Car & Bus Exposure Studies
Air pollution from vehicles can harm human health whether it is vehicle exhaust that enters a vehicle from outside or pollutants off-gassing from materials inside the vehicle. Pollutants from vehicle exhaust include carbon monoxide (CO), nitrogen dioxide (NO2), particulate matter (PM) such as PM 2.5 and ultrafine particles (UFP), and volatile organic compounds (VOCs). VOCs and semi-volatile organic compounds (SVOCs), such as phthalates and polybrominated diphenyl ethers (PBDEs), are sometimes emitted from interior vehicle materials. Health effects from vehicle pollution can include respiratory and cardiovascular diseases, as well as increased risk of cancer. Children can be exposed to high levels of air pollutants from buses when they are either standing nearby or inside buses.
This page contains information about studies funded by CARB to determine the levels of pollutants inside vehicles such as cars and school buses and ways that those levels can be reduced. Some exposure reduction measures that have been identified from this research include the use of high efficiency filters for passenger cabin air, and not idling or caravanning school buses.
Measuring Concentrations of Selected Air Pollutants Inside California Vehicles (1997)
To learn more about exposures to pollutants while driving on California's busy roadways, in 1997 CARB funded one of the first comprehensive studies of pollutant concentrations inside cars. Investigators outfitted cars with a variety of air monitors and measured pollutant levels while driving on freeways and arterial roadways during rush-hour and non-rush-hour periods in Los Angeles and Sacramento. They found that VOC and particle levels inside vehicles and along the roadside were generally much higher than pollutant levels measured at the nearest ambient monitoring station. This study was one of the first to confirm elevated on-road pollutant levels and to identify commuting inside one's vehicle as a high exposure condition.
Volatile and Semi-Volatile Organic Compounds in the Cabins of Passenger Vehicles under Static Conditions (2008)
Air pollutants inside vehicles can come from many sources. Cabin interior materials and standing emissions (from the engine, gas tank and hoses when the car is not operating) may be the most important sources of organic chemical pollutants. In 2008 CARB investigated concentrations of VOCs such as benzene and acrolein, as well as SVOCs such as PBDEs (present in relatively new vehicles), to learn more about Californians' exposures to organic chemicals inside vehicle cabins. Measurements were obtained after the vehicles had been parked in direct sunlight for about five hours. This study condition was selected to obtain information on all VOCs and SVOCs that might be emitted inside the car, including those emitted only on the hottest days. It also represents a high exposure scenario such as when an occupant first gets into their car on a hot day, when VOC concentrations are expected to be at their highest. Thus the results do not necessarily reflect typical exposure levels, because average concentrations during typical commutes are likely to be lower.
This study showed that concentrations of several VOCs exceeded their health benchmarks in at least one vehicle. The VOCs included acrolein, acrylonitrile, bromomethane, 1,3-butadiene, benzene, ethylbenzene, carbon tetrachloride, 1,2-dichloroethane, p-dichlorobenzene, vinyl chloride, perchloroethylene and chloroform. Therefore, exposure to in-vehicle VOCs still poses a health risk to vehicle occupants, at least under the high temperature conditions tested in this study.
Also, for this study, vehicle cabin dust and passenger cabin air filters were tested for SVOCs. A few samples of cabin dust and all of the used cabin filters had measurable levels of phthalates and PBDEs. Vehicles manufactured during and after 2008 had elevated levels of deca-bromodiphenyl ethers (deca-BDEs) in the dust samples, indicating that deca-BDEs may be used more often as flame retardants since penta- and octa-BDEs were prohibited in California products.
