Droplets Produced by Medical Nebulizers: Discussion (Part 3)
The nebulizer solution equilibrates to a lower steady temperature Ts when the dry gas from a compressed gas cylinder is used to generate the aerosol, compared with the air compressors. The droplet solute concentration thus increases to a greater extent (relative to the value at ambient temperature) as a result (Table 3).
The ambient air inhaled along with the aerosol that makes up the inspiratory flow is likely to have a lower relative humidity than that corresponding to the nebulizer solution; therefore, it has the effect of drying the aerosol droplets as it mixes before inhalation. This concentrating effect is quite marked, especially when the generation flow and the output of solution from the nebulizer is low. The consequence of the low output is that there is only a small volume of water present in the droplets to resaturate the aerosol stream (see results for Up-Draft/Flatus and especially the UpDraft/Aerosol-One systems, Fig 5 and Table 1). The ambient relative humidity was comparatively high during these experiments (65 to 75 percent), and a lower ambient relative humidity, such as that found in air-conditioned rooms, would be expected to greatly enhance these effects.
The ultrasonic nebulizer has a higher output and larger droplet size, so more water is available for saturation of the dilution air and the concentration change of the droplet solution hence is much smaller than that of the jet nebulizers. The droplet concentration therefore starts off greater than isotonic, but as the tubing becomes saturated, this is able to supply the vapor necessary to saturate the dilution air. Although the nebulizer solution temperature is increasing, the droplets do not become hypotonic due to water vapor condensing on them as the aerosol stream cools; rather, the excess vapor will condense on the walls of the conducting tubing.