The respiratory system is a complex of epithelial tissues, immune cells, and lung parenchyma. Due to the potentially long exposure periods, permeability and sensitive nature of airway regions, the tissues spanning the nasal and oropharynx to the deep lung are exceptionally susceptible to insult and damage by toxicants. Manifestations can range from asymptomatic or mild upregulation of immune mediators/cytokines and decreased cellular function to fulminant disease states (such as asthma, COPD and fibrosis) to complete system failure. As a result, knowledge of how the body responds to an inhaled material is critical toxicological information.