Relation between humidity and size of exhaled particles.

Background: Aerosol particles are generated in human airways, and leave the body with exhaled air. These particles may carry indicators of various lung conditions. To fully utilize the information provided by endogenously produced exhaled particles, it is important to understand their formation mechanism and physical properties. The scope of this work was to measure number size distributions of exhaled aerosol particles at various surrounding relative humidities (RH) in order to gain some knowledge of the size distribution at the point of particle generation.

Methods: Number size distributions of exhaled particles were measured at various RHs, using an optical particle counter. Breathing with airway closure was employed.

Results: A relation between particle volume and RH was fitted to experimental data and used to predict how exhaled droplets behave at RHs not easily accessible by experiments. The diameter of an exhaled particle is reduced by a factor of 0.42 when the RH is changed from 99.5 to 75% at 309 K. Calculations also show that the droplets are concentrated solutions near saturation at 75% RH.

Conclusions: It is concluded that the particles are supersaturated liquid particles, rather than crystalline solids, in ambient air with RH below 75%. A size distribution related to the aerosol at the moment of formation is given. A successful detailed formation mechanism should be able to accommodate the size distribution predicted at 99.5% RH.