Henry F. Smyth, Jr. Award Lecture: The Challenge of Setting Occupational Exposure Limits for Engineered Nanomaterials
May 23 |
2:00 PM – 3:00 PM
Exposure Banding/OEL Process, Nanotechnology, Special
Presenter: M. Ellenbecker, University of Massachusetts Lowell, Lowell, MA.
Engineered nanomaterials (ENMs), ranging from the exotic (carbon nanotubes, quantum dots, targeted drug delivery nanoparticles) to the prosaic (nanometer-sized silica, titanium oxide, and carbon black), have been widely hailed as an exciting frontier that will benefit industries ranging from advanced electronics and medicine to clothing and sports equipment. However, as this industry grows and develops, as has happened far too often in the past, concerns emerge as to possible adverse health and environmental impacts from these new materials. A prominent example is carbon nanotubes (CNTs), which in rodent studies have been found to cause a range of acute (e.g., lung inflammation) and chronic (e.g., lung fibrosis) health effects. Of even more concern are recent studies finding the induction of lung cancer and mesothelioma in mice, leading the International Agency for Research on Cancer to designate certain multi-walled CNTs as 2B suspect human carcinogens.
The occupational hygiene community has been slow to react to this new hazard, likely in part because the extent of the danger is not fully known, but perhaps also because traditional methods to evaluate and control aerosol exposures are difficult to apply to nanometer-sized particles. Exposure assessment is particularly difficult; airborne concentrations of a few thousand nanoparticles per cm3 may be of toxicological significance, and standard mass-based measurement techniques will not be able to detect such low concentrations. Specialized equipment to count and size nanoparticles has been developed, but currently-available instruments are expensive, large, and difficult to use in the field.
Closely related to measurement issues is the development of occupational exposure limits (OELs) for ENMs, since an OEL must have available at least one method to measure against it. Almost all available aerosol OELs are mass-based. For one substance, asbestos, most if not all OELs are based on number concentration. At first glance, this approach may seem to be appropriate for CNTs, which cause the same diseases as asbestos, but unfortunately the optical microscopy techniques used to count asbestos fibers will not detect CNTs that are tens of nanometers in diameter. It is largely for this reason that NIOSH has published a mass-based recommended exposure limit for CNTs. This approach is fraught with difficulties, as will be discussed.
This presentation will focus on the broad issue of the need to work safely with a new material such as ENMs, and the specific issue of setting OELs for ENMs, using CNTs as a prime example. Hopefully, this subject will honor the career of Dr. Smyth, who devoted a great deal of attention to the subject of assessing the hazards of new materials. To quote from an article of his written in July, 1946:
“It is clearly the duty of a manufacturer to delay production of a chemical until the health hazards are well enough defined so that protection of his workers is possible. It is also his duty not to sell a chemical for an application in which it would endanger the health of the public, and to inform customers, by proper labelling and otherwise, of the hazards of the compounds they buy.”
This approach is not being followed with regard to engineered nanomaterials. It is hoped that discussion and activities can be generated to help rectify this unfortunate situation.