Lung Generated Aerosols

Introduction
In addition to coughing and sneezing, the ‘silent’ tidal breathing can also generate aerosols. This might play a more important role in disease transmission, especially in intensive care and emergency care units. The main objectives of the present study were to establish and optimize an exhaled breath aerosol monitoring system, and then using this system to characterize the aerosol emission from human lung.

Methods
An Exhaled Breath Aerosol (EBA) Monitoring system was composed of a pneumotachograph, a condensation particle and an aerosol-free chamber with flow rate over 200 L/min. The subject(s) were asked to respire though a mouth piece and a pneumotachograph connected the aerosol-free chamber. A particle counter was used to monitor the EBA concentration. A nose clip was used to force the respiration through mouth only. 

Results
The results supported the hypothesis of ‘Bronchiole Fluid Film Burst’ through a breath-holding technique. The EBA count per breath increased with increasing tidal volume. However, breathing frequency did not affect EBA generation. The between-subject variation is much higher than the within-subject variation, indicating that the mucus properties and the respiratory tract structure vary more significantly among subjects. The EBA of all tested subjects showed similar size distribution. The mode is around 0.3 micron, with up to 90% in the sub-micro-meter-sized range. 

Discussion
The EBA per breath was not affected by the breathing frequency, indicating that the number of closing-up and re-opening terminal bronchioles increased with increasing tidal volume. The functional residual capacity might also be an important factor, but it is difficult to request subject to maneuver with a fixed FRC. When subjects were sick, they tended to generate more exhaled breath aerosols, showing 20 to 50% increase in count per breath, a bad news to health-care workers.