Ozone (O 3) is an air pollutant produced by sunlight-driven reactions involving the oxides of nitrogen and volatile organic compounds. The population of many large metropolitan areas in the US is exposed to high levels of O 3, particularly in the summer months. Individuals exposed to O 3 levels in human experiments at higher than common ambient levels develop reversible reductions in lung function often associated with symptoms, such as airway hyperreactivity and lung inflammation. Animal models have helped characterize potential mechanisms of lung injury from O 3 exposure. Defining the adverse effects of chronic exposure to ambient levels of O 3 on lung function and disease have been challenging, in part due to the presence of co-pollutants, such as particulate matter. The US Environmental Protection Agency's 1997 revised standard for O 3 (0.08 ppm averaged over 8 hours) is designed to provide better protection to susceptible individuals. The revised standard is being implemented following the failure of court challenges.