Introduction to Nanoscience and Nanotechnology. Chris Binns

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Figure 2.2 Size distribution of urban aerosol. The concentration of airborne particles (aerosol) as a function of their diameter in a typical urban environment using three different measures. The lower curve shows the total volume of particles in cubic μm per cubic centimeter of air. The middle curve shows the total surface area of the particles in square μm per cubic centimeter of air. The upper curve gives simply the number of particles per cubic centimeter of air. It is evident that while the total volume (and also the mass) contained in the particles per cubic centimeter is concentrated in large particles outside of the nanoworld. In terms of particle numbers per unit volume, the distribution is dominated by nanoparticles with a typical size of 10–20 nm diameter.

      Source: Reproduced with the permission of Wiley from J. J. Seinfeld and S. N. Pandis [3].

A common process that produces nanoparticles in the atmosphere is gas‐to‐particle conversion (GPC). When a vapor is rapidly cooled, for example, in combustion where hot gases meet cool air, the atoms or molecules in the vapor condense to form particles. Alternatively, a vapor produced by some process may chemically react with the atmosphere to produce a less volatile compound, which then condenses into solid or liquid particles. Just about any combustion will generate a cloud of nanoparticles. For example, Figure 2.3 shows the particles generated by a “nightlite” type candle. Burning petrol and diesel also generate nanoparticles so in urban areas this is a major source. A particular issue is diesel engines, which generate most of their nanoparticles from sulfur contained in the fuel. A massive worldwide effort to reduce the level of sulfur in diesel and thus the number of nanoparticles produced has brought down the concentration from 5000 ppm before 1993 to 10 ppm today.

Figure 2.3 Nanoparticles produced by candles. The size distribution of particles produced by combustion from a standard “nightlite” type candle. The distribution was measured using an aerosol particle sizer and if plotted as the number density, consists mostly of nanoparticles with diameters around 10 nm. These are predominantly carbon particles. The method used to measure the size distribution (differential mobility analyzer) is described in Chapter 5, Section 5.1.7.

2.2 Atmospheric Nanoparticles and Health

      The effect on the body of exposure to nanoparticles is a hot topic in the debate on the benefits/hazards of nanotechnology, but there is limited hard information to inform the discussion. It is worth emphasizing that life on this planet evolved in a dense cloud of naturally occurring nanoparticles and we have been exposed to nanoparticles produced by human activity since the invention of fire. In addition, industrial activity based on certain types of nanoparticles, for example, carbon black, has been around since early civilizations. The difference now is that new nanotechnology industries will produce nanoparticles composed of a wide range of different materials as well as other forms of “nanomaterial” such as carbon nanotubes (see Chapter 3). Whichever way the nanoparticles enter the body, there is a highly developed immune response to clear them in the same way that any foreign invaders, including bacteria and viruses are removed. The clearance mechanisms are described in Chapter 8, Section 8.1.2, but the end result is that nanoparticles are passed out of the body via the feces or the urine. As we will see in Chapter 8, which describes medical applications of nanoparticles, keeping them in circulation for long enough to carry out their therapeutic or diagnostic function can be a problem. Here, the potential health risks of accidentally ingested nanoparticles will be discussed. There are three general surfaces in the body that act as interfaces, that is, the lungs, intestines, and skin and each will be considered.

      2.2.1 Entry Via the Lungs

The lungs have to deal with the biggest nanoparticle load as they are designed to exchange material directly with the environment. Within the lungs, a system of finer and finer tubes terminates at clusters of tiny air sacs called alveoli (Figure 2.4a), typically 200 μm (0.2 mm) across. There is a capacity for a huge number of these and an adult lung contains of the order of 500 million alveoli thus presenting an area of about 100 m² in adults (about half a tennis court) to the environment. The alveoli are encased in blood vessels and the barrier separating the flowing blood and air in the alveoli is about 300 nm, which is thin enough to allow gas molecules to diffuse through. Particles that enter the lungs, if they are smaller than a few microns can pass right through the tubular system and into the alveoli. Nanoparticles, with diameters less than 100 nm almost all end up in the alveoli.