Nanomaterials a risk to health at work? First International Symposium on Occupational Health Implications of Nanomaterials 12-14 October 2004, Buxton, Derbyshire, UK. Buxton, Derbyshire, UK: Health and Safety Laboratory, 2004 Oct; :72-77
Introduction: Nanotechnology is a broad-based enabling technology that holds the promise of major advances in many areas. The next few years will see increasing commercialisation of products that exploit the unique properties of nanoscale materials and devices. However, these same properties present tough new challenges to understanding, predicting and managing potential adverse health effects following exposure. Among the many challenges being faced is the need to be able to monitor exposure to nanomaterials in the workplace in terms of relevant material characteristics. Historically, the mass and bulk chemical composition of materials entering the body have been used to estimate health impact. However, research over the past 15 years has indicated that the size, physical structure and surface chemistry of nanostructured materials play an important role in determining biological response (Donaldson et al. 2000; Oberdorster 2000; Tran et al. 2000; Brown et al. 2001; Oberdorster et al. 2004). Background: Although nanomaterials may potentially enter the body via a number of routes, most toxicology and epidemiology data related to nanometer-diameter particles to date have focused on inhalation exposure. The potential exists to inhale nanostructured materials when working with suspensions or slurries of nanostructured materials (through sprays and atomisation), nanoscale powders and airborne nanoscale materials. While the composition and chemistry of insoluble inhaled nanomaterials are likely to be important factors in determining biological response, the ability to measure physical characteristics such as size, number and surface-area will be key to appropriate in situ exposure monitoring. A critical step towards developing appropriate airborne exposure monitoring approaches is the definition of particle sizes of interest. Nanotechnology generally refers to the creation and use of sub-100 nm structures, and the exploitation of the unique properties associated with these structures. A wide range of nanostructured materials are formed as powders, suspensions or solutions are comprised of primary particles with diameters of less than 100 nm. Consequently, there has been a tendency to discuss airborne exposure to 'nanoparticles' or 'ultrafine particles' in terms of discrete sub-100 nm diameter airborne particles. However, the unique properties of nanostructured materials are not confined to discrete nanometer diameter particles. In the context of health risk it is important to consider whether the nanostructure of a material leads to a specific or enhanced biological response, and whether the material can interact with the body in such a way that the nanostructure is bio-available. Under these criteria, the size of discrete particles only becomes important where biological activity is associated with individual particles, and where size governs location following inhalation and subsequent translocation in the body. For instance, the increased inflammatory response to ultrafine TiO2 reported by Oberdorster et al. (Oberdorster et al. 1994) was most likely associated with nanostructured agglomerates larger than 100 nm in diameter (Maynard 2002), and other studies have demonstrated a correlation between surface-area and inflammatory response for particles significantly larger than 100 nm in diameter (Lison et al. 1997; Tran et al. 2000). These and other studies suggest that in some cases nanostructure alone (represented by surface-area) can provide an indication of biological activity. However studies indicating size-dependent particle translocation from the respiratory system to other organs (Nemmar et al. 2001; Nemmar et al. 2002; Oberdorster et al. 2004) suggest that there will be cases where discrete particle diameter strongly influences impact. If particle nanostructure rather than diameter is the primary driver behind biological response, exposure monitoring most likely needs to be carried out with respect to the impacted areas of the respiratory system, in line with the inhalable, thoracic and respirable sampling conventions. However, where discrete particle size potentially drives translocation and biological response, size-selective sampling/monitoring methods beyond these conventions are most likely required. For an equivalent mass of material, aerosol surface-area varies inversely with particle diameter, and particle number varies inversely with the cube of particle diameter (assuming spherical particles). Thus, even at low mass concentrations, the surface-area and particle number associated with airborne nanoparticles may be substantial. Critical challenges to nanoparticle monitoring include the use of mass-based methods to reflect increasing number and surface-area with decreasing particle diameter, the use of number concentration as an exposure metric and measurement of aerosol surface-area in situ.
Nanomaterials a risk to health at work? First International Symposium on Occupational Health Implications of Nanomaterials 12-14 October 2004, Buxton, Derbyshire, UK