Chapter 11 - Air Quality

What Does This Topic Include?

This topic covers the regulatory framework and recommended procedures for performing an air quality analysis for both Caltrans and local agency transportation projects. Preparation of the air quality section of the environmental document and supporting technical report are discussed in detail. There is also discussion of air quality requirements throughout the project delivery process, from transportation conformity determinations at the regional planning stage to project requirements during construction.

Laws, Regulations, and Guidance

See also SER, Chapter 1, "Federal Requirements" and Chapter 2, "State Requirements" for laws and regulations that apply to all environmental analysis activities.

The primary laws related to air quality issues are the federal and California Clean Air Acts, federal Planning regulations and enabling legislation, U.S. EPA conformity regulations, and local and air district ordinances and regulations related to specific activities. Air quality is a required subject for the environmental analysis pursuant to the National Environmental Policy Act (NEPA) and the California Environmental Quality Act (CEQA).

NEPA Assignment

California participated in the "Surface Transportation Project Delivery Pilot Program" (Pilot Program) pursuant to 23 USC § 327, for more than five years, beginning July 1, 2007, and ending September 30, 2012. MAP-21 (P.L. 112-141), signed by President Obama on July 6th, 2012, amended 23 USC § 327 to establish a revised and permanent Surface Transportation Project Delivery Program. As a result, the Department entered into a memorandum of understanding pursuant to 23 USC § 327 (NEPA Assignment MOU) with FHWA. With NEPA Assignment, FHWA assigned and the Department assumed all of the United States Department of Transportation (USDOT) Secretary's responsibilities under NEPA. This assignment includes projects on the State Highway System (SHS) and Local Assistance Projects off the SHS within the state of California, except for categorical exclusions that FHWA assigned to the Department under the 23 USC § 326 CE Assignment MOU, projects excluded by definition, and specific project exclusions. Under the 23 USC § 327 MOU, FHWA's air quality conformity responsibilities may not be assigned to the Department. However, if a project under 23 USC § 327 is clearly exempt from the conformity requirements under 40 CFR 93.126 or 40 CFR 93.128 a conformity determination from FHWA is not needed. Additionally, a project-level conformity determination from FHWA is not needed for any project exempt from regional conformity (40 CFR 93.127) if the project is located in an area NOT subject to CO, PM2.5, and/or PM10 hot-spot requirements (i.e. ozone-only areas). This is documented in the Air Quality Conformity Findings Checklist. For projects under 23 USC § 327 requiring interagency consultation, FHWA participates in the interagency consultation because FHWA continues to make the project-level conformity determination for those projects.

Air quality conformity determinations for CEs assigned to the Department under 23 USC § 326 MOU have been assigned to the Department. FHWA does not participate in the interagency consultation process for projects assigned under 23 USC § 326; the Department carries out that function.

See the SER, Vol. 1, Chapter 38, "NEPA Assignment" for more information

Federal Air Quality Laws and Regulations

State Air Quality Laws and Regulations

Policy Memos

Key Web Sites

Air Quality Guidance and Information

Contact Lists and Related Information


Regulatory Framework

Nearly all transportation projects have some effect on air quality. On-road motor vehicles are the major source of air pollutant emissions contributing to violations of most air quality standards. Projects that affect the capacity or location of major roads or other elements of the transportation system can cause both adverse and beneficial air quality impacts. The impact is often positive, a capacity or alignment change may improve traffic flow enough, at least for a period of time, to reduce pollutant emissions compared to existing conditions or no-build alternative. In the long term, overall emissions from motor vehicles will drop due to improved vehicle emission controls, but future traffic growth will, to some degree, counteract these reductions. For some pollutants, vehicle-related emission controls may not have a major effect; for example, re-entrained road dust emissions are roughly proportional to vehicle miles traveled. Also, short-term emissions from construction processes and equipment may be an issue if not adequately accounted for in existing regional air quality plans and regulations.

Most of California's transportation air quality requirements come from the federal Clean Air Act (FCAA) and the California Clean Air Act (CCAA). The FCAA, as amended in 1990, sets nationwide air quality standards, called National Ambient Air Quality Standards (NAAQS). The U.S. Environmental Protection Agency (U.S. EPA) administers the FCAA, sets the specific air quality and emissions standards, and delegates certain responsibilities to other federal agencies and to the states. U.S. EPA sets and updates both primary (health) and secondary (welfare) standards for the six "criteria pollutants" - carbon monoxide, lead, nitrogen dioxide, ozone, sulfur dioxide, and particulate matters (PM10 and PM2.5).

In addition to the criteria air pollutants for which there are National Ambient Air Quality Standards (NAAQS), U.S. EPA and the California Air Resources Board (ARB) also regulate Toxic Air Contaminants (TACs, or air toxics). Most air toxics originate from human-made sources, including on-road mobile sources, non-road mobile sources (such as airplanes), areas sources (e.g., dry cleaners), and stationary sources (e.g., factories or refineries). Mobile Source Air Toxics (MSAT) make up a subset of the 188 (187 as of May 2014) air toxics defined by the FCAA. MSAT are compounds emitted from highway vehicles and non-road equipment. FHWA released updated Interim Guidance on Mobile Source Air Toxic Analysis in NEPA Documents on January 18, 2023.

