United States

Environmental Protection

Agency

Air and Radiation EPA420-D-00-003

July 2000

Draft Technical Support

Document: Control of

Emissions of Hazardous

Air Pollutants from

Motor Vehicles and

Motor Vehicle Fuels

 Printed on Recycled Paper
EPA420-D-00-003

July 2000

Draft Technical Support Document: Control of

Emissions of Hazardous Air Pollutants from

Motor Vehicles and Motor Vehicle Fuels

Assessment and Standards Division

Office of Transportation and Air Quality

U.S. Environmental Protection Agency

NOTICE

This technical report does not necessarily represent final EPA decisions or positions.

It is intended to present technical analysis of issues using data that are currently available.

The purpose in the release of such reports is to facilitate the exchange of

technical information and to inform the public of technical developments which

may form the basis for a final EPA decision, position, or regulatory action.
2

Executive Summary

A range of compounds known as hazardous air pollutants are emitted from motor

vehicles and fuels and are known or suspected to have serious health impacts. This document

describes in greater detail the technical reasoning behind our proposed program to address

emissions of hazardous air pollutants from mobile sources. 

We begin with background information in support of the regulatory decisions for control

of mobile source air toxics under Section 202(l)(2) of the Clean Air Act and, more specifically, a

review of our mobile source emission control programs that relate to mobile source air toxics

control. This is followed by a discussion of how we identified our list of Mobile Source Air

Toxics. Selection was based on identifying those compounds that we know are emitted by motor

vehicles and comparing this list to EPA’s Integrated Risk Information System (IRIS) database. 

IRIS is a database of compounds that identifies EPA’s consensus scientific judgment on the

characterization of the potential adverse health effects that may result from a lifetime exposure to

various substance. This process resulted in a list of 21 compounds. Chapter 3 contains

important health and environmental information for each of those MSATs.

Before we can evaluate whether additional mobile source air toxics controls are

appropriate, we must evaluate the effectiveness of current controls in reducing on-highway

emissions of these MSATs. Our analysis, contained in Chapter 4, shows that the programs we

currently have in place, including our reformulated gasoline (RFG) program, national low

emission vehicle (NLEV) program, Tier 2 motor vehicle emissions standards and gasoline sulfur

control requirements (Tier 2), and our recently proposed heavy-duty engine and vehicle standards

and on-highway diesel fuel sulfur control requirements (HD2007 rule), are expected to yield

significant reductions of mobile source air toxics. Between 1990 and 2020, these programs are

expected to reduce on-highway emissions of benzene, formaldehyde, 1,3-butadiene, and

acetaldehyde by 75 percent or more. In addition, we expect to see on-highway diesel PM

emission reductions of over 90 percent. Nonroad engines and equipment also contribute

substantially to levels of MSATs emissions and have not been subject to the same stringent

controls as highway sources.

Chapter 5 reviews what we know about ambient concentrations and exposures associated

with emissions of mobile source air toxics. We look at monitoring and modeled data on ambient

concentrations of five of the 21 mobile source air toxics. These compounds are benzene, 1,3-

butadiene, formaldehyde, acetaldehyde, and diesel PM. We also review results of an highway

vehicle inhalation exposure assessment prepared by EPA. The exposure estimates for gaseous

air toxics are compared to estimates of the highway vehicle contribution to modeled ambient

concentrations. We also discuss what we know about inhalation exposures from microenvironmental sources. 

We next consider whether there are additional air toxics controls that should be put in
3

place at this time to further reduce on-highway MSAT inventories. Chapter 6 provides our

analysis with respect to vehicle-based controls. With regard to vehicle-based controls, we

conclude that the it is not appropriate at this time to propose more stringent standards than the

technology forcing standards found in our recently adopted Tier 2 and recently proposed HD2007

rule standards. Chapter 7 provides our analysis for fuel-based controls. With regard to fuelsbased controls, we are proposing a gasoline benzene control program that requires refiners to

maintain the current levels of over-compliance with RFG and anti-dumping toxics requirements. 

Because the proposed standards for each refiner as the same as the 1998/9 average gasoline

benzene level for that refiner, the proposed standards are expected to impose only negligible

costs, if any. 

