Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration
Office of Planning, Environment, & Realty (HEP)

Strategic Workplan for Air Toxics Research


Table of Contents


Acronyms and Abbreviations

  1. Characterization
    1. Introduction
    2. Workshop Discussion Summary
    3. Priority Research Recommendations
  2. Ambient Monitoring
    1. Introduction
    2. Workshop Discussion Summary'
    3. Priority Research Recommendations
  3. Analysis
    1. Introduction
    2. Workshop Discussion Summary
    3. Priority Research Recommendations
  4. Control Strategies/Measures
    1. Introduction
    2. Workshop Discussion Summary
    3. Priority Research Recommendations
  5. Conclusions

    Appendix A

    Appendix B

    Appendix C



This strategic workplan for air toxics research is designed to provide direction for the research on air toxics being undertaken by and on behalf of the transportation community. It identifies a set of four research focus areas and describes program areas where research is needed to most effectively develop needed information and tools and to target resources. Sponsored by the Federal Highway Administration (FHWA), this plan was developed by FHWA in cooperation with atmospheric scientists, air quality experts, environmental and transportation planners from State departments of transportation, metropolitan planning organizations, air quality agencies, industry, and academia.

In some instances, the research objectives noted in this workplan overlap with other Federal agency objectives, most notably the U.S. Environmental Protection Agency (EPA). Thus, the report recommendations are expected to be used as a tool to communicate with these agencies, and identify leveraging opportunities. For the purposes of the workshop, and this report, the transportation community was broadly defined to include the federal, state, and local agencies and related organizations that are involved in transportation-air quality analyses and evaluations. The air toxics-related research that FHWA ultimately decides to sponsor may differ from that described in this research plan. In addition, there may be instances where FHWA decides to contribute to the accomplishment of studies which are being directed by other agencies and organizations. There are also expected to be instances where FHWA is the primary research sponsor, and the project is performed collaboratively with the contributions of other agencies and organizations. The research priorities in this report are presented without regard to who the primary research sponsor might be.

The research ideas in this workplan are based on suggestions made by the participants at a one-day workshop that was held on May 12, 2003 in Rosemont, Illinois. There were 55 workshop participants, including the facilitators. Workshop participants represented a broad spectrum of research areas, interests, and organizations. Names and contact information for the workshop participants are included in Appendix A.

Workshop sessions were organized into four primary research or focus areas. These focus areas were: (1) vehicle emissions characterization, (2) ambient monitoring, (3) analysis, and (4) control strategies/measures. Each workshop participant attended three sessions lasting one hour apiece and provided their recommendations to the session facilitators about which research efforts the transportation community should undertake in the near future. For the purposes of this workshop and this associated report, the transportation community is defined to be those responsible for on-road vehicles, associated roadways, and roadway construction equipment. Those recommendations are the focus of this report. The session organization is summarized in Appendix B.

This report is organized with the first four chapters describing the priority research areas discussed at the corresponding workshop sessions. Each chapter contains a brief introduction, a summary of the current information base by topic area from the companion literature review (FHWA, 2003), a summary of the discussion that occurred during the workshop sessions, and concludes with 3 or 4 proposed program areas for research. Chapter 5 of this report summarizes the priority research areas for all sessions/topics combined.

Air toxics are pollutants known to cause - or that are suspected of causing - cancer or other serious health effects, such as reproductive problems or birth defects. Many toxics are known to cause respiratory, neurological, immune system, or reproductive problems, particularly in more susceptible and sensitive populations, such as children.

One of the difficult issues for air toxics is to decide which individual air toxic compounds to focus on. The 1990 Clean Air Act Amendments listed 188 hazardous air pollutants (HAPs) for EPA to regulate. However, not all of these compounds are emitted by motor vehicles. Because EPA is charged with determining whether mobile source air toxics controls are technologically feasible and setting vehicle-based air toxics controls, the evaluations that EPA had performed as of 2001 are summarized in EPA's Technical Support Document for Control of Emissions of Hazardous Air Pollutants from Motor Vehicles and Motor Vehicle Fuels. In this technical support document, EPA identifies 21 of the 188 compounds that should be considered mobile source air toxics (MSATs), and examines the mobile source contributions to national inventories of these compounds, and the impacts of existing and newly promulgated mobile source control programs.

