Gases and vapors differ in that, at the temperature of interest, gases do not condense, whereas vapors can be in equilibrium with their liquid state. This difference, however, has little impact on the behavior of these substances in the respiratory tract.

Gaseous forms are dispersed as individual atoms or molecules, most often in the presence of other gaseous mixtures such as air and water vapor. Because gases are molecularly dispersed, they are not subject to gravitational and inertial forces to the same degree as particles. Thus, the gases do not deposit on surfaces by inertia and sedimentation. Gas molecules undergo kinetic motion and can deposit on surfaces of the respiratory tract by passive diffusion.

To evaluate potential harm to the respiratory tract from inhaled radioactive gases and vapors, one must first estimate uptake of the gases and vapors in the epithelial surface of the airways. The quantity of gas molecules transferred from inhaled air to the epithelium surfaces depends upon the total volume of air inhaled during the exposure episode and on the molecular diffusivity of the gas molecules.

The uptake of gases and vapors by the epithelium of the respiratory tract is influenced by factors that control the rate of transfer of molecules of the inhaled substance through the liquid/gas interface and epithelial membranes. This includes the air-to-tissue and air-to-blood partition coefficients. The partition coefficients depend upon the solubility of the inhaled substance in body fluids.

The metabolic fate of absorbed gas or vapor molecules will determine the concentration in lung tissue and how much of the agent ends up in other organs and tissue in the body.

Pharmacokinetic models are used to evaluate the inhalation uptake, distribution, metabolism, and excretion of vapors and gases by humans. In some cases the models were derived based on laboratory animal data and were adapted for application to humans.