Finally, in Chapter 8 we describe our current nonroad engine emission control programs

and present our estimates of the impacts of these programs on future air toxics inventories. In

this chapter, we also highlight the significant uncertainty and several of the data gaps that exist

with respect to toxics emissions from nonroad engines. 
4

List of Acronyms

μg/m3 micrograms per cubic meter

AIRS Aerometric Information Retrieval System

API American Petroleum Institute

ASPEN Assessment System for Population Exposure Nationwide

ATSDR Agency for Toxic Substances and Disease Registry

CAA or the Act Clean Air Act

CalEPA California Environmental Protection Agency

CARB California Air Resources Board

CASAC Clean Air Scientific Advisory Committee

CEP Cumulative Exposure Project

CG conventional gasoline

CHAD Comprehensive Human Activitiy Database

CMB chemical mass balance

CO carbon monoxide

CPIEM California Population Indoor Exposure Model

DHHS Department of Health and Human Services

DOC diesel oxidation catalyst

EPA or the Agency U.S. Environmental Protection Agency

FAA Federal Aviation Administration

FTP federal test procedure

g/bhp-hr grams per brake-horsepower-hour

GVWR gross vehicle weight rating

HAP hazardous air pollutant

HAPEM Hazardous Air Pollutant Exposure Model

HAPEM-MS Hazardous Air Pollutant Exposure Model for Mobile Sources

HC hydrocarbon

HD2007 heavy-duty engine and vehicle standards and diesel sulfur controls

HD-FTP heavy-duty federal test procedure

HDE heavy-duty engine

HDV heavy-duty vehicle

HLDT heavy light-duty truck

I/M inspection/maintenance
5

IARC International Agency for Research on Cancer

ICAO International Civil Aviation Organization

IRIS Integrated Risk Information System

LDT light-duty truck

LDV light-duty vehicle

LEV low emission vehicle

LLDT light light-duty truck

MARPOL International Convention on the Prevention of Pollution from Ships

MDPV medium-duty passenger vehicle

MRL minimum risk level

MSAT mobile source air toxic

MTBE methyl tert butyl ether

NAAQS National Ambient Air Quality Standards

NATA National Air Toxic Assessment

NESCAUM Northeast States for Coordinated Air Use Management

NIPER National Institute for Petroleum and Energy Research

NLEV national low emission vehicle

NMHC non-methane hydrocarbons

NMOG non-methane organic gases

NOx oxides of nitrogen

NPC National Petroleum Council

NPRA National Petrochemical & Refiners Association

NPRM Notice of Proposed Rulemaking

NTE not-to-exceed

NTI national toxics inventory

OAQPS Office of Air Quality Planning and Standards

OBD on-board diagnostics

OMB Office of Management and Budget

ORD Office of Research and Development

ORVR on-board refueling vapor recovery

OTAQ Office of Transportation and Air Quality

PADD Petroleum Administrative Districts for Defense

PAH polycyclic aromatic hydrocarbon compounds

PCM powertrain control module
6

PM particulate matter

POM polycyclic organic matter

PTD product transfer document

R&D research and development

REL reference exposure level

RFA Regulatory Flexibility Act

RfC reference concentration for noncancer effects

RfD reference dose for noncancer health effects

RFG reformulated gasoline

RVP Reid vapor pressure

SAB Scientific Advisory Board

SBA U.S. Small Business Administration

SBARP or the Panel Small Business Advocacy Review Panel

SBREFA Small Business Regulatory Enforcement Fairness Act

SCAQMD South Coast Air Quality Management District (California)

SFTP supplemental federal test procedure

SI spark ignited 

SIC Standard Industrial Classification

SIGMA Society of Independent Gasoline Marketers of America

SIP State Implementation Plan

SOF soluble organic fraction

SRP scientific review panel

TAC toxic air contaminant

TEAM Total Exposure Assessment Methodology study

THC total hydrocarbons

Tier 2 tier 2 motor vehicle emission standards and gasoline sulfur controls

TOG total organic gases

TSCA Toxic Substances Control Act

TSD technical support document

TWC three-way catalyst

UATS Integrated Urban Strategy (also called Urban Air Toxics Strategy)

