What is the main factor in determining absorption of particulate matter in the lung?

• A. Particle size
• B. Drug color
• C. Drug polarity
• D. Solubility in lung fluids

Answer: (A)

Particle size controls where inhaled particles deposit in the respiratory tract, which largely sets their opportunity for absorption. Larger particles—typically above about 5–10 micrometers—impact the nose and upper airways and are cleared by mucociliary action rather than being absorbed. Intermediate-sized particles in the roughly 1–5 micrometer range can reach the bronchioles and alveolar regions, where absorption into the bloodstream is most feasible. Ultrafine particles may be exhaled or deposit by diffusion, but those that reach the alveolar surface have the best chance to dissolve and cross into systemic circulation. While solubility and polarity influence how quickly a deposited particle dissolves and transfers across the epithelium, they come into play after deposition; the primary determinant of absorption location is size.