The National Ambient Air Quality Standard for ozone is based primarily on the evaluation of pulmonary function in individuals exposed to ozone during a couple of hours of moderate physical activity. Decrements in forced expired spirometry parameters observed at ambient levels of ozone were reproducible within subjects, but a significant variation occurred between subjects. We hypothesize that this intersubject variability in response is due in large part to a corresponding variability in ozone uptake into the lower conducting airways, the site most likely responsible for changes in FEV1. To test this hypothesis, we plan to study a group of 30 young male and 30 young female nonsmokers whose anatomy will be characterized by using aciustic reflectometry to measure the volume of the upper airways, and nitrogen dilution to determine the volumes of the lower airways and respiratory airspaces. During each research session, a subject will breathe orally through an apparatus that monitors respired flow and ozone concentration while he or she exercises for one hour at a moderate level. The change in forced expiratory volume, forced vital capacity, and forced expired flow between 25 and 75% of vital capacity will be measured between the beginning and the end of each one hour exercise period. Each subject will participate in three types of research sessions. During the first session, the distribution of ozone uptake will be measured by the bolus inhalation method. During the second session, overall ozone uptake will be measured on a breathe-by-breathe basis while a subject continuously breathes air containing 0.3 ppm O3. During the third session, intended to be a control study, each subject will breathe clean air. The specific aims of this part of the study are to determine: 1) the effect of lung anatomy on the distribution of ozone; 2) the effect of lung anatomy on overall uptake of ozone; 3) the relationship between forced expiratory response and overall ozone uptake; and 4) the relationship between forced expiratory response and regional ozone uptake.