In order to be used for inhalation administration, particles or droplets must be smaller than what size in diameter?

• A. 0.5 micrometer
• B. 5 micrometers
• C. 0.5 millimeter
• D. 5 millimeters

Answer: (B)

Inhaled therapies rely on particles that are small enough to reach the lungs. The practical upper limit for respirable particles is about 5 micrometers in aerodynamic diameter. Particles smaller than this can travel into the lower airways and potentially the alveoli, while larger particles tend to deposit in the upper nasal and throat regions and are not suitable for lung delivery. Extremely small particles (much less than 1 μm) may be exhaled and not deposit efficiently, so the key threshold used is 5 micrometers. Therefore, to be used for inhalation administration, particles must be smaller than 5 micrometers in diameter. The other sizes listed are too large to be effectively inhaled as aerosols, or are not the standard boundary used for determining inhalable particles.