You are here:
THE EFFECTS OF INHALED OXIDANTS AND ACID AEROSOLS ON PULMONARY FUNCTION
Impact/Purpose:
Oxidants gases, such as ozone and nitrogen dioxide, and acid aerosols, such as sulfuric and nitric acids, are components of photochemical smog. Epidemiologic studies suggest that episodes of asthma are exacerbated by smog events. However, controlled human exposure studies have not been able to determine the specific pollutants that are responsible for these findings, nor do people with asthma appear to respond more intensely to ozone or nitrogen dioxide than healthy people in laboratory settings. However, some evidence indicates that people with asthma may be more sensitive to acid aerosols than healthy subjects.
Current risk estimates for air pollutants are calculated largely on the basis of the effects observed in humans exposed to individual gases in laboratory studies. Little is known about any interactive effects of multiple pollutants, or whether a pollutant can sensitize the airways and exacerbate an individual's response to a subsequent pollutant exposure. HEI sponsored the two studies, summarized here, to examine the effects of exposing healthy subjects and subjects with asthma to combined oxidant and acid pollutants.
Description:
Drs. Koenig and Utell each conducted studies in which human volunteers received either combined or sequential exposures to oxidant gases and acid aerosols. In each case, standard pulmonary function tests were performed and symptoms were recorded. Dr. Koenig exposed 28 adolescents with asthma to each of the following substances in a random sequence: filtered air (control exposure); 0.12 parts per million (ppm) ozone + 0.3 ppm nitrogen dioxide; 0.12 ppm ozone + 0.3 ppm nitrogen dioxide + 68 ?g of sulfuric acid per cubic meter of air; 0.12 ppm ozone + 0.3 ppm nitrogen dioxide + 0.05 ppm nitric acid. Each exposure lasted for 90 minutes while the subject exercised intermittently; exposures to the different combinations of substances were separated by at least one week.
Dr. Utell examined the effects of sequential exposures to sulfuric acid and ozone on pulmonary function in 30 subjects with asthma and 30 healthy subjects between the ages of 18 and 45. Subjects were exposed first to either sulfuric acid or sodium chloride (as a control exposure) at a concentration of 100 ?g/m3. Twenty-four hours later they were exposed to ozone (0.08, 0.12, or 0.18 ppm) for three hours, including intermittent exercise. Each two-day exposure protocol was separated from the next one by at least two weeks.
Dr. Koenig found no significant effects of exposure to any combination of the test atmospheres on the pulmonary function of the adolescents with asthma. Interpretation of these negative results is limited, however, because 6 of the 28 subjects were not able to complete the study. These individuals had moderate to severe asthma, and may represent a particularly sensitive subgroup of subjects with asthma. Dr. Utell found a statistically significant decrease in one pulmonary function measurement (forced vital capacity) in subjects with asthma exposed to 0.18 ppm ozone. This decrease was greater with exposure to sulfuric acid before exposure to ozone, but no evidence of aggravation of asthma was apparent. Exposure to ozone or sulfuric acid aerosol caused no change in symptoms or pulmonary function in healthy subjects.
These results need to be interpreted in the context of the investigators' protocols, which were directed toward simple mixtures of two or three components and do not reflect the complex mixtures found in ambient air. Results from both studies suggest that subjects with asthma are heterogeneous in their responses to air pollutants. As a group, they did not show consistently enhanced responses to air pollutants; however, some subpopulations of individuals with asthma may be more susceptible to certain pollutant exposures. Future studies should be directed toward identifying and studying such subpopulations.