The great majority of inhaled particles between 5-10 µm diameter are deposited on the tracheobronchial surface. Deposition at the alveolar level occurs mainly for particles between 0.5-5 µm. Approximately 50% of 0.5 µm particles are retained in the alveoli, the remainder are exhaled.
One lesson to be learned from this is that the nose, the pharynx and lower airways are very effective in clearing the air of suspended particles, so that the volume of large particles to be cleared by the alveoli is minimal. Short-circuiting the nose by mouth breathing, such as during exercise, will increase the particle load to the intrathoracic airways. Particles deposited in intrathoracic airways are removed by mucociliary activity, or by phagocytosis. Non-inert substances may interact with their host and elicit a response. The nature of the response to the non-inert substance is reviewed in the section on airway response (pre- and postjunctional stimulation). Even if the particle is inert, it may be the carrier for aggressive substances which adhere to its surface.
Exposure to flowering grass and their pollen.
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