On the Formation and Physical Behaviour of Exhaled Particles

Doctoral thesis, 2011

Aerosol particles are generated in human airways and leave the body with exhaled air. The particles originate from the respiratory tract and contain non-volatile compounds that potentially may be used as biomarkers for various medical conditions. To utilise any information provided by the particles, they must be characterised. The work performed in this thesis focuses on measuring concentrations and size distributions of endogenous particles in exhaled air. The results give indications on how and where in the airways particles are formed. Number size distributions of exhaled particles, obtained from different individuals and breathing techniques, were measured. The results show that the inter-individual variation in number concentration is very high, while the size distribution peaks at about the same diameter for all subjects. The distribution peaks at a smaller particle diameter for tidal breathing than it does for breathing eaching airway closure. Moreover, deep exhalations result in significantly higher particle concentrations than tidal breathing. Exhalation followed by a few seconds breath hold amplifies the number of particles emitted in the subsequent exhalation, whereas breath hold after inhalation reduces the concentration. It was also found that a particle quickly shrinks when it leaves the saturated environment inside the human body for a surrounding with lower relative humidity. The results obtained support the theory that exhaled particles are created through a film rupture process. Following exhalation, airway closure and subsequent inhalation, fluid films are spanned across the airways. These films burst and form particles that leave the body with the next exhalation. It is suggested that tidal breathing activates the very smallest airways and produces the smallest particles. As exhalation depth increases, additional, larger airways close and larger particles are formed. Breath hold at low lung volume results in extensive airway closure and increased particle concentration, while breath hold at high lung volume causes deposition and loss of particles. It is further suggested that the particles are supersaturated liquid droplets shortly after leaving the human body.