ABSTRACT
Humans can live in a great variety of habitats. Some may be natural but possess a polluted atmosphere, while others are transiently extreme, such as in the air, where airline passen gers breathe pressurized air, under water, where scuba divers breathe compressed gases, or in an intensive care unit, where patients on assisted ventilation breathe oxygen-enriched air. Whatever the challenge, the respiratory system is amazingly adaptable and efficient in oxygenating blood, eliminating waste gases, and protecting itself at various environmental extremes. This task of eliminating or minimizing adverse effects from inhaled airborne particles or toxic gases is a breath-to-breath effort involving components spaced along the entire respiratory tract from the portals of air entry to the alveolar epithelial surface (Fig. 1). Pollutants are not always external but can be present in nasooropharyngeal secre tions that may be aspirated into the airways or as microbial toxins carried in blood that perfuses the lung’s vasculature. Nonetheless, injury to the lung from airborne toxins or from proliferation of bacteria on the mucosal surface of the airways is surprisingly infre quent for healthy people. Exposure to microbes, however, is incessant.
