The respiratory system serves as the interface for an organism with its gaseous environment. As a consequence, the respiratory system is the target for airborne pathogens, especially viruses and bacteria and inhaled toxic gases, including air pollutants such as ozone and environmental tobacco smoke. Because the respiratory system also receives the entire output from the right side of the heart, the respiratory system is a target for blood-borne toxicants and pathogens and a site of sequestration for metastasizing tumors