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Last Updated: Monday, May 5, 2008 2:52 PM

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Air Toxics Analysis Tools

Toxic Air Contaminants (TACs, or Air Toxics) are a relatively new field of investigation for transportation projects. In general, Air Toxics are compounds that have a 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 specific impacts of air toxics related to transportation facilities. Criteria Pollutant identification is also based on human health and welfare effects, but they are called out specifically in legislation and have ambient air quality standards. Air Toxics are identified based on regulation, and generally have no ambient air quality standard - the threshold for adverse effect is typically zero.

Many Federal and State laws 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 most transportation projects under programs such as Proposition 65 and the "AB 2566" Toxic Hot Spots permitting program.

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 January 2006 for performing Air Toxics studies. The FHWA guidance 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.
  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 quantitive emission analysis (not including dispersion modeling and Health Risk Analysis) is required to document the relative impacts of project alternatives.
  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 serious effect, of the project regarding Air Toxics.

The FHWA Guidance deals with operational emissions, which are usually the largest part of project emissions. Quantification of construction-related emissions is not required.

California's EMFAC, unlike the MOBILE model used elsewhere in the country and on which the FHWA guidance is based, does not directly produce estimates of the key mobile-source air toxic (MSAT) compounds. A separate "speciation" step is therefore required using factors provided by the Air Resources Board.

The "template" language in the FHWA Guidance is in the process of being modified to reflect use of the EMFAC model in California rather than the MOBILE model. When available, the revised language will be posted here.

Download FHWA Guidance:

UC Davis Spreadsheet Tool

To assist with performing an Air Toxics analysis, UC Davis developed a spreadsheet to help structure the use of the EMFAC model. "Speciation factors" supplied by the Air Resources Board for determining toxics from EMFAC outputs are incorporated into the UCD spreadsheet.

Notices

UCD Spreadsheet requires Microsoft Excel™ or compatible software for use. Free Excel™ Viewer software can be downloaded from the Microsoft™ web site, but may not be fully compatible with the multiple-page layout of the spreadsheet.

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