The U.S. Environmental Protection Agency has traditionally examined the effects of single pollutant exposure on human health outcomes; however, to provide a better assessment of pollutant exposure-associated effect on human health the Agency is moving toward a multi-pollutant approach. Ozone (O3) and nitrogen dioxide (NO2) are national ambient air quality standards (NAAQS) criteria pollutants that are major constituents of ambient air pollution. This study will address the Agency's goals by investigating the cardiopulmonary health effects of sequential exposure to O3 and NO2 in healthy adult human volunteers. The findings of this study will provide data that will inform risk assessment models for O3 and NO2 exposures. Additionally, the findings will provide insight into how the human health effects of multi-pollutant exposures differ from those of single pollutant exposures.