A Model to Estimate the Delivered Doses of Substances in Liquid and Gaseous Phases

A conceptual model is presented for assessing the dose of a chemical substance delivered to individual lung cells when exposure can occur through inhalation of the substance as either a gas or an aerosol of droplets. Assuming a threshold value for the concentration of a toxic species in a single cell, the model shows that the physical state of the substance may affect its biologic activity. Exposure to aerosol droplets of an irritating or reactive substance will deliver much higher surface concentrations to single cells than will a similar exposure to a gas. Under certain conditions, the surface concentration can be between 100 and 10¹0; times higher when the exposure involves an aerosol. Indirect support for the model is found in the observations that threshold limit values for aerosols are generally lower than those for gases.