1. A surface-active agent, including substances commonly referred to as wetting agents, surface tension depressants, detergents, dispersing agents, emulsifiers, and quaternary ammonium antiseptics.
2. Those surface-active agents forming a monomolecular layer over pulmonary alveolar surfaces; lipoproteins, lecithins, and sphingomyelins that stabilize alveolar volume by reducing surface tension and altering the relationship between surface tension and surface area.

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