This study is a blinded randomized crossover efficacy trial (N=172 households consisting of 207 participants) of High Efficiency Particulate Air (HEPA) filtration in near-highway homes that lack mechanical air-handling systems. Households will be randomized to 30 days of either filtration or sham filtration followed by a 30 washout period with a subsequent 30-day period of the alternative assignment. Room air filters that are commercially available will be placed in the bedroom and living room of each home. The investigators will measure UFP and PM2.5 concentrations in 20% of the homes during filtration and sham periods and assess personal exposure in a subset of participants. The investigators will also assess chemical composition of particulate air pollution in 10 homes/year for exploratory purposes that could lead to future lines of research. The primary health endpoints will be participants' hsCRP and peripheral blood pressure, measures that the investigators have used in multiple observational studies of UFP as well as in pilot filtration intervention studies. Secondary biological measures that contribute to understanding biological pathways will be IL-6 (inflammation), D-dimer (coagulation), metabolome, central pressure and arterial stiffness. The primary intention to treat analysis will compare outcomes between HEPA filtration to sham filtration. The investigators will have 80% power to detect a difference of 0.6 mg/L in change in hsCRP and a difference in reduction in systolic blood pressure of 3.5 mmHg compared to participants who receive no filtration. Having participants serve as their own controls in the within-subject comparisons of intervention effectiveness increases statistical power and eliminates the possibility of baseline imbalances in demographic and clinical characteristics. A social science evaluation will inform final adjustments to the investigators' approach at the start and also assess participant acceptance and experience with the intervention at the end. The investigators' primary innovation is that this will be the first near highway HEPA intervention trial that is large enough and careful enough to be policy-relevant.