The carbon dioxide (CO₂) expirogram provides useful information concerning the response of the pulmonary system to the inhalation of air pollutants such as O₃. The dead space (V_D) reflecting the size of the conducting airways and the alveolar slope (S_N) characterizing the efficiency of gas transport in the respiratory zone are particularly useful parameters associated with the expirogram. In previous studies of 47 healthy non-smokers (25 men, 22 women), measurements of the expirogram immediately before and 10-minutes after a one-hour exposure to 0.3 ppm O₃ revealed percent changes (mean ± SD) of -4.2 ± 5.1 (P <0.001) in V_D and 17.5 ± 15.4 (P <0.001) in S_N. The objective of this study was to determine whether changes in V_D and S_N can be observed on a minute-by-minute basis during controlled O₃ exposures while the subjects are exercising.

Thirty healthy non-smokers (17 men, 13 women) participated in two sessions (control and ozone) in which they exercised for one hour on a cycle ergometer at a work load sufficient to maintain a minute volume of 15 L per square meter of body surface. Throughout each of these sessions, measurements of respiratory flow, O₃ concentration, and CO₂ concentration were continuously recorded. In the ozone session, the subjects orally inhaled air containing 0.258 ± 0.012 ppm O₃. The concentration of O₃ in the room air inhaled during the control session was measured at 0.001 ± 0.001 ppm. For purposes of comparison, pre-exposure and post-exposure measurements of CO₂ expirograms were also collected by training the subjects to follow a prescribed breathing pattern.

The pooled data obtained during the O₃ sessions indicated that V_D decreased at a rate of 0.067 ± 0.035 mL/min (P = 0.06), corresponding to an overall decrement of 2.1 ± 1.1% during the entire exposure. The value of S_N increased during the O₃ sessions at a rate of 0.57 ± 0.094 10^-3L^-1min^-1 (P < 0.001), corresponding to an overall increase of 11.8 ± 1.9%. The dynamic changes in expirogram parameters where greater than those found from pre to post-exposure CO₂ expirogram results, which showed no significant changes for either V_D or S_N. During the control session, the value of V_D did not change significantly with time (P = 0.72), whereas S_N increased at a rate of 0.29 ± 0.11 10^-3L^-1min^-1 (P = 0.011), corresponding to an overall increment of 5.6 ± 2.1% over the hour of exercise. Thus, exposure to O₃ appears to cause a progressive reduction in the volume of the conducting airways and a progressive impairment of gas transfer in the respiratory airspaces. However, changes in breathing pattern were also observed during the exposures, and this may have contributed to the observed changes in V_D and S_N.