ULEV ultra-low emission vehicles

VMT vehicle miles traveled

VOC volatile organic compound
7

List of Tables

Table I.B-1: Hydrocarbon (HC) Exhaust Emission Standards for Light-Duty

Vehicles (gpm) 

Table I.B-2: Heavy-Duty Standards for Diesel and Gasoline Engines (g/bhp-hr)

Table I.B-3: Heavy-Duty Gasoline Vehicle Standards Proposed for 2004

Table I.B-4: New Urban Bus Standards (g/bhp-hr)

Table I.B-5: Current Light Duty Vehicles, Light Duty Truck, and Heavy-Duty

Gasoline Vehicle Evaporative Hydrocarbon and Refueling Spitback

Standards 

Table I.B-6: On-Board Vapor Recovery for Gasoline Vehicles Phase-In Periods

and Standard

Table II.B-1: List of Mobile Source Air Toxics (MSATs)

Table IV.A-1: Annual Emission Summary for the Total U.S. for Selected Toxics OnHighway Vehicles Only (short tons per year)

Table IV.A-2: 1996 On-Highway and Nonroad Emission Inventories of MSATs 1996

NTI (short tons)

Table IV.B-1: Metropolitan Areas and Regions Included in Toxic Emissions

Modeling

Table IV.B-2: Example of Data File Format for Toxic Adjustment Factors

Table IV.B-3: Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only - Pre-Tier 2 Control Scenario(thousand

short tons per year)

Table IV.B-4: Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only - Pre-Tier 2 Control Scenario

Table IV.B-5: Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only - Tier 2 Control Scenario (thousand short

tons per year)

Table IV.B-6: Annual Emissions Summary for Selected Toxics for the Total U.S.
8

On-Highway Vehicles Only - Tier 2 Control Scenario

Table IV.B-7: Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only - Heavy-Duty 2007 Control Scenario

(thousand short tons per year)

Table IV.B-8: Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only - Heavy-Duty 2007 Control Scenario

Table IV.B-9: Annual VOC Emissions Summary for the Total U.S. On-Highway

Vehicles Only - Tier 2 Control Scenario

Table IV.B-10: Annual VOC Emissions Summary for the Total U.S. On-Highway

Vehicles Only - Heavy-Duty 2007 Control Scenario

Table IV.B-11: Annual Diesel PM Emissions Summary for the Total U.S. OnHighway Vehicles Only - Tier 2 Control Scenario

Table IV.B-12: Annual Diesel PM Emissions Summary for the Total U.S. OnHighway Vehicles Only - Heavy-Duty 2007 Control Scenario

Table IV.C-1: VMT Fractions used in the 1999 EPA Motor Vehicle Air Toxics Study

Compared with proposed fractions for MOBILE6

Table IV.C-2: U.S. Annual average toxic emission factors (mg/mi) from the 1999

EPA Motor Vehicle Air Toxics Study 

Table V.A-1: Monitored 1996 ambient concentration estimates nationwide from

AIRS

Table V.A-2: Monitored average ambient concentration estimates (μg/m3

), and

estimated highway and nonroad contributions, in the South Coast Air

Basin and in Minnesota.

Table V.A-3: Ambient Diesel Particulate Matter Concentrations from Receptor

Modeling and Elemental Carbon Measurements

Table V.A-4: Average estimated nationwide concentrations of selected air toxics in

1990 from the Cumulative Exposure Project (μg/m3)

Table V.A-5: Draft average estimates of mobile source contributions to nationwide

concentrations of selected air toxics in 1996 from the NATA national

scale assessment
9

Table V.A-6: Annual average diesel particulate matter concentrations predicted

from dispersion modeling

Table V.B-1: California annual average diesel PM exposure estimates for all mobile

sources from the California Population Indoor Exposure Model

Table V.B-2: Comparison of 1990 average exposure attributable to on-highway

vehicle emissions (HAPEM-MS3) to 1990 ambient concentration

estimates attributable to on-highway vehicle emissions (CEP)

Table V.B-3: Comparison of 1996 annual average exposures attributable to onhighway vehicles (HAPEM-MS3) and the on-highway vehicle portion

of 1996 modeled ambient concentrations (National Scale Assessment)

