What follows relates to airborne radioactive particles. Where inhaled particles are likely to deposit in the respiratory tract depends on the aerodynamic properties of the particles and the geometry of the respiratory tract of the person of interest.

A key aerodynamic property is the aerodynamic size. For airborne particles, the aerodynamic size depends on the shape and density of the particle. The shapes and densities of airborne radioactive particles depend on the exposure scenario considered.

The aerodynamic size is different from the actual size and accounts for the aerodynamic behavior of an aerosol. Usually aerodynamic equivalent diameter (also called aerodynamic diameter) is used to indicate the aerodynamic size.

The aerodynamic equivalent diameter is the diameter of a sphere, with density = 1 g/cm³, that has the same terminal settling velocity under gravity as the airborne particle considered. Computer programs have been developed that use aerodynamic diameter to calculate expected particle deposition in the respiratory tract.

There are different ways by which airborne particles can deposit in the respiratory tract. The five most important ways are sedimentation, impaction, Brownian diffusion, interception, and electrostatic precipitation. The five ways of deposition are summarized in Figure 3 and are discussed in the sections that follow.

3.1 Sedimentation. Sedimentation represents deposition caused by gravity. The chance of particle deposition in the respiratory tract by sedimentation increases as the particle size, particle density, and length of time (residence time) spent in the airway increase. Airway residence time increases as the breathing rate slows.

Respiratory tract deposition by sedimentation is important for particles with an aerodynamic diameter greater than 0.5 µm that reach the medium-sized to small bronchi and bronchioles, where air velocity is relatively low.

3.2 Impaction. When the aerodynamic diameter is larger than 1 µm, inhaled particles can deposit in the nose, pharynx, and mouth. Deposition by impaction can occur when the particle momentum is too large for it to change directions in an area where there is a rapid change in the direction of the bulk airflow. The chance of impaction increases as the air velocity, particle size, and particle density increase. Air velocity increases as the breathing rate increases. The breathing rate increases as the level of physical activity increases.

3.3 Brownian Diffusion. For particles with an aerodynamic diameter less than 1 µm, Brownian diffusion is a major way for deposition in airways where the bulk flow is very low or absent (e.g., bronchioles and alveoli). With Brownian diffusion, airborne particles acquire a random motion because of their bombardment by surrounding air molecules; this motion can result in particle contact with an airway wall.

The displacement sustained by a particle depends on a parameter called the diffusion coefficient, which increases as the particle size decreases. Deposition by Brownian diffusion is especially important for particles with aerodynamic diameters less than 0.2 µm. Particles in this size range may also deposit by diffusion in the upper respiratory tract, trachea, and larger bronchi. Deposition in the respiratory tract by Brownian diffusion is unimportant for relatively large particles.

3.4 Interception. Particle deposition in the respiratory tract can occur when the edge of the particle contacts the airway wall. For elongated particles (e.g., fibers), interception is an important respiratory tract deposition mechanism. The chance of particle interception increases as the airway diameter becomes smaller.

3.5 Electrically Charged Particles. Inhaled particles can be electrically charged. If so, then they can exhibit greater regional deposition over what would be expected based on their size, shape, and density.