Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Office of Planning, Environment, & Realty (HEP)
PlanningEnvironmentReal Estate

HEP Events Guidance Publications Awards Contacts

Prairie Parkway Study Fine Particulate Matter (PM2.5) Project Level Hot-Spot Analysis

4.0 Background of PM2.5 Requirements

Airborne particulate matter (PM) consists of many different substances suspended in air in the form of particles (solids or liquid droplets) that vary widely in size. The particle mix in most U.S. cities is dominated by fine particles (less than 2.5 micrometers in diameter) generated by combustion sources, with smaller amounts of coarse dust(between 2.5 and 10 micrometers in diameter). Particles less than 10 micrometers in diameter include both fine and coarse dust particles. These particles pose the greatest health concern because they can pass through the nose and throat and get into the lungs. Particles larger than 10 micrometers in diameter that suspend in the air are referred to as total suspended particulates (TSP). These larger particles can cause irritation to the eyes, nose and throat in some people, but they are not likely to cause more serious problems since they do not get down into the lungs.

Motor vehicles (i.e., cars, trucks, and buses) emit direct PM from their tailpipes, as well as from normal brake and tire wear. In addition, vehicles cause dust from paved and unpaved roads to be reentrained, or resuspended, in the atmosphere; and highway and transit project construction may cause dust. Finally, gases in vehicle exhaust may react in the atmosphere to form PM. Particles come in a wide variety of sizes and have been historically assessed based on size, typically measured by the diameter of the particle in micrometers. PM2.5, or fine particulate matter, refers to particles that are 2.5 micrometers in diameter or less. (Note: A human hair is about 70 micrometers in diameter and a grain of sand is about 90 micrometers in diameter). The National Ambient Air Quality Standards (NAAQS) for fine particulate matter, as revised by EPA in 2006, include an annual standard (15.0 micrograms per cubic meter (μg/m3)) and a 24-hour standard (35 μg/m3). However, for conformity purposes, only the 1997 PM2.5 NAAQS (15.0 μg/m3 for the annual standard and 65 μg/m3 for the 24-hour standard) applies at this time based on EPA's April 2007 guidance4. The annual standard is based on a 3-year average of annual mean PM2.5 concentrations; the 24-hour standard is based on a 3-year average of the 98th percentile of 24-hour concentrations.

4.1 Statutory and Regulatory Requirements

On March 10, 2006, EPA issued amendments to the Transportation Conformity Rule to address localized impacts of particulate matter: "PM2.5 and PM10 Hot-Spot Analyses in Project-level Transportation Conformity Determinations for the New PM2.5 and Existing PM10 National Ambient Air Quality Standards" (71 FR 12468). This rule amendment requires the assessment of localized air quality impacts of Federally-funded or approved transportation projects in PM10 and PM2.5 nonattainment and maintenance areas deemed to be projects of air quality concern5. This assessment of localized impacts (i.e., "analysis") examines potential air quality impacts on a scale smaller than an entire nonattainment or maintenance area. 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.

Qualitative hot-spot analysis is required for these projects before EPA releases its future quantitative modeling guidance and announces that quantitative PM2.5 hot-spot analyses are required under 40 CFR §93.123(b)(4). EPA requires hot-spot findings to be based on directly emitted PM2.5, since secondary particles take several hours to form in the atmosphere giving emissions time to disperse beyond the immediate area of concern. The Conformity Rule requires PM2.5 hot-spot analyses to include road dust emissions only if such emissions have been found significant by EPA or the state air agency prior to the PM2.5 SIP or as part of an adequate PM2.5 SIP motor vehicle emissions budget (40 CFR §93.102(b)(3)). Emissions resulting from construction of the project are not required to be considered in the hot-spot analysis if such emissions are considered temporary according to 40 CFR §93.123(c)(5).

The PM2.5 and PM10 hot-spot requirements in the final rule became effective April 5, 2006. A qualitative PM2.5 hot-spot analysis that meets the final rules requirements must be completed for project-level determinations for projects of air quality concern completed on or after April 5, 2006.

4.2 PM2.5 Regional Conformity Determination

Section 176(c) of the Clean Air Act and the federal conformity rule require that transportation plans and programs conform to applicable state air quality implementation plans (SIPs) and Section 174 and 176(c) and (d) of the Clean Air Act (42 U.S.C. 7504, 7506(c) and (d)). The Prairie Parkway project was included in the 2030 Regional Transportation Plan (RTP) and FY 2007 - 2012 Transportation Improvement Program (TIP), endorsed by the Policy Committee of the Chicago Area Transportation Study (CATS), the Metropolitan Planning Organization (MPO) for the region in which the project is located. On October 16, 2006, the FHWA and the FTA determined that the RTP and TIP conformed to the State Implementation Plan (SIP) and made a PM2.5 conformity determination. These findings were in accordance with 40 CFR Part 93, "Criteria and Procedures for Determining Conformity to State or Federal Implementation Plans, Programs, and Projects Funded or Approved under Title 23 USC or the Federal Transit Act."

The Prairie Parkway Preferred Alternative design concept and scope is consistent with the project information used for the RTP and TIP conformity analysis. The northeastern Illinois 2030 RTP and FY 2007-2012 TIP comply with the interim PM2.5 emissions tests required by the conformity rule and the project complies with the project-level PM2.5 hot-spot analysis requirements of the conformity rule. This project's TIP number is #09-02-9033. The Prairie Parkway project was included in the regional emissions analysis and there have been no significant changes in the project's design concept or scope, as used in the conformity analyses. Therefore, the project comes from a conforming plan and program in accordance with 40 CFR §93.115.



4 EPA, "Transportation Conformity and the Revised 24-hour PM2.5 Standard", April 16, 2007.
5 Criteria for identifying projects of air quality concern is described in 40 CFR 93.123(b)(1), as amended.
Updated: 07/06/2011
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000