The term mobile source air toxics, or MSATs, was developed by EPA to signify those air toxics emitted by nonroad engines and on-highway motor vehicles. Section 202(l) of the Clean Air Act, which addresses controls for HAPs from motor vehicles and motor vehicle fuels, does not specify which pollutants are to be evaluated as air toxics, other than benzene, formaldehyde, and 1,3-butadiene. EPA developed a list of 21 MSATs by comparing the lists of compounds identified in motor vehicle emission data bases and studies with the toxic compounds listed in the Integrated Risk Information System (IRIS). This list is shown in Table 1. The purpose of the list is to provide a screening tool that identifies those compounds emitted from motor vehicles or their fuels for which further evaluation is appropriate. Six mobile source air toxics are of primary concern according to EPA: benzene, 1,3-butadiene, formaldehyde, acetaldehyde, acrolein, and diesel particulate matter plus diesel exhaust organic gases. These compounds are higlighted in bold in Table 1. EPA's National-Scale Air Toxics Assessment identified these pollutants as posing the greatest health risks.

Table 1. List of Mobile Source Air Toxics
Acetaldehyde Diesel Particulate Matter + Diesel Exhaust Organic Gases Methyl Tertiary Butyl Ether (MTBE)
Acrolein Ethylbenzene Naphthalene
Arsenic Compounds Formaldehyde Nickel Compounds
Benzene n-Hexane Polycyclic organic matter (POM)
1,3-Butadiene Lead Compounds Styrene
Chromium Compounds Manganese Compounds Toluene
Dioxin/Furans Mercury Compounds Xylene

The IRIS listing and the summary of cancer and noncancer health effects described in the final Health Assessment Document for Diesel Exhaust are based on studies linking serious adverse health effects to whole diesel exhaust exposure, using diesel particulate matter (DPM) as the measure of dose. Available science, while suggesting an important role in toxicity for the particle phase component of diesel exhaust, cannot rule out a role for the gas phase components such as semi-volatile organics that are partly in both the gas and particle phases.

EPA defines diesel particulate matter (DPM) plus diesel exhaust organic gas (DEOG) as a primary mobile source air toxic. This definition was adopted because it was believed to focus on the components of diesel exhaust expected to contribute to observed cancer and noncancer health effects. Research studies do not separate the health effects of the particulate and gaseous components of diesel exhaust. DPM+DEOG is a particular type of emission which is composed of many listed HAPs, including chemicals that fall into the group of polycyclic organic matter (POM) chemicals, as well as some HAP metals and volatile organic compounds (VOCs). POM includes organic compounds with more than one benzene ring, and which has a boiling point greater than or equal to 100oC.

A group of seven polynuclear aromatic hydrocarbons (7-PAH), which have been identified by EPA as probable human carcinogens [benz(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, chrysene, dibenz(a,h)anthracene, and indeno(1,2,3-cd)pyrene] are sometimes used as surrogates for the larger group of POM compounds. Another common PAH grouping is referred to as 16-PAH. This grouping includes the following compounds in addition to the 7-PAH group: acenaphthene, acenaphylene, anthracene, benzo(g,h,i)perylene, fluoranthene, fluorene, naphthalene, phenanthrene, and pyrene.

MSATs come from many sources. First, some air toxics are present in fuel and are emitted to the air when the fuel evaporates or passes through the engine unburned. Benzene, for example, is a component of gasoline. Cars emit small quantities of benzene in unburned fuel, or as vapor when gasoline evaporates. Second, MSATs are formed through engine combustion processes. For instance, a significant amount of gasoline-vehicle benzene comes from the incomplete combustion of compounds in gasoline such as toluene and xylene that are chemically very similar to benzene (EPA, 2000). Like benzene, these compounds occur naturally in petroleum and become more concentrated when petroleum is refined to produce high-octane gasoline. DPM + DEOG emissions, as well as formaldehyde, acetaldehyde, and 1,3-butadiene, are also by-products of incomplete combustion. Third, some compounds, like formaldehyde and acetaldehyde, are also formed through a secondary process when other mobile source pollutants react in the atmosphere. Fourth, metal air toxics result from engine wear or from impurities in oil or gasoline. They can also be present in fuel and lubricant additives. Metals also appear in emissions that are related to vehicle operation-brake and tire wear, mechanical deterioration of catalysts and other exhaust system components, and resuspension of road dust. Finally, burned and unburned oil emitted via the tailpipe are sources of HAPs.

