Several lines of thought help to convince us that a low EBC pH reflects low pH of the fluid lining the airways. It is expected that acidification of the airway at any level will allow protonation, volatilization, and exhalation of acids that would otherwise be trapped in the airway as conjugate anionic bases. Exhaled water-soluble acids are then trapped in EBC and lower its pH. In separate studies, EBC pH from healthy subjects has been found to be very similar to tracheobronchial secretion pH and the airway lining fluid pH when measured by pH probe inva-sively. The airway has been directly measured to have a profoundly low pH in the presence of laryngopharyngeal acid reflux, revealing a ready ability to aspirate acid into the lungs and drop pH of the tracheal lining fluid to as low as 2. Our study population ages are skewed toward the younger decades, which is similar to the skewing of the healthy population near our medical center, which is overwhelmingly dominated by the students of the University of Virginia. health and care mall
The ability to collect data from this large study population was enhanced by the brevity of the subject time commitment; for this purpose, the only enrollment exclusion criteria were based on a brief health questionnaire. Our study was limited because we were not able to obtain data regarding spirometry, bronchial hyperreactivity, allergy skin tests, chest radiographs, exercise tolerance, or esophageal pH probes/manometry to help ensure lack of disease in those who were not excluded by the questionnaire. The wider interquartile range and lower 5% cutoff in the young age study groups may reflect a true difference in their normal range, or speculatively may result from artifacts from oral ingestion or acid reflux not as well reported by these subjects.
Gas standardization by bringing EBC into equilibrium with a CO2-free gas has been employed in many of the studies of EBC pH, but not all. Technical validation has been primarily performed using gas standardization, however. The gas-standardized pH of EBC is a very simple assay of an easy-to-collect fluid. It is immune to storage, transport, timing of assay, and other characteristics that could otherwise make data collection difficult. EBC pH is not affected by the age of the subject.