For anyone who’s been looking for a guide on handling carbon nanotubes, it seems the Australians released a relevant publication back in March 2012. There’s some really good information in it (happily, they covered a few items I have long been curious about). The Aug. 1, 2012 news item on Nanowerk provides more details about the document,
The potential risks from exposure to carbon nanotubes have been identified and examined in research studies. To help people work safely with carbon nanotubes, Safe Work Australia commissioned the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to develop the guidance document Safe Handling and Use of Carbon Nanotubes (pdf).
I went to look at the 42-page PDF document and found this description of single-walled and multi-walled carbon nanotubes (CNT) along with a description of the health concerns as outlined in a US NIOSH (National Institute of Occupational Safety and Health) document on p. 4 print version, p. 6 PDF,
SINGLE-WALLED AND MULTI-WALLED CARBON NANOTUBES
In general, there are two groups of CNTs:
- Single walled carbon nanotubes (SWCNTs) are a single cylinder of carbon atoms forming a tube. They are normally around 1nm in diameter, but may be up to 5nm.
- Multi walled carbon nanotubes (MWCNTs) consist of two or more concentric layers of carbon nanotubes with a hollow core typically 2-30nm in diameter. For example, double-walled carbon nanotubes have two concentric layers. MWCNTs may be stiffer than SWCNTs and may potentially be of greater health and safety risk due to the possibility of piercing the body’s pleural tissue.
Potential health concerns
The toxicity of CNTs is the subject of much discussion and experimentation. This document does not aim to consider or analyse this literature in detail. However, CNTs can be bio-persistent and have the potential to exist as fibre-like structures.
NIOSH (2010) reports that currently there are no studies reported in the literature of adverse health effects in workers producing or using CNTs. However use is not yet widespread, and there can be a long latency before the development of disease. The concern about worker exposure to CNTs arises from results of animal studies, showing adverse lung effects including pulmonary inflammation and fibrosis.
NIOSH (2010) also reports that animal studies have also shown asbestos-type pathology associated with exposure to longer, straighter CNT structures. Mesothelial tumors have been reported in a susceptible strain of mice after intraperitoneal injection of longer MWCNTs (10-20 μm in length) but not by short MWCNTs (<1 μm in length).
In a recent review, Toxikos (2009) reports: “Evidence leads to a conclusion that as a precautionary default: all biopersistent CNTs, or aggregates of CNTs, of pathogenic fibre dimensions could be considered as presenting a potential fibrogenic and mesothelioma hazard unless demonstrated otherwise by appropriate tests…” (Toxikos 2009).
There is also evidence that CNTs and structures of CNTs that are not of fibre-like shape may also be hazardous.
They give guidance on on two methods for risk management (pp. 5 – 7 print version, pp. 7-9 PDF),
Risk management methods — Overview
Risk management, including work with CNTs, is focused on preventing incidents, injury, illness, property damage, and environmental harm.
The general risk management process, which is applicable to working safely with CNTs, is illustrated in Figure 1. It shows that risks may be controlled with or without conducting a detailed risk assessment. If, after identifying a hazard, you already know the risk and how to control it effectively, you may implement the controls without further assessment.
Guidance on the general risk management process is available in the Code of Practice: How to Manage Work Health and Safety Risks.
This document provides guidance on two options to manage the risks.
Method 1 — Carbon nanotubes risk management with detailed hazard analysis and exposure assessment 1
This approach should be used when it is necessary to gather and evaluate information on characteristics of the carbon nanotubes or structures of carbon nanotubes and/or on potential levels of exposure throughout the process and associated work, to assess risk. The approach involves:
collecting relevant information to identify the hazards
assessing the risks
implementing appropriate control measures, and
monitoring and reviewing the effectiveness of control measures.
Information can be collected from external sources, including the manufacturer and supplier. This will include information on:
physical and chemical characteristics of carbon nanotubes
potential health effects
Specialised knowledge of the production processes, analysis methods and controls will be required to undertake a full risk management process.
Method 2 — Carbon nanotubes risk management by Control Banding
Control banding for CNTs involves a simplified form of the risk management approach, where specific controls are recommended based on process risk. The CNTs are considered to be hazardous, therefore the controls are based on the potential level of exposure. Control banding can be used when production and manufacturing processes are well understood, potential exposure routes are known and safe work procedures are developed.
As with Method 1 above, this approach involves implementing appropriate controls for specific processes and monitoring and reviewing control effectiveness.
After my encounter with Canadian firefight Peter McBride (he disagreed vociferously with some of my comments in an April 25, 2012 posting), I’ve been interested in any fire safety issues posed by nanomaterials. There’s not much in this report but here it is (p. 10 of the print version, p. 12 PDF),
Safety hazards are considered in the CSIRO’s safety data sheet for MWCNTs (CSIRO 2009). CNTs are not considered to be dangerous goods. In relation to fire and explosion hazards the following points are noted:
- CNTs are difficult to combust and ignite.
- However in general, accumulations of fine dust (420 microns or less) may burn rapidly and fiercely if ignited; once initiated larger particles up to 1400 microns diameter will contribute to the propagation of an explosion.
This is the last bit I’m excerpting from the report and it’s an example of how ventilation practices were changed to bring exposure rates to airborne CNTs below recommended levels,
REDUCING EXPOSURE TO AIRBORNE CNTS — EXAMPLE
An assessment of airborne exposure to MWCNTs in a research laboratory manufacturing and handling MWCNTs found a total particulate concentration of 430 μg/m3 for a blending process in the absence of exposure controls (Han 2008). The implementation of ventilated enclosure of the blending process reduced airborne concentrations of MWCNTs from 172.9-193.6 tubes/cm3 to 0.018-0.05 tubes/cm3. At airborne levels of 0.018-0.05 tubes/cm3, the airborne MWCNTs concentration is significantly below the NIOSH REL of 7 μg/m3.
I included that last bit as it demonstrates the possibilities for minimizing risk. Unfortunately, there’s no way yet of ascertaining whether the minimum levels for exposure have been set correctly.
So, here’s my final word on this guide, it provides some good introductory material, guidelines for analyzing the best safety practices, a helpful bibliography, and a reminder that we still don’t know much about the risks of handling CNTs. For those who won’t make their way through a 40-page document, there’s an information sheet.