|dc.description.abstract||Nanoparticles are particles with a maximum size of 100 nm in at least one dimension. Nanoparticles are released by both natural and anthropogenic sources. Up until now only exposure limit values are present for bulk size substances. However, nanoparticles might have different health effects and or dose-response relationships. It is believed that small particles might have greater reactivity potential, e.g. inflammatory effects per unit mass. Since nanoparticles might have different health effects and or dose-response relationships compared with bulk size (micrometer size and above) particles, there is a need to evaluate the feasibility of deriving exposure limit values for these type of particles.
Inhalation exposure to nanoparticles can cause, among others, local inflammatory effects. These effects are partly dependent on location of deposition of the particles in the lungs, which dependents on particle size. Physical phenomena like agglomeration and aggregation of nanoparticles can change the particle size and thereby the location deposition. Agglomeration/aggregation can also include several different substances, increasing the complexity of the exposure, but also the complexity of the health effect.
Exposure limits are determined by combining exposure and hazard parameters. It is essential that both factors are available. Hazardous substances are identified and the effect is quantified in a dose-response relationship. Exposure assessment focuses on the possible concentrations present during exposure scenarios. The research of both exposure assessment and toxicology for nano particles is in the developing stage. First attempts are made to derive exposure limit values for nano sized materials.
Surface area is a metric which is believed to be important when nanoparticles are involved. However, more than one type of surface area metrics can be defined, which may vary in biological relevancy and complexity, dependent on several biological processes. Lung deposited surface area concentration and lung fluid available surface area seem to be the most relevant metrics. The concept of surface area is only believed to be relevant for insoluble particles, as soluble particles will dissolve in the lung fluid, making mass concentration the only relevant metric.
At this moment, there is no consensus about surface area as a dose metric or an exposure metric. In toxicology, surface area as a dose metric is not yet accepted to be the most relevant dose metric, leaving only few studies with surface area as the dose metric. Toxicological studies using this type of dose metric often do not use a surface area per volume metric, but only total or calculated surface area, or surface area, normalized for lung weight.
Surface area as the dose metric in exposure limits has potency as efforts are made to increase knowledge on dose-response relationships and exposure assessment methods based on surface area. At present however, data lacks in order to derive exposure limits with surface area as the dose metric.||