EPA420-B-06-902 March 2006

Transportation Conformity 

Guidance for Qualitative 

Hot-spot Analyses in 

PM2.5 and PM10

Nonattainment and 

Maintenance Areas
EPA420-B-06-902 March 2006

Transportation Conformity Guidance for Qualitative 

Hot-spot Analyses in PM2.5 and PM10

Nonattainment and Maintenance Areas

Office of Transportation and Air Quality

U.S. Environmental Protection Agency

Office of Natural and Human Environment

Federal Highway Administration
1 

Table of Contents 

Chapter 1: Introduction

1.1. What is the purpose of this guidance? 

1.2. What is a hot-spot analysis? 

1.3. What projects in PM2.5 and PM10 areas are addressed by this guidance? 

1.4. How is this guidance structured? 

1.5. Which parts of this guidance apply to PM2.5 hot-spot analyses and which parts of this 

guidance apply to PM10 hot-spot analyses? 

1.6. Who can I contact for more information? 

1.7. Does this guidance create new requirements? 

Chapter 2: Overview of Transportation Conformity Requirements

2.1. What are the primary requirements for assessing the impacts of projects in PM2.5 and PM10

nonattainment and maintenance areas? 

2.2. What is a project of air quality concern? 

2.3. When is a PM2.5 or PM10 hot-spot analysis required? 

2.4. What air quality standards are evaluated in PM2.5 or PM10 hot-spot analyses? 

2.5. What is the definition of causing a new violation or increasing the frequency or severity of 

an existing air quality violation? 

2.6. What are the interagency consultation requirements for PM2.5 and PM10 hot-spot analyses? 

2.7. What are the roles and responsibilities of different agencies in project-level conformity 

determinations? 

2.8. What are the public participation requirements for PM2.5 and PM10 hot-spot analyses? 

Chapter 3: Analytical Requirements

3.1. What are the general analytical requirements for PM2.5 and PM10 hot-spot analyses? 

3.2. What emissions are considered in PM2.5 and PM10 hot-spot analyses? 

3.3. When is re-entrained road dust considered in PM2.5 or PM10 hot-spot analyses? 

3.4. When are construction emissions considered in PM2.5 or PM10 hot-spot analyses? 

3.5. What time frame and analysis years should be used in hot-spot analyses? 

Chapter 4: Developing a Qualitative PM2.5 or PM10 Hot-spot Analysis

4.1. What methods can be used for performing qualitative PM2.5 and PM10 hot-spot analyses? 

4.2. What should be documented for a qualitative PM2.5 or PM10 hot-spot analysis? 

4.3. What are some of the factors that may be considered in describing existing conditions absent 

the proposed project? 

4.4. How would changes in existing conditions be evaluated for future scenarios with the 

proposed project? 

4.5. What are the potential measures to mitigate PM2.5 or PM10 air quality concerns? 

Appendix A – Examples of Projects of Air Quality Concern 

Appendix B – Examples of Qualitative PM2.5 or PM10 Hot-spot Analyses 

Appendix C – Potential Mitigation Measures 
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Chapter 1: Introduction

1.1. What is the purpose of this guidance?

On March 10, 2006, the Environmental Protection Agency (EPA) published a final rule that 

establishes the transportation conformity criteria and procedures for determining which 

transportation projects must be analyzed for local air quality impacts in PM2.5 and PM10

nonattainment and maintenance areas (“areas”) (71 FR 12468). The final rule also provides 

flexibility so that state and local resources are used efficiently. The EPA and the Federal 

Highway Administration (FHWA) have developed this guidance to help state and local agencies 

meet the final rule’s hot-spot analysis requirements. 

Transportation conformity is required under Clean Air Act section 176(c) (42 U.S.C. 7506(c)) to 

ensure that federally supported highway and transit project activities are consistent with 

(“conform to”) the purpose of the state air quality implementation plan (SIP). Conformity to the 

purpose of the SIP means that transportation activities will not cause new air quality violations, 

worsen existing violations, or delay timely attainment of the relevant national ambient air quality 

standards (NAAQS or “standards”). EPA’s transportation conformity rule (40 CFR 51.390 and 

Part 93) establishes the criteria and procedures for determining whether transportation activities 

conform to the SIP. 

From this date forward, future qualitative PM2.5 and PM10 hot-spot analyses should be based on 

today’s new guidance, which supersedes FHWA’s existing September 12, 2001, “Guidance for 

Qualitative Project-Level ‘Hot Spot’ Analysis in PM10 Nonattainment and Maintenance Areas.” 

However, any PM10 hot-spot analysis that was started prior to the release of EPA and FHWA’s 

new guidance may be completed with the previous 2001 guidance. Any PM2.5 hot-spot analysis 

that was started prior to the release of EPA and FHWA’s new guidance must meet the March 

2006 final rule’s requirements, and should meet the new guidance whenever possible. 

1.2. What is a hot-spot analysis?

A hot-spot analysis is defined in 40 CFR 93.101 as an estimation of likely future localized PM2.5 

or PM10 pollutant concentrations and a comparison of those concentrations to the relevant air 

quality standards. A hot-spot analysis assesses the air quality impacts on a scale smaller than an 

entire nonattainment or maintenance area, including for example, congested roadway 

intersections and highways or transit terminals. Such an analysis is a means of demonstrating 

that a transportation project meets Clean Air Act conformity requirements to support state and 

local air quality goals with respect to potential localized air quality impacts. When a hot-spot 

analysis is required, it is included within the project-level conformity determination that is made 

by FHWA or the Federal Transit Administration (FTA). 

EPA and FHWA are issuing guidance at this time for qualitative hot-spot analyses. Quantitative 

PM2.5 or PM10 hot-spot analyses will be required when appropriate methods and modeling 
3 

guidance are available. Qualitative hot-spot analyses involve more streamlined reviews of local 

factors such as local monitoring data near a proposed project location. 

1.3. What projects in PM2.5 and PM10 areas are addressed by this guidance?

This guidance provides information to meet hot-spot analysis requirements for projects in PM2.5 

and PM10 areas. See Chapter 2 and Appendix B for more specific information. 

For PM2.5 areas

For all PM2.5 areas, this guidance would be used to complete qualitative PM2.5 hot-spot analyses 

only for “projects of air quality concern” as defined in the final rule by 40 CFR 93.123(b)(1). 

The final rule specifies that projects of air quality concern are certain highway and transit 

projects that involve significant levels of diesel traffic, or any other project that is identified by 

the PM2.5 SIP as a localized air quality concern. 

A qualitative PM2.5 hot-spot analysis is not required for projects that are not an air quality 

concern. For these types of projects, state and local project sponsors should briefly document in 

their project-level conformity determinations that Clean Air Act and 40 CFR 93.116 

requirements were met without a hot-spot analysis, since such projects have been found to not be 

of air quality concern under 40 CFR 93.123(b)(1). 

For PM10 areas without approved conformity SIPs

For these PM10 areas, this guidance would also be used to complete qualitative PM10 hot-spot 

analyses only for “projects of air quality concern” as defined by 40 CFR 93.123(b)(1). 

A qualitative PM10 hot-spot analysis is not required for projects that are not an air quality 

concern. For these types of projects, state and local project sponsors should briefly document in 

their project-level conformity determination that Clean Air Act and 40 CFR 93.116 requirements 

were met without a hot-spot analysis, since such projects have been found to not be of air quality 

concern under 40 CFR 93.123(b)(1). 

