User:Mdahm85/sandbox

Toxicity
See also: Fullerene § Safety and toxicity

The toxicity of carbon nanotubes has been an important question in nanotechnology. As of 2007, such research had just begun. The data is still fragmentary and subject to criticism. Preliminary results highlight the difficulties in evaluating the toxicity of this heterogeneous material. Parameters such as structure, size distribution, surface area, surface chemistry, surface charge, and agglomeration state as well as purity of the samples, have considerable impact on the reactivity of carbon nanotubes. However, available data clearly show that, under some conditions, nanotubes can cross membrane barriers, which suggests that, if raw materials reach the organs, they can induce harmful effects such as inflammatory and fibrotic reactions.[73][74]

Under certain conditions CNTs can enter human cells and accumulate in the cytoplasm, causing cell death.[75]

Results of rodent studies collectively show that regardless of the process by which CNTs were synthesized and the types and amounts of metals they contained, CNTs were capable of producing inflammation, epithelioid granulomas (microscopic nodules), fibrosis, and biochemical/toxicological changes in the lungs.[76] Comparative toxicity studies in which mice were given equal weights of test materials showed that SWCNTs were more toxic than quartz, which is considered a serious occupational health hazard when chronically inhaled. As a control, ultrafine carbon black was shown to produce minimal lung responses.[77]

Carbon nanotubes deposit in the alveolar ducts by aligning lengthwise with the airways; the nanotubes will often combine with metals.[78] The needle-like fiber shape of CNTs is similar to asbestos fibers. This raises the idea that widespread use of carbon nanotubes may lead to pleural mesothelioma, a cancer of the lining of the lungs, or peritoneal mesothelioma, a cancer of the lining of the abdomen (both caused by exposure to asbestos). A recently published pilot study supports this prediction.[79] Scientists exposed the mesothelial lining of the body cavity of mice to long multiwalled carbon nanotubes and observed asbestos-like, length-dependent, pathogenic behavior that included inflammation and formation of lesions known as granulomas. Authors of the study conclude:

Although further research is required, the available data suggest that under certain conditions, especially those involving chronic exposure, carbon nanotubes can pose a serious risk to human health.[73][75][77][79]

In 2014, experts from the International Agency for Research on Cancer (IARC) assessed the carcinogenicity of CNTs, including SWCNTs and MWCNTs. No human epidemiologic or cancer data was available to the IARC Working Group at the time, so the evaluation focused on the results of in vivo animal studies assessing the carcinogenicity of SWCNTs and MWCNTs in rodents.

The Working Group concluded that there was sufficient evidence for the specific MWCNT type “MWCNT-7”, limited evidence for the two other types of MWCNTs with dimensions similar to MWCNT-7, and inadequate evidence for SWCNTs. Therefore, it was agreed to specifically classify MWCNT-7 as possibly carcinogenic to humans (Group 2B) while the other forms of CNT, namely SWCNT and other types of MWCNT, excluding MWCNT-7, were considered not classifiable as to their carcinogenicity to humans (Group 3) due to a lack of coherent evidence.[80]

Epidemiology and Risk Management
Currently, there is a lack of epidemiological evidence linking exposure to CNT to human health effects. To date, there have been only a handful of published epidemiological studies that have solely examined the health effects related to the exposure of CNT, while several other studies are currently underway and yet to be published. With the limited amount of human data, scientists are more reliant on the results of current animal toxicity studies to predict adverse health effects, as well as applying what is already known about exposures to other fibrous materials such as asbestos or fine and ultra-fine particulates. This limitation of human data has lead to the use of the precautionary principal, which urges workplaces to limit exposure levels to CNT as low as possibly achievable in the absence of known health effects data.

To date, several international government agencies, as well as individual authors, have developed occupational exposure limits (OEL) to reduce the risk of any possible human health effects associated with workplace exposures to CNT. The National Institute for Occupational Safety and Health (NIOSH) conducted a risk assessment using animal and other toxicological data relevant to assessing the potential non-malignant adverse respiratory effects of CNT and proposed an OEL of 1 μg/m3 elemental carbon as a respirable mass 8-hour time-weighted average (TWA) concentration. Several individual authors have also performed similar risk assessments using animal toxicity data and have established inhalation exposure limits ranging from 2.5 to 50 ug/m3.

Safety and Exposure Prevention
Occupational exposures that could potentially allow the inhalation of CNT is of the greatest concern, especially in situations where the CNT is handled in powder form which can easily be aerosolized and inhaled. Also of concern are any high-energy processes that are applied to various CNT preparations such as the mixing or sonication of CNT in liquids as well as processes that cut or drill into CNT based composites in downstream products. These types of high-energy processes will aerosolize CNT which can then be inhaled.

Guidance for minimizing exposure and risk to CNT have been published by several international agencies which includes several documents from the British Health and Safety Executive titled "Using nanomaterials at work Including carbon nanotubes and other bio-persistent high aspect ratio nanomaterials" and the "Risk Management of Carbon Nanotubes"  Safe Work Australia has also published guidance titled "Safe Handling and use of Carbon Nanotubes" which describes two approaches to managing the risks that include risk management with detailed hazard analysis and exposure assessment as well as risk management by using Control Banding. The National Institute for Occupational Safety and Health has also published a document titled "Current Intelligence Bulletin 65: Occupational Exposure to Carbon Nanotubes and Nanofibers" describes strategies for controlling workplace exposures and implementing a medical surveillance program.

These guidance documents generally advocate instituting the principles of the Hierarchy of Hazard Control which is a system used in industry to minimize or eliminate exposure to hazards. The hazard controls in the hierarchy are, in order of decreasing effectiveness:
 * Elimination
 * Substitution
 * Engineering Controls
 * Administration
 * Personal Protective Equipment