5. How might a definition work in practice?
Although many other measurements may provide useful information, size remains the key element in any definition. A credible working definition of "nano-sized” materials covers those having at least one external dimension between 1nm and 100nm, or an internal structure of that size. For dry solid powders, the volume specific surface area is a useful complement to the definition.
The distribution of sizes over a sample is also relevant. In addition, the science itself does not identify any specific size associated with properties linked to the nanoscale.
That suggests a broader approach to size, taking into account the entire nanoscale – 1-999nm. In practice, applying a tiered approach might be useful to determine what would prompt a specific risk assessment. The working definition would then mean that 100nm was treated as a lower threshold which would indicate that extensive information relevant to nanomaterials would have to be gathered to feed into a case-by-case risk assessment.
If initial measurements indicate a size above 500nm, on average, the smallest sizes in any sample are unlikely to fall below the 100nm threshold. Size distribution measurements may be called for to confirm this.
For average sizes below 500nm but above 100nm, measurement of the size distribution would come into play. If some specified percentage of the material, say 0.15% or more, turns out to be below 100nm in size, then any risk assessment needed for nanomaterials would be required.
Finally, materials between 100nm and 1nm would be classified as nanomaterials without further ado, as before. This category may be defined by direct size measurements or by measuring the volume specific surface area.