To define multi-pollutant exposures that are relevant to real world scenarios we consulted experts in the EPA Office of Air and Radiation (OAR), who advised us to study the effects of sequential exposure to NO2 and O3, two ubiquitous NAAQS criteria pollutants. Ambient diurnal profiles of these two pollutants indicate that levels of NO2 often peak in the evening and morning hours, which are followed by peak ambient O3 concentrations during mid-day. Using this information we designed the study described here to determine whether sequential exposure to NO2 and O3, or O3 and NO2, will result in greater pulmonary and cardiovascular effects than exposure to either pollutant alone. Ozone is a major component of photochemical smog and is one of the most thoroughly studied gaseous pollutants. Controlled human exposure studies have been critical in demonstrating that it can cause airway inflammation, including increases in neutrophil infiltration into the lung and the production of pro-inflammatory mediators, and ultimately decrements in lung function. More recent studies have shown that ozone can also increase vascular inflammation, as well as alter autonomic nervous system control of heart rate. Nitrogen dioxide is an oxidant that is produced by natural and anthropogenic processes. The majority of man-made NO2 results from large-scale combustion-related processes, such as automobile emissions and the generation of electricity. Although traffic-related exposures account for the majority of NO2 emissions. Emissions from natural gas cooking appliances and kerosene-fueled space heaters with inadequate ventilation can serve as a significant source of human exposure to NO2 indoors. Previous studies have shown that NO2 concentrations can reach 600ppb in the area surrounding an operating gas stove, and peak levels may exceed 2000ppb. Controlled human exposure studies have indicated that exposure to NO2 alone (ranging from 110-2000ppb) results in little to no observable decrement in lung function; however, NO2 exposure has been associated with increases in airway hyper-responsiveness, susceptibility to pulmonary infection, and increased pulmonary inflammation. More recently, exposure to 500ppb NO2 has been associated with changes in cardiac electrophysiology. Recent epidemiological data indicate that exposure to NO2 from vehicle emissions were associated with both respiratory and cardiovascular-related mortality. Previous studies have shown that sequential exposure to NO2 and O3 (at concentrations similar to those proposed in this study) results in greater lung function decrements and increased non-specific airway responsiveness compared to O3 exposure preceded by clean air exposure in young women. Additional studies have demonstrated that sequential exposure to ozone, separated by 24 hours, resulted in greater lung function decrements, assessed as forced expiratory volume in the first second of exhalation (FEV1), following the second exposure than was observed after the first. Ozone exposure has also been shown to have a priming effect for subsequent exposure to sulfur dioxide (SO2) in adolescent asthmatics and allergen-induced responses of perennially allergic asthmatics. Additionally, ongoing research at the EPA Human Studies Facility has demonstrated that sequential exposure of humans to diesel exhaust and ozone can result in greater lung function decrement than exposure to either pollutant alone. Given the complex nature of pollutant exposure, we are interested in determining if exposure to one pollutant can sensitize a person so that subsequent exposure to a second pollutant would cause a more pronounced response than would be expected based on exposure to just the second pollutant alone. Thus, in this study we will examine two exposure scenarios involving sequential exposures of NO2 and O3. The first involves determining whether an initial exposure to NO2 will "prime" an individual to a subsequent O3 exposure. The second involves determining whether an initial exposure to O3, at a concentration that results in small cardiopulmonary changes that resolve within 24 hours, will augment a subsequent exposure to NO2. Generally speaking, exposure to NO2 alone is not associated with robust changes in metrics of cardiopulmonary function; however, we believe that it can modify, and be modified by, ozone exposure. Specifically, this study will test two general hypotheses. First, we hypothesize that pre-exposure to a relatively low concentration of NO2 will "sensitize" individuals to a subsequent O3 exposure and lead to greater changes in cardiopulmonary function compared to O3 exposure preceded by clean air exposure. Second, we hypothesize that pre-exposure to O3, at a concentration that has been previously associated with small changes in cardiopulmonary function, will prime individuals to have a greater response to NO2 compared to pre-exposure to clean air. The information obtained during the course of this study will enable the EPA to better evaluate the risks associated with sequential multi-pollutant exposure and potentially provide advice on activities to mitigate the effects.