ABSTRACT
The respiratory tract has a unique proximity to the environment. The same thinness and delicacy that qualify the air-blood barrier for the rapid exchange of oxygen and carbon dioxide reduce its effectiveness as a barrier to inhaled microorganisms, allergens, carcinogens, and a wide variety of toxic particles and noxious gases. The surface area of the more than 300 million alveoli is typically 100-150 m2. The thickness of the air-blood barrier-the distance between inspired air and circulating blood-is less than 1 μm. This is small compared to the barrier between blood and the external environment in the gut or in the skin. Clearly, any discussion of toxicology must include the respiratory tract as an important site for interactions between the internal and external environments.