For PM10 areas with approved conformity SIPs

In areas where EPA has already approved conformity SIPs that include PM10 hot-spot provisions 

from previous conformity rulemakings, the revised PM10 hot-spot requirements in the March 10, 

2006 final rule will only be effective when a state either: 

• withdraws the existing provisions from its approved conformity SIP and EPA approves 

the withdrawal, or 

• includes the revised PM10 hot-spot requirements in a SIP revision and EPA approves that 

SIP revision. 
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For more information on revising approved conformity SIPs, please see the February 14, 2006 

EPA and DOT guidance entitled, “Interim Guidance for Implementing the Transportation 

Conformity Provisions in the Safe, Accountable, Flexible, Efficient Transportation Equity Act: 

A Legacy for Users (SAFETEA-LU).”

1

 

Therefore, for all non-exempt federally funded or approved projects, PM10 areas with approved 

conformity SIPs must continue to follow the PM10 hot-spot procedures in their existing 

conformity SIPs until the SIP is updated and subsequently approved by EPA. PM10 areas with 

approved conformity SIPs most likely are required to complete a qualitative PM10 hot-spot 

analysis for every project-level conformity determination, since these were the federal 

conformity requirements prior to the March 10, 2006 final rule. 

1.4. How is this guidance structured?

This guidance is in the form of questions and answers for basic components of PM2.5 and PM10

hot-spot analyses. The guidance addresses many issues such as: 

• What requirements must be met under the March 10, 2006 final rule? 

• When must the analysis be performed? 

• What are the different agencies involved in PM2.5 and PM10 hot-spot analyses and 

project-level conformity determinations? 

• What information should be included in a qualitative hot-spot analysis? 

Following the question and answer section are three appendices that provide examples of: 

• Projects that are or are not an air quality concern, 

• Approaches for qualitative PM2.5 and PM10 hot-spot analyses, and 

• Potential project-level mitigation measures. 

These examples demonstrate different levels of inquiry that may be used to qualitatively 

consider the local air quality impacts of projects in a given PM2.5 or PM10 nonattainment or 

maintenance area. This guidance is not definitive for any specific project but rather is general 

guidance for all relevant projects. 

Additional assistance is available from: 

• EPA regional and headquarters offices, 

• FHWA division and headquarters offices, and 

• FTA regional and headquarters offices. 

See Question 1.6 for specific contact information. 

 

1 SAFETEA-LU is Public Law 109-59. EPA and DOT’s interim conformity guidance is available at either 

http://www.epa.gov/otaq/stateresources/transconf/420b06901.pdf, or 

http://www.fhwa.dot.gov/environment/conformity/sec6011guidmemo.htm. 
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1.5. Which parts of this guidance apply to PM2.5 hot-spot analyses and which parts of this 

guidance apply to PM10 hot-spot analyses?

The criteria and procedures for hot-spot analyses will be generally the same for both PM2.5 and 

PM10 areas, except for PM10 areas with approved conformity SIPs as noted elsewhere in this 

guidance. Questions and answers in this guidance address PM2.5 and PM10 together where the 

requirements or analytical methods and data are the same. Separate answers are provided where 

the answers differ. 

1.6. Who can I contact for more information?

For specific questions concerning a particular nonattainment or maintenance area, please contact 

the transportation conformity staff person responsible for your state at the appropriate EPA 

regional office, FHWA division office, or FTA regional office. 

 

• Contact information for EPA regional offices can be found at: 

http://www.epa.gov/otaq/stateresources/transconf/contacts.htm. 

• Contact information for FHWA division offices can be found at: 

http://www.fhwa.dot.gov/field.html. 

• Contact information for FTA regional offices can be found at: 

http://www.fta.dot.gov/about/offices/4978_ENG_HTML.htm. 

General questions about this guidance can be directed to: 

• Meg Patulski at EPA’s Office of Transportation and Air Quality, patulski.meg@epa.gov, 

(734) 214-4842; 

• Joe Pedelty at EPA’s Office of Transportation and Air Quality, pedelty.joe@epa.gov, 

(734) 214-4410; 

• Cecilia Ho at FHWA’s Office of Natural and Human Environment, 

cecilia.ho@fhwa.dot.gov, (202) 366-9862; or 

• Abbe Marner at FTA’s Office of Planning and Environment, abbe.marner@fta.dot.gov, 

(202) 366-4317. 

1.7. Does this guidance create new requirements?

No, this guidance explains how to implement the hot-spot analysis requirements of the March 

10, 2006 final rule, and does not create any new requirements. 
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The regulations described in this document contain legally binding requirements. This document 

is not a substitute for those provisions or regulations, nor is it a regulation itself. Thus, it does 

not impose legally binding requirements on EPA, FHWA, FTA, states, or the regulated 

community, and may not apply to a particular situation based upon the circumstances. EPA, 

FHWA, and FTA retain the discretion to adopt approaches on a case-by-case basis that may 

differ from this guidance, but still comply with the Clean Air Act and the transportation 

conformity regulations. Any decisions regarding a particular conformity determination or hotspot analysis will be made based on the statute and regulations, after appropriate public input. 

This guidance may be revised periodically without public notice. 
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Chapter 2: Overview of Transportation Conformity Requirements

2.1. What are the primary requirements for assessing the impacts of projects in PM2.5 and

PM10 nonattainment and maintenance areas?

Clean Air Act section 176(c)(1)(B) is the statutory criterion that must be met by all projects in 

nonattainment and maintenance areas that are subject to transportation conformity. Section 

176(c)(1)(B) states that federally-supported transportation projects must not “cause or contribute 

to any new violation of any standard in any area; increase the frequency or severity of any 

existing violation of any standard in any area; or delay timely attainment of any standard or any 

required interim emission reductions or other milestones in any area.” 

To meet statutory requirements, the March 10, 2006 final rule requires PM2.5 and PM10 hot-spot 

analyses to be performed for projects of air quality concern. Qualitative hot-spot analyses would 

be done for these projects before appropriate methods and modeling guidance are available and 

quantitative PM2.5 and PM10 hot-spot analyses are required under 40 CFR 93.123(b)(4). In 

addition, through the final rule, EPA determined that projects not identified in 40 CFR 

93.123(b)(1) as projects of air quality concern have also met statutory requirements without any 

further hot-spot analyses (40 CFR 93.116(a)). Please see Questions 1.3 and 2.3 for information 

on when the new PM10 hot-spot analysis requirements can be used in PM10 areas with and 

without approved conformity SIPs. 

2.2. What is a project of air quality concern?

EPA specified in 40 CFR 93.123(b)(1) of the final rule that projects of air quality concern are 

certain highway and transit projects that involve significant levels of diesel vehicle traffic, or any 

other project that is identified in the PM2.5 or PM10 SIP as a localized air quality concern. See 

the preamble of the March 10, 2006 final rule for further information regarding how and why 

EPA defined projects of air quality concern (71 FR 12491-12493). 

The final rule defines the projects of air quality concern that require a PM2.5 or PM10 hot-spot 

analysis in 40 CFR 93.123(b)(1) as: 

 

“(i) New or expanded highway projects that have a significant number of or 

significant increase in diesel vehicles; 

(ii) Projects affecting intersections that are at Level-of-Service D, E, or F with a 

significant number of diesel vehicles, or those that will change to Level-of-Service D, E, 

or F because of increased traffic volumes from a significant number of diesel vehicles 

related to the project; 

(iii) New bus and rail terminals and transfer points that have a significant number 

of diesel vehicles congregating at a single location; 

(iv) Expanded bus and rail terminals and transfer points that significantly increase 

the number of diesel vehicles congregating at a single location; and 
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(v) Projects in or affecting locations, areas, or categories of sites which are 

identified in the PM2.5 or PM10 applicable implementation plan or implementation plan 

submission, as appropriate, as sites of violation or possible violation. 

