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NONINVASIVE DETERMINATION OF RESPIRATORY OZONE ABSORPTION: THE BOLUS-RESPONSE METHOD
Impact/Purpose:
Ozone is a ubiquitous irritant air pollutant that is a major constituent of photochemical smog. When inhaled, it can react with cellular biomolecules. Human and animal studies show that exposure to sufficiently high concentrations of ozone causes decreases in lung function and increases in markers of airway inflammation. The U.S. Environmental Protection Agency (EPA) has classified ozone as a criteria pollutant, and established a National Ambient Air Quality Standard of 0.12 parts per million (ppm) as an hourly average. This Standard is currently being reevaluated by the EPA.
For regulators to establish appropriate standards for ozone, they need to know the relations among the ambient concentration of the gas (exposure), the amount of gas absorbed in the respiratory tract and its tissues (dose), and the subsequent health effects (responses). For ozone, the relation between exposure and response is well established. However, few data are available that link exposure to dose, and dose to response because of the technical difficulties of making such measurements. This study, sponsored by the Health Effects Institute, sought to develop methods to quantify ozone dose and the efficiency of ozone absorption in different regions of the respiratory tract.
Description:
Dr. James Ultman and colleagues 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. This caused the bolus of ozone to be inhaled to a desired volumetric depth in the lungs. 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. Ozone concentrations from 0.5 to 4 ppm were used. These data were used in a theoretical model of ozone absorption to estimate the ozone dose rate to different regions of the respiratory tract.
The investigators made substantial improvements in the technology to measure ozone absorption in the respiratory tract of human subjects by developing a fast-responding ozone analyzer and incorporating this instrument into a computer-controlled bolus inhalation system. When they measured the distribution of ozone in different regions of the respiratory tract, they found that with quiet mouth breathing, 50% of the ozone was absorbed in the mouth and oropharynx, and the remainder was absorbed within the conducting airways. When breathing nasally, about 80% of the ozone was absorbed in the upper airways, showing that the nose protects the lungs from ozone exposure. With increasing flow rates, more ozone reached and was absorbed by the lower airways and gas-exchange tissues in the lungs. During exercise, which entails both oral breathing and high flow rates, the dose rate of ozone to the lower airways and gas-exchange tissues would be more than three times the dose rate when at rest. These investigators have provided a valuable research tool for studies that measure doses of ozone and other gases, and their results have advanced our understanding of how this pollutant is absorbed by the respiratory tract.