What are the risks of nanoscale particles?
Naturally-occurring nanoscale materials have been in the environment for millions of years, yet little is known about their occurrence and inputs. With an emerging nanotechnology industry, a proper appraisal of their potential risk is well overdue.
Nanoparticles – all dimensions under 100nm – and nanomaterials – with at least one dimension under 100nm – are relatively new terms within the scientific community. However, naturally occurring nanomaterials including volcanic ash, oceanspray, magnetotactic bacteria, and mineral composites have long been present in the environment. Indeed, engineered nanomaterials (ENMs) were used 2,000 years ago to make dichroic glass such as the Lycurgus cup held in the British Museum. Nevertheless, there is trepidation over the use of nano-scale materials.
There are more than 1,000 consumer products on the market that specifically state that they contain ENMs. UK manufacturers make less than 43 ENM-containing products, including face cream, badminton racquets, car cleaning products, fuel catalysts, clothing and shower filters. This figure may be unrepresentative, as companies are not required to declare if products contain ENMs. The most commonly occurring ENMs in consumer products are silver, carbon (in the form of carbon black and carbon nanotubes), silicon oxide, titanium dioxide, zinc oxide and gold.
While small size and increased specific surface area of ENMs are beneficial in many fields, the same properties can also be detrimental. Even those chemicals that are considered to be inert in their bulk form, including silver, can become chemically active at the nano scale. The chemical properties of nanoscale particles are dependent on the size, shape, crystallinity, surface properties of area, charge, modification, porosity and agglomeration state, in addition to the chemistry of the compound:
These properties will be different to the bulk properties due to the size of the particle, resulting in changes in the bond characteristics inside the particle and the surface area to volume ratio. These physicochemical properties will also change the interaction between the compound and the surrounding environment.
Humans may be exposed to ENMs via inhalation, ingestion, injection or absorption through the skin. Once ENMs are taken into the body, their size, shape, and solubility help determine which organ is initially targeted. However, experimental data about this process is limited and research has concentrated on determining which organs contain radio-labelled ENMs after exposure. Studies also suggest that ENMs can translocate in the body and pass from the primary target organ into a secondary one, dependent on the material and time.
Within the body, there are a number of possible toxicological end points:
These toxicological end points can be related to:
i) physicochemical properties of the particle – for example carbon nanotubes [CNT] that have been likened to asbestos fibres, the size allows them to pass into the lungs where their inert nature prevents their dissolution by protective cell processes, resulting in further damage to the lung tissue and the development of scar-like tissue.
ii) the inherent toxicity of the chemical, for example cadmium. Other toxicological processes are linked to the reactive nature of the particle, which may cause free radical generation leading to inflammation, DNA damage and cell damage.
In general, when ENMs are held in a matrix (for example CNT/titanium composite in tennis racquets), they are considered to be low risk, as there is no exposure to the environment. However, there is still a possibility of environmental exposure to these particles during the lifecycle of the product. Loose ENMs are present during manufacture and could be released during use or disposal of the product. Loose ENMs are released into the environment from:
i) industrial sources via smoke plumes or effluent discharge.
ii) consumer sources via waste water – for example nanosilver in socks released during washing cycle, nano TiO2 in sunscreen released during swimming
iii) agricultural land via run-off from roads and accidental spills (see Figure 3 below).
The physical state of ENMs within the environment may be as individual particles, aggregated particles, or bound to natural organic matter (NOM).
Figure 3: Schematic showing potential environmental reactions and sources of engineered nanomaterials (ENMs) that can be released into the environment from industrial, consumer and agricultural sources and dispersed in air, soil and water, either as single particles, in ionic form (Mn+) or bound to natural organic matter (NOM). The physical state of the particle is linked to the method of dispersion
Risk and regulation of ENMs
As with any chemical assessment, the risk assessment of ENMs requires knowledge about the harmful impacts, probability of exposure and behaviour within a specific environment. There is little information about the relationship between environmentally-relevant doses of ENMs and the potential toxicological responses in humans and the environment. Therefore risk management decisions for ENMs involve a high degree of uncertainty.
Currently, ENMs are considered within the context of chemicals legislation as there are no specific guidelines for the distinction of nanoscale chemicals from bulk forms. Within the UK and EU, ENMs are covered by the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Directive (EC 1907/2006) that regulates the production and importing of chemical substances. However, REACH relies on weight triggers (more then one tonne per annum) to prompt assessment, which may not be suitable for ENM production. It should be noted that other statutory instruments cover specific uses of ENMs, including: consumer products, medical directives, and waste and pollution controls.
The international approach to regulation with ENMs has been focused on:
i) development of an evidence base
ii) commissioning regulatory gap reviews to identify where there are gaps in current regulations
iii) the initiation of voluntary reporting and stewardship schemes for industry.
While these approaches address knowledge gaps, voluntary schemes such as benchmarking against devised industry standards attempt to ensure the responsible use of ENMs. Such codes, along with the precautionary principle, and their use with ENMs, are a matter of current debate.
It is known that many nanomaterials are considered to pose no risks, however, others do, and for many more the risks are simply unknown. Many researchers consider the current risk assessment procedures as inadequate and therefore continue to drive towards an ideal risk assessment of ENMs. There have been a number of well-reported studies that have identified knowledge gaps within this area, including Royal Society/Royal Academy of Engineering, Royal Commission on Environmental Pollution and Scientific Committee on Emerging and Newly-Identified Health Risks. In general, these studies have concluded that the toxicological frameworks were generally appropriate although missing eco-toxicological data and had widespread disagreement regarding the significant characteristics that must be included in the risk assessment of ENMs. While the significant characteristics required for the appropriate risk assessment of ENMs are undetermined, their applicability and the potential risks they pose to the environment remain uncharacterised, meaning there is a greater reliance on the responsibility of researchers, manufacturers and consumers to take appropriate steps to safeguard environmental health.
NOTE: Since this article was written additional guidance has been issued by the EC
Sophie Rocks is a Lecturer in nanotoxicology at Cranfield University.