Appendix A of this guidance includes the final rule’s examples of projects that are most likely to 

be an air quality concern, as well as examples of projects that are not considered an air quality 

concern (and therefore do not require a PM2.5 or PM10 hot-spot analysis). However, as described 

in Questions 1.3 and 2.3, a PM10 hot-spot analysis is required for any project-level conformity 

determination in PM10 areas with approved conformity SIPs, until such SIPs are revised and 

approved by EPA. 

2.3. When is a PM2.5 or PM10 hot-spot analysis required?

In general, a hot-spot analysis would be done for required projects when a project-level 

conformity determination is completed. This is typically done during the environmental review 

process for the National Environmental Policy Act (NEPA). There can be limited cases, as 

described below, when transportation conformity requirements initially apply in a nonattainment 

area after the NEPA process has been completed for a project, but a project-level conformity 

determination is required for a subsequent federal approval. 

The following paragraphs provide more specific information for PM2.5 and PM10 areas. 

PM2.5 areas

The March 10, 2006 final rule requires a qualitative PM2.5 hot-spot analysis to be completed for 

project-level conformity determinations for projects of air quality concern completed on or after 

April 5, 2006, when PM2.5 conformity requirements apply and the final rule is effective.2 

Prior to April 5, 2006, FHWA or FTA could voluntarily make a project-level conformity 

determination that includes a PM2.5 hot-spot analysis that meets the final rule’s requirements. 

If a project still requires a FHWA or FTA approval or authorization, a project-level conformity 

determination will be required prior to the first such action on or after April 5, 2006, even if the 

project has already completed the NEPA process. After project-level conformity is determined 

for a project, a new conformity determination is only required under the scenarios discussed in 

40 CFR 93.104(d).3 

 

2 On January 5, 2005 (70 FR 943), EPA designated areas as attainment and nonattainment for the PM2.5 standards. 

These designations became effective on April 5, 2005. As a result, conformity for the PM2.5 standards will apply to 

newly designated nonattainment areas on April 5, 2006. 

3

 40 CFR 93.104(d) states, “FHWA/FTA projects must be found to conform before they are adopted, accepted, 

approved, or funded. Conformity must be redetermined for any FHWA/FTA project if one of the following occurs: 

a significant change in the project’s design concept and scope; three years elapse since the most recent major step to 

advance the project; or initiation of a supplemental environmental document for air quality purposes. Major steps 

include NEPA process completion; start of final design; acquisition of a significant portion of the right-of-way; and, 

construction (including Federal approval of plans, specifications and estimates).” 
9

A project-level conformity determination and hot-spot analysis will not be required for projects 

that have already completed the NEPA process and require no further FHWA or FTA approval 

or authorization on or after April 5, 2006. A project-level conformity determination would only 

be required for such projects under the scenarios discussed in 40 CFR 93.104(d). 

PM10 areas without approved conformity SIPs

The revised PM10 hot-spot requirements in the final rule are not effective until April 5, 2006. A 

qualitative PM10 hot-spot analysis that meets the final rule’s requirements must be completed for 

project-level determinations for projects of air quality concern completed on or after April 5, 

2006. 

Prior to April 5, 2006, any project-level conformity determination made by FHWA or FTA in 

these PM10 nonattainment and maintenance areas must meet the previous conformity rule’s 

requirements for PM10 hot-spot analyses. 

 

PM10 areas with approved conformity SIPs

As described above, PM10 areas that have approved conformity SIPs that include PM10 hot-spot 

provisions from previous rulemakings cannot take advantage of the March 10, 2006 final rule 

until the conformity SIP is revised and approved by EPA. 

Prior to that time, all project-level conformity determinations in these PM10 areas must include a 

PM10 hot-spot analysis that meets the requirements in the approved conformity SIP. 

2.4. What air quality standards are evaluated in PM2.5 or PM10 hot-spot analyses?

The Clean Air Act and transportation conformity regulation require that conformity be met for 

all national ambient air quality standards (NAAQS or “standards”) for a given pollutant. 

Therefore, a conformity determination must address all relevant standards for a given pollutant, 

unless meeting conformity for the controlling standard would ensure that Clean Air Act 

requirements are met for all standards. This conformity approach is consistent with how SIPs 

are developed for pollutants with multiple standards. 

The following paragraphs provide more specific information on the current 24-hour and annual 

standards that must be addressed in respective PM2.5 and PM10 hot-spot analyses. 4 

PM2.5 areas

PM2.5 nonattainment and maintenance areas are required to attain and maintain two standards: 

 

4 This guidance document implements conformity under the current PM2.5 and PM10 air quality standards. EPA 

proposed revisions to the current PM2.5 and PM10 air quality standards on January 17, 2006 (71 FR 2620). 
10

• 24-hour standard – 65 μg/m3 , and • annual standard – 15.0 μg/m3 

The current 24-hour standard is based on a 3-year average of the 98th percentile of 24-hour PM2.5 

concentrations; the current annual standard is based on a 3-year average of annual mean PM2.5 

concentrations. 

A PM2.5 hot-spot analysis must consider both standards, unless it is determined for a given area 

that meeting the controlling standard would ensure that Clean Air Act requirements are met for 

both standards. The interagency consultation process should be used to discuss how the 

qualitative PM2.5 hot-spot analysis meets statutory and regulatory requirements for both PM2.5 

standards, depending on the factors that are evaluated for a given project. 

PM10 areas

PM10 nonattainment and maintenance areas are required to attain and maintain two standards as 

well: • 24-hour standard – 150 μg/m3

, and • Annual standard – 50 μg/m3 

The 24-hour PM10 standard is attained when the average number of exceedances in the past three 

calendar years is less than or equal to 1.0. An exceedance occurs when a 24-hour concentration 

of 155 μg/m3 or greater is measured at a site. The annual PM10 standard is attained if the average 

of the annual arithmetic means for the past three calendar years is less than or equal to 50 μg/m3. 

 

A PM10 hot-spot analysis must consider both standards, unless it is determined for a given area 

that meeting the controlling standard would ensure that Clean Air Act requirements are met for 

both standards. The interagency consultation process should be used to discuss how the 

qualitative PM10 hot-spot analysis meets statutory and regulatory requirements for both PM10

standards, depending on the factors that are evaluated for a given project. 

2.5. What is the definition of causing a new violation or increasing the frequency or 

severity of an existing air quality violation?

A PM2.5 or PM10 hot-spot analysis assesses potential new or worsened future violations due to 

the project in combination with changes in background air quality concentrations. The 

interagency consultation process would be used to determine if new violations or increases in the 

frequency or severity of existing violations are anticipated based on the hot-spot analysis. 

40 CFR 93.101 already defines when a new or worsened air quality violation is determined to 

occur: 

“Cause or contribute to a new violation for a project means: 
11

(1) To cause or contribute to a new violation of a standard in the area substantially 

affected by the project or over a region which would otherwise not be in violation of the standard 

during the future period in question, if the project were not implemented; or 

(2) To contribute to a new violation in a manner that would increase the frequency or 

severity of a new violation of a standard in such area.” 

“Increase the frequency of severity means to cause a location or region to exceed a 

standard more often or to cause a violation at a greater concentration than previously existed 

and/or would otherwise exist during the future period in question, if the project were not 

implemented.” 

