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To determine the influence of exercise on pulmonary dose of inhaled pollutants, we compared biomarkers of inhaled ozone (O 3 ) dose and toxic effect between exercise levels in humans, and between humans and rats. Resting human subjects were exposed to labeled O 3 ( 18 O 3 , 0.4 ppm, for 2 hours) and alveolar O 3 dose measured as the concentration of excess 18 O in cells and extracellular material of nasal, bronchial, and bronchoalveolar lavage fluid (BALF). We related O 3 dose to effects (changes in BALF protein, LDH, IL-6, and antioxidant substances) measurable in the BALF. A parallel study of resting subjects examined lung function (FEV 1 ) changes following O 3 . Subjects exposed while resting had 18 O concentrations in BALF cells that were 1/5th of those of exercising subjects and directly proportional to the amount of O 3 breathed during exposure. Quantitative measures of alveolar O 3 dose and toxicity that were observed previously in exercising subjects were greatly reduced or non-observable in O 3 exposed resting subjects. Resting rats and resting humans were found to have a similar alveolar O 3 dose.