3. Particle Deposition.
For an inhaled particle to enter the lung it must be inhaled, pass through the upper and lower airways and enter the alveoli. Once there it may have either local or systemic adverse effects. The size, shape and mass of particles affects their deposition and hence their possible site of damage. Large particles bigger than 5-10 µm will usually not remain airborne for very long, or if they are inhaled will deposit in the nasal passages or other large airways. They are large enough and have sufficient mass to have momentum and so when they try to navigate their way in the turbulent flow around the nasal structures they will usually deposit by a process of impaction on one or other of the mucosal surfaces. Smaller particles, of around 1-5 µm, may pass the nose but will then usually deposit in the trachea or larger areas. These smaller particles have less momentum than the larger particles, but still have sufficient mass to be affected by gravity and so tend to settle onto mucosal surfaces by a process of sedimentation as airways branch and flow rates decrease. Particles of around 1 µm and smaller have little mass or momentum and may stay airborne for long periods. They are however very dynamic, constantly moving due to Brownian motion' and this process of diffusion inevitably leads to some particles depositing on lung surfaces.
While this simplified model is useful for estimating deposition of particles, it is not altogether accurate for allowing prediction of adverse effects. Most particles are not spherical and so the functional size of particles is usually quantified as mass median aerodynamic diameter' (MMAD), the particles equivalent size if it were a sphere, and this determines deposition. Particles may change in size as they pass through the respiratory tract due to evaporation or condensation of water. Further, particles which are not spherical may behave in complex ways.