These definitions apply whether air quality information at the project location is used or when a 

monitor not in the geographic area of the project is used because it is located near a different 

project with similar characteristics (i.e., a “surrogate”). 

 

In addition, as discussed in the preamble to the November 24, 1993, transportation conformity 

rule, EPA believes that “a seemingly new violation may be considered to be a relocation and 

reduction of an existing violation only if it were in the area substantially affected by the project 

and if the predicted [future] design value for the “new” site would be less than the design value 

at the “old” site without the project – that is, if there would be a net air quality benefit” (58 FR 

62213). 

2.6. What are the interagency consultation requirements for PM2.5 and PM10 hot-spot 

analyses?

The interagency consultation process is an important tool to completing project-level conformity 

determinations and hot-spot analyses.5 Interagency consultation must also be used to evaluate 

and choose associated methods and assumptions to be used in PM2.5 and PM10 hot-spot analyses 

(40 CFR 93.105(c)(1)(i)). 

The different agencies that can be involved in the interagency consultation process include the 

project sponsor, other state and local transportation and air quality agencies, EPA, FHWA, and 

FTA. 

2.7. What are the roles and responsibilities of different agencies in project-level conformity 

determinations?

Roles and responsibilities of different agencies for meeting the transportation conformity 

requirements are addressed in 40 CFR 93.105 or in the approved conformity SIP. The following 

 

5

 Throughout this document, the term “interagency consultation process” is intended to mean that process required 

by 40 CFR 93.105 for transportation conformity determinations. 
12

paragraphs provide more information on the potential roles and responsibilities in implementing 

the PM2.5 and PM10 hot-spot analysis requirements. 

Project Sponsor

The project sponsor is the agency responsible for implementing the project. Typically, the 

project sponsor is a local government, transit operator, or state department of transportation. The 

project sponsor is responsible for providing the PM2.5 and/or PM10 qualitative hot-spot analysis 

addressed in this guidance and meeting consultation requirements described in 40 CFR 93.105 or 

the approved conformity SIP. The interagency consultation process is critical to completing 

project-level conformity determinations and qualitative PM2.5 and PM10 hot-spot analyses. The 

project sponsor, in cooperation with federal agencies, is also responsible for conducting the 

environmental analysis and review to comply with NEPA as required by the Council on 

Environmental Quality regulations (40 CFR 1500-1508) and the FHWA/FTA Environmental 

Impact and Related Procedures (23 CFR Part 771). 

FHWA and FTA 

 

FHWA and FTA are responsible for determining that the requirements of the transportation 

conformity rule are met. PM2.5 or PM10 hot-spot analyses would generally be included in 

documents prepared to meet NEPA requirements. Such documents may include: 

• an Environmental Impact Statement (EIS) with a Record of Decision (ROD); 

• an Environmental Assessment (EA) with a Finding of No Significant Impact (FONSI); or 

• a Categorical Exclusion (CE) determination. 

It is the responsibility of either FHWA or FTA to review and approve these NEPA documents 

for their certain actions. 

EPA 

EPA is responsible for promulgating transportation conformity regulations and related guidance, 

and as such, provides general and specific policy and technical assistance to federal, state, and 

local conformity implementers. EPA is also an active member of the interagency consultation 

process regarding conformity determinations. Additionally, EPA reviews submitted SIPs and 

makes adequacy or other findings as appropriate for conformity purposes, and provides policy 

and technical support with air quality modeling and monitoring issues.
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State and local air agencies 

State and local air quality agencies are part of the interagency consultation process and aid in air 

quality and transportation modeling. These agencies may provide much of the data required to 

perform a qualitative PM2.5 or PM10 hot-spot analysis, as described in Questions 4.4 and 4.5). 

The state air quality agency also operates the air quality monitoring network and is responsible 

for developing SIPs for PM2.5 and PM10 nonattainment and maintenance areas. 

2.8. What are the public participation requirements for PM2.5 and PM10 hot-spot 

analyses?

Affected agencies developing project-level conformity determinations (and any associated PM2.5 

or PM10 hot-spot analysis) need to establish a proactive public involvement process that provides 

opportunity for public review and comment. The NEPA public involvement process can be used 

to satisfy these public participation requirements, since project-level conformity determinations 

are usually conducted as part of the NEPA process. If a project-level conformity determination 

that includes an associated hot-spot analysis is done after NEPA is completed, as described in 

Question 2.3, a public comment period is also to be provided. 
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Chapter 3: Analytical Requirements

3.1. What are the general analytical requirements for PM2.5 and PM10 hot-spot analyses? 

In the March 2006 final rule, EPA retained for PM10 areas and extended for PM2.5 areas the 

general requirements in 40 CFR 93.123(c) for all hot-spot analyses (71 FR 12496-12497). These 

requirements are as follows: 

• Analyzing the total emissions burden of direct PM2.5 or PM10 emissions which may result 

from the implementation of the projects (including re-entrained road dust and 

construction emissions as appropriate, as described below), summed together with future 

background concentrations; 

• Analyzing the entire transportation project, after the identification of major design 

features which will significantly impact local concentrations; 

• Using consistent assumptions with those used in regional emissions analyses for inputs 

that are required for both analyses (e.g., temperature, humidity); 

• Assuming the implementation of mitigation or control measures only where written 

commitments for such measures have been obtained (40 CFR 93.125(c)); and 

• Not considering temporary emissions increases from construction-related activities which 

occur only during the construction phase and last five years or less at any individual site. 

For a project-level conformity determination, the design concept and scope of the project must 

be consistent with that included in the conforming transportation plan and transportation 

improvement program (TIP). Any significant change in a project’s design concept or scope will 

require a reevaluation of regional emissions (i.e., a new plan/TIP conformity determination) and 

a new project-level conformity determination and hot-spot analysis. 

PM2.5 and PM10 hot-spot analyses must also be based on the latest planning assumptions. In 

addition, FHWA or FTA, as applicable, must obtain from the project sponsor and/or operator 

enforceable written commitments to implement any required project-level control or mitigation 

measures, prior to making a project-level conformity determination (40 CFR 93.125(c)). 

3.2. What emissions are considered in PM2.5 and PM10 hot-spot analyses?

Hot-spot analyses under this guidance must be based only on directly emitted PM2.5 or PM10

emissions. Tailpipe, break wear, and tire wear PM2.5 or PM10 would always be considered in a 

project’s hot-spot analysis. See Questions 3.3 and 3.4 for further information regarding when reentrained road dust and construction emissions would be considered in a PM2.5 or PM10 hot-spot 

analysis. 

PM2.5 and PM10 precursors are not considered in respective hot-spot analyses. Secondary 

particles formed through PM2.5 and PM10 precursor emissions from a transportation project take 

several hours to form in the atmosphere giving emissions time to disperse beyond the immediate 

project area of concern for localized analyses. 
15

3.3. When is re-entrained road dust considered in PM2.5 or PM10 hot-spot analyses?

For PM2.5 nonattainment and maintenance areas

Re-entrained road dust must only be considered in PM2.5 hot-spot analyses if EPA or the state air 

agency has made a finding that such emissions are a significant contributor to the PM2.5 air 

quality problem in a given area (40 CFR 93.102(b)(3)). See the July 1, 2004 final conformity 

rule for further information (69 FR 40004). Please refer to the EPA regional office for 

information on whether a finding of significance for re-entrained road dust was made for a given 

PM2.5 nonattainment or maintenance area. 

For PM10 nonattainment and maintenance areas

Re-entrained road dust must be included in all PM10 hot-spot analyses. EPA has historically 

required road dust emissions to be included in all conformity analyses of direct PM10 emissions -

- including hot-spot analyses. See the March 2006 final conformity rule for further background 

(71 FR 12496).

