Nanoparticle risk management

by Michael Smith
NBS Information Specialist

A new technology revolution is under way, based on nanotechnologies. The very small size of engineered nanoparticles, less than 100 nanometres (nm) in diameter, confers unique properties not found in larger products composed of the same chemicals. Most universities and research teams are already working on the design of new applications. Many companies are in the early phase of operation, or have already incorporated nanoparticles into processes aimed at improving their products' performance.

It is widely expected that the trend will be accentuated in the coming years. Internationally, in 2007, more than 500 nanoparticle technologies were commercially available; the future world market for these will grow exponentially, causing major impacts in every field of economic and social activity.

The aim of this article is to overview the risks associated with nanoparticle production, highlighted in guidance given in a recent report, Best practices guide to synthetic nanoparticle risk management, issued by the Canadian Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail (IRSST). While this is not a UK report, it is one of the first to seriously investigate the issues around health and nanoparticle processing.

Purpose of nanotechnology

The nanotechnology field is developing extremely rapidly and the applications currently envisioned should allow spinoffs in every industrial sector. Nanoparticles (NP) radically transform the properties of different finished products: increased strength, better electrical conductivity, unique optical properties, better resistance, etc. These unique NP properties are not found in larger-scale substances with the same chemical composition.

By 2012, it is forecast that annual worldwide sales of nanoparticle products will exceed $1,000 billion. With such potentially lucrative rewards, all industrialised countries have ambitions of capturing some of the market share, with many producing complex development plans in this field.

Types of nanoparticles

Nanoparticles can be classified in various ways; one way is to classify them into two categories: particles that only exist in nanometre dimensions, and particles that also exist in larger scales but are produced as nanoparticles to take advantage of their unique properties on this scale.

Carbon nanotubes, fullerenes, quantum dots and dendrimers are the main particles that exist solely in nanometre dimensions; however, many inorganic products (e.g. metals, metal oxides, ceramics) and organic products (e.g. PVC, latex) can be synthesised in these sizes also. It has been proven, in fact, that nearly every solid product can be reduced to nanometre dimensions, though not all would necessarily exhibit commercially interesting properties.

Health effects

The greatest absorption of dusts in the work environment normally occurs through breathing. The leading particularity of nanoparticles is their pulmonary deposition mode; the deposit site is highly dependent on the particle size. Whereas particles of a few nanometres diameter will be deposited mainly in the nose and throat, more than 50% of nanoparticles of 15-20 nm will be deposited at the alveolar level, in the lungs.

Because of their extremely small size, nanoparticles can pass through organs while remaining solid. This involves translocation through the pulmonary epithelial layers to the blood and lymph systems and through the olfactory nerve endings and along neuronal pathways to the brain. Nanoparticles reaching the blood system circulate throughout the body and there is clear evidence that they can be retained by various organs. Several toxic effects have been documented for different organs, dependent on the nature of the nanoparticles.

Skin absorption is another accepted exposure route for workers handling nanoparticles, particularly those prepared and used in solution, since these can also end up in the circulatory system. Absorption can be further facilitated by skin damage or when exposure conditions in the work environment (e.g. humidity) are conducive to it.

Insoluble nanoparticles can thus end up in the blood, bypassing the body's protection mechanisms and then spreading throughout the body, including the brain.

More importantly, nanoparticles show a propensity to pass through cell barriers. Once they have penetrated cells, they interact with the sub-cellular structures, leading to induction of oxidative stress as the main damage mechanism.

Toxicity of microscopic particles is usually correlated to the mass of the toxic substance. However, the situation is totally different in the case of nanoparticles. Studies show that toxicity for a specific substance varies substantially according to particle size for the same nanoparticle mass. The most significant parameters for nanoparticle toxicity seem to be chemical composition, specific surface area and the number and size of particles.

The main parameters most often reported that are capable of influencing nanoparticle toxicity are:

• Specific surface area
• Number of particles
• Size and granulometric distribution
• Concentration
• Chemical composition
• Surface properties
• Zeta charge/potential, reactivity
• Functional groupings
• Presence of metals/redox potential
• Potential to generate free radicals
• Surface coverage.

Other reported parameters capable of influencing nanoparticle toxicity are:

• Solubility
• Shape and porosity
• Degree of agglomeration/aggregation
• Biopersistence
• Crystalline structure
• Hydrophilicity/hydrophobicity
• Pulmonary deposition site
• Age of particles
• Producer, process and source of the material used.
   

Safety risks

Explosion
An explosive dust cloud can be formed from organic or metallic materials and compounds. One of the main factors influencing the ignition energy and violence of an explosion is particle size or area. Many nanoparticles meet these criteria because of their chemical composition and their very small size.