While the FHWA Interim Guidance and U.S. EPA's emission control programs provide for the national level, California's vehicle emission control and fuel standards are more stringent than federal standards. The state's standards are also effective sooner, so the effect on air toxics of combined state and federal regulations is expected to result in greater emission reductions, more quickly, than the FHWA analysis on which the Interim Guidance is based shows. The FHWA analysis, with modification related to use of the California-specific EMFAC model rather than the MOVES model, would therefore be conservative.

The California Clean Air Act sets up a structure managed by ARB similar to the federal one for air toxics. All federal air toxics are incorporated into the California lists by reference. In addition, California regulates a large number other substances not currently on the federal list. Key California-only air toxics for consideration in transportation projects include diesel exhaust particulate matter and naturally occurring asbestos.

Note that not all of the ARB toxics programs apply to transportation projects; consult with technical or Legal staff before getting into a lot of detail.

Under the FCAA, U.S. EPA has designated planning areas throughout the country. Areas are classified as being in "attainment" for a given pollutant if they meet the prescribed standards. If an area does not meet the standard, it is designated as a "nonattainment" area for that pollutant. Areas that were previously designated as non-attainment areas but have now met the standard (with U.S. EPA approval of a suitable air quality plan) are called "maintenance" areas.

The FCAA requires that states produce improvements in air quality over time. This means reducing air pollution to healthful levels in nonattainment areas and developing controls to ensure the air remains healthful in subsequent years. The State Implementation Plan (SIP) process is the means by which states develop a collection of regulations and plans to demonstrate this effort to the federal government. In order to meet their transportation planning goals, Metropolitan Planning Organizations (MPO) and Regional Transportation Agencies (RTPA) create long-range plans and programs, such as Regional Transportation Plans (RTP) and Regional Transportation Improvement Programs (RTIP), which include proposed transportation projects. The projects included in these plans and programs must be consistent with (or conform to) the approved SIP, and hence the requirements of the FCAA. This process is called "Transportation Conformity." If a project would contribute to the violation of a standard, it cannot be included in the conforming plan and cannot be built. For more about how air quality is addressed at the planning stage, see the SER, Vol. 1,Chapter 4, "Environmental Considerations During Transportation Planning."

In nonattainment or maintenance areas, transportation conformity applies if projects will be funded by the FHWA, Federal Transit Administration (FTA), or any agency that has been delegated project approval by these agencies. It also applies if projects are determined to be regionally significant as defined at 40 CFR 93.101 and are approved by a regular recipient of federal highway or transit funds, such as Caltrans and most local transportation agencies.

A further demonstration of transportation conformity–at the project level–is required if a project is located in a nonattainment or maintenance area. The basic demonstration of conformity consists of showing that the project is listed in and consistent with a conforming RTP and TIP. In addition, a microscale or "hot-spot" analysis for conformity is required if a project is located in a nonattainment area for CO, and/or PM2.5 and PM10. Emission reduction measures may be required to ensure that the project will not cause or contribute to new violations of a standard. Caltrans cannot approve a project that would cause or contribute to violation of a federal air quality standard. FHWA and U.S. EPA released a national guidance memo on March 29, 2006, for doing qualitative PM2.5 and PM10 hot-spot analysis. See the attachment at the bottom of this memo.

The California Clean Air Act (CCAA) must be addressed in CEQA documents for transportation projects. Although state air standards for some pollutants are more stringent than federal standards, there is no conformity process under the state law. Over time, as air quality improves under the federal conformity process, it is expected that there will be progress in meeting state standards as well. The California Air Resources Board (ARB) is responsible for establishing state air quality standards and enacting regulations for statewide air pollution control programs, and for submitting the SIP to the U.S. EPA. Local air pollution control districts and air quality management districts directly regulate stationary and some area sources of emissions, but not mobile sources, under the structure set up by the CCAA. The air districts perform most of the air quality planning in California, including the development of SIP submittals under the FCAA.

Various Pollutants

Environmental documents for FHWA Federal-aid projects (on and off the SHS) must analyze a variety of air pollutants in order to assess a project's potential to impact the environment. The following is a brief breakdown of the most common pollutants.

