[Deposition of particles in the respiratory system]

Aerosols consist of two phases: a gas phase and particle phase, either liquid or solid. Mathematical modeling of the three principal processes (impaction, sedimentation and diffusion) which determine the amounts of deposition of particles at various levels of the respiratory system is in agreement with the majority of experimental data, the upper airways behaving as a very effective filter for particles with an aerodynamic diameter greater than 2 micron. Below such a diameter, particles are capable of being deposited at all levels along the tracheo-bronchial system or in the terminal respirator units. This scale of micronic aerodynamic diameters furthermore corresponds to atmospheric aerosols with a long duration (monodispersed aerosols). Such findings have obvious consequences both in terms of the physiopathology of respiratory disorders as well as the therapeutic or diagnostic use of aerosols. In order to better define the deposition of infra-micronic particles, we used the Liu and Whitby analyser for measurement of particle concentration for diameters of between 0.0042 micron and 0.75 micron. The aerosol inhaled was ordinary atmospheric air with a stable profile. The respiratory tract of the subject was free of any particles after 5 respiratory cycles through a filter. Overall, expired particle concentrations decreased exponentially with the increase in apnea at the end of inspiration. This decrease was all the more marked when pulmonary capacity was smaller. These findings were apparently related to the increased probability of deposition by diffusion as the dimension of the air spaces decreases.