In healthy individuals, gastric acid aspiration is common and usually does not present with symptoms. More common when sleeping, about half of all adults aspirate at nighttime. The volume of aspiration and underlying medical comorbidities determine the extent of symptoms related to aspiration.

Symptomatic aspiration occurs when there is (1) a compromise in the airway defenses (eg, cough reflex, clearing mechanisms), and (2) a substance and amount aspirated is directly toxic to the lower airways. Chemical pneumonitis (sometimes called Mendelson syndrome) is caused by the regurgitation of stomach contents and aspiration resulting in inflammation of the lung. This syndrome is typically confused with bacterial aspiration pneumonia. In aspiration pneumonitis, there is rapid development of symptoms resulting from pulmonary inflammation with fluid accumulation in the alveolar spaces. Aspiration pneumonia, on the other hand, is caused by bacteria from the oral and nasal pharynx and generally is more indolent in presentation. For both conditions, symptoms range from mild shortness of breath to sudden onset of tachypnea and hypoxia.