  • Ozone impacts are regional in nature and cannot be ascribed to any single project. Ozone is formed by reactions, in the presence of sunlight and heat, primarily between organic compounds and nitrogen oxides. These precursor compounds are emitted by all combustion sources, and in many regions mobile sources including motor vehicles are the major source. Ozone violations most often occur in summer (April-October), since strong sunlight and heat are necessary for its formation. Projects that are included in the RTP and TIP have been included in a regional conformity analysis and require no further conformity analysis for ozone at the Project initiation Document (PID) or subsequent project phases. Additional ozone analysis may be needed for CEQA, however.
  • Particulate Matter (PM10 and PM2.5) refers to airborne particles that are less than 10 microns in diameter (PM10) and less than 2.5 microns in diameter (PM2.5), respectively. Particulate matter is both a regional and project-level issue. Particulate matter is both directly emitted and, especially for PM2.5, a result of secondary formation based on nitrogen oxides (NOx), volatile organic compounds (VOC), sulfates (SOx), and ammonia (NH3). As with ozone, secondary pollution forms some distance away from the precursor emission sources, and up to several hours later. Regional PM is primarily a winter nighttime product, since cool, damp, stable weather is needed to support the chemical reactions that produce it. Directly-emitted PM2.5 and PM10 has been determined to be a conformity issue in California.  In areas that are subject to conformity requirements for PM10 or PM2.5, projects determined to be of Air Quality Concern (POAQC) must perform a project-level PM analysis following the EPA Quantitative Analysis Guidance (2021). For projects determined not to be of Air Quality Concern, Project-level PM emissions analysis should follow the EPA's Qualitative Analysis Guidance (2006). NOTE: POAQC determinations are made through Interagency Consultation. PM from diesel engine exhaust (mostly in the PM2.5 size range) is also a toxic air contaminant under California regulations and one of the Mobile Source Air Toxics (MSAT) listed by U.S. EPA.
  • Carbon monoxide is emitted directly from vehicles and is a major issue at the project level. Transportation projects in nonattainment or maintenance areas for CO are evaluated using approved emissions models such as the Caltrans/UCD CO Protocol and  the CALINE4 model.
  • Asbestos is an air quality, as well as a hazardous materials, issue. If structural demolition is needed, such as removal of an existing bridge, the release of friable asbestos may be an abatement issue. In areas where naturally occurring asbestos is an issue, geologic studies, mandatory air district notices, special construction provisions, and other activities may be needed. Structural asbestos (demolition) is regulated by federal and related state/air district regulations (NESHAP). Naturally occurring asbestos is regulated under ARB regulations and worker-safety programs.
  • Construction stage impacts are an issue in some areas of the state. This is especially true if sensitive land uses (such as schools, health- and child-care facilities) are nearby. Dust emissions are an important consideration, but are largely mitigated by standard dust control techniques required by Caltrans' Standard Specifications (Section 14-9.02, 2018 version). In a growing number of areas, more stringent regulations requiring specific work practices and plans for dust control are contained in air district or local rules; these regulations apply to all work using Caltrans specifications and special provisions based on Section 14-9.02 of the Caltrans Standard Specifications, 2018 version. Federal conformity regulations require analysis of construction impacts for projects when construction activities will last for more than five years; analysis for shorter construction periods may be needed for both NEPA and CEQA if sensitive land uses are nearby or construction emissions are likely to be large.
  • The Clean Air Act identifies 188 (187 as of February 2015) air toxics, also known as hazardous air pollutants. The U.S. EPA has identified 21 of these toxics as MSAT. They have also identified a subset of MSAT that are known as Priority MSAT. These are: 1,3-butadiene, acetaldehyde, acrolein, benzene, diesel particulate matter (diesel PM), ethylbenzene, formaldehyde, naphthalene, and polycyclic organic matter. Diesel Exhaust Particulate Matter is a growing area of interest, although no single analysis method, or criterion for determining when detailed analysis is needed, has yet been defined by the air quality regulatory community. Quantitative health risk analysis is generally not done for transportation projects, but there is a growing emphasis on identifying and avoiding impacts to sensitive receptors, such as schools. FHWA has updated their Interim Guidance on Air Toxic Analysis for NEPA documents several times.

Interagency Coordination

Early Coordination Meeting

Air quality is one of the more complex of the resource areas treated in environmental documents. For many projects there is a substantial amount of interagency work done at the transportation planning stage. When a project moves into the PID stage, environmental planners and air quality specialists should attend any early coordination meetings held for air quality.

For major projects, the local air district should be involved in at least one early meeting during project initiation. Also, the local air district should be invited to any public or agency scoping meeting held at the Project Approval and Environmental Document (PA&ED) phase.

Here are some good questions to consider at this stage:

  • Is the project in an area where federal Clean Air Act conformity requirements apply? If so, does the project come from a conforming Regional Transportation Plan?
  • Are any project-level conformity issues present? Will a hot-spot assessment or analysis be needed for conformity?
  • Is adequate traffic information available to determine the Level of Service (LOS), vehicle fleet mix, and traffic volumes both on the project (for Baseline, the no-build alternative, and viable build alternatives) and for nearby major intersections?
  • If an air quality analysis has already been prepared, does it adequately document project effects on localized emissions of CO and/or PM 2.5 and PM10?
  • Could asbestos be an issue? Asbestos may be naturally occurring, or can be from structural modification. Is any demolition or structural modification necessary?
  • What standard minimization and avoidance measures, such as dust control during construction, could be part of the project?
  • Are there schools, health facilities, or other sensitive receptors in the project area?

Interagency Consultation for Conformity

Conformity requirements apply in areas that are designated nonattainment or maintenance for federal air quality standards. For projects that affect highway capacity, or other projects where questions arise regarding conformity issues, a regional interagency consultation process is available to discuss and gain consensus on conformity issues. Areas in California that are subject to conformity requirements are shown at the Division of Environmental Analysis Air Quality Conformity web page. A map and a table of affected areas (with links to relevant transportation planning and air quality agencies) are available.

Interagency consultation requirements are defined in the U.S. EPA Conformity Rule at 40 CFR 93.105. Most metropolitan areas have adopted consultation procedures involving local, state, and federal transportation planning, transit, and air quality agencies. Interagency consultation for conformity and air quality planning is managed by the MPO in most areas.

FHWA retains air quality conformity determinations under the 23 USC § 327 NEPA Assignment MOU. Caltrans has prepared an Air Quality Conformity Findings Checklist, the FHWA Submittal Package Checklist, conformity language in annotated outlines for the various types of NEPA documents, and an annotated outline for an Air Quality Conformity Analysis report that supports requests to FHWA for project-level conformity determinations. These items are available at the SER Forms and Templates web site. Additional information can be found at the FHWA Transportation Conformity website.


