The US EPA Clean Air Multiyear research program is moving toward a multi-pollutant approach to the assessment of air pollution in response to recommendations by the NRC 2004 and the BOSC in 2005. Such an approach better reflects the complexity of real-world air pollution problems and parallels evolving scientific and regulatory considerations. Ozone (O3) and diesel exhaust (DE) generally are major and important components of ambient air pollution. This proposed study will address the agency's goals by investigating the cardiopulmonary health effects in healthy human subjects co-exposed to O3 and DE. The findings derived from these exposures will provide NCEA findings for risk assessments of O3 and DE, as well as the Office of Air and Radiation (includes OTAQ and OAQPS) with information relevant to possible modulation of PM-induced health effects and responses by a gaseous co-pollutant for potential standard setting. Additionally the findings will address the fundamental driving principle of the Clean Air Research strategy related to reduction of health due to air pollutant exposures.