TEM comparison of chrysotile (asbestos) nanotubes and carbon nanotubes

Fibrous materials, both natural and synthetic, have been an important part of global technologies for decades. Many of these materials in their elemental forms are nanomaterials with unusual properties, especially crystalline whiskers and fibers used to make an array of composite materials systems. Around 1970, asbestos, especially chrysotile (Mg3Si2O5(OH)4), and crocidolite (Na2(Fe 2+ 3 , Fe 3+ 2 )Si8O22(OH,F)2) had been used to fabricate nearly 80 000 miles of asbestos–cement (AC) water pipe in the United States, alone, hundreds of millions of tons of asbestos insulation, brake-linings, AC sheet (commonly referred to as transite), and a host of other commercial products. But by the early 1970s, the health effects of asbestos mining, milling, and manufacturing began to emerge [1–3], and asbestos was observed in beverages and drinking water [4, 5]. This of course led to a moratorium on AC pipe and other asbestos products, including brake-linings, especially in the United States. Other studies of ingested mineral fibers, including asbestos, suggested that asbestos fibers longer than 20 μm but thinner than 3 μm were more carcinogenic than fibers of greater diameter, regardless of length, or those shorter than 20 μm, regardless of diameter. Of course, these were actually fiber bundles. It is now well known that the geometry and surface chemistry of particulates can play an important role in causing lung toxicity, especially chrysotile asbestos exposure [6]. Renwick et al. [7] have demonstrated that ultrafine particulates impair macrophage phagocytosis to a greater extent than fine particles (compared on a mass basis). Schwartz et al. [8] have shown that episodes involving high concentrations of coarse particles are not associated with increased mortality while Oberdorster et al. [9] correspondingly showed that ultrafine particles contribute to acute mortality. In addition, crystalline particulates in contrast to amorphous particulates in the same size range are more damaging to human lungs [10], and Momarca et al. [11] have shown that ultrafine crystalline particulates are major contributors to adverse health effects. There is mounting evidence that ultrafine airborne particulates with mean diameters <100 nm are far more toxic than expected and pose considerable health risks, including mortality, asthma complications, chronic bronchitis, respiratory tract infections, ischaemic heart diseases, and stroke. A recent joint industry, government, and private sector, multicity analysis found that a daily increase of 20 μg/m3 in inhalable particulate matter <10 μm increased the death rate by about 1% [12], while a 25 μg/m3 increase in average lifetime concentration of fine particles (<2.5 μm diameter) increased the overall total annual death rate by some 15% [13]. The start of the 21st century heralded the U.S. National Nanotechnology Initiative [14], following a decade of materials discovery and development, notably carbon nanotubes [15], which have demonstrated a wide range of remarkable properties [16]. This has led to predictions of annual production quantities of single-wall carbon nanotubes (SWCNTs) in thousands of tons within the first decade of the 21st century [17]; however, large quantities of multiwall carbon nanotubes (MWCNTs) are already in commercial use [18]. In a recent summary of American Chemical Society Symposium on Nanomaterials and Nanotechnology, Dagani [19] concluded that “early results suggest that some nanoparticles, such as carbon nanotubes, may pose health risks.” Correspondingly, Lam et al. [20] have recently shown that SWCNTs are more toxic than carbon black or silica (SiO2) particulates (on an equal weight basis) in mouse lungs, while Warheit et al. [21] have demonstrated that pulmonary exposures to SWCNTs in rats produced evidence of a foreign tissue body reaction. Recent exposure assessment studies conducted at workplace sites where carbon nanotubes are either manufactured or utilized reported low airborne particulate exposure levels of respirable SWCNTs, not exceeding 100 μg/m3 [22, 23]. Moreover, it has recently been observed that carbon nanotubes, particularly MWCNTs are ubiquitous in the atmosphere, both indoor and outdoor, and are produced by a wide range of fuel gas combustion sources, including natural gas and propane gas kitchen stove tops, gas furnaces and hot water heaters, electric power generation stations, etc. [24–26]. These airborne MWCNT particulates are actually complex aggregates often composed of hundreds or thousands of carbon nanotubes and related carbon nanocrystal particles. While specific quantitation has not been measured, estimates of carbon nanocrystal aggregate concentrations can exceed 100 μg/m3. While the technological and health-related issues involving carbon nanotubes have emphasized SWCNTs, it is apparent that the most prevalent occurrence of carbon nanotubes is MWCNTs and especially complex aggregates of MWCNTs and related carbon nanocrystal forms. Harris [27] has noted that chrysotile