When it comes to assessing the occupational health hazards of exposure to nanoparticles, what can we learn from other small particles and fibers such as asbestos?
That question was the subject of an Oct. 5 presentation made by Fionna Mowat, Ph.D., managing scientist for the Health Sciences Practice of Menlo Park, Calif.-based Exponent, at the Second International Symposium on Nanotechnology and Occupational Health in Minneapolis.
While Mowat's presentation, like many others at the symposium, raised more questions than answers, she concluded that current knowledge of materials such as asbestos, welding fumes and ultrafine particulate matter may be useful in the assessment of the toxicity of nanomaterials.
Drawing a possible parallel to asbestos, Mowat noted that asbestos once was considered a "miracle mineral" before it was discovered to be a human health risk at certain doses.
"Materials like asbestos show us we really need to use caution when developing novel materials and [we need to] use the information we have to develop a health risk assessment paradigm," Mowat said. She noted that information on materials such as asbestos could help to shape future toxicity testing and health assessment of nanomaterials particularly nanotubes, as asbestos and nanotubes share a common geometry (both are long, thin fibers).
Wide Range of Applications Poses Wide Opportunity for Exposure
Nanotechnology which is the manipulation of matter at the molecular level for the purpose of creating extremely small-scale (less than 100 nanometers) materials with unique properties may be a relatively recent concept, but Mowat noted that human exposure to nanomaterials is not. Forest fires, cooking, vehicle exhaust emissions and the ash from volcanic eruptions are examples of sources of nanoscale particles, she explained.
However, the wide range of applications in which nanomaterials are being used or considered for from cancer treatment to cell phone batteries "confers wide opportunity for exposure to humans."
"As for now, workers are experiencing the highest exposures while using the raw materials and synthesizing them [for product applications]," Mowat said. "But we also must think about the end-users and the environment, which is the ultimate sink for these materials."
Size Matters, But So Do Other Characteristics
Based on recent research on nanoparticles and past research on substances such as coal, nuisance dusts, man-made mineral fibers, fumed silica and ultrafine particulate matter, several concerns over exposure to nanoparticles have emerged.
Those concerns include nanoparticles' ability to become airborne because of their small size and to cross biological membranes and barriers, according to Mowat.
Other health concerns involve nanoparticles' reactivity. Since nanoparticles are small and small size equals a large surface area some experts suspect that "even relatively inert substances may become more reactive, and thus toxic," Mowat explains in the abstract to her presentation.
But while size is the first characteristic that comes to mind when discussing nanoparticles, Mowat added that smaller does not always mean more toxic. Citing the "lessons learned from asbestos and other small particles," Mowat explained that size isn't the only factor that can predict the toxicity of nanoparticles: Chemical composition, aggregation potential, surface charges, dosimetry, particle shape, structure and surface properties or coatings are among the other factors must be considered.
"Differences in chemical composition and durability, such as those seen when comparing chrysotile and amphibole asbestos fibers, also play a large role in potential toxicity," Mowat explains in the presentation abstract. "Recent studies indicate that biopersistence plays a large role in disease potential for asbestos, and will need to be investigated for nanomaterials."
Current research on asbestos also suggests that assumptions about the health hazards of nanoparticles based on their size may be a bit counterintuitive. Such research indicates "that it is actually the longer fibers that cause harm, as the smaller fibers are easily cleared or dealt with by the body," according to Mowat.
Mowat mentioned in her presentation that asbestos research may be "particularly relevant" to nanotubes. Nanotubes are extended tubes of rolled graphite sheets favored for their strength, flexibility and conductivity which typically are a few nanometers in diameter and several micrometers to centimeters long.
In concluding her presentation, Mowat said:
- Toxicity of particles may be related to surface area more than mass.
- Toxicity is affected by many characteristics.
- Test substances need to be adequately characterized.
- Realistic dose levels and appropriate routes of exposure must be evaluated.
- Actual risks will depend on specific exposure and type of nanomaterial.
- Studies of chronic health effects should be undertaken.
- Information from related fields and materials might be useful for assessment of nanomaterials.