Comparison of EPAG and Respironics Next Generation Impactor Test Protocols using Cooled and Ambient Conditions

Summary A recent EPAG study 1 reported a significant difference in MMAD between a cooled and ambient Next Generation Impactor (NGI) when air was entrained directly in to the impactor without first passing through the nebulizer. Stapleton and Finlay 2 calculated that air is saturated at the exit of a nebulizer mouthpiece. We therefore included a seal between nebulizer and impactor to ensure all air passed through the nebulizer prior to entering the impactor. Results show the inclusion of a seal produces a less significant difference in MMAD between cooled and ambient conditions. We conclude the air exiting the mouthpiece of a nebulizer is substantially saturated under ambient conditions and droplet downsizing in the impactor itself is potentially not as great as reported when air is entrained after the mouthpiece.