Here's a picture from "Using Microfine Formulations of Inhaled Corticosteroids to Treat Asthma" (by SRxA Institute for Professional Education, Inc.) showing the difference that different particle sizes of corticosteroids makes when the meds enter the lung. Smaller particles penetrate further into the lung. Compare this toa picture of the different size of particles coming from different corticosteroid inhalers. There are is insufficient data to say that the meds with smaller particle sizes are clearly preferred because they produce significantly better results. The FDA and pharmaceutical industry appears to be headed in this direction, because the newer products tend to have smaller and more uniform particle sizes. Small airways are those 2 mm or smaller. More as data becomes available. "The Allergy Dude."