Thirty smokers and thirty nonsmokers each participated in two research sessions. During an O3 exposure session, subjects exercised on a cycle ergometer for one hour while breathing room air containing 0.3 ppm O3. By making periodic adjustments in the ergometer workload, the experimenter targeted a minute ventilation (MV) of 15 L/min/m2 of body surface. During a control session, carried out at least one week prior to the O3 exposure session, the subject followed the same procedure while breathing room air not containing O3. During both sessions, the subject breathed through an oral mask fitted with sensors that monitored respiratory flow and O3 concentration throughout the one-hour session.
The fractional uptake efficiency (UE), defined as the ratio of retained amount of O3 to inhaled amount of O3, was determined for each breath and averaged at five minute increments. UE decreased for both smokers and nonsmokers during the one-hour exposure, and the smoking subjects had a somewhat higher UE than the nonsmoking subjects. An ANCOVA of UE using time as the covariate and smoking status as the fixed variable revealed that smoking status was indeed a significant predictor of uptake efficiency (p=0.02).
Respiratory flow data was processed to obtain MV, frequency (FR), and tidal volume (VT). Minute-by-minute values of the MV exhibited minimal variation within either the control or O3 exposure sessions, and the session-averaged MV values were not significantly different between the control and exposure sessions or between the smokers and nonsmokers. Relative to the control session, there was an increase in FR and a decrease in TV associated with O3 exposure for both smokers and nonsmokers. Smokers breathed at a slightly higher VT and lower FR than the nonsmokers. We hypothesized it was these variations in breathing pattern that were responsible for the observed variations in UE.
In previous work, the UE values observed in health nonsmoking subjects during a one-hour O3 exposure were simulated with a single-path respiratory transport model. Simulations were performed for individual subjects by incorporating a square wave respiratory flow pattern with VT and FR varying in accordance with their observed minute-by-minute values. By adjusting the rate constant for O3 reaction in the mucous layer (kr) on a subject-by-subject basis, these simulations closely matched the measured UE for all subjects.
To test our hypothesis, the single-path model was employed to separately simulate the current data grouped into four subpopulations: male nonsmokers, female nonsmokers, male smokers, and female smokers. The squared error between the observed and simulated UE was minimized by adjustment of kr for each subpopulation. The final values of kr were 1.06E6 s-1 for female nonsmokers, 2.25E6 s-1 for male nonsmokers, 2.55E6 s-1 for female smokers, and 2.60E6 s-1 for male smokers. Thus, by taking breathing pattern alone into account, it was possible to simulate differences in the observed UE between the smokers and nonsmokers.