Dr. Laskin and her colleagues at the Environmental and Occupational Health Sciences Institute at Rutgers University tested the hypothesis that oxidants in ambient air, such as hydrogen peroxide, may be transported by fine particulate matter into the lungs and thus contribute to lung tissue injury. The investigators used ammonium sulfate particles because of their prevalence in the ambient air of the eastern United States and their reportedly low toxicity in animals and humans. Rats inhaled ammonium sulfate, hydrogen peroxide, or combinations thereof, for 2 hours on a single occasion to assess lung tissue injury and presence of inflammatory markers in the lung. They also assessed activation of alveolar macrophages, which are involved in the first line of defense against foreign materials that enter the lung. Exposures with 18O-labeled hydrogen peroxide were conducted to measure deposition in the lung. Additional experiments assessed lung injury and inflammation after rats inhaled organic peroxide, and investigated hydrogen peroxide formation in an indoor environment.