Size of particle for aerosol therapy is?

Reference: KDT, Essentials of Medical Pharmacology, 7th ed., Pg. 232. Efficacy of aerosolized drug depends on the particlesize: 1–5 μm diameter particles deposit on the bronchioles and effectively deliver the drug. Larger particles settle on the oropharynx, while very fine particles do not settle anywhere and are exhaled out. On an average only ~10% of the inhaled drug reaches the site of action.