A conceptual model is presented for assessing the dose ofa chem-
ical 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 droplet of an irritating or reactive sr~hstance
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 lot0 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.