Table V.B-4: Highway vehicle portion of nationwide average inhalation exposures

to benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and diesel PM

for three demographic groups in 1996, based on HAPEM-MS3

Table V.B-5: On-Highway vehicle portion of 1996 benzene exposure estimates for

10 urban areas, and urban and rural areas nationwide, based on

HAPEM-MS3 exposure modeling

Table V.B-6: Annual Average Ambient CO Levels as a Function of Population

Density, 14 Cities

Table V.B-7: Seasonal Average Nationwide Exposures (μg/m3

) Attributable to OnHighway Vehicle Emissions, for the General Population, 1996

Table V.C-1: Micro-environmental concentrations of benzene (μg/m3

)

Table V.C-2: Micro-environmental exposure to formaldehyde (μg/m3)

Table VI.A-1: Gaseous MSATs

Table VI.A-2: Gaseous MSATs in Typical Gasoline-Fueled Vehicle Exhaust

Table VI.A-3: Gaseous MSATs in Typical Gasoline-Fueled Evaporative Emissions

Table VII.A-1: Refinery Count and 1998 Gasoline Volumes by PADD

Table VII.A-2: 1998 Imported Gasoline

Table VII.A-3: PADD-Average Refinery Benzene Levels for Gasoline in 1998
10

Table VII.B-1: On-highway Baseline Benzene Inventories from Sierra Report (50-

state Tons)

Table VII.B-2: Relative Importance of Toxics Emissions from Gasoline-powered

Vehicles

Table VIII.B-1: Annual Toxics Emissions Summary for Selected Air Pollutants for the

Total U.S. Nonroad Mobile Sources from 1990 to 2020 (thousand

short tons per year)

Table VIII.B-2: Summary of Percent Emission Reductions in 2007 and 2020 for

Selected Air Pollutants for the Total U.S. from 1990 or 1996 Nonroad

Mobile Sources

Table VIII.B-3: Annual VOC Emissions Summary for the Total U.S. Nonroad Mobile

Sources

Table VIII.B-4: Annual Diesel PM Emissions Summary for the Total U.S. Nonroad

Mobile Sources
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List of Figures

Figure IV.B-1: Example Plot of Target Fuel Benzene Versus Baseline Fuel TOG

under FTP Conditions

Figure V.A.-1: National trend in annual average benzene concentrations in

metropolitan areas, 1993–1998 (Source:1998 Air Quality Trends

Report)

Figure V.B-1: Average and 95th percentile benzene exposures (attributable to onhighway vehicles) in New York City, 1996

Figure V.B-2: Exposure levels for four gaseous toxics and diesel PM under

currentlyplanned controls and with 2007 Standards for Heavy Duty

Engines (ug/m3)

Figure VII.A-1: Location of PADDs in the Contiguous U.S. 

Figure VII.D-1: Chemical Benzene Demand in the U.S. (Honeywell Hi-Spec Solutions

forecast)
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Table of Contents

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Chapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

A. Roadmap of This Document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

B. Description of Motor Vehicle Air Pollution Control Programs . . . . . . . . . . . . . . 17

1. Mobile Source Control Programs and the Clean Air Act . . . . . . . . . . . . 17

2. Passenger Car Tail Pipe Emission Controls . . . . . . . . . . . . . . . . . . . . . . 18

3. Heavy-Duty Truck Tail Pipe Emission Controls . . . . . . . . . . . . . . . . . . . 19

4. Emission Control Programs for Buses . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5. Evaporative Emission Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6. Fuel Control Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Chapter 2: Identification of Mobile Source Air Toxics (MSATs) . . . . . . . . . . . . . . . . . . . . . 24

A. The Methodology Used to Identify Our List of Mobile Source Air Toxics . . . . . 24

B. How we Applied the Methodology to Identify our List of Mobile Sources Air

Toxics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1. Identifying Pollutants Emitted from Mobile Source . . . . . . . . . . . . . . . . 25

2. Using IRIS to Identify Pollutants with Potential Adverse Health Effects

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3. List of Mobile Source Air Toxics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

C. How Our List of MSATs Compares to Other Lists or Sources of Data on Toxics

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1. Comparison to Other Lists of Toxics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Chapter 3: Health Effects of Mobile Source Air Toxics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

