Car and Bus Exposure Studies

This page last reviewed February 6, 2015

woman sitting inside a car

Air pollution from vehicles can harm human health whether it is vehicle exhaust that enters a vehicle from outside or pollutants off-gassing 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.

This web page contains information about various studies funded by ARB 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.

Reducing Air Pollution Exposure in Passenger Vehicles and School Buses

image of a high efficiency vehicle cabin air filter

ARB funded a project in 2012 entitled “Reducing Air Pollution Exposure in Passenger Vehicles and School Buses” to explore the application of high efficiency filtration to reduce in-cabin exposure to particulate pollution, including PM2.5 and ultrafine particles (UFP). In-cabin exposure is due to infiltration of vehicle particle emissions into the cabin. Equipping automobiles and school buses with high efficiency filters rated MERV (Minimum Efficiency Reporting Value) 16 or higher, or HEPA (High-Efficiency Particle Arrestance) may provide a potential mitigation strategy for reducing in-cabin exposure to particles in the short-term, while ARB fleet regulations are implemented over the long-term to reduce vehicular emissions. The project’s principal investigator is Dr. Yifang Zhu of the University of California, Los Angeles. The final report is expected to be available in spring 2015.

Volatile and Semi-Volatile Organic Compounds in the Cabins of Passenger Vehicles under Static Conditions

Air pollutants inside vehicles can come from many sources. For organic chemicals, 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. In 2008 the ARB investigated the concentrations of VOCs, such as benzene and acrolein, and SVOCs, such as PBDEs, present in relatively new vehicles (up to 3 years old) 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 have been prohibited in California products.

Characterizing the Range of Children's Exposures to Pollutants During School Bus Commutes

kids in school bus

To learn more about children's exposure to air pollution during their school bus commute, the ARB sponsored the Children's School Bus Exposure Study which was conducted in the spring of 2002. The objective of this study was to obtain pollutant measurements inside school buses to provide data to quantify children's exposure to diesel exhaust particulate matter (PM) and other pollutants during their school bus commutes. Investigators used school buses with different types of fuel and obtained measurements inside the buses and at school bus stops under different driving and traffic conditions. The researchers found that minimizing commute times, using the cleanest buses for the longest routes, and reducing bus caravanning and idling time would reduce children’s exposure to bus-related pollutants. The results of this study supported strengthening of ARB's School Bus Idling Air Toxic Control Measure (ATCM) which has been in effect since July 2003.

In 2006 ARB sponsored another study titled "Evaluation of Mechanisms of Exhaust Intrusion into School Buses and Feasible Mitigation Measures." The researchers in this study attempted to clarify how and when self-pollution of school buses occurs and tested various methods to reduce it. They also investigated how exhaust enters a school bus from a leader bus (one bus that is being driven or idled in front of another bus). The results of the study showed that one method tested for determining the overall tightness of a bus was effective and should be used to check tightness as buses age. The results also showed that self-pollution of buses could be significantly reduced by placing the exhaust outlet above the bus. Lastly, the researchers found that exhaust from a leader bus can be more significant than self-pollution and thus, this research further strengthened the recommendation that school buses should not caravan close together or follow closely behind other diesel-powered vehicles.

Measuring Concentrations of Selected Air Pollutants Inside California Vehicles

To learn more about Californians' exposures to pollutants while driving on California's busy roadways, the ARB funded one of the first comprehensive studies of pollutant concentrations inside cars. In 1997, 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.