A one millimeter side bulk material cube offers 6 mm2 of exchange surface. In cutting it out following 3 axis, the exchange surface doubles at every cutting operation. After 16 operations, the exchange surface area is 6.6.104 times greater. For a same volume, the "reactivity" of a material, therefore, depends significantly on its granulometry. Moreover, the visible color of certain materials is changed depending on the size of the particles.

Size, movement and density of air molecules.

The air also contains nanoparticles 102 to 103 times larger than the molecules. What is the density of nanoparticles? Is this number stable? Can we distinguish the origin of nanoparticles? What can we say about their behavior? How can we specifically detect them?

The diffusion is characterized by a diffusion coefficient (expressed in m2/s) inversely proportional to the square of the particle diameter. The smaller the particle is, the higher its mean displacement and speed will be. But this diffusion method can not move the particles over distances up to a fraction of a millimeter.

Speed and mean displacement of nanoparticles (10, 30 and 50 nm) under the influence of thermal diffusion.

Illustration of the movements of 3 particles (10, 30 and 50 nm) subject only to thermal diffusion, then to a convection movement and to thermal diffusion.

Working atmospheres are generally subject to ventilation, movement of people, extreme temperatures. In a working space, the average air velocity is about 0,3 m.s-1 [P. Gorner et al., Annals of Occupational Hygiene 54 (2010) 165-187]. The coefficient of diffusion equivalent Deq being at least a thousand times larger than the thermal diffusion coefficient.

Visualization of the mechanism of coagulation kinetics of nanoparticles (10, 30 and 50 nm).

The more the size of the aggregate increases, the less sensitive to the thermal diffusion it is: speed and mean displacement decrease. Beyond 5x102 nm, the force of gravity is no longer negligible: an agglomerate of one micron could be deposited on the ground in a few tenths of seconds.

The probability of deposit depends on the thermal agitation, but also on the condition of the wall.

The temperature differential and especially the wall electric charge are favorable parameters to the deposit.

Deposit on a nanoparticles wall (10, 30, 50 nm): (1) only subject to thermal diffusion, (2) attracted by a cold wall, (3) attracted by an electrostatically charged wall.

In a stream of air, thermal diffusion of nanoparticles promote their capture by the filter.

Important speeds (from 10 to 100 m/sec) and average paths of nanoparticles are responsible for trapping by the filter. The probability of contact between a nanoparticle and a fiber of the filter is very high: paradoxically, especially as the particle is small.

Depending on the particle size, the location and the percentage of deposit of the particles vary during an inspiration/expiration cycle.

The very small particles (1 nm) do not settle in the alveolar area but are stopped in the upper airways. The alveolar fraction deposited is the most important at around 20 nm.

The deposit location determines the future of nanoparticles in the organism.