Cardiovascular Health Effects of Fine and Ultrafine Particles During Freeway Travel (2010)
This was a 24-month study of human response to two-hour exposures to freeway air in Southern California. A nine-passenger van was modified with a high-efficiency filtration system that delivered filtered or unfiltered air to an exposure chamber inside the van. State-of-the-art instruments were used to measure concentration and size distribution of fine and ultrafine particles and the concentration of other pollutants associated with motor vehicles. Nineteen volunteer subjects (average age 71 years) rode for two hours each in filtered and unfiltered air on two freeways, I-405 and I-710. Double-blind health assessments included 24-hour ambulatory ECG, blood biochemistry, blood pressure, and lung function. Mean unfiltered particle number concentration was 107,500 particles/cm3 for I-710 and 77,800 particles/cm3 for I-405; mean PM-2.5 mass was 51.4 and 44.5 μg/m3 respectively. Filtration reduced particle count > 95% but did not remove gases. Atrial ectopic beat incidence during and after exposure decreased 20% on average with filtered air compared to unfiltered air (P < 0.05). Individual responses related most strongly to particle count (P=0.01). Blood markers NT pro-BNP and VEGF decreased 30% on average in filtered air compared to unfiltered air (P < 0.05). This study documents a cardiac and vascular response associated with freeway travel.
In-Vehicle Air Pollution Exposure Measurement and Modeling (2012)
On-road concentrations of traffic-related pollutants are typically much higher than concentrations measured at ambient monitoring stations. This results in in-vehicle microenvironments contributing disproportionately to the total exposure with exposures frequently being as high as on-road concentrations. However, under conditions of low air exchange rate, pollutants with significant in-vehicle losses, such as particles, can have in-vehicle concentrations that are significantly lower than those outside the vehicle. We tested a large sample of vehicles selected to be representative of the California fleet for air exchange rate (AER) at various speeds and found that AER is a predictable function of vehicle age or mileage, speed, and ventilation setting choice (outside air, recirculation, or open windows). We demonstrated that AER is the dominant factor in determining the inside-to-outside ratio for pollutants like ultrafine particles. Models were developed that explain over 79% of the variability in AER and ultrafine particle indoor/outdoor ratios across the California fleet and across the expected range of normal driving conditions. To better determine on-road concentrations, we also conducted extensive on-road measurements using a mobile platform hybrid vehicle with real-time instrumentation. Models were developed and validated to estimate on-road traffic-related pollutant concentrations (variance explained was 37% to 73% depending on the air pollutant and modeling method). Models developed in this study can be combined with subject information about their vehicle, ventilation choices, and commute route to estimate in-vehicle exposures in future studies.
Reducing Air Pollution Exposure in Passenger Vehicles and School Buses (2015)
Exposures to vehicle-emitted PM2.5, black carbon (BC), and ultrafine particles (UFPs), have been associated with adverse health effects. As a potential strategy to mitigate in-cabin exposure, the authors developed a novel high efficiency cabin air (HECA) filter for passenger vehicles and an on-board HECA filtration system for school buses. Their performance was evaluated in twelve passenger vehicles and six school buses, respectively. UFP number concentration and size distribution as well as BC and PM2.5 levels were concurrently monitored inside and outside of each vehicle under three driving conditions: stationary, on local roadways, and on freeways. For passenger vehicles, data were collected with no filter, the in-use original equipment manufacturer (OEM) filter, and two prototypes of HECA filters (i.e., HECA A and B filters). For school buses, data were collected with and without operating the filtration system equipped with HECA B filters. For passenger vehicles, the HECA B filters offered in-cabin concentration reductions of 90 ± 8% for UFPs on average across all driving conditions, much higher than the OEM filters (50 ± 11% on average). Similarly, the HECA B filters offered an 81 ± 15% reduction for BC and 66 ± 28% for PM2.5 across all driving conditions. In comparison, across all driving conditions, in-use OEM filters only provided 31 ± 17% and 29 ± 20% reduction for BC and PM2.5, respectively. For school buses, across all driving conditions, in-cabin UFP and BC levels were reduced by 88 ± 6% and 84 ± 5% on average, respectively, when the on-board HECA filtration system was operating. The HECA system achieved 55 ± 22% reductions on average for PM2.5 and successfully kept its levels below 12 μg/m3.
In California, about one in eight children take a school bus between home and school. Even though the time children spend in school bus commutes account for less than 10% of a child's day, the commutes contribute 33% of a child's daily exposure to some air pollutants. To understand this issue and reduce children's exposure to air pollutants in school buses, CARB has funded studies to examine such school bus exposures and possible mitigation approaches.