Questions on Nanomaterials
The answers to these questions are a faithful summary of the scientific opinion produced in 2010 by "The Scientific Committee on Emerging and Newly Identified Health Risks" (SCENIHR):
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1. What are nanomaterials?
Nanotechnologies involve designing and producing either very small objects or materials which contain very small structures or particles. The scale of these objects or structures is 100 nanometres (100 millionth of a millimetre) or less. Nanotechnologies operate by manipulating individual atoms or molecules.
Nanomaterials are one of the main products of nanotechnologies – as nano-scale particles, tubes, rods, or fibres. Nanoparticles are normally defined as being smaller that 100 nanometres in at least one dimension. As nanotechnologies develop, nanomaterials are finding uses in many sectors such as healthcare, electronics, cosmetics, textiles, information technology and environmental protection.
There are many different types of nanomaterials. They may be produced by processing bulk materials, by chemical synthesis or by self-assembly from smaller components. They often have multiple components, occur in aggregates, and have diverse internal or external structures.
All these features may affect their properties so that the way they interact, chemically or physically, often differs from larger scale materials. This poses challenges for risk assessment as the uses of such materials expand.
2. How have nanomaterials been defined?
A number of definitions have been proposed. All use size limits. The proposed upper size limits have varied from 100 nm up to 1000nm for pharmaceuticals. Lower size limits have also varied, from 1nm, to 0.1 nm.
For individual particles or agglomerates, the external dimensions can be measured. The measurement of internal dimensions applies to aggregates of nanoparticles. Applying them more widely leads to including materials with nano-sized pores in the general classification of nanomaterials.
In some cases, other physical or chemical characteristics or properties have been used to define nanomaterials. These have not yet been applied consistently.
3. What are the key criteria for defining nanomaterials?
Size remains the best starting point as it is universally applicable. A 100nm upper limit is useful, but nevertheless excludes some things which need to be considered as nanomaterials, such as clumped materials and some which have acquired a coating or envelope. They can be covered by a suitable definition if it extends to internal structures.
The lower size limit has to contend with molecular sizes, which may be greater than 1nm, and with nanomaterials taking the form of tubes or fibres which are less than 1nm across but have lengths above 100nm.
Few nanomaterials have a uniform size, so the size distribution also has to be taken into account.
A useful additional parameter to define nanomaterials is the volume specific surface area (VSSA) – usually expressed in square metres per cubic centimetre (m2/cm3). This parameter includes internal surfaces if they exist.
A commonly used threshold for a nanomaterial is a VSSA greater than or equal to 60 m2/cm3
4. What other measurable properties of nanomaterials might be relevant?
A range of other characteristics of nanomaterials can be defined. They include:
- Surface modification
- Crystal structure
- Chemical oxidising or reducing potential
- (Photo)catalysis and formation of reactive free radicals
- Surface charge
- Water solubility
- Partition coefficient in different solvents
Any of these may be important to know for risk-assessment. However, none is relevant to the whole class of nanomaterials, so their use in any overarching definition is unlikely to be helpful.
5. How might a definition work in practice?
A working definition of "nano-sized” materials could include those with at least one external dimension, or an internal structure, between 1nm and 100nm. The volume specific surface area is a useful additional parameter.
While the reference to a size range between 1 nm and 100nm is useful, scientific evidence does not pinpoint any specific size associated with properties linked to the nanoscale. Therefore, regulators and risk assessors need to examine materials over the entire nanoscale – 1-999nm to determine which materials are likely to have properties specific of the nanoscale. Not all would deserve a full assessment. A tiered approach, based on size thresholds, may be the best initial approach.
For example, for sizes below 500nm but above 100nm, sample 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 a specific risk assessment might be needed.
6. Should there be variations or exceptions to the working definition?
The 1nm-100nm lower and upper limits both need careful interpretation in some cases. For example, the lower limit of 1nm implies that many molecules would be classified as nanomaterials. On the other hand the upper limit of 100nm could lead to excluding some multi-component nanomaterials used in medicine and cosmetics. In such cases, the size of the components may need to be taken into account.