Evaluating and Mitigating Residential Fine and Ultrafine Particle Exposure via an Open-Source Condensation Particle Counter and Low-Cost Sensor Suite

Indoor air quality (IAQ) standards and public health guidance have traditionally relied on mass-based particulate matter metrics, most notably PM2.5. However, a growing body of evidence indicates that ultrafine particles (UFP) represent a distinct and clinically relevant exposure class that is poorly captured by mass-based measurements. UFPs contribute negligibly to particle mass yet dominate particle number concentration and surface area, properties associated with enhanced biological reactivity and systemic transport. Despite this, UFPs are rarely monitored in residential environments due to the cost and operational complexity of conventional condensation particle counters (CPCs). This study presents a two-week field deployment of the OpenCPC®, a low-cost, fully open-source CPC, in an occupied single-family home. UFP number concentrations were measured concurrently with PM2.5 mass, optical particle counts (>0.3 μm), and CO2 using a distributed sensor network spanning two floors. A daily crossover design was used to evaluate the effectiveness of portable air cleaners (PACs) operated in alternating 24-hour periods. Results show that PAC operation reduced mean UFP concentrations by 55.5% in the primary living area and 90.7% in an upstairs bedroom, with corresponding PM2.5 reductions of 82.3% and 93.2%, respectively. Time-resolved data revealed consistent inter-floor transport of particles with a 15 to 20-minute lag, indicative of stack-driven vertical migration. Correlation analysis demonstrated strong agreement among co-located optical sensors but a marked decoupling between PM2.5 and UFP concentrations, particularly across zones. These findings confirm that mass-based metrics are insufficient proxies for UFP exposure in residential settings. The study demonstrates that accessible CPC technology enables routine UFP monitoring and that distributed portable filtration provides an effective, layered mitigation strategy for both ultrafine and accumulation-mode particles in naturally ventilated homes.