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Last Updated: Friday, January 19, 2018 2:08 PM

Air Toxics Analysis Tools

Toxic Air Contaminants (TACs, or Air Toxics) are chemicals that have a serious (usually cancer-related) health impact on individuals based on direct emissions, either acutely or chronically, but are not Criteria Pollutants like carbon monoxide (CO). Research is still going on to determine the cause of and appropriate mitigation for impacts of air toxics related to transportation facilities, but there is a clear correlation between proximity to highway traffic (especially trucks), and rail operations, and adverse impacts related to air toxics. Criteria Pollutant standards are based on human health and welfare effects with a margin of safety, and are specified as a concentration in ambient air. Air Toxics generally have no ambient air quality standard - the threshold for potential adverse effect is typically assumed to be zero.

Many Federal and State laws and regulations apply to Air Toxics. Most that have specific analysis requirements are targeted at industrial and other facilities that must have air district, State air agency, and/or EPA permits to construct and operate. Transportation facilities in general are exempt from permitting requirements related to Air Toxics, so detailed analysis methods and requirements, including formal Health Risk Analysis, are not required for them under programs such as Proposition 65 and the "AB 2566" Toxic Hot Spots permitting program.

For transportation projects and plans, the focus is usually on the "priority" Mobile Source Air Toxics (MSAT). These are the TACs that cause most of the health risk related to mobile sources like motor vehicles. U.S. EPA Exit Caltrans website issued regulations in 2001 and 2007 specifying priority MSAT, which are addressed in the FHWA Interim Guidance discussed below. Priority MSAT identified in the 2007 EPA rules are summarized in this PDF file. The California Air resources Board (ARB) lists Diesel Exhaust Particulate Matter (DPM) as a TAC, and in California it is normally used in place of DPM + Diesel Exhaust Organic Gas as listed by EPA.

Federal Highway Administration Guidance

The Federal Highway Administration (FHWA) has identified Air Toxics as a matter to be analyzed in National Environmental Policy Act (NEPA) environmental documents for highway projects. The FHWA issued interim guidance in 2006 (updated in 2009) or performing Air Toxics studies. The FHWA guidance generally describes a 3-tier process for evaluating a project's Air Toxics effects:

  1. Is the project covered by a Categorical Exclusion under NEPA? If so, its impacts are de minimus and no further Air Toxics analysis is required. In areas subject to conformity requirements, the project must also be exempt from conformity analysis requirements (see 40 CFR 93.126 and 40 CFR 93.128). For CE projects that are subject to hot spot analysis requirements (40 CFR 93.127 list) for conformity in PM10 and PM2.5 areas, the project should be "Not a Project of Concern" or "Not a POAQC" to fit this category.
  2. Does the project affect a facility with 140,000 AADT or more in any analysis year through the design year of the project (usually at least 20 years from opening)? If so, a quantitative emission analysis (limited to MSAT emission burden, not including dispersion modeling and Health Risk Analysis) is required to document the relative impacts of project alternatives. This level of analysis may be considered at lower traffic volumes in California due to publication of the California Air Resources Board's Land Use Handbook Exit Caltrans website; the Handbook specifies that freeways, urban non-freeways with AADT > 100,000, and rural non-freeways with AADT > 50,000 are of serious concern for MSATs, so detailed analysis under the FHWA Guidance may be considered if sensitive receptors are nearby.
  3. A project that fits in neither the first nor the second groups would normally have a qualitative analysis prepared that documents the potential effect, or lack of substantial effect, of the project regarding Air Toxics.

The FHWA Guidance deals with operational emissions, which are usually the largest part of project-related MSAT emissions. It does not require quantification of construction-related emissions.

California's EMFAC, unlike U.S. EPA's emission model (MOVES Exit Caltrans website) used elsewhere in the country, does not directly produce estimates of the key mobile-source air toxic (MSAT) compounds. A separate "speciation" step is required using factors provided by the Air Resources Board Exit Caltrans website and the Office of Environmental Health Hazard Assessment Exit Caltrans website. Speciation factors for most organic priority MSAT listed by EPA, and diesel particulate matter, are included in the CT-EMFAC tool available at this web site. For some MSAT, post-processing is needed using a spreadsheet provided with CT-EMFAC 4.1. Speciation factors in a format that is usable directly with EMFAC for the additional MSAT identified in 2007 are not available from ARB.

The "template" language in the FHWA Guidance may need to be modified to reflect use of the EMFAC model in California rather than the MOBILE model, and to cover other specific issues such as dealing with the ARB Land Use Handbook. For instance, California project analysts may wish to use criteria described in the ARB Land Use Handbook instead of FHWA's 140,000-150,000 AADT to determine when a detailed (quantitative or emission) anlaysis is needed.

FHWA staff has informally indicated that its guidance does not preclude more detailed analysis, including quantitative Health Risk Analysis (HRA). However, they expect that situations requiring such analysis should be rare when MSAT analysis is considered in a NEPA (alternatives analysis) context. Studies using more detailed analysis should describe the unusual or unique aspects of a project require a level of analysis beyond what the FHWA guidance provides for. Caltrans and FHWA do not at this time have guidelines or supported tools for studies that go beyond FHWA guidance.

Download FHWA Guidance:

Air Districts

Some air districts place major emphasis on MSAT analysis in their CEQA Guidance documents and local thresholds of significance. In many cases, air district guidance may appear to mandate a quantitative Health Risk Assessment involving detailed traffic, land use, dispersion, and other modeling. Project analysts should use screening methods such as the FHWA guidance (above) or other procedures before committing to a detailed modeling-based analysis. A roadway-oriented MSAT (diesel exhaust PM-oriented) screening tool that may be useful is published by Sacramento AQMD:


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