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The SCENIHR opinion states:
3.5. Sources of free nanoparticles
Nanoparticles are formed through the natural or human mediated disintegration of larger structures or by controlled assembly processes. The associated processes occur either in the gas phase, in a plasma, in a vacuum phase or in the liquid phase, eventually followed by the intentional or unintentional transfer into one or more relevant fluid media and then to an individual receptor in an exposure setting.
Source & ©: SCENIHR 
3.5 Nanoparticles: Sources of Free Nanoparticles, p. 14
3.5.6 Conclusions
Nanoparticles are produced by natural phenomena, and many human industrial and domestic endeavours, such as cooking, material fabrication and transportation utilising internal combustion and jet engines, unintentionally release nanoparticles into the atmosphere. In recent years a new type of source of nanoparticle has been introduced, within the sphere of intentionally engineered nanoscale components of consumer products and advanced technologies. It is not yet clear just how significant is the increase in exposure to nanoparticles associated with these new products, either in the workplace or in the context of consumers of nanotechnology-based products.
Source & ©: SCENIHR
3.5.6 Conclusions, p. 17
3.3.3 The essentials of Nanostructure Generation: Top-Down vs. Bottom-up Chemical and Physical Self Assembly
Nanotechnology is dependent on nanostructures that require creation and characterization. Two fundamentally different approaches for the controlled generation of nanostructures have evolved. On one hand there is growth and self assembly, from the bottom up, involving single atoms and molecules. On the other hand there is the top- down approach in which the powerful techniques of lithography and etching start with large uniform pieces of material and generate the required nanostructures from them. Both methods have inherent advantages. Top down assembly methods are currently superior for the possibility of interconnection and integration, as in electronic circuitry.
Bottom-up assembly is very powerful in creating identical structures with atomic precision, such as the supramolecular functional entities in living organisms. In many different fields of nanoscale science, e.g. the production of semiconductor quantum dots for lasers, the production of nanoparticles by self organization, and the generation of vesicles from lipids, self organization is used for the generation of functional nanometre sized objects. To date, man made self organised structures (Niemeyer 2001) remain much simpler than nature’s complex self organised processes and structures.
Source & ©: SCENIHR
3.3.3 The essentials of Nanostructure Generation: Top-Down vs. Bottom-up Chemical and Physical Self Assembly, p. 12
The SCENIHR opinion states:
3.5.1 Formation of nanoparticles in the liquid phase
Defined bottom-up production of nanoparticles in the liquid phase with respect to particle size, chemical composition, surface and charge properties occurs mainly through controlled chemical reactions (Frens 1973), and self limiting self assembly processes have evolved by controlling growth conditions. In view of the ecological cycling of nanomaterials, some emphasis has to be given to the corrosion and disintegration of bulk materials, where little knowledge is currently available (Oberdörster G et al 2005]. Naturally occurring processes generating nanosized structures in the liquid phase include erosion and chemical disintegration of organic (plant or microorganism debris) or geological (e.g. clays) parent materials. In all these types of disintegration process, the surface properties and their change through chemical reaction are critical in determining whether individual nanoparticles will be formed in the respective medium (Boyle et al 2005).
Source & ©: SCENIHR
3.5.1 Formation of nanoparticles in the liquid phase, p.15
The SCENIHR opinion states:
3.5.2 Formation of nanoparticles suspended in the gas phase
The main route of bottom-up formation of nanoparticles in the gas-phase is by a chemical reaction leading to a non-volatile product, which undergoes homogeneous nucleation followed by condensation and growth. Recently, this has become an important pathway for the industrial production of nanoparticle powders, which may be of metals, oxides, semiconductors, polymers and various forms of carbon, and which may be in the form of spheres, wires, needles, tubes, platelets or other shapes. This is also the unintentional pathway by which nanoparticles are formed following the oxidation of gas-phase precursors in the atmosphere, in volcanic plumes, in natural and man-made combustion processes, or in fumes associated with any man-made process involving volatilizable material at elevated temperature, such as welding or smelting, polymer fabrication, or even cooking .
