My humble answer:
Consider the following:
1. Greater than 10 um deposits in the nose
2. Greater than 5 um deposits in the mouth
3. 5-10 um deposits in the first six generations of bronchi (large airways)
4. 1-5 um deposits in the last 5-6 generations
5. 0.8-2 um are optimal for alveolar deposition
For aerosolized particles to have a therapeutic effect they must make it past the oralpharyngeal airway, with particles 6 um or larger landing in the oral airway.
To be able to reach the lungs the particle size must be between 0.5 and 5 um. Greater than 3 um has a tendency to impact in the conducting airways, with 5 um unable to make it to the peripheral air passages. Less than 2 um will reach the alveolar regions and be too small to be therapeutic.
Particles less than 1um have a tendency to be exhaled.
Therefore, "In order for drugs to reach their target receptors, they must penetrate the mucous layer and
airway mucosa. Ultimately, the greatest effect of lung dose, the mass of drug delivered to the
lung, is dependent on the rate of drug clearance from the airway and the medication site ofaction."
Source: Elliot, Deborah, Patrick Dunne, "A Guide to Aerosolized Drug Delivery," American Association of Respiratory Care, page 9