Procedures (Methods): In this observational panel study, subjects will come to the U.S. EPA Human Studies Facility for up to 5 sessions, each consisting of 2 consecutive visit days. The following endpoints will be collected: blood pressure, heart rate variability measurements, blood biomarkers, endothelial cell function, retinal venule and arteriole diameter, and lung function. Air pollution exposure will be assessed using area-specific air quality data derived from local air monitoring stations interfaced with activity monitoring and GPS tracked location for each subject.