A . Acetaldehyde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

B. Acrolein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

C. Arsenic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

D. Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

E. 1,3-Butadiene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

F. Chromium Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

G. Dioxin/Furans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

H. Diesel Exhaust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

1. Cancer Effects of Diesel Exhaust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2. Noncancer Effects of Diesel Exhaust . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
13

3. Diesel Exhaust and Diesel Particulate Matter . . . . . . . . . . . . . . . . . . . . . 47

I. Ethylbenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

J. Formaldehyde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

K. n-Hexane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

L. Lead Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

M. Manganese Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

N. Mercury Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

O. MTBE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

P. Naphthalene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Q. Nickel Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

R. POM (Polycyclic Organic Matter) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

S. Styrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

T. Toluene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

U. Xylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Chapter 4: Impacts of Motor Vehicle Emission Control Programs on MSAT Emissions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

A. Baseline Inventories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

1. The 1999 EPA Motor Vehicle Air Toxics Study . . . . . . . . . . . . . . . . . . . 61

2. The 1996 National Toxics Inventory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

B. Impacts of Motor Vehicle Emission Controls on Emission Inventories . . . . . . . 65

1. Overview of Inventory Methodologies . . . . . . . . . . . . . . . . . . . . . . . . . . 65

2. 1999 EPA Motor Vehicle Air Toxics Study . . . . . . . . . . . . . . . . . . . . . . 67

3. VOC Emissions Inventory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4. Diesel PM Inventory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

C. Limitations and Uncertainties in the Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

1. 1999 EPA Motor Vehicle Air Toxics Study . . . . . . . . . . . . . . . . . . . . . . 83

2. VOC Inventory Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3. Diesel PM Inventory Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Chapter 5: Mobile Source Air Toxic Ambient Concentrations and Exposures . . . . . . . . . . . . . . 88

A. Survey of Data Ambient Concentrations of Mobile Source Air Toxics. . . . . . . . 88

1. Ambient Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

2. Modeled Ambient Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

B. Modeled Inhalation Exposures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

1. Methodology for Modeling Inhalation Exposures to Benzene,

Formaldehyde, Acetaldehyde, 1,3-Butadiene and Diesel PM: HAPEMMS3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

2. Comparison of Exposure Modeling Results to Modeled Ambient

Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

3. Variance in Exposures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

4. Impact of Current On-Highway Vehicle Control Programs on Toxics

Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
14

5. Sensitivity Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

C. Exposures in Micro-environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

1. Diesel PM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

2. Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

3. Acetaldehyde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

4. Formaldehyde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5. 1,3-Butadiene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Chapter 6: Motor Vehicle-Based Controls of Mobile Source Air Toxics . . . . . . . . . . . . . . . . 124

A. Vehicle-Based Technologies that Control Air Toxics . . . . . . . . . . . . . . . . . . . . 124

1. Gaseous Organic Toxics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

2. Diesel Exhaust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

3. Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

B. Emission Control Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

1. Tier 2 Standards for Light-Duty Vehicles . . . . . . . . . . . . . . . . . . . . . . . 132

2. Heavy-Duty Engines and Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

C. Potential for Further Reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Chapter 7: Fuel Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

A. Industry and Product Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

1. Description of entities subject to the proposed benzene standards . . . . 136

2. Gasoline benzene level variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

B. Benzene Emissions and Inventory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

C. Technological Feasibility of the Proposed Program . . . . . . . . . . . . . . . . . . . . . 142

1. Requirements for refiners and importers . . . . . . . . . . . . . . . . . . . . . . . . 142

2. Requirements for those without complete 1998-1999 benzene data . . 143

3. Refinery technologies for controlling gasoline benzene . . . . . . . . . . . . 143

D. Costs and Benefits of the Proposed Program . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

E. More Stringent Control Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

Chapter 8: Nonroad Mobile Source Air Toxics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

