Nitrogen dioxide is an ubiquitous air pollutant that can react with and damages lung cells when inhaled at high levels. Although outdoor and indoor levels of nitrogen dioxide are usually below the annual standard of 0.053 ppm, peaks can occur that reach up to 10 times this standard. Dr. Mark Utell and coworkers at the University of Rochester examined the human health impacts of higher (peak) levels of nitrogen dioxide that exceed the annual standard. The investigators exposed healthy, nonasthmatic, human volunteers to either nitrogen dioxide or filtered air for three hours. The subjects were exposed continuously to 0.05 ppm nitrogen dioxide, except for three 15-minute periods when the nitrogen dioxide concentration was raised to 3 ppm. The investigators conducted lung function tests to evaluate whether exposure to nitrogen dioxide affected the subjects' breathing patterns, as well as airway reactivity by testing the subjects with carbachol, a drug that causes airways to constrict. Finally, the investigators collected fluids and cells from the subjects' lungs to look for indicators of lung damage and for functional alterations in the infection-fighting cells.