Dr. James Ultman and colleagues at Pennsylvania State University used a fast-responding ozone measurement system, which they had developed with previous HEI support, to noninvasively measure the absorption of inhaled ozone in different regions of the respiratory tract of healthy adult men. While the subject was breathing through the measurement apparatus, a narrow 10-mL bolus of ozone was introduced into the inhaled air at a predetermined point. By comparing the amounts of ozone inhaled and exhaled, they calculated the cumulative efficiency of ozone absorption (called the bolus-response analysis method). By delivering the bolus to other depths and then relating these depths to anatomical regions of the respiratory tract, they quantified the absorption efficiency of ozone in the upper and lower airways, and in the gas-exchange region of the lungs. To mimic different exposure scenarios, they measured ozone absorption while the subjects breathed through the mouth or nose.