Acute exposure to acid fog: Effects on mucociliary clearance

Submicrometric sulfuric acid (H₂SO₄) aerosol can affect mucociliary clearance without eliciting irritative symptoms or changes in pulmonary function. The effect of larger fog droplets containing H₂SO₄ on mucociliary clearance is unknown. We quantified mucociliary clearance from the trachea (n = 4) and small airways (n = 7) of young healthy male adults after an acute exposure to H₂SO₄ fog (MMAD = 10.3 μm; pH = 2.0; liquid water content = 481 ± 65 mg/m³; osmolarity = 30 mOsm). Acid fog (AF) or saline fog (SF) (10.9 μm; 492 ± 116 mg/m³; 30 mOsm) was administered for 40 min of unencumbered breathing (no mouth-piece) at rest and for 20 min of exercise sufficient to produce oronasal breathing. Fog exposures were followed by a methacholine (MCh) challenge (a measure of airway reactivity) or inhalation of technetium-99M radioaerosol (MMAD = 3.4 μm) on 2 study days each. Changes in symptoms and forced ventilatory function were also assessed. Clearance was quantified from computer-assisted analyses of gamma camera images of the lower respiratory tract in terms of %removal/min of the radiolabel from the trachea 25 min after inhalation and from the outer zone of the right lung after 1.9 to 3 h. Symptoms, forced ventilatory function, and MCh response were unaffected by either fog. Tracheal clearance was more rapid in four of four subjects after AF (0.83 ± 1.58% removal/min) compared with that after SF (-0.54 ± 0.85% removal/min). Outer zone clearance was more rapid in six of seven subjects after AF (0.22 ± 0.15% removal/min) compared with that after SF (0.01 ± 0.09% removal/min). The association between outer zone clearance rate and the estimated amount of hydrogen ion inhaled orally was significant (r(s) = 0.79, p = 0.05). The mechanism(s) underlying these changes in regional clearance is unknown.