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Mobile Toxics: Exposure and Risk
Comments – Eric Fujita

    The use of CO to estimate exposure to air toxics has several limitations. The approach is intended for estimating long-term average risk and does not directly account for the range of real-world exposures that include many intermittent elevated exposures, including evaporative emissions. Estimates of exposure to PM are highly uncertain because the correlation between exhaust CO and PM is poor, especially with diesel exhaust. The method does not account for chemical and physical transformation in the atmosphere, which results in both underestimation and overestimation of exposure. EPA has acknowledged these limitations, but it is not clear how and to what extent they will be addressed.

    The greatest uncertainty in current exposure estimates is with PM. The MATES II Study by the South Coast Air Quality Management District assigns 70 percent of the cumulative risk from air toxics to diesel PM, while EPA and other state programs have not yet assigned a risk to this source. While other mobile source air toxics, such as benzene and 1,3-butadiene can be directly measured, there is no direct measure of diesel PM in the ambient air. SCAQMD used elemental carbon (EC) as a surrogate for diesel PM and estimated ambient concentrations of diesel PM by applying a factor of 1.04 to the measured EC concentrations. This factor assumes that 64 percent of the diesel PM consists of EC (with organic carbon [OC] accounting for the remainder) and that diesels account for 67 percent of the ambient fine particulate EC. These factors are based upon measurements that were taken nearly two decades ago, and should be updated. Assuming that these factors are reasonably accurate, diesel PM accounts for 3.4 m g/m3 of the eight-site annual average total PM2.5 carbon of 9.8 m g/m3. The source of two-third of the ambient PM2.5 carbon is not identified in the MATE II report and no risk is attributed to this residual particulate carbon. The PM emission inventory does not reconcile with the ambient carbon measurements. Secondary organic aerosols may be significant during the summer, their contributions are relatively small during the fall and winter when ambient concentrations of directly emitted pollutants are highest.

    While improvement have been made during the past decade in the estimation of on-road CO, HC, and NOx emissions, the corresponding models for PM emissions have not been significantly updated since their initial development in the mid-1980s. PART5 exhaust carbon emissions from light duty vehicles depend on technology class and go up to a maximum of only 30 mg/mile for pre-1970 non-catalyst vehicles. PART5 does not account for cold starts, modal emissions, or high emitters. Results from the Northern Front Range Air Quality Study indicate that sources with emissions similar to light-duty gasoline vehicles contributed about 60% of PM2.5 carbon at urban Denver sites, and these contributions were 2.5 to 3 times the diesel exhaust contributions (http://nfraqs.cira.colostate.edu/). Are the risks from spark-ignition PM emissions really negligible relative to diesel PM?

    Primary emissions from motor vehicles and other combustion sources are very complex mixtures containing thousands of organic and inorganic constituents in the gas and particulate phases. Gases include criteria gases, hazardous air pollutants, and volatile and semi-volatile organic compounds that are precursors to ozone, organic aerosols, and hazardous air pollutants (e.g., formaldehyde, acetaldehyde, and nitrated polycyclic aromatic hydrocarbons). Carbonyl compounds are also produced by photooxidation of biogenic emissions and during organic decay. Many organic compounds are emitted at elevated temperatures as a result of combustion and condense rapidly upon cooling to ambient temperatures forming ultrafine and nuclei range particles. The "nuclei" range consists of particles less than 0.08 m m in aerodynamic diameter that are emitted directly from combustion sources. In polluted areas, the lifetime of particles in the nuclei range is relatively short because they rapidly coagulate with large particles or serve as nuclei for cloud or fog droplets. This size range is generally detected only when fresh emissions sources are close to the measurement site.

    Estimating nationwide long-term average risk of mobile source air toxics based upon ambient CO levels is a reasonable first approximation given the data that were available when this effort was initiated. However, sensitivity analyses are required to determine the range of intermittent elevated exposures, especially during heavy traffic, residential and public parking garages, and during refueling. The CO-based estimates of the ambient levels of mobile air toxics should be compared with recent data from the Urban Air Toxics Monitoring Program and Photochemcial Assessment Monitoring Stations (PAMS). CO is not an appropriate surrogate for combustion PM and more reliable alternatives are required to assess the risks posed by PM emissions from both diesel- and gasoline-powered vehicles. Possible ambient measurements include continuous PM mass, OC/EC analyzers, aethalometers and nephelometers. Special studies are required to characterize the evolution of PM composition and size distribution as mobile source emissions are transported downwind. Despite the limitations of exposure estimates derived from ambient measurements, they are preferable to modeled estimates based upon emission inventories because the uncertainties in the magnitude and spatial variability of emissions cannot be evaluated statistically.

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