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Inhalation OCTOBER2016 31 Figure 3 From Module 3, Practice of APSD Testing Figure 4 From Module 4, Data Analysis carefully constructed images to illus- trate the concept of differing airway sizes in the human respiratory tract. This visual approach is intended to help users appreciate the importance of aerodynamic diameter for the fate of an inhaled particle. The second module, Theory of APSD Testing, explains the physical princi- ples behind inertial impaction and multi-stage cascade impactors. In the example shown in Figure 2, the nar- rator explains how the dimensions of the nozzles impact the effective cut- off diameters of stages in a cascade impactor. This module also provides an overview of impactor configura- tions and components commonly used in the pharmaceutical industry. The third module, Practice of APSD Testing, shows how the theory intro- duced in the second module is put into practice for aerodynamic particle size distribution (APSD) testing of orally inhaled products. Best practices are detailed for the users in a step-by- step fashion. Attention is paid to important practical details such as stage cleaning and mensuration. (See Figure 3.) The fourth module, Data Analysis, reviews the common metrics derived from cascade impactor measure- ments. A primary goal of this module is to prompt users to think critically about those metrics that are most use- ful in characterizing aerosols and how they should be calculated. Detailed examples, as shown in Figure 4, are used to underscore the importance of understanding how metrics such as mass median aerodynamic diameter (MMAD) are calculated. Where can the free tutorial be accessed? The tutorial modules are free and available to the public on the IPAC- RS website at the following web address: http://ipacrs.org/strategic- initiatives/cmc-product-develop ment-test/cascade-impaction-ci/cas cade-impaction-tutorial-modules. New modules will be made available online as they are completed. In the meantime, topics can be explored in greater depth through publications by the CI working group, which can be accessed through the IPAC-RS website. Interested parties may also contact the CI working group through the "Contact Us" button on the IPAC-RS website. We welcome feedback on the content and format of the tutorial as well as suggestions for future module topics. Adrian P. Goodey, PhD is a Principal Scientist at Merck & Co, Inc., Jolyon P. Mitchell, PhD, FRSC(UK), CChem, CSci is owner of Jolyon Mitchell Inhaler Consulting Services, Inc., Terrence P. Tougas, PhD is a Science Advisor for the IPAC-RS Secretariat, Dave Christo- pher, MSc, is Executive Director, Statis- tics, Merck & Co.and Svetlana A. Lya- pustina, PhD is a Senior Science Advi- sor for the IPAC-RS Secretariat. Corre- sponding author: Svetlana Lyapustina, PhD, 1500 K Street NW, Washington, DC, 20005, US, Tel: +1 202 230- 5607, Svetlana.Lyapustina@dbr.com. Website: www.ipacrs.org.