As with the liquid phase case, disintegration processes of parent materials provide a pathway which only leads to nanoparticles suspended in the gas phase under special conditions. While in the liquid phase the presence of emulsifying agents accompanying an erosion or chemical disintegration process could support the suspension process, the dispersion of nanoparticles into a gas from liquid emulsions or dry powders is severely limited by the strong adhesive forces between individual nanoparticles. Therefore, any mechanically induced stress on the parent material mostly leads to particles in the micrometer range and above. Only under accidental conditions, e.g. in the case of uncontrolled release of a powder or an emulsion from a highly pressurized vessel could strong shear forces overcome these adhesive forces (Reeks and Hall 2001). In contrast, the spraying of liquids containing nanoparticles or soluble material at very low concentrations, followed by drying of the solvent, can lead to the resuspension of nanoparticles or to the formation of new nanoparticles from the solutes. This can lead to redistribution of nanoparticles, biological material or toxic substances into nanoparticulate airborne form.
Source & ©: SCENIHR
3.5.2 Formation of nanoparticles suspended in the gas phase, p. 15
The SCENIHR opinion states:
3.5.3 Environmental Sources of airborne Nanoparticles
The amount of nanoparticles in the air can be surprisingly similar in urban and rural areas, with as much as 106 to 108 nanoparticles per litre of air depending on conditions. In rural areas, nanoparticles mostly originate from the oxidation of volatile compounds of biogenic or anthropogenic origin, including secondary organic aerosols. In urban areas, the primary sources of these particles are diesel engines (Schneider et al 2005) or cars with defective or cold catalytic converters (Zhiqiang et al 2000). Photo-oxidation processes also lead to significant numbers of nanoparticles in urban areas. Real-time measurements show that exhaust aerosol concentrations range between 104 to 106 particles.cm-3, with the majority of the particles by number being less than 50 nm in diameter. The highest particle number concentrations and smallest particle size are associated with high-speed road traffic, presumably due to the subtle conditions during concomitant cooling and dilution of the exhaust gases. Emission factors for gasoline vehicles ranged from 1.9 to 9.9x1014 particles.km-1 and 2.2x1015 to 1.1x1016 particles.kg-1 fuel (Kittelson et al 2003 a,b). The awareness that combustion processes significantly contribute to the nanoparticle load by number has been rising recently and has provided a new motivation for airborne particle research (Donaldson et al 2001a).
Source & ©: SCENIHR
3.5.3 Environmental Sources of airborne Nanoparticles, p. 15
Wieser and Gaegauf (2000) have evaluated different wood combustion systems with respect to emissions. Particle sizes were mainly in the range of 30 to 300 nm. Particles of less than 300 nm did not add much to the total particle count in flue gas. The particle distribution of manually operated appliances varied during a burn cycle, while continuous fed wood combustion systems show a fairly constant particle size distribution. Total particle numbers for automatically fed burners were smaller than with manual operation. Around 95% of the particles were smaller than 400 nm. The most frequent size of the particle number concentrations for batch operated appliances is approximately 110 nm, whereas the particle distribution changes significantly during the burn cycle.
3.5.4 Occupational Sources of airborne Nanoparticles
Inhaled nanoparticles may represent a potential health risk. Aerosols in workplace environments may be derived from a wide variety of sources, depending on the type of activity and processes taking place. Nanoparticle aerosols arising from mechanical processes (e.g. the breaking or fracture of solid or liquid material) are unlikely to be formed. Grinding and surface finishing typically releases micrometre and submicrometre particles, possibly down to 100 nm but rarely below this. Most plasma and laser deposition and aerosol processes are performed in evacuated or at least closed reaction chambers. Therefore exposure to nanoparticles is more likely to happen after the manufacturing process itself, except in those cases of failures during the processing (Luther 2004). In processes involving high pressure (e.g. supercritical fluid techniques), or with high energy mechanical forces, particle release could occur in the case of failure of sealing of the reactor or the mills. Nanoparticles exhibit increased diffusivity with decreasing size and therefore show delayed sedimentation in the earth’s gravitational field, which translates into potentially increased lifetimes for nanoparticulate impurities at low concentration. In the presence of larger microparticles, as with the wide size distribution in aerosols such as smoke, the highly diffusive character of nanoparticles may lead to faster agglomeration or impaction on the larger particles. Furthermore, many particles, including metallic particles, are highly pyrophoric and there is a considerable risk of dust explosions.
Source & ©: SCENIHR
3.5.3 Occupational Sources of airborne Nanoparticles, p. 16
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