Other comprehensive studies besides EPA's MSAT study that have examined the importance of mobile sources to measured air toxic concentrations have included EPA's National Air Toxics Assessment and the South Coast Air Quality Management District (SCAQMD) Multiple Air Toxic Exposure Study-II. EPA's National Air Toxics Assessment collects and evaluates monitoring data from around the United States and estimates the contributions of various source types to measured concentrations (EPA, 2002a). EPA compiled 1996 ambient monitoring data for 33 toxic air pollutants and estimated source contributions. On-road mobile sources contributed 30 to 65 percent of the total measured concentrations for the six priority MSATs. Mobile sources (primarily non-road engines/vehicles) were also responsible for 16 percent of nickel concentrations, 8 percent of chromium, and 5 to 6 percent of arsenic and manganese levels.

The SCAQMD conducted the Multiple Air Toxic Exposure Study-II during 1998 and 1999, which involved monitoring and modeling over 30 different pollutants across the South Coast Air Basin, including most of the 21 MSATs identified by EPA. Using monitored concentrations, the SCAQMD estimated that MSAT (DPM, 1,3-butadiene, benzene, formaldehyde, and acetaldehyde) contributed 89 percent of the total excess cancer risk from the more than 30 pollutants measured. It was also estimated that on-road vehicles were responsible for about 48 percent of the total excess cancer risk based on the on-road vehicle fraction of total MSAT emissions for each pollutant evaluated. Whether the results of this study are borne out by future research is still in question - nevertheless, it is important research.

Transportation agencies have an interest in air toxics because of the potential health risks of exposures to individual HAPs from mobile sources and because of the relationships between air toxics and other current atmospheric concerns. The interrelationship among air toxics and other pollutants is summarized in Figure 1. Interactions between air toxics and ozone are an important concern since one of ozone's precursors, VOCs, include many of the EPA MSATs. Some particulates are also toxic, such as diesel particulate matter plus diesel exhaust organic gases, therefore reductions in the mass of these particulates may also reduce toxic air contaminants. In addition, particulate matter (PM) indirectly affects global climate change by increasing light scattering and the number of particles available for cloud droplet formation.

The figure illustrates the inter-relationships between particulate matter, ozone, mobile source air toxic compounds, and the precursor chemical compounds that are common to all of these. Also noted are additional relationships to regional haze and climate change effects that aren’t directly harmful to humans but influence the environment. The figure illustrates that the compounds can react with one another influencing the concentrations of these in the atmosphere and on the atmospheric conditions causing haze and potential climate change.
Figure 1. Relationship of Air Toxic to Other Atmospheric Concerns

Air toxics can be divided into those that are primarily organics and those that are particulates. The organic HAPs react in the atmosphere (each at different rates) to form ozone - so they are ozone precursors. Similarly, particulate HAPs are PM2.5 and PM10 precursors. Some areas are also finding organic compounds, including secondary organic aerosols, to be an important contributor to measured PM2.5 ambient concentrations. Particulates contribute to regional haze, as well. Therefore, the importance of mobile source air toxics in ozone and PM10/PM2.5 formation, or to regional haze, is a factor to consider if and when air toxic control strategies are formulated.

Table 2 provides additional detail about which of the 21 MSATs are primarily organic versus inorganic (i.e., metals). Note that DPM plus DEOG as well as POM are both organic and inorganic. Of the inorganic air toxics, arsenic, chromium, lead, manganese, mercury, and nickel are metals. Among the organic air toxics, benzene is stable in the atmosphere, but formaldehyde, acetaldehyde, acrolein, and 1,3-butadiene are reactive. Formaldehyde, acetaldehyde, and acrolein are emitted directly by motor vehicles and also formed secondarily in the atmosphere.

Table 2. Mobile Source Air Toxics that are Primarily Organic Versus Inorganic
Specific Organic Compounds Compounds Containing Inorganics Pollutant Categories
Arsenic Compounds
Chromium Compounds
Lead Compounds
Manganese Compounds
Mercury Compounds
Nickel Compounds

Air toxics differ from criteria air pollutants in that air toxics are not subject to a national ambient air quality standard (NAAQS). Without an ambient standard, research on air toxics includes the following risk assessment steps: emissions, ambient concentrations, exposure, and adverse health effects. EPA's MOBILE6.2 model has some capability to estimate air toxic emission factors. Available air quality dispersion models have sometimes been adapted for toxic assessments. Tools for modeling air toxics exposure and characterizing risk are available, but are less familiar to the transportation practitioner.

