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

Abstract

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.

Development of a Standard Testing Method for Vehicle Cabin Air Quality Index

“…This suggests a long operation of recirculation mode can lead to poor ventilation. Similar results were found by Jung et al [20]. They determined AER as a function of recirculation door opening percentage.…”

“…The slope is equal to -Q l /V c , where Q l is the body leakage flow and V c is the cabin volume. In addition, Jung et al [20] showed…”

“…CAQI CO2 did not show any difference between one and two passengers during AC off/fresh air condition while CAQI CO2 reported a substantially higher value with two passengers during the AC on recirculation condition. It appears none of the test vehicles adopted either passive or active recirculation air control technology such as the one suggested by Grady et al [13] and Jung et al [20]. Cabin air recirculation condition should be used only intermittently with discretion to prevent adverse health effects from increased cabin CO 2 concentrations.…”

“…It is a measure of the air volume added to or removed from a space divided by the volume of the space [19]. On the other hand, the CO 2 exhaled from the passengers can accumulate and reach high concentrations over time [5,6,13,20,21]. Increasing vehicle speed can affect the pressure difference between in-cabin and external air, which can increase the penetration of particulate and gaseous pollutant concentrations [22].…”

Thermal Comfort in the Passenger Compartment Using a 3-D Numerical Analysis and Comparison with Fanger’s Comfort Models

“…Since automotive industries aim to decrease energy consumption and the thermal environment of the vehicle has a direct influence on the energy consumption of the HVAC system. A study on quality improvement of vehicular HVAC system was made by Jung et al [8] by performing several air recirculation scenarios. Fojtlin et al [9] developed an automatic control system to accurately capture the thermal environment for the asymmetric and dynamic nature of a car cabin.…”