Effect of Humidity on Constant Output and Breath Enhanced Nebulizer Designs When Tested in the EN 13544-1 EC Standard

Aqueous aerosols produced by nebulizers used in clinical situations can rapidly change size as the patient inhales. This is caused by air with a relative humidity (RH) lower than inside the nebulizer being entrained and mixed with nebulized aerosol during the inhalation maneuver. A way to assess the change in size is to measure the aerosol in a test method that reflects the clinical situation. The EC standard, EN 13544-1, offers a first step towards this assessment. In this paper we have tested two nebulizer designs, one conventional constant output nebulizer and one breath-enhanced nebulizer, using the proposed standard in order to assess the effect of the relative humidity of entrained ambient air on nebulized aerosol size properties. The results indicate that aerosol size from the conventional nebulizer is greatly affected by the RH of the entrained air, while the breath-enhanced nebulizer is not affected. The results agree with theoretical expectations of how the entrained air interacts with nebulized aerosol. In the breath-enhanced nebulizer, the air is passed through the nebulizer interior and becomes saturated with moisture drawn from the relatively large nebulizer reservoir solution. With the conventional constant output nebulizer, ambient air is drawn over the nebulizer and draws its moisture from the relatively small volume of nebulized aerosol released from the nebulizer. For the conventional nebulizer design, we found a large decrease in measured nebulized aerosol size with decreasing relative humidity--3.1 w m MMAD at 75% RH fell to 1.9 w m MMAD at 20% RH. For the breath-enhanced nebulizer design, the MMAD was stable between a similar humidity range. The results indicate that aerosol size is dependent on relative humidity of the entrained air for the constant output jet nebulizer design that has no air entrainment through the nebulizer. We found no significant effect of ambient humidity of entrained air on nebulized aerosol size from the breath-enhanced nebulizer design.