A drug is to be delivered by a nebulizer. The size of a droplet for its humidification is:

Because of their settling velocities in air, paicles >10-15 mm in diameter do not penetrate beyond the nose and throat. Paicles <10 mm in size are deposited below the larynx. These paicles are divided into three size fractions on the basis of their size characteristics and sources. Paicles ~2.5-10 mm (coarse-mode fraction) contain crustal elements such as silica, aluminum, and iron. These paicles mostly deposit relatively high in the tracheobronchial tree. Ref Harrison 19th edition pg 1688