There may be several distinct air quality analyses or studies performed in support of the project development process, both at the PID and the PA&ED phases. For many projects, a technical report is completed to summarize studies, outline conformity information, and provide other needed information for the environmental document.

Determining the Need for a Technical Report

This should be determined at the scoping stage of the project and documented in the PID. The project will probably NOT need a technical report on operational emissions analysis if:

  • it is exempt from all conformity analysis requirements (see the "Table 2" exemption list in 40 CFR 93.126 or 40 CFR 93.128 AND
  • it has no potential air quality impacts subject to NEPA analysis according to the initial screening procedure of the CO Protocol, and qualifies for a NEPA Categorical Exclusion pursuant to 23 USC 326 (except for 771.117[c][22] or 771.117[c][23]), AND
  • it qualifies for a Categorical Exemption under CEQA.

A construction greenhouse gas emissions analysis is required for all projects.

For projects in areas subject to conformity, the conformity documentation and the Air Quality Conformity Findings Checklist must be prepared and filed with the NEPA CE even if no other air quality analysis is determined to be necessary. The documentation should also cover consultation with FHWA/FTA, the MPO, and air agencies, where needed.

For most other projects, an air quality technical report should be prepared, although the report can be fairly short for projects where air quality issues are minimal and can be avoided or minimized with standard measures. Here are some examples where a comprehensive report is recommended:

  • The environmental document will be a higher level document with many complex issues, including air quality.
  • Early scoping, air analyses, or the PID indicate that air quality mitigation beyond standard impact minimization and avoidance measures may be needed.
  • The project is regionally significant for conformity analysis purposes, regardless of the type of environmental documentation to be prepared. All capacity-increasing State Highway System (SHS) projects, including auxiliary lanes, passing lanes, and many truck climbing lanes, are considered regionally significant by definition under federal conformity regulations.

The Air Quality Conformity Findings Checklist (DOCX) summarizes and documents the conformity analysis and determination for projects. The checklist must be completed for all projects on and off the SHS for which a NEPA environmental document or determination will be prepared. The checklist is completed and signed by the District/Region air quality specialist, indicating that the analysis is complete and documented in the project file. If the project is exempt from conformity analysis (Steps 1 and 2 only), then the Environmental Planner/Generalist may sign the checklist.

For projects processed pursuant to 23 USC 326 CE Assignment (except for 771.117(c)(22), (c)(23), (c)(26), (c)(27), or (c)(28)1) for which an Air Quality Conformity Analysis is required), the Senior Environmental Planner's signature on the Categorical Exclusion (CE) formally documents the conformity determination for the project. If hot-spot or other analysis is done to back up checklist entries, such as use of the CO Protocol to determine whether modeling is needed, that analysis must be retained in the project file.

For projects processed pursuant to 23 USC 327 NEPA Assignment, the Department will document the air quality conformity analysis and submit it to FHWA who will make the formal conformity determination. The air quality conformity analysis and request for air quality conformity determination must be submitted by the Department to FHWA. The Air Quality Conformity Analysis annotated outline is available on the SER Forms and Templates page. The FHWA conformity determination must be received before NEPA can be completed. FHWA's June 21, 2007 letter and “Transportation Conformity and NEPA Assumption Questions and Answers” describe FHWA's expectations for the 23 USC 327 project-level air quality conformity determination process. The FHWA determination shall be included in the environmental document.

Report Content

An air quality study report should include:

  • A basic project description documenting project design assumptions-such as baseline or existing year, future (design-year) traffic and interim years such as open-to-traffic and RTP horizon that are relevant to the air quality analysis. The project description should include a brief description of the alternatives under consideration. Make sure it is consistent with the environmental document.
  • Regional meteorology and climate discussion. Include a description of the area's topography as well, if pertinent to the discussion.
  • Regulatory Framework. Provide a description of the applicable regulations as well as the agencies, such as air districts and MPOs that are involved with air quality in the area or region. Also outline regulations on the federal, state, local levels. Include a table outlining air quality standards on both the federal and state levels.
  • Project area and affected environment: Include monitor locations and data, area conformity designations and classifications, location of sensitive receptors.
  • Results of air quality analyses. Include a description of methodologies and assumptions used.
  • Potential impacts of the project on air quality: direct, indirect, temporary, and permanent.
  • Potential avoidance, minimization and/or mitigation measures.
  • Project-level conformity documentation, if the project is in an area subject to conformity requirements under the FCAA.
  • An appendix with the results of any emissions modeling done for the analysis and interagency consultation documentation.
  • Discussion of lead and asbestos, if needed, include by reference hazardous waste reports.

Preparer Qualifications

Air quality reports should be prepared by an air quality specialist.

If asbestos is an issue, that portion of the report should be coordinated with a hazardous materials specialist. Certain asbestos studies must be done, by law or regulation, by or under the direction of a Registered Geologist, a Certified Industrial Hygienist (CIH), or a Certified Asbestos Consultant (CAC).

Processing and Approval

The air quality study report should be reviewed, at a minimum, by the District air quality specialist (for consultant-prepared documents) and the Environmental Planner/Generalist responsible for the environmental document. The air quality study report is approved by the air quality specialist's supervisor (District Branch Chief). Approval should be completed before circulation of the draft environmental document.