3.4. When are construction emissions considered in PM2.5 or PM10 hot-spot analyses?

Construction-related PM2.5 or PM10 emissions due to a particular project are not required to be 

included in hot-spot analyses, if such emissions are considered temporary as defined in 40 CFR 

93.123(c)(5) (i.e., emissions which occur only during the construction phase and last five years 

or less at any individual site). 

While, for most projects, it is anticipated that construction emissions would not be included in 

PM2.5 or PM10 hot-spot analyses, there may be limited cases where a large project is constructed 

over a longer time period where it may be appropriate to include construction emissions, when 

an analysis year is chosen during project construction. For example, PM2.5 or PM10 emissions, as 

applicable, would be considered for projects that take more than five years to build at any 

individual site. See Question 3.5 for further information on analysis years for PM2.5 or PM10 hotspot analyses. 

3.5. What time frame and analysis years should be used in hot-spot analyses?

The March 2006 final rule does not change the time frame and analysis years required when 

PM2.5 or PM10 hot-spot analyses are conducted. As discussed in the July 1, 2004, final 

conformity rule (69 FR 40056-40058), hot-spot analyses in metropolitan nonattainment and 

maintenance areas must consider the full time frame of an area's transportation plan at the time 

the analysis is conducted. Hot-spot analyses for projects in isolated rural nonattainment and 

maintenance areas must consider the full time frame of the area's 20-year regional emissions 
16

analysis since these areas are not required to develop a transportation plan under DOT's 

statewide transportation planning regulations. Although SAFETEA-LU and Clean Air Act 

section 176(c)(7) now allow the election of changes to the time horizons for transportation plan 

and TIP conformity determinations, these changes to do not affect the time frame and analysis 

requirements for hot-spot analyses. 

 

To ensure that conformity requirements are being satisfied, areas should examine the year(s) 

within the transportation plan or regional emissions analysis, as appropriate, during which: 

• peak emissions from the project are expected, and 

• a new violation or worsening of an existing violation would most likely occur due to the 

cumulative impacts of the project and background concentrations in the project area. 

EPA believes that conformity requirements are met if areas demonstrate that no new or worsened 

violations occur in the year(s) of highest expected emissions – which includes the project’s 

emissions in addition to background regional emissions. If such a demonstration occurs, then no 

adverse impacts would be expected to occur in any other years within the time frame of the 

transportation plan or regional emissions analysis. See the July 2004 final rule for further 

information on this topic. 
17

Chapter 4: Developing a Qualitative PM2.5 or PM10 Hot-spot Analysis

This chapter provides general information on the methods and data that can be used to meet 

qualitative PM2.5 and PM10 hot-spot requirements. The interagency consultation process would 

be used to determine what is needed for a particular project. 

4.1. What methods can be used for performing qualitative PM2.5 and PM10 hot-spot 

analyses? 

This guidance highlights two methods for completing qualitative PM2.5 and PM10 hot-spot 

analyses. These methods are provided as examples only, and there may be other methods. 

Elements of both methods may also be combined for a given hot-spot analysis. The method 

chosen will be affected by the characteristics of a particular project, the project location, and 

available information. 

The data and method used, whether one of those below or an alternate method, must be selected 

and documented through the interagency consultation process (40 CFR 93.105(c)(1)(i)). 

A. Comparison to another location with similar characteristics

This method is a simple approach for demonstrating that a new project will meet statutory 

conformity requirements. It involves reviewing existing highway or transit facilities that were 

constructed in the past and built in locations similar to the proposed project and, whenever 

possible, near an air quality monitor (a “surrogate”) to allow a comparison of PM2.5 or PM10 air 

quality concentrations. See Examples A, C, and D in Appendix B for suggestions of when this 

method can be used. 

The interagency consultation process would be used to determine what project(s) and air quality 

monitor(s) are appropriate to be used as a surrogate for the air quality impacts of the proposed 

project. The project sponsor would document in the project-level conformity determination the 

reasons for picking a surrogate project and air quality monitor, including similarities to and 

differences between the surrogate and proposed project and location. See Question 4.3 for more 

information on what other documentation should be included for a hot-spot analysis. 

B. Air quality studies for the proposed project location 

Air quality information from many sources may be available for the proposed project’s location. 

 See Examples B, C, and D in Appendix B for suggestions of when this method can be used. 

The SIP can be an important tool to be referenced when conducting qualitative hot-spot analyses, 

especially for PM10 nonattainment and maintenance areas that already have SIPs in place. PM2.5 

nonattainment areas may use, as appropriate, any preliminary data or modeling from a PM2.5 SIP 

under development. The SIP contains specific information on the air quality conditions of a 

given nonattainment or maintenance area. Such information may include monitoring data and 
18

modeling data for past or future years at or near a project’s location. Even if a state has not yet 

begun work on its PM2.5 SIP, the air agency would be able to supply data from air quality 

monitors that may be useful in a given hot-spot analysis. 

In some cases, the state or local air agency or a university may also have performed an air 

quality study near the location of a proposed project. In addition, other scientific studies may be 

appropriate to understand the potential air quality impact from certain projects.6 

The interagency consultation process would be used to determine what air quality information 

from a SIP or other air quality study is appropriate for assessing the air quality impacts of the 

proposed project. The project sponsor would document within the project-level conformity 

determination the air quality information used and why it is appropriate. See Question 4.3 for 

more information on what other documentation should be included for a hot-spot analysis. 

4.2. What should be documented for a qualitative PM2.5 or PM10 hot-spot analysis?

The hot-spot analysis should include sufficient documentation to justify the conclusion that a 

proposed project meets conformity hot-spot analysis requirements in 40 CFR 93.116 and 93.123. 

The amount of documentation needed and method of analysis chosen will vary depending on 

individual circumstances (e.g., local background PM2.5 or PM10 concentrations, the size and 

nature of the project, etc.). 

The hot-spot analysis should include a summary of the method and data that were used, such as: 

• A description of the proposed project,7 including where the project is located, the project’s 

scope (adding an interchange, widening a highway, expanding a major bus terminal, etc.), 

when the project is expected to be open to traffic, and what part of 40 CFR 93.123(b)(1) is 

applicable. 

• A description of the method chosen to conduct the hot-spot analysis (see Question 4.1); 

• A description of the type of PM2.5 or PM10 emissions from the proposed project that are 

considered in the qualitative hot-spot analysis (see Questions 3.2, 3.3, and 3.4). 

• A description of existing conditions pertaining to the project and project location (see list of 

factors that may be considered in Question 4.3). 

• A description of the changes in these factors that will result from the project for future 

scenarios, including changes in the surrounding environment that will affect PM2.5 or PM10

air quality, changes in traffic and emissions trends (see Question 4.4); 

• A description of the analysis year(s) that is examined (see Question 3.5). 

• A discussion of any mitigation measures that will be implemented and their expected effects; 

and 

 

6 EPA will be providing a summary of scientific studies that have been completed on the potential impacts of 

transportation projects. See EPA’s website for further information: 

http://www.epa.gov/otaq/stateresources/transconf/policy.htm. 7 The appropriate section of the NEPA document can also be referenced when relevant. 
19

• A conclusion for how the proposed project meets 40 CFR 93.116 and 93.123 conformity 

requirements for the PM2.5 and/or PM10 air quality standards. 

4.3. What are some of the factors that may be considered in describing existing conditions

absent the proposed project?

