A thin fluid layer that covers and protects the lung’s airways, from the large airways down to the alveoli.

The ELF is made of both a liquid and a mucus layer and forms a barrier against the external environment, therefore protecting the airway mucosa from irritants and pathogens by promoting mucociliary clearance. the ELF is obtained by an invasive procedure called bronchoalveolar lavage. The concentration of antibiotics can be measured in the ELF and is thought to reflect the antibiotic concentration at the site of infection. This information is key to determining which antibiotics can be used to treat pulmonary infections caused by extracellular bacterial pathogens.