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.