An accurate description of existing conditions and factors that may influence PM2.5 or PM10

concentrations in the proposed project area should be provided. Analysis of those conditions and 

how they are projected to change over time with the addition of the proposed project is the basis 

of the hot-spot analysis. 

While the following list is not intended to be exhaustive or prescriptive, factors that are relevant 

to PM2.5 or PM10 levels may include: 

A. Air Quality 

Existing and future air quality information should be considered to assess the probability of the 

project causing or contributing to an air quality violation. Analysts and reviewers should be 

aware of the existing air quality conditions so that they can understand the relative impact that 

the proposed project is likely to have. The description of existing air quality information may 

include the following:

• Summarize PM2.5 or PM10 design values from nearby monitors in the nonattainment or 

maintenance area. Determine if a monitoring station is near the project that will provide data 

on local air quality conditions, including PM2.5 and PM10 concentrations. Monitors closer to 

the project location, but still within the nonattainment or maintenance area are preferable to 

those further away. In the absence of a nearby monitor, other appropriate monitors in the 

nonattainment or maintenance area can also be used. Interagency consultation would be used 

to select appropriate monitors for a given project, when monitoring information is necessary 

for a hot-spot analysis. 

• Consider reviewing data from monitoring stations located in other PM2.5 or PM10 

nonattainment or maintenance areas that may have similar traffic and environmental 

conditions to the proposed project and location. 

• Describe future estimated air quality for the attainment year, years beyond the attainment 

year, and any changes in PM2.5 or PM10 concentrations needed to meet attainment and 

maintenance schedules. Expected changes in air quality at the project location may result in 

changes in the background concentration and the likelihood that a given project may create 

or worsen an air quality problem. 

• Consider PM2.5 or PM10 source apportionment studies when available. 

• Consider future emissions trends that could affect air quality concentrations at the project’s 

location, such as a stationary source, port, or other new source of PM2.5 or PM10 emissions. 

• It is appropriate to also cite published scientific studies or other information regarding 

regional or local trend data on PM2.5 or PM10 concentrations, when such data is available and 

applicable to a given project and location. 
20

Sources: State/local air quality agencies or public health departments would have monitoring 

data and modeling results included in a nonattainment or maintenance area’s SIP or recent 

monitoring, modeling, or other data. Universities or other sources may have completed 

independent air quality studies for the project or similar location. Air quality information may 

also be useful from other nonattainment and maintenance areas with similar types of projects and 

locations. 

B. Transportation and traffic conditions 

Available traffic information such as current volumes and expected volumes should be included, 

including any information regarding the types of percentages of diesel and other vehicles on the 

affected roadway(s). Planned or expected development that will affect traffic volume growth 

rates should be taken into consideration. 

Understanding whether vehicle miles traveled (VMT) are increasing or decreasing, or how a 

project would change the mix of vehicles on the road will assist in judging the project=s air 

quality impacts. For example, it would be important to consider the PM2.5 or PM10 air quality 

impacts of any increase in diesel truck or bus traffic due to the proposed project or other 

activities at the project location. Also, increased VMT and how re-entrained road dust emissions 

are impacted would be considered in PM10 areas and PM2.5 areas where re-entrained road dust is 

found to be significant (40 CFR 93.102(b)(3)). 

Other relevant information may include transportation modes, volumes, speed, congestion, 

trends, etc. When the project analysis is incorporated in a NEPA document, this description 

should largely reference other sections of the NEPA document that address traffic and 

transportation issues in greater depth. 

Sources: Project sponsor, state department of transportation, local planning agency or 

metropolitan planning organization. 

C. Built and natural environment

This description would include whether the character of the project area is urban, suburban or 

rural, and whether adjacent buildings or topography create barriers to dispersal of PM2.5 or PM10. 

 Relevant development trends and land use patterns should be addressed if they have a bearing 

on potential PM2.5 or PM10 emissions and concentrations in the vicinity of the project (e.g., a new 

area or stationary emissions source, increased rail traffic resulting from a rail terminal, increased 

truck traffic due to a port or intermodal freight terminal, or due to industrial or agricultural 

purposes). 

Sources: State department of transportation, the project sponsor, local planning agency or 

metropolitan planning organization. 
21

D. Meteorology, climate and seasonal data 

This description could address atmospheric inversions, prevailing wind direction and speed, as 

they impact PM2.5 or PM10 concentrations in the project area, if appropriate. 

Sources: State/local air quality agencies, review of the applicable PM2.5 or PM10 SIP, and the 

National Weather Service. 

E. Retrofit, anti-idling or other adopted emission control measures 

Emission control measures, such as retrofit or anti-idling measures, may mitigate any potential 

increase in PM2.5 or PM10 emissions at the proposed project’s location. The impact of phase-in 

of national rules and regulations that EPA has promulgated, such as heavy-duty diesel rules, that 

are currently being implemented should also be considered. 

Source: State/local air agency, EPA, review of the applicable PM10 or PM2.5 SIP. 

4.4. How would changes in existing conditions be evaluated for future scenarios with the 

proposed project? 

Many factors may change air quality in the future and whether increases or decreases in PM2.5 or 

PM10 levels are expected should be documented in the project-level conformity determination. 

Examples of changes in factors that may lead to changes in PM2.5 or PM10 concentrations in the 

project are listed below. Potential sources for this type of information are similar to those in 

Question 4.4. 

PM2.5 and PM10

• Changes in traffic volumes and VMT, broken out by estimated diesel fraction and diesel 

vehicle class; 

• Changes in traffic congestion and traffic flow; 

• Changes in diesel truck or bus routes; 

• Changes in truck weight limits; 

• Retrofit projects, idling policies, truck hoteling electrification infrastructure, or other 

emission reduction policies; 

• Date the project is expected to open; 

• Effect on phase-in of heavy-duty diesel emission; and 

• Changes in the built and natural environment that may change existing PM2.5 or PM10 

dispersion patterns. 

PM10 (unless fugitive dust is included in a PM2.5 area – see Questions 3.3 and 3.4) 

• street sanding/sweeping practices. 
22

As described in Question 3.5, the future (build) scenario should consider whether the proposed 

project would be expected to increase or decrease PM2.5 or PM10 concentrations at the project 

location over the time frame of the area’s transportation plan or, in the case of an isolated rural 

area, over the 20-year period covered by the area’s regional emissions analysis. The hot-spot 

analysis should address the expected air quality changes resulting from the proposed project, and 

address whether the build scenario(s) would be expected to result in new or worsened air quality 

violations of the PM2.5 or PM10 standards. 

4.5. What are the potential measures to mitigate PM2.5 or PM10 air quality concerns? 

Where the proposed project may lead to a potential new PM2.5 or PM10 violation or increase the 

severity or frequency of an existing PM2.5 or PM10 violation, mitigation measures would be 

considered to reduce project emissions and any local air quality impact. In these cases, written 

commitments for project-level mitigation or control measures must be obtained from the project 

sponsor and/or operator prior to making a project-level conformity determination (40 CFR 

93.125(a)). A table including a menu of available options is included in Appendix C; however, 

many others may be possible. 
23

APPENDIX A 

EXAMPLES OF PROJECTS OF AIR QUALITY CONCERN 
24

Examples of Projects of Air Quality Concern

Note: EPA noted in the March 2006 final rule that the examples below are considered to be the 

most likely projects that would be covered by 40 CFR 93.123(b)(1) and require a PM2.5 or PM10

hot-spot analysis (71 FR 12491). 