Because of the potential importance of DPM + DEOG as a mobile source air toxic, and the similar importance of evaluating the effect of diesel and gasoline engine emissions on ambient particulate (PM10 and PM2.5) concentrations, there are a number of instances in this work plan where PM is discussed and specific recommendations made related to PM. These PM references are an acknowledgment of the difficulty in separating the research needs for DPM + DEOG as a mobile source air toxic from those for diesel and gasoline vehicle emissions as a criteria air pollutant. Nevertheless, because this is an air toxics research plan, its orientation is towards research needed to better understand and evaluate air toxics. However, it is expected that particulate air toxics research and criteria pollutant PM research will ultimately be coordinated efforts that provide valuable information to both research areas.

There are acknowledged weaknesses in the basic analytic tools and data for air toxics analyses. In particular, air pollution control agency staff charged with responding to concerns about exposures to air toxics are challenged by factors such as a limited understanding of the spatial distribution and atmospheric reactions of toxic air pollutants, inaccurate and incomplete emissions inventories, and inadequate emissions models. Attempts to require or apply common stationary source air toxic analysis techniques have to date been inconclusive or impractical to implement. The gaps in information necessitate targeted research focused specifically on the needs of the transportation community, including tools to assess MSAT risks for specific toxics, as often requested by air pollution control agency staff.

FHWA's Role in Air Toxics Research

The FHWA's 1998 National Strategic Plan establishes the Agency's mission "to continually improve the quality of our Nation's highway system and its intermodal connections." It identifies five strategic goals for achieving this mission, one of which is to protect and enhance the natural environment and communities affected by highway transportation.

Building on the National Strategic Plan, FHWA established an environmental research program as a core component of the Agency's environmental stewardship responsibilities. The Agency's broad environmental research goals are identified in FHWA's 1998 Strategic Plan for Environmental Research. Air quality research is one of eight program goals established in the Strategic Plan for Environmental Research.

This strategic workplan for Air Toxics Research draws on FHWA's previous strategic planning initiatives to provide direction and focus for the Agency's role in air pollution research. It establishes a two-fold vision for conducting research that establishes a transportation focus in air pollution research and ensures that research results are relevant to the needs of transportation policymakers.

Integrating Research Focus Areas and Policy Questions

Figure 2 summarizes the priority mobile source air toxics research areas based on the discussion at the May 2003 workshop. This diagram is the blueprint for the workplan, and it identifies how each priority project helps to answer one or more of the transportation community's critical questions.

Most emphasis, however, has been placed on research to help understand the contribution of transportation sources to air toxics, because this is a critical research gap of primary importance to the transportation community. The blueprint shows how the research priorities in this plan form a research path that integrates findings across focus areas to answer the transportation community's critical research questions.

FHWA acknowledges that the technical complexity and broad scope of the projects contained in the workplan will require a coordinated, multi-agency approach. The multi-disciplinary range of projects outlined in Figure 2 will likely require coordination among FHWA, State Departments of Transportation, and metropolitan planning organizations, involvement by academic and applied research organizations, as well as State and Federal air quality agencies, and industry groups. Dialogue among all these groups is encouraged to facilitate speedy resolution of issues critical to implementing this workplan, particularly, equitable distribution of research leadership, development of detailed project scope information, and funding responsibilities for individual projects.

Appendix C lists recent, relevant air toxics research papers and reports submitted by the workshop participants at the time of the workshop. These research works are not included in the companion literature review that was prepared and distributed to participants prior to the workshop.

