Workshop Report 07 – Workshop on Testing Strategies to Establish the Safety of Nanomaterials

WR 07 : Workshop on Testing Strategies to Establish the Safety of Nanomaterials | September 2006

Nanotechnology produces an increasing number of engineered nanoparticles. A better understanding of the tests available to assess exposure levels of nanoparticles in the occupational setting, to the consumer and to evaluate the potential health and environmental impact is necessary.  It is also necessary to agree how thee tests should be applied and where new developments are needed.

A workshop was held to develop testing strategies to establish the safety of nanomaterials.  It brought together about 70 scientific and clinical experts from industry, academia, governmental agencies and one environmental non-governmental organisation.  The primary questions to be addressed were the following:  What can we do today?  And, what do we need for tomorrow?  The three major themes of the workshop were: 1) the need for enhanced efforts in nanomaterial characterisation; 2) methodologies for the assessment of airborne and internal exposures to nanomaterials; and 3) evaluation of the hazard potential, primarily through pulmonary or dermal routes of exposures.

The major summary conclusions of the workshop included the following:

For the development of nanoparticle characterisation, the working definition of nanoparticles was agreed as < 100 nm in one dimension.  In addition, it was suggested by some that the criteria be expanded to < 1000 nm to include aggregates and agglomerates.  Moreover, it was concluded that although many physical factors can influence the functional, toxicological and environmental characteristics of nanoparticles, their impact is largely determined by:

• composition;
• dissolution;
• surface area and other surface characteristics;
• size;
• size distribution (including aggregation and agglomeration state); and
• shape.

Most of the information on potential systemic effects has thus far been derived from combustion-generated particles with a major focus on the cardiovascular system.

With respect to the assessment of external exposures and metrics appropriate for nanoparticles, the general view of the participants was that it is not currently possible to select one form of dose metric (i.e. mass, surface area or particle number) as the most appropriate.  However, it was clear that the metric, namely surface area, was likely to be of interest and needed further development.  Standardisation of methods for quantifying dose metrics will be necessary.  In addition, there is a clear need to develop monitoring instruments which are smaller, more portable and less expensive than the state of the art instrumentation currently available.

Overall, few occupational exposure data are currently available.  Since exposure and hazard data form the integral components of risk assessment processes, it will be necessary to develop the workplace exposure data in a systematic and reproducible fashion.  Detailed characterisation of nanoparticle exposure methodologies should be documented and provided.

With regard to a general testing approach for human health hazard evaluation of nanoparticles the following was concluded:

• A first step would include a prioritisation-type in vitro screening strategy to assess the possible reactivity, biomarkers of inflammation and cellular uptake of nanoparticles.  This strategy would determine likely potency but should ultimately be validated using in vivo techniques.
• A Tier 1 in vivo testing strategy would include a short-term inhalation or alternate route such as intratracheal instillation of nanoparticles as the route of exposure in the lungs of rats or mice.  The effects that should be assessed include endpoints of lung inflammation, cytotoxicity, oxidative stress as well as cell proliferation and histopathology of the respiratory tract and the major extra-pulmonary organs.
• For Tier 2 in vivo testing for hazard identification, a longer term inhalation study is recommended, and this would include more substantive mechanistic endpoints such as determination of particle deposition, translocation and disposition.

At present, there is little evidence that nanoparticle aggregates and agglomerates at a size exceeding 100 nm penetrate through the skin barrier into the living tissue.  The penetration of nanoparticles at a size less than 100 nm should be a topic of further investigation.

When analysing the dermal exposure and the hazard potential of nanoparticles, it must be taken into consideration that the dermal uptake of nanoparticles will be an order of magnitude, or more, smaller than the uptake by inhalation or the oral route.  For the evaluation of the health risk of nanoparticles, it has to be determined whether they are harmful to living cells and whether, under realistic and practical conditions, they penetrate through the stratum corneum of the skin into the living tissue.  Cell culture experiments are broadly used for toxicological assessments.  Three methods were mentioned that are available for the evaluation of skin penetration.

Environmental safety testing, applications of nanoparticles for medical purposes and pathways of inhaled nanoparticles to the central nervous system were also briefly addressed during this workshop.  It has become clear that these topics should be subject of separate workshops.