(718r) Fractional Uptake Efficiency of Ozone in Human Subjects

Ozone (O₃) is an urban outdoor pollutant primarily generated by a photochemical reaction of atmospheric oxygen with oxides of nitrogen and unburned hydrocarbons from automobile emissions. The inhalation of O₃ is associated with health effects that include decrements in pulmonary function, pulmonary inflammation, and alterations in lung development. The purpose of the current research was to determine the influence of smoking on the retention of O₃ in the adult human lung.

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 (RR), and tidal volume (VT). During the O₃ exposure sessions, there was a significant increase in RR and a decrease in VT for both smokers and nonsmokers. Smokers breathed at a slightly higher VT and lower RR 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 RR equivalent to the session averaged value. By adjusting the rate constant for O₃ reaction in the mucous layer (k_r) on a subject-by-subject basis, these simulations closely matched the measured UE for the subjects.

To test our hypothesis, the single-path model was employed to separately simulate the current data on a subject-by-subject basis. The squared error between the observed and simulated UE was minimized by adjusting a multiplier applied to a base value of k_r (3.00 E6 s^-1) for each subject. The population mean k_r multiplier values were 6.8 +/- 19.1 (mean +/- SD) for nonsmokers and 32.6 +/- 107.6 for smokers. A two-sample t-test resulted in no significant differences in the average k_r values between nonsmokers and smokers.

Sixty-four percent of the subject specific values of k_r are within an order of magnitude of the base value and 95% of the subject specific values are within two orders of magnitude of the base value. The subjects that have k_r multipliers greater than 10 skew the population mean value upward. The median population values of k_r were 2.90 E6 s^-1 for nonsmokers and 2.13 E6 s^-1 for smokers. These results are consistent with the subject specific k_r values previously determined for healthy nonsmokers.

The subject specific values of k_r do not indicate differences in the uptake rate of O₃ between smokers and nonsmokers. Thus, by taking breathing patterns into account with the single-path model, it was possible to simulate differences in the observed UE between the smokers and nonsmokers.