Key Questions: Do we have adequate emission measurement methods for MSATs? How do the existing ambient monitoring sites/networks assist in providing data about the mobile source contribution to ATs? What improvements to existing models, or new modeling techniques, need to be developed to provide viable assessments of AT levels (emissions and concentrations) in situations where direct measurements are not available? Which are the most cost-effective control strategies for transportation sources?
Proposed Research Programs and Projects: P1: Fund research for improvements in emission measurement technology that are needed to measure the lower emission levels of air toxic compounds expected with improved emission control technologies and lower sulfur fuels expected in the 2005 to 2007 time frame. P4: Fund research to identify what additional monitoring information needs to be collected to enhance existing monitoring networks to meet transportation sector needs. P7: Develop a protocol on the appropriate methods to be used in emissions modeling, atmospheric dispersion modeling, and exposure modeling at varying spatial scales applicable to transportation projects (e.g., project-level, urban scale). P11: Performing studies of potential control measures and their cost effectiveness is predicated on there being observed harmful levels which can be ameliorated by reducing motor vehicle emissions. Therefore, any programmatic initiatives to reduce air toxic emissions need to be informed by research on existing ambient air toxic concentrations, estimated associated risks, and the mobile source contributions to them.
  P2: Design and initiate experiments to examine near roadway concentration patterns (especially for highways). There is evidence that air toxic concentrations near roadways are appreciably higher than those 300 to 500 meters or more downwind. Experiments to examine these gradients are likely to assist in steering future research toward examining pollutant emissions, fate, and transport in the immediate vicinity of highways compared with examining regional scale chemistry and transport. P5: Fund research to understand existing, and develop new, practical tools for local and regional organizations to assess MSAT impacts and evaluate results. P8: Conduct research and develop methods on variability and uncertainty analysis of on-road emission estimates. Identification of key sources of uncertainty can be used to target resources to reduce uncertainty. Sub-Area 1: Research the expected multi-media MSAT benefits or dis-benefits of the control measures that are expected to be the leading candidates for adoption in upcoming 8-hour ozone and PM2.5 nonattainment plans.
  P3: Conduct research to expand the available set of information about air toxic emissions and activity patterns for the on-road and off-road vehicle types with the most significant contributions to ambient air toxic concentrations of concern and the greatest uncertainty in their emission estimates. Given the current state-of-knowledge and planned research projects in this area, two sub areas within this programmatic initiative have been identified as likely first priorities for study. P6: Determine whether the existing knowledge about transportation air toxics can be enhanced by further examining current data. P9: Develop improved inputs for emissions and receptor modeling. Some improvements are expected via incorporating already available data. Gap filling research may be needed in selected topics. Sub-Area 2: Determine the emissions and potential emission reductions for measures that could be applied to mitigate the emissions from non-road construction equipment that is typically used in constructing/widening highways.
  Sub-Area 1: There are needs to investigate the differences in fleet populations and travel characteristics for important sub-categories of trucks. These sub-categories can be defined as: long-range fleet, regional fleet (300 to 500 miles), and local fleet (primarily delivery trucks).   P10: Research the atmospheric chemistry of mobile source air toxics. Incorporate this information into distributed modeling tools. Sub-Area 3: Research the effectiveness of control strategies on toxics. This initiative specifically focuses on identifying the most cost effective control strategies available for reducing air toxic emissions. What would provide the best and most cost effective results where the most cost effective air toxics control strategies could differ from the most cost effective criteria pollutant options?
  Sub-Area 2: For off-road engines/vehicles, diesel-powered construction equipment is a major emissions source (of PM) with large uncertainties in emission estimates.      
Focus Areas: Characterization Ambient Monitoring Analysis Control Measures/Strategies

Figure 2. Connection Between Transportation Issues and Research Agenda


Air Resources Board
compression ignition
compressed natural gas
carbon monoxide
carbon dioxide
Coordinating Research Council
diesel exhaust organic gas
Department of Energy
diesel particulate matter
elemental carbon
U.S. Environmental Protection Agency
Federal Highway Administration
Federal Test Procedure
geographic information system
hazardous air pollutants
heavy-duty diesel vehicles
Health Effects Institute
inspection and maintenance
Integrated Risk Information System
Los Angeles 1992 unified driving cycle
light-duty gasoline vehicle
liquefied petroleum gas
Motor Vehicle Emission Simulator
mobile source air toxics
methyl tertiary butyl ether
National Ambient Air Quality Standards
National Air Toxics Trends Stations
Northern Front Range Air Quality Study
oxides of nitrogen
National Renewable Energy Laboratory
polynuclear aromatic hydrocarbons
Photochemical Assessment Monitoring Stations
portable emission measurement systems
particulate matter
polycyclic organic matter
parts per million
Regulatory Modeling System for Aerosols and Deposition
reformulated gasoline
South Coast Air Quality Management District
spark ignition
State Implementation Plan
State and Territorial Air Pollution Program Administrators/Association of Local Air Pollution Control Officials
vehicle miles traveled
volatile organic compounds
vehicle specific power
Updated: 3/17/2015
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000