This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Anatomy
A drug is to be delivered by a nebulizer. The size of a droplet for its humidification is: 
Select and explain. 
A. <5 u, B. 10-May, C. 15-Oct, D. 15-20
A: <5 u
Answer: A: <5 u 
Explanation: 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