The investigators will enroll 50 healthy nonsmoking adults (20-65 years old) into a protocol involving 2 separate 5 day long "study blocks". Each block will consist of 5 separate 2 hour-long "exposure periods" conducted 5 days in a row (Monday - Friday; start: 8 am to end: 10 am) (Table 2: Study flow chart). Investigators will enroll 2 patients to undergo the trial study blocks simultaneously (one subject starting block 1 in the active intervention by wearing the facemask and the other wearing no facemask to begin block 1). Day 1 (Monday) of each study block will occur in a non-polluted clean indoor setting (Research Exam Room, Suite A 1310; Lobby A, Dominos Farms). All subsequent exposure periods (days 2-5, Tuesday through Friday) of each study block will all take place at the same designated "exposure site", an Ann Arbor Transportation Association (AATA) parking lot area nearby several busy roadways in Ann Arbor MI. To complete the entire study, each subject will perform 2 separate study blocks. Minimal total study duration is 2 weeks (washout period = 2 days over weekend) to a maximal period of 6 weeks (wash out period = 5 weeks). During one block, subjects will wear an N-95 respirator facemask (active intervention) during all exposure periods (days 1-5, even during day 1 indoor exposures). Subjects will remove the mask at the end of the study day after all testing is completed. Active versus control intervention ordering will be performed in a randomized single-blind cross-over order. During the other block subjects will wear no facemask (control) during exposure periods. While subjects will necessarily be aware of the intervention type, the study investigators responsible for evaluating the results will be blinded to the intervention type during all data analyses (i.e., single blind trial). Throughout all exposure periods, subjects will wear ambulatory BP monitors (ABPM) and ECG Holter monitors to determine brachial BP/aortic hemodynamics measured every 10 minutes and heart rate variability (HRV) metrics measured continuously and averaged into 10 minute epochs corresponding to same time periods as the BP levels, respectively. Air pollution exposures (PM2.5, black carbon (BC), and ultrafine particle (UFP) count), ambient temperature levels, and noise intensity will be measured continuously at the near-roadway exposure site during all exposure periods (Days 2-5). All environmental exposure metrics will also be averaged into 10 minute epochs during the exposure periods corresponding to the health outcome times. Following completion of exposure periods on day 4 (Thursday), subjects will have endothelial function measured by endoPAT (RHI) immediately post-exposures at the near-roadway site, after lying supine resting 5 minutes on a portable support. Patients will wear facemask throughout RHI testing when in the active intervention block. On the final day 5 of each study block, all subjects will have blood drawn. A sub-group (n=20) of subjects (enrolled on a first-come basis) will have SNS activity measured by MSNA. After blood draws on each study visit day subjects will remove the facemasks and can thereafter carry on with their routine daily activities. However, subjects will be asked to not deviate from their usual lifestyle during each study block. Subjects will be specifically directed to not significantly change their lifestyle (e.g., diet, sleep patterns) and/or any routine activity (e.g., exercise regimen, work, commutes, travel) throughout each study block.