ABSTRACT Cardiovascular disease (CVD) is the leading cause of death in the US. There has been an increasing recognition of the important contribution of fine particulate pollution (PM2.5) to CVD morbidity and mortality. Mounting evidence suggests that the use of indoor air filtration to reduce PM2.5 exposure from days to weeks leads to improvement in acute atherothrombosis biomarkers. However, no trials have assessed whether these short-term effects are sustained or further improved with prolonged filtration. The proposed study is the longest intervention trial investigating the effect of using low-cost air purifiers in residences on ameliorating atherothrombosis progressions in vulnerable individuals. The study will examine atherothrombosis responses to HEPA filtration across and within four subgroups of male or female and Hispanic or non-Hispanic participants. We will leverage the diverse population in Los Angeles to investigate the effect of indoor PM2.5 filtration on both levels and slopes of change in atherothrombosis biomarkers including arterial stiffness, blood pressure, targeted proinflammatory and prothrombotic markers, and novel proteomic makers, as well as CVD risk score. The focus on the slopes of change over time is novel and will provide better prediction of future CVD events and mortality than biomarker level changes assessed in previous studies. We have an experienced team of investigators at the University of Southern California (USC) and Duke University and will leverage the rich patient resource from the USC KECK electronic medical records and the linked census-track PM2.5 data to enroll 100 participants of 65-84 years old non-smokers with a history of ischemic heart disease (clinically stable for at least 6 months) who have lived in the census track with higher ambient PM2.5 exposure in the past years. Participants will be block- randomized to two intervention arms (HEPA or sham filtrations, each of 9-month duration) and crossed over to the other intervention arm after a 3-month washout period. This design allows us to conduct the two intervention arms in the same calendar months of two consecutive years. Three home visits will be conducted during each intervention arm to set-up and calibrate air monitors, measure health outcomes, and collect biospecimens. Lifestyle and health covariates will be collected through online questionnaires. The crossover design and timing- controlled interventions will reduce the influence of confounding factors since participants serve as their own controls in the within-person comparison. Indoor and outdoor PM2.5 and co-pollutants will be measured; and meteorological data as well as wildfire events will be recorded throughout the trial. These covariates will be adjusted for in the statistical analyses. Our aims match the goal of NOT-HL-20-788 calling feasibility trials to investigate the effects of portable air cleaners on reducing PM2.5 exposure and CVD biomarkers including novel omics markers...