and chemical properties raising high expectations for a variety of applications. One of these new materials is carbon nanotubes (CNTs), which have commercial expectations in different manufacturing sectors.

However, epidemiologic studies of air pollution suggest that particulate matter has a strong association with cardiopulmonary diseases (Pope et al. 2004). Research has shown that nanoparticles may enter the human body more easily and be more biologically active because of their larger surface area per mass unit compared with that of larger particles (Oberdorster G et al. 2005). The prospective widespread use of engineered nanoparticles in consumer products may increase environmental, occupational, and public exposures dramatically. Consequently, different stakeholders have raised serious concerns regarding health effects of engineered nanoparticles (Helland et al. 2006). Recent review articles on the toxicity potential of nanoparticles (Nel et al. 2006; Oberdorster G et al. 2005) conclude that the toxicity of nanoparticles depends on specific physiochemical and environmental factors. Thus, the toxic potential of each type of nanoparticle has to be evaluated individually.

Here we review the currently available literature on the potential risks of CNTs to human health and the environment. We also investigated the life cycle of CNTs, as release into different environmental compartments may occur at the production stages as well as at the product's use and disposal stages, which may directly or indirectly lead to human exposure. However, the published literature revealed many unanswered questions. Therefore, we also systematically interviewed seven leading world-class scientists and integrated their contemporary knowledge into this review (see Supplemental Material; http:// www.ehponline.org/docs/2007/9652/suppl. pdf). This assisted us in identifying questions and developing recommendations. The scientists interviewed were key authors or project leaders who have investigated and reported the potential impacts of CNTs on human health or environment. Through this combined approach we are able to present an updated and contemporary knowledge base for scientific discussion.

In this review, we use the term "carbon nanotubes" when addressing the general aspects of the material, which includes singlewalled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). Here "multi" is defined as two or more walls. The terms "SWCNTs" and "MWCNTs" are used in a specified manner.

Exposure to Carbon Nanotube Material

Exposure in occupational settings. Procedures for the handling of CNTs can result in aerosol release of these materials into the surroundings (Maynard et al. 2004). MWCNT aerosols generally have diameters between 20 nm to > 200 nm, lengths from 1,000 nm to > [10.sup.6] nm, and different shapes (straight, partly rigid, bent, curled, and partly flexible) that may appear single or in clumps or ropes (Donaldson et al. 2006).

Only one published study has investigated the potential for SWCNTs to become airborne. A laboratory study by Maynard et al. (2004) investigating the physical nature of the aerosol formed during mechanical agitation was complemented by a field study of SWCNT release during handling of unrefined SWCNTs. The authors found that sufficient agitation of unrefined SWCNT material can release fine particles into the air, but the concentrations generated while handling the material in the field were very low (< 53 [micro]g/[m.sup.3]). The laboratory study also revealed that different SWCNT production methods produced different types of aerosols. …