The special characteristics of nanoparticles (type, chemical and surface composition, size, combustibility, etc.) and the environmental conditions (temperature, humidity, pressure) influence the exposable range. Several organics and metals, including aluminium, magnesium, zirconium and lithium, and also some inorganic substances, are particularly high-risk.

Fire
While there is little specific information available on the fire potential of nanoparticles, it is possible to rely on general knowledge concerning larger-sized particles or substances. The risks of encountering favourable conditions are higher in the presence of an ignition source. A fire raging in a room containing a sufficient quantity of nanoparticles can trigger a deflagration, and furthermore can provoke various effects on workers' health, such as asphyxia and serious burns, in addition to equipment damage.

Catalytic reaction
Given their large surface area, nanoparticles may also have catalytic potential that can translate into an uncontrolled reaction.

Catalytic reactions will depend on nanoparticle composition and structure. Consequently, because of their small sizes, they could initiate an unanticipated catalytic reaction, increasing the fire potential. Nanoparticle leaks and spills can contribute to the formation of deflagrations followed by an explosion or fire within a system component, depending on the type and quantity of particles released and the ambient conditions. This could further expose workers by inhalation or skin contact.

Other risks
Other risks are also likely to be linked to the instability or chemical reactivity of nanoparticles, which could be incompatible and create a dangerous reaction when they come into direct contact with other products. This could trigger a reaction with energy release, or be corrosive and cause damage to the contact site. The processes involved in the synthesis of nanoparticles could also represent specific risks that must be taken into account.

Occupational exposures can also occur when there is little or no ventilation, or during cleaning with an inappropriate method, for example compressed air.

Environment risks

Synthetic nanoparticles are likely to be present in the environment due to factory releases (e.g. air, wastewater, solid wastes), through leaks or spills during transportation, and during materials use, destruction or degradation.

Once in the environment, nanoparticles can interact with other particles present, be transformed, and differ in size and composition from their point of origin. They then will be dispersed in the different media (water, air, soil) affecting them and the living organisms in them. In general, the environmental effects of synthetic nanoparticles are little known. While studies of ultrafine particles of dimensions similar to nanoparticles have been studied for a long time, the assessment of the consequences for the environment are not possible to infer from these. Because of their very small size, nanoparticles are extremely mobile in the environment and have a strong tendency to aggregate and agglomerate. In air, water and soil, they can contaminate flora and fauna, ending up in the human food chain.

It is difficult to document the route and quantity of nanoparticles in the environment, because to date no effective methods exist for monitoring and measuring them specifically.

Risk analysis

Because of their diversity of reaction, use and consequent risk, a case by case approach is preferred for risk analysis. In the absence of nanoparticle-specific data, it is initially possible to estimate the risks based on those known for the same larger-scaled substance.

The overall risk assessment approach is summarised in the following section. The same methodology is also applicable to the calculation of environmental risk.

Risk analysis methodology

The first step of the risk assessment is to gather all the available written information allowing identification of the health and safety risk factors in the workplace; the Best practices guide to synthetic nanoparticle risk management, that this review is based upon, outlines the guidelines relating to risk factors that can lead to accidents, fires or explosions. Although this form of quantitative assessment must be performed case by case, the main obstacle currently is the lack of specific data available for nanoparticles, particularly in terms of their dust potential.

After gathering and interpreting all the available information on nanoparticle toxicity and on the occupational exposure conditions prevailing in the work environment, it should be possible to estimate the toxic risk. However, to be able to implement safe, realistic means of control in relation to the risk, a new approach, developed in Great Britain, is becoming increasingly widespread. Control Banding has already been successfully applied in various workplaces, but the Paik research team (2008) are the first to propose its use in relation to nanoparticles. Control Banding makes it possible to take into account all the available information (toxicity, exposure level) and to develop logical hypotheses on the missing information.

Report conclusions

The nanotechnology field is rapidly expanding and the number of workers potentially exposed to nanoparticles is constantly increasing, including exposure to those involving dangers of fire or explosion or dangers to workers' health. The synthesis and production of these new materials currently raise many questions and generate concerns, due to the fragmentary scientific knowledge of their health and safety risks. In general, nanoparticles are more toxic than equivalent larger-scale chemical substances. Their distribution in the organism is differentiated and it is not currently possible to anticipate all the effects of their presence.

In this context, quantitative risk assessment is all but impossible; however Control Banding offers an alternative in determining some minimum prevention measures.

Related NBS information:

Articles:

• Nanotechnology in construction

Written October 2009

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