Simultaneously reducing CO₂ and particulate exposures via fractional recirculation of vehicle cabin air

Prior studies demonstrate that air recirculation can reduce exposure to nanoparticles in vehicle cabins. However when people occupy confined spaces, air recirculation can lead to carbon dioxide (CO₂) accumulation which can potentially lead to deleterious effects on cognitive function. This study proposes a fractional air recirculation system for reducing nanoparticle concentration while simultaneously suppressing CO₂ levels in the cabin. Several recirculation scenarios were tested using a custom-programmed HVAC (heat, ventilation, air conditioning) unit that varied the recirculation door angle in the test vehicle. Operating the recirculation system with a standard cabin filter reduced particle concentrations to 1000 particles/cm³, although CO₂ levels rose to 3000 ppm. When as little as 25% fresh air was introduced (75% recirculation), CO₂ levels dropped to 1000 ppm, while particle concentrations remained below 5000 particles/cm³. We found that nanoparticles were removed selectively during recirculation and demonstrated the trade-off between cabin CO₂ concentration and cabin particle concentration using fractional air recirculation. Data showed significant increases in CO₂ levels during 100% recirculation. For various fan speeds, recirculation fractions of 50-75% maintained lower CO₂ levels in the cabin, while still reducing particulate levels. We recommend fractional recirculation as a simple method to reduce occupants' exposures to particulate matter and CO₂ in vehicles. A design with several fractional recirculation settings could allow air exchange adequate for reducing both particulate and CO₂ exposures. Developing this technology could lead to reductions in airborne nanoparticle exposure, while also mitigating safety risks from CO₂ accumulation.