Local Assistance Projects - Air quality study reports prepared in support of local agency Federal-aid transportation projects "off" the SHS have the same review and approval requirements but are processed through the District Local Assistance Engineer (DLAE). The report shall be reviewed by the District air quality specialist, the District hazardous waste technical specialist and the District Local Assistance Environmental Planner/Generalist, and approved by the air quality specialist's supervisor (District Branch Chief).

Recommended Methodologies

The methods for analyzing air quality impacts can be quite complex. This section is meant to guide the environmental planner in scoping and/or reviewing the technical report, and then writing the environmental document. The analyses are typically done by an air quality specialist. Note that avoidance, minimization, and/or mitigation measures are discussed in the environmental document section of this chapter.

Direct Impacts

See "Information for the Environmental Document" for a discussion of permanent and temporary direct impacts.

Indirect and Cumulative Impacts

See "Information for the Environmental Document" for a discussion of indirect and cumulative impacts.


Conformity is both a project-level and a regional planning issue. Extensive modeling is required at the planning stage to do a regional conformity analysis for RTPs and TIPs, both of which must describe the specific projects that are included. See the overview section for more on this topic.

Note: Compliance with conformity requirements may be adequate to cover federal criteria pollutant cumulative impact issues but does not address state criteria pollutant or non-criteria-pollutant issues (MSAT, asbestos, etc.) as might be covered under CEQA. Additional analysis may be needed for state-level issues.

23 USC § 327

Air quality conformity determinations are excluded from NEPA Assignment [(see 23 USC § 327(a)(2)(b)]. Conformity determinations, both regional conformity and project-level conformity, will remain the responsibility of FHWA California Division for all projects under 23 USC § 327. For Environmental Assessments (EAs) and Environmental Impact Statements (EISs) under 23 USC § 327, FHWA will make the conformity determinations after the preferred alternative is identified and prior to completion of the final environmental document (FED). For CEs under 23 USC § 327, FHWA will make the conformity determinations prior to the approval of the CE. Both the regional and project-level conformity determinations must be documented in the air quality section of the final environmental document. A copy of the FHWA conformity determination letter must also be included in the final EA or EIS. The Department may not issue a Finding of No Significant Impact (FONSI) or Record of Decision (ROD) without the conformity determination(s) from FHWA.

For conformity determinations under NEPA Assignment, the Department will send FHWA, separate from the environmental review process, a request for conformity determinations. In this regard, FHWA will be treated much like other federal resource agencies. FHWA's preference is that the air quality conformity documentation submitted to FHWA be a stand-alone report. The air quality conformity documentation must include adequate summary information such that conformity determinations can be made on the preferred alternative without having to refer to the environmental document. An Air Quality Conformity Analysis Annotated Outline can be found on the SER Forms and Templates page. The conformity documentation should not include air quality issues that are not required for transportation conformity purposes (e.g., air toxics, asbestos, etc.).

NOTE: Conformity determination requests must be submitted to the Caltrans Division of Transportation Planning (DOTP) - Air Quality office and the Caltrans Division of Environmental Analysis (DEA) - Air Quality office for review and approval prior to sending to FHWA. Please direct technical questions about conformity to DOTP-AQ. Headquarters staff will coordinate with FHWA on behalf of the district. Current contact information can be found on the Air Quality Conformity Findings Checklist

23 USC § 326

For projects covered by the 23 USC § 326 CE Assignment MOU, both regional and project-level air quality conformity determinations for individual projects have been assigned to the Department and those determinations are made by the district/region. For additional information, see the SER, Vol. 1, Chapter 38, "NEPA Assignment."

Project-Level Conformity

At the project level, existence of a valid regional conformity analysis must be documented, and in some cases more specific "hot-spot" analysis and other findings may be needed. Here are some good resources on transportation conformity.

Project-Level Conformity Process Flowchart

View the Project-Level Conformity Process Flowchart (PDF) | Project-Level Conformity Process Flowchart (ADA version)

Components of a Project-Level Conformity Analysis

The project-level conformity documentation is done by the air quality specialist and documented in the technical report. The environmental document preparer will summarize the results in the environmental document. Ensuring conformity and documenting it is a step-by-step process. Here are the basic steps:

  • Determine if the project area is subject to conformity requirements (within a nonattainment or maintenance area for a federal criteria air pollutant).
  • If so, determine if the project exempt from conformity, or at least from regional conformity analysis, requirements

If not exempt, confirm:

  • Status of Regional Transportation Plan (RTP) and Transportation Improvement Program (TIP) conformity, or if an Isolated Rural area the date and status of the most recent regional conformity analysis.
  • That the project is listed in the latest conforming RTP and TIP, or in a previous regional emission analysis if in an Isolated Rural area.
  • If not, include the results of a regional emission analysis done for the project (usually only in Isolated Rural areas).
  • If in a carbon monoxide (CO) area include results of "hot-spot" analysis with respect to conformity requirements.
  • If in a particulate matter (PM2.5, PM10) area include Interagency Consultation materials confirming the project is not a project of air quality concern, or the quantitative “hot-spot” analysis if the project is determined to be a project of air quality concern, in addition to the Interagency Consultation materials.
  • Determine whether the project implements a Transportation Control Measure (TCM) identified in the applicable air quality plan (SIP) and RTP; if not a TCM, confirm that the project does not interfere with implementation of any TCMs.

More details on some of the key concerns in this documentation outline are provided below. See the air quality specialist if there are questions on conformity.

