An apparatus that simulates a respiratory system for characterizing particle delivery within lungs by an inhaler within lungs is shown and described. The respiratory system simulator comprises a simulated oral cavity to receive an aerosol, a simulated oropharynx cavity, a simulated lung airway system, and a breath simulator. The simulated oral cavity is configured to receive a flow of particles and direct the flow to the simulated oropharynx cavity. The simulated oropharynx cavity is configured to receive a flow from the simulated oral cavity and direct the flow to a simulated trachea cavity. The simulated trachea cavity directs the flow to the simulated lung airway system. The simulated lung airway system comprises a plurality of bronchial airway generations simulating bronchial airway generations of a lung or lungs. The respiratory system simulator may be maintained at or near humidity and temperature levels within a respiratory system. The breath simulator interface is in fluid communication with the rest of the respiratory system simulator and configured to control a flow through the simulated oropharynx cavity, the simulated trachea airway cavity, and the simulated lung airway system.