Effects of Vehicle Cabin Filter Efficiency on Ultrafine Particle Concentration Ratios Measured In-Cabin and On-Roadway

Abstract

Understanding the in-cabin microenvironment of vehicles is important for assessing human exposure to ultrafine particles (UFPs) of vehicular origin. Filtration through the cabin filter is one of the processes that determine the ratio of in-cabin to on-roadway (I/O) UFP concentrations. In this study, two filter test systems were used to measure the particle filtration efficiencies of fine, ultrafine, and coarse particles. Two types of particles (diesel exhaust UFPs and Arizona test particles) were used to represent the particle types expected in the on-roadway environment. The most penetrating particle size was around 300 nm with filtration efficiency lower than 20%. As the filter face velocity increased from 0.1 to 0.5 m s⁻¹, the filtration efficiency decreased by 10–20%. For vehicles that were frequently driven under heavy traffic conditions (65,000–72,000 vehicles day⁻¹) the pressure drop across the cabin filter increased up to 45 Pa within 10 months. It took 20 months to achieve the same pressure drop under moderate traffic conditions (10,000–24,000 vehicles day⁻¹) and 30 months under light conditions (700–2,000 vehicles day⁻¹). When the vehicle ventilation fan was on and the recirculation was off, it took approximately 10 months under heavy traffic conditions for UFP I/O ratios to increase by 40%. Explicit relationships between UFP I/O ratios and filter usage under various conditions were derived to facilitate cabin filter change decisions based on individual preferences.