Conformity Exemption

Certain types of projects are exempt from conformity requirements. These project types are found by U.S. EPA to be neutral from an air quality or emissions standpoint, and are listed in the Conformity Regulations at 40 CFR 93.126, 40 CFR 93.127, and 40 CFR 93.128, or “projects that correct, improve or eliminate a hazardous location or feature” based on the Clarifications on Exempt Project Determinations.  If a project fits one of the listed types, it may need little or no conformity analysis, and does not need to be individually listed and considered in regional conformity modeling.

Regional Conformity

If a project is not exempt from conformity requirements, and is regionally significant (see definition at 40CFR 93.101), it must come from the latest conforming RTP and TIP for the region. Documentation of the nonattainment or maintenance status of the area, including SIP submittal/approval dates, is needed. Also, documentation of the RTP and TIP conformity status (most recent amendment dates, date of last full and updated conformity determinations) is needed. The design concept and scope of the project must match the design concept and scope used in the RTP and FTIP project listing. "Design concept" means the type of facility that is proposed, such as a freeway as opposed to an expressway or arterial highway. "Design scope" refers to those aspects of the project that would clearly affect capacity and thus any regional emissions analysis, such as the number of lanes and the length of the project. The "open-to-traffic" delivery date of the project must be within the same conformity analysis time period that the project is listed in for the RTP and FTIP conformity analysis. For Draft NEPA documents, at least one viable alternative must match the RTP and TIP project listing; for the Final NEPA document, the selected alternative design scope and design concept must match the RTP and TIP project listing.

If the project is in an “isolated rural” nonattainment area—where there is no MPO in the nonattainment area—there will be no RTP and TIP conformity to which to refer. In this case for regionally significant projects, regional analysis must be done and documented for the project itself using procedures (including interagency consultation and public involvement) and criteria similar to those used by an MPO. See 40 CFR 93.109(g) for more information.

Hot-Spot Analysis

If a project is located in a CO and/or PM 2.5 and PM10 non-attainment or maintenance area, a "hot-spot" assessment and possible “hot-spot” analysis is required for conformity purposes. The analysis must confirm that the project will not cause or worsen a localized violation of the standard. Ordinarily, the hot-spot analysis done for the environmental document will be adequate to allow for conformity hot-spot findings. In CO areas, a project is analyzed using the Caltrans/UCD CO Protocol and modeled if necessary using the CALINE4 model. See the Division of Environmental Analysis Air Quality Analysis web page for details on current acceptable methodologies. Also refer to the U.S. EPA PM Hot-Spot Analysis Guidance.

In any area (attainment or nonattainment), a project must not cause an exceedance of the federal NAAQS for CO, PM10, or PM2.5. In CO, PM2.5, and PM10 non-attainment areas, a project must not cause an exceedance, OR contribute to increasing an existing exceedance (make it worse). For the project to conform, the build design-value must be less than or equal to the no-build design-value at each receptor in the build scenario that exceeded the NAAQS. If one or more of these tests fails, the project cannot proceed. In such a situation the project must be modified to produce changes in traffic operation that allow the hot-spot tests to be met, which in some cases may require additional work at intersections in the vicinity that otherwise might not be part of the project. The conformity hot-spot test is required only if the project area is determined to be a project of air quality concern and it is in an area of non-attainment or maintenance for the federal CO and/or particulate matter standards. CO and PM hot-spot consideration is needed in all areas, not just nonattainment/maintenance areas, to ensure for NEPA and CEQA purposes that the project would not create a violation that could put the area into nonattainment.

SIP Control Measures

All projects in areas subject to conformity must demonstrate that they do not interfere with implementation of Transportation Control Measures (TCMs) listed in the State Implementation Plan for the area. (TCMs are regional measures used to reduce emissions. They are a broad array of strategies and can range from specific traffic control measures to the incorporation of carpool programs. If a project comes from a conforming RTP and TIP the RTP and FTIP conformity analyses would have documented system-level timely implementation and non-interference with TCMs.)

Suggested Language for Conformity Documentation

CE Documentation: Attach a sheet to the standard CE/CE form with conformity documentation. Backup materials such as a localized (CO, PM 2.5, PM10) impact analysis or screening, as needed, should be included in the project file. Projects that are processed with CEs under CEQA and NEPA will usually, but not always, be exempt from regional conformity requirements as well. Include a copy of the Air Quality Conformity Findings Checklist in the project file.

EA or EIS: Use the flowchart included in the environmental document annotated outlines (AOs) in the SER. Where additional documentation is needed, consult with the air quality specialist for content for the environmental document. Include a copy of the Air Quality Conformity Findings Checklist in the project file.

Information for the Environmental Document

Caltrans uses a standard format for impact discussions in its environmental documents. See the SER forms and Templates page for the following annotated outlines:

  • IS/EA outline with annotated guidance, including an air quality section
  • EIR/EA outline with annotated guidance, including an air quality section
  • EIR/EIS outline with annotated guidance, including an air quality section
  • EIS outline with annotated guidance, including an air quality section
  • EA outline with annotated guidance, including an air quality section

The use of the joint NEPA/CEQA AOs or the NEPA-only AOs is required for any project receiving FHWA Federal-aid funds. In addition, the AOs are required for projects on the SHS. The use of the joint NEPA/CEQA AOs is highly recommended for all other projects. This section is meant to complement and, where needed, expand on the information and guidance provided in the outlines.