Some examples of projects of air quality concern that would be covered by 40 CFR 

93.123(b)(1)(i) and (ii) are: 

$ A project on a new highway or expressway that serves a significant volume of diesel 

truck traffic, such as facilities with greater than 125,000 annual average daily traffic 

(AADT) and 8% or more8

 of such AADT is diesel truck traffic; 

$ New exit ramps and other highway facility improvements to connect a highway or 

expressway to a major freight, bus, or intermodal terminal; 

$ Expansion of an existing highway or other facility that affects a congested intersection 

(operated at Level-of-Service D, E, or F) that has a significant increase in the number of 

diesel trucks; and, 

$ Similar highway projects that involve a significant increase in the number of diesel 

transit busses and/or diesel trucks. 

Some examples of projects of air quality concern that would be covered by 40 CFR 

93.123(b)(1)(iii) and (iv) are: 

$ A major new bus or intermodal terminal that is considered to be a “regionally significant 

project” under 40 CFR 93.1019; and, 

$ An existing bus or intermodal terminal that has a large vehicle fleet where the number of 

diesel buses increases by 50% or more, as measured by bus arrivals. 

 

8

This percentage is the national average of truck vehicle miles traveled (VMT) to total VMT, based on 

FHWA’s Highway Statistics publication which can be found at: 

http://www.fhwa.dot.gov/policy/ohim/hs04/index.htm. EPA’s MOBILE6.2 motor vehicle emissions model also uses 

8% truck VMT as a national default. 

9

40 CFR 93.101 defines a “regionally significant project” as “a transportation project (other than an exempt 

project) that is on a facility which serves regional transportation needs (such as access to and from the area outside 

of the region, major activity centers in the region, major planned developments such as new retail malls, sports 

complexes, etc., or transportation terminals as well as most terminals themselves) and would normally be included in 

the modeling of a metropolitan area’s transportation network, including at a minimum all principal arterial highways 

and all fixed guideway transit facilities that offer an alternative to regional highway travel.”
25

Examples of Projects of That Are Not an Air Quality Concern 

Note: The March 2006 final rule also provided examples of projects that would not be covered 

by 40 CFR 93.123(b)(1) and would not require a PM2.5 or PM10 hot-spot analysis (71 FR 12491). 

However, as noted elsewhere in this guidance, PM10 nonattainment and maintenance areas with 

approved conformity SIPs that include PM10 hot-spot provisions from previous rulemakings 

must continue to follow those approved conformity SIP provisions until the SIP is revised. 

The following are examples of projects that are not an air quality concern under 40 CFR 

93.123(b)(1)(i) and (ii): 

$ Any new or expanded highway project that primarily services gasoline vehicle traffic 

(i.e., does not involve a significant number or increase in the number of diesel vehicles), 

including such projects involving congested intersections operating at Level-of-Service 

D, E, or F; 

$ An intersection channelization project or interchange configuration project that involves 

either turn lanes or slots, or lanes or movements that are physically separated. These 

kinds of projects improve freeway operations by smoothing traffic flow and vehicle 

speeds by improving weave and merge operations, which would not be expected to create 

or worsen PM2.5 or PM10 violations; and, 

• Intersection channelization projects, traffic circles or roundabouts, intersection 

signalization projects at individual intersections, and interchange reconfiguration projects 

that are designed to improve traffic flow and vehicle speeds, and do not involve any 

increases in idling. Thus, they would be expected to have a neutral or positive influence 

on PM2.5 or PM10 emissions. 

Examples of projects that are not an air quality concern under 40 CFR 93.123(b)(1)(iii) and (iv) 

would be: 

$ A new or expanded bus terminal that is serviced by non-diesel vehicles (e.g., compressed 

natural gas) or hybrid-electric vehicles; and, 

$ A 50% increase in daily arrivals at a small terminal (e.g., a facility with 10 buses in the 

peak hour). 
26

 

APPENDIX B 

EXAMPLES OF QUALITATIVE PM2.5 OR PM10 HOT-SPOT ANALYSES 

Note: The information in Appendix B is intended to briefly summarize the types of methods and 

data that can be considered in qualitative PM2.5 or PM10 hot-spot analyses. An actual qualitative 

PM2.5 or PM10 hot-spot analysis would include more documentation regarding the proposed 

project, the analysis method and data considered, and the analysis’ final conclusion. 
27

Example A: Comparison of a New Bus Terminal to 

Another Site Based on Monitoring Data

Proposed project

• A new major bus terminal is proposed to be built along a public transit route in a rapidly 

growing suburban area. The proposed project would significantly increase diesel bus 

traffic at the project’s location. 

• The project would be located in a PM2.5 nonattainment area. 

• A PM2.5 hot-spot analysis is required for this project since it is covered by 40 CFR 

93.123(b)(1)(iii). 

Analysis method

• The interagency consultation process is used to decide that the hot-spot analysis would 

rely on a comparison to an existing project with similar characteristics as the proposed 

project, as discussed in Question 4.2. 

 

Data considered

• The hot-spot analysis would not consider PM2.5 road dust emissions, since a finding of 

significance has not been made by EPA or the state air agency. 

• A nearby air quality monitor indicates that the proposed project’s location is significantly 

below the 24-hour PM2.5 standard (50 μg/m3

) and close to the annual PM2.5 standard (14.5 

μg/m3). 

• A monitor in the vicinity of an existing bus terminal in another part of the PM2.5 

nonattainment area has recorded data near the 24-hour PM2.5 standard (60 μg/m3) and a 

violation of the annual PM2.5 standard (15.1 μg/m3

). 

• The existing bus terminal also has significant levels of diesel bus traffic and other similar 

traffic characteristics as the proposed project. 

• Two measures were added to the project to mitigate potential local air quality impacts. 

These measures were an anti-idling policy for diesel buses and retrofitting older buses 

that were committed to be implemented at the project location. 

Conclusion

• The interagency consultation process concluded that additional mitigation measures for 

the new bus terminal would be beneficial and should allow concentrations to be lower 

than the standards compared to the air quality monitoring data found by the existing 

terminal that did not have the mitigation measures that were near or slightly over the 24-

hour and annual PM2.5 standards. 

• These measures allowed the project to meet the conformity hot-spot requirements in 40 

CFR 93.116 and 93.123. 
28

Example B: Consideration of a Highway Project and Nearby Monitoring Data

Proposed project

• The project entails a major modification to a highway interchange connecting a primary 

route to an interstate. A significant number of diesel vehicles are expected to use the 

interchange. 

• The project would be located in a suburban portion of a larger metropolitan city. The 

project is located in a nonattainment area for PM2.5 and PM10 standards. 

• PM2.5 and PM10 hot-spot analyses are required pursuant to 40 CFR 93.123(b)(1)(i). 

Analysis method

• The interagency consultation process was used to decide that the hot-spot analysis would 

rely on air quality data at the proposed project location, as discussed in Question 4.2. 

Data considered

• Air quality information supplied by the state air quality agency found the project’s 

location did not have any current violations and was significantly below the annual and 

24-hour PM2.5 and PM10 standards. This information also showed that PM2.5 and PM10

emissions from existing sources were decreasing in the project area into the future. 

• The hot-spot analysis would not consider PM2.5 road dust emissions, since a finding of 

significance has not been made by EPA or the state air agency. Road dust emissions 

would be considered for the PM10 hot-spot analysis. 

• The traffic change resulting from the project was estimated. It was found to be consistent 

with VMT increases in the metropolitan area generally where no increase in PM2.5 and 

PM10 emissions or concentrations has been noted. 

