How are inhaled drug particles classified by size and where do they reach?

• A. Large: >5 μm stay nasopharyngeal region; Medium: 2-4 μm reach bronchi; Small: ≤1 μm reach alveoli
• B. Small: ≤1 μm reach alveoli; Large: >5 μm reach alveoli; Medium: 2-4 μm reach nasopharyngeal
• C. Large: >5 μm reach alveoli; Medium: 2-4 μm reach nasopharyngeal; Small: ≤1 μm stay bronchi
• D. Large: >5 μm stay bronchi; Medium: 2-4 μm stay nasopharyngeal; Small: ≤1 μm reach alveoli

Answer: (A)

Particle size determines where inhaled drug particles deposit in the respiratory tract because different aerodynamic forces govern how they move with the airstream. Larger particles have more inertia and tend to impact the airway walls near the entrance, so they stay in the nasopharyngeal region. Mid-sized particles can bypass the upper throat but are still subject to impaction in the larger airways, so they tend to deposit in the bronchi. Very small particles are carried deeper into the lungs and deposit mainly in the alveolar region, largely by diffusion.

Thus, large particles (>5 μm) stay in the nasopharyngeal region, medium particles (~2–4 μm) reach the bronchi, and small particles (≤1 μm) reach the alveoli. This aligns with the inhalation deposition pattern used to tailor particle sizes for targeted delivery in therapies.