The air quality section of the environmental document should discuss the following items, as appropriate:

  • Regulatory Setting
  • Affected Environment
  • Environmental Consequences (including permanent, temporary, direct and indirect)
  • Avoidance, Minimization and/or Mitigation Measures
  • Cumulative Impacts

Construction impacts can be included or treated as a separate section in the environmental document.

Summarize the information in the technical report, as needed, to draft the environmental document. Remember to write the document for the general reader. Technical detail should be kept to a minimum. If needed, refer the reader to the technical report.

Regulatory Setting

Use the standard (boilerplate) language in the approved AO if applicable. If not using one of the AOs, include a brief discussion of the regulatory framework for the analysis, including local, state, and federal laws. Provide a summary of regional transportation planning.

Affected Environment

  • Briefly describe climate and meteorology, focusing on what will most affect air quality.
  • Describe current air quality status of the area for both state and federal criteria pollutants: ARB ambient air quality standards.
  • List air quality monitoring stations and recent data (generally the last 5 years) in the project area, and any other localized air quality-related information pertinent to the air quality analysis.
  • Document if naturally occurring asbestos is found in the project area.

Environmental Consequences

Note: Be sure to address impacts of the no-build and ALL alternatives in the environmental document.

Conformity: In areas where conformity applies, include a brief discussion and documentation of project-level conformity. Refer the conformity discussion in the Reporting Section for details.

Direct Permanent Impacts

The following are permanent impacts directly and locally attributable to implementation and operation of the project. The analyses for CO, PM2.5, and PM10 are also used to document project-level conformity, where required.

  • CO (carbon monoxide) hot-spot analysis: the air quality specialist should use the CO Protocol. All projects affecting traffic flow should be evaluated for potential CO impacts. Modeling may be needed. If there are questions on CO models, see the Caltrans Headquarters Air Quality Analysis Page.
  • PM2.5 and PM10 (particulate matter/dust) analysis for conformity: If the project is determined, usually through interagency consultation, to be a project of air quality concern, quantitative (with dispersion modeling) analysis is required. For more information, consult the U.S. EPA PM Hot-Spot Analysis Guidance.
  • Mobile Source Air Toxics (MSAT): For guidance on how to address mobile source air toxics in an environmental document, please refer to the FHWA Interim Guidance on Addressing MSAT.
    • Mobile Source Air Toxics (MSAT) in the NEPA Process for Highways

Flowchart for Analyzing Mobile Source Air Toxics (MSAT) in the NEPA Process for Highways (PDF) 

  • Diesel exhaust: This a particular issue on facilities with large volumes of truck traffic. It is known that exposure to diesel exhaust over time can have effects on health. Criteria and quantitative methods for assessing diesel impacts are not yet developed at the regulatory level. However it is important to document any sensitive land uses in the vicinity of the project.

Temporary, Construction-Related Impacts

These are temporary impacts related to construction activities.

  • Dust Control. Visible emissions of all types including dust must be controlled. See Standard specifications 10-5 and Section 18. Local and air district regulations may also apply. Standard Specification 14-9.02 requires compliance with all applicable air quality laws and regulations including local and air district ordinances and rules. Construction impact analysis should document the PM 2.5 and PM10 (dust) impacts expected.
  • Structural Asbestos: Document the need for (or results of) asbestos inspection for structures that will be demolished or substantially renovated. Formal air district or ARB/U.S. EPA notification (depending on delegation status of the area) not less than 15 working days before starting demolition work is required by federal NESHAP and some air district regulations.
  • Naturally Occurring Asbestos: Document the need for (or results of) site surveys regarding naturally occurring asbestos. This impact is largely construction-related, but may occur through ongoing maintenance activities as well. Notification of the air district before starting work is required by ARB and some air district regulations. Cross reference with the hazardous materials chapter as appropriate.
  • Provide results of studies aimed at any location-specific issues that may occur. In San Joaquin Valley, South Coast, and Coachella Valley areas (as of 3/2005), summarize how project design and operational/construction practices will comply with PM2.5 and PM10 regulations.
  • If construction will last more than 3 years, or will substantially affect traffic due to detours, closures, and temporary terminations, then the CO and PM10 hot-spot impacts of the disturbed traffic flow should be analyzed. In areas subject to transportation conformity requirements, a construction impact analysis is required if construction will last more than 5 years at any individual site.

Indirect and Cumulative Impacts

Air pollutant emissions from a project may have an indirect or cumulative effect at a regional scale, since they combine with all other emissions in a region to produce certain pollutant concentrations. If a project is included in a RTP and TIP that contributes to attainment or maintenance of applicable air quality standards–as documented in a program level environmental document or a conformity analysis–it would not typically have a cumulative adverse impact on regional air quality.

There may be cases where a separate methodology is needed for assessing indirect and/or cumulative impacts.

See the SER for guidance on cumulative impact analysis and growth-related indirect impact analysis.

Avoidance, Minimization, and/or Mitigation Measures

For direct, operational impacts, summarize any avoidance, minimization and/or mitigation measures. Include, for example, project design features such as paved shoulders, which will reduce PM2.5 and PM10 emissions from re-entrained road dust. Include also any design changes that were included to avoid CO exceedances. Include design changes made to avoid or minimize exposure of sensitive receptors.

For construction-related, temporary impacts:

  • Summarize avoidance, minimization and/or mitigation measures for issues such as naturally occurring or structural asbestos.
  • Describe any standard or special provisions that will be included in the project to minimize dust exposure during construction.