• The meteorology at the project location can generally be categorized as variable, since 

the wind varies during the day. There is often some wind that acts to disperse PM2.5 and 

PM10 emissions at the site. Temperature, humidity, and rainfall do not seem to influence 

the level of PM2.5 and PM10 pollution at this site. 

• A nearby monitor has not registered any violations, and through the consultation process, 

it was determined that emissions from the project would not result in a new violation as 

any increased emissions that might affect concentrations would be offset by the 

decreasing PM2.5 and PM10 emissions and concentrations at the project location. In other 

words, any increase in the emissions due to traffic changes associated with the project, 

would be offset by decreases in the emissions from the transportation facility due to 

decreasing on-road vehicle emissions trends, as well as decreasing background 

concentrations. This conclusion was supported by scientific journal articles about the air 

quality impact of similar projects, which were discussed through the consultation process 

and cited in the final hot-spot analysis. 

Conclusion

• For the reasons described above, future new or worsened PM2.5 and PM10 violations of 

any standards are not anticipated, and therefore, the project meets the conformity hot-spot 

requirements in 40 CFR 93.116 and 93.123 for both PM2.5 and PM10. 
29

Example C: Comparison of New Highway Project to Similar Project Location in the SIP

Proposed project

• A new 6-lane freeway interchange is proposed to be built at the edge of an urban area. 

This interchange would lead to a significant increase in diesel vehicle traffic from both 

additional travel on the new connecting road, and from commercial and industrial 

development planned for the vicinity of the interchange. 

• The project would be located in a PM10 maintenance area. 

• A PM10 hot-spot analysis is required for this project since it is covered by 40 CFR 

93.123(b)(1)(i). 

Analysis method

• The interagency consultation process is used to decide that the hot-spot analysis would 

rely on a hybrid of the two methods discussed in Question 4.2, which include a 

comparison to another location with similar characteristics and air quality studies. 

Data considered

• Through the interagency consultation process, it was determined that the approved PM10

maintenance plan included a modeled demonstration of maintenance for the 24-hour 

PM10 standard extending out to the year 2015. The SIP also included a modeled 

demonstration that the annual PM10 standard would be met as long as the 24-hour PM10

standard was met. Therefore, consistent with the SIP’s demonstration, conformity 

requirements can also be achieved by evaluating only the 24-hour PM10 standard in this 

particular area. 

• The interagency consultation group decided to evaluate the new interchange by 

comparing it to an existing interchange that is within the PM10 maintenance plan=s 

modeling domain. The interagency consultation group located an existing interchange 

that was located near another edge of the urban area that was similar in terms of 

meteorological conditions, and had higher diesel traffic volumes and more intensive 

surrounding development than that expected at the new interchange location. 

• This existing interchange was within the PM10 maintenance plan’s modeling grid that 

was predicted to experience concentrations of approximately 110 μg/m3

. The current 24-

hour PM10 standard is 150 μg/m3

. 

Conclusion

• Since this existing interchange was not predicted to experience new or worsened 

violations of the 24-hour and annual PM10 standards, and the new interchange would see 

lower traffic volumes and less development, the interagency consultation group 

concluded that the new interchange met the conformity hot-spot requirements in 40 CFR 

93.116 and 93.123. 
30

Example D: Determination of Screening Threshold for Multiple Projects

Proposed projects to be considered

• The state department of transportation (DOT) for a PM10 nonattainment area anticipates a 

large number of new highway interchange projects involving significant levels of diesel 

traffic that would require a qualitative PM10 hot-spot analysis in the next few years. 

• These projects would be considered of air quality concern by 40 CFR 93.123(b)(1)(i). 

Analysis method

• The interagency consultation process is used to decide that a screening method that 

would support future qualitative PM10 hot-spot analyses for such projects. The screening 

method is based on a hybrid of the two methods discussed in Question 4.2, which include 

a comparison to another location with similar characteristics and air quality studies. 

Data considered

• The interagency consultation group agreed that it would be the most efficient use of their 

resources to develop an analysis for a hypothetical project to which individual projects 

could be compared. 

• The state DOT retained a consultant to conduct an air quality analysis of some 

hypothetical interchange projects that were representative of those the State may wish to 

construct in the future. This PM10 nonattainment area’s on-road mobile source inventory 

is dominated by re-entrained road dust. 

• The consultant conducted an air quality modeling exercise, using typical project 

configurations and the highest background values typically experienced in the 

metropolitan area, and concluded that a project would have to generate 500,000 daily 

VMT within a one-square-mile area in order to cause a potential violation of the PM10

standard. 

• After discussing the situation with the interagency consultation group, it was decided that 

certain projects, depending on their characteristics, could be constructed without 

triggering a violation of the PM10 standard. 

 

Conclusion

• Any applicable future project would meet the conformity hot-spot requirements in 40 

CFR 93.116 and 93.123 by referencing the study and providing project-specific 

information for comparison. 

• Under this example, if a future project has less than 500,000 VMT/day, no further hotspot analysis and no mitigation is required. 

• If a future project has more than 500,000 VMT/day, further hot-spot analysis is required, 

and possibly mitigation measures. 

 
31

APPENDIX C 

POTENTIAL MITIGATION MEASURES
32

Potential PM2.5 or PM10 Project-level Mitigation Measures: Diesel Emissions 

Options to Reduce PM Pollution 

Suspected Source 

of PM2.5 or PM10

Problem 

Type of PM 

primarily 

controlled 

Mitigation Measure Comments 

Diesel emissions 

in general from a 

highway or transit 

facility 

PM2.5 or PM10 Provide a “retrofit” program 

for older, higher emitting 

vehicles 

Anti-idling requirements or 

policies (e.g., restrictions on 

idling, truck stop 

electrification) 

Routing existing traffic 

away from populated areas 

(e.g., truck restricted zone) 

Replace a significant 

number of older buses with 

cleaner busses (e.g., those 

meeting 2007 heavy-duty 

diesel standards, as 

practical, hybrid-electric 

vehicles, etc.) 

Retrofits could be used on truck or 

bus fleets to install newer engines 

or technologies known to have 

lower emissions 

Anti-idling polices are relevant 

where significant numbers of 

diesel vehicles congregate for 

extended periods of time 

Routing traffic away from 

populated areas may change an 

area’s VMT 

Cleaner buses will reduce 

localized PM2.5 and PM10

emissions for these types of transit 

projects 
33

Potential PM2.5 or PM10 Project-level Mitigation Measures: Fugitive Dust Emissions

Options to Reduce PM Pollution 

Suspected Source 

of PM2.5 or PM10

Problem 

Type of PM 

primarily 

controlled 

Mitigation Measure Comments 

PM10 Truck cover laws May require greater enforcement 

effort in some areas 

PM10 Street cleaning program Includes vacuuming and flushing 

PM10 Site watering program Regular program will reduce dust 

Fugitive Dust 

PM10 Street and shoulder paving; 

Runoff and erosion control Should reduce significant quantities 

of dust material 

PM10 Changes in highway weight 

and length restrictions for 

trucks 

May change an area’s fugitive dust 

emissions or change the number of 

trucks on the road 

Snow and Ice 

Control 

PM10 Reduce the quantity of sand Use harder material that is not prone 

to grinding into finer particles or 

additional chemical treatments 

Note: The above table focuses on measures for mitigating PM10 fugitive dust emissions because 

all PM10 areas must include these emissions in their PM10 hot-spot analyses. However, as 

described in Questions 3.3. and 3.4., there may be PM2.5 areas that also could take advantage of 

the above measures if re-entrained road dust or construction dust is required for a PM2.5 hot-spot 

analysis.