  • Note that compliance with any applicable local, air district, state, and federal air quality laws and regulations applicable to construction activity is required by Standard Specifications Section 14-9.02.
  • List/summarize any air quality permits or notifications expected to be needed for construction.

Information Needed for Project Delivery

Regional Transportation Plan

This information should be documented in the environmental document prepared by the MPO or RTPA for the RTP and serve as a building block in subsequent decision making.

  • The RTP should establish a purpose and need for regionally significant projects.
  • The RTP should include a listing of all regionally significant projects assumed to be constructed and open to travel in each of the analysis periods used for the air quality analysis. Individual project listings should include a brief description of the design concept and scope of the project.

Project Initiation Document

The following information should be included in the Preliminary Environmental Analysis Report (PEAR) prepared as part of the PID.

  • Verify air quality information from RTP stage. If the project was specifically listed in the RTP conformity analysis, verify design concept and scope information and make sure any new alternatives are included. If design concept and scope have changed substantially from that used in the RTP Conformity analysis, a RTP amendment may be needed to complete the programming process. Determine whether conformity applies, whether the project is exempt from conformity, whether the project is listed in the conforming RTP if not exempt, whether regional significance questions are relevant, etc.
  • Determine whether conformity applies, and if so whether a conformity exemption applies.
  • Plan any needed early coordination meetings.
  • Plan the cost, scope, and schedule for any air studies to be completed at the environmental document stage. 

Draft Project Report

This is the stage where the bulk of air quality analysis is done. Detailed analyses for all viable alternatives and the no-build alternative are completed. If field air quality monitoring is done (exceptional cases only), it would be completed at this stage, but might need to start during the PID phase due to timing requirements (CO monitoring must be done for the entire period between October and April; PM monitoring, if done, must occur for several months during the period when known exceedances of the PM standards occur). The following information should be included in the Draft Environmental Document (DED) or used as supporting documentation for a CE, as appropriate.

  • Make sure the design concept and scope (40 CFR 93.101) of at least one alternative matches the latest conforming RTP and TIP project descriptions.
  • Make sure the purpose and need and project descriptions of the DED and draft project report match.
  • The potential impacts of each alternative must be analyzed and discussed in the environmental document, along with appropriate avoidance, minimization, and/or mitigation measures.
  • Confirm the project-level conformity status of the project.
  • The technical report and all supporting studies must be completed before the draft project report and DED can circulate.

Project Report

This information should be presented in the Final Environmental Document (FED).

  • Prepare updated air quality report/analysis responding to comments made during DED circulation.
  • Determine whether analysis requires updating to address selected alternative; update if needed.
  • Determine and document whether the selected alternative is contained, with substantially the same design concept and scope, in the current conforming RTP and TIP. If RTP and TIP amendments are needed to deal with design concept and/or scope changes during the environmental process, they must be completed and a revised regional conformity determination made by FHWA and FTA before the FED can be signed and a ROD or FONSI issued. Include the final project-level conformity determination, determination of impacts, and specify and incorporate into the project required avoidance, minimization, and/or mitigation measures for the selected alternative.
  • A copy of the FHWA conformity determination letter must also be included in the FED.
  • Provide cost estimates for mitigation measures, as defined, for the Mitigation Cost Tracking System.

Environmental Reevaluation (during Design and Right of Way phases of a project)

Air issues should be re-examined and updated as necessary whenever an Environmental Reevaluation is done. If regional conformity lapses during later phases, such as Design, work on the project may not be able to proceed to the next phase. A regional conformity "freeze" can prevent changes that affect the design concept and scope of the project. In areas where conformity requirements are newly applied after the environmental process is complete, it may be necessary to perform the first conformity analysis, and obtain a conformity determination from FHWA and FTA, at the time the Plans, Specifications, and Estimates (PS&E) is submitted. In order to retain the conformity determination made for the FED, no more than 3 years may elapse between each of the following milestones: Final NEPA approval; start of Design work; acquisition of "a substantial portion" of the needed right of way; and completion of PS&E.

Permits/Approvals Required

Local permits and approvals are often required at the construction stage. They are usually the responsibility of the Contractor. Where applicable, they should be listed in the PS&E for the project. The Resident Engineer is responsible for ensuring that all air quality permits are complied with during construction.

Here are some typical local permits:

  • In some air districts, an Indirect Source permit or fee may be required for major construction projects.
  • In some air districts, a dust control plan, with plan and inspection fees, may have to be filed for construction activities.
  • Asbestos Notification (for demolition and renovation activities, including removal of asbestos discovered during construction) to the local air district and U.S.EPA.
  • Asbestos notification (for naturally occurring asbestos) to the local air district is required where work is in an area that may have it. Local (typically air district) permit or notification for removal of lead-based paint or other sandblasting operations.
  • Local air district permits or formal notification for Portable Equipment to be used during construction. A statewide equipment registration program administered by the California Air Resources Board is available, and is commonly used by contractors and equipment suppliers.
  • If construction includes work subject to Permit to Construct or Permit to Operate requirements under air district rules (such as a stationary engine/generator set, or long-term operation of Portable Equipment like a batch plant), those permits must be obtained prior to starting construction.

1Please note that certain activities covered by these categorical exclusions may require that Caltrans prepare an Air Quality Conformity Analysis rather than documenting the conformity determination with the Senior Environmental Planner’s signature on the Categorical Exclusion form.

(Last content update: 02/18/22 JH)