A. Overview of Current Nonroad Engine Emission Control Programs . . . . . . . . . 149

1. Land-Based Nonroad Diesel Engines . . . . . . . . . . . . . . . . . . . . . . . . . . 151

2. Small Land-Based SI Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

3. Large Land-Based Spark-Ignition Engines . . . . . . . . . . . . . . . . . . . . . . 152

4. Marine Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

5. Locomotives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

6. Aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

7. Recreational Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

8. Fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

B. Impacts of Nonroad Control Programs on Air Toxics . . . . . . . . . . . . . . . . . . . . 156

1. Nonroad MSAT Baseline Inventories . . . . . . . . . . . . . . . . . . . . . . . . . . 156

2. Emission Reductions from Current Programs . . . . . . . . . . . . . . . . . . . . 157
15

C. Data Gaps and Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
16

Chapter 1: Introduction

The purpose of this Technical Support Document ( TSD) is to provide background

information in support of the regulatory decisions for control of mobile source air toxics under

Section 202(l)(2) of the Clean Air Act. Chapter 1 begins with a “roadmap” of the organization of

this document. The remainder of Chapter 1 reviews our mobile source emission control

programs that relate to mobile source air toxics control. 

A. Roadmap of This Document

Unlike the provisions for the control of toxic air pollutants from stationary sources found

in §112 of the Act, §202(l)(2) does not specify which compounds should be controlled. Chapter

2 of this TSD describes the approach we took to identify a list of 21 mobile source air toxics

(MSATs). Chapter 3 provides background information about the potential health impacts of

these compounds, including how they enter the environment and how they can affect human

health. 

Chapter 4 provides our estimates of highway motor vehicle emissions (“emission

inventories”) of these compounds. We present baseline inventory information for 1990 and 1996

and compare motor vehicle inventories to overall national inventories of emissions from both

mobile and stationary sources. This chapter also contains our estimates of the reductions in

emissions we can expect when our highway emission control programs are fully phased-in. We

estimate that highway inventories of certain key MSATs will decrease by as much as 70 percent

by 2020. This chapter also describes the methods we used to estimate these emission

inventories. 

In Chapter 5 we review existing information on ambient concentrations of toxic

compounds, from both monitoring and modeling efforts. We also present our estimates of the

human exposure to highway motor vehicle emissions of these compounds. This chapter also

describes the methods we used to estimate the ambient concentrations and exposure, along with

associated limitations and uncertainties.

The next two chapters of this TSD contain our analysis of MSAT controls beyond those

mobile source emission controls that are already in place or are proposed for the near future. 

Chapter 6 addresses vehicle-based controls and concludes that our recently finalized Tier

2/Gasoline Sulfur program and our Heavy-Duty Engine/Diesel Sulfur (2007) proposal represent

the most stringent controls feasible for motor vehicles air toxics control at this time. Chapter 7

contains our analysis of fuel-based controls. In it we present our rationale for our proposed

provisions relating to benzene in gasoline. We also discuss the challenges to fuel-based air

toxics control.

Finally, in Chapter 8 we describe our current nonroad engine emission control programs

and present our estimates of the impacts of these programs on future air toxics inventories. In
17

this chapter, we also highlight the significant uncertainty and several of the data gaps that exist

with respect to toxics emissions from nonroad engines. 

B. Description of Motor Vehicle Air Pollution Control Programs

In this section, we present a short history of some of EPA’s key programs addressing car,

truck, and bus tail pipe emission controls, evaporative emission controls, and fuel controls. 

1. Mobile Source Control Programs and the Clean Air Act

Our national mobile source emission control program began in the early 1970s, when we

issued the first sets of motor vehicle standards to reduce air pollution. These early standards

focused on emissions of hydrocarbons (HC), nitrogen oxides (NOx), and carbon monoxide (CO). 

While they were not designed to address toxics emissions specifically, these standards

nevertheless helped to reduce those emissions. Catalytic converters designed to reduce HC and

CO emissions also reduce gaseous air toxics, and the removal of lead from gasoline to permit the

use of catalytic converters (used to meet the HC and NOx standards) led to a significant

reduction in the inventory of toxic lead emissions.

More recently, the 1990 Clean Air Act Amendments added new elements to our mobile

source emission control programs. Like the earlier versions of the Act, the primary focus of the

mobile source provisions, contained in Title II of the Act, continues to be directed at attainment

of the National Ambient Air Quality Standards (NAAQS), primarily for ozone, CO, nitrogen

dioxide, and particulate matter (PM). The CAA Amendments led to a set of EPA programs that

focus on highway motor vehicles (passenger cars, light-duty trucks, and heavy duty trucks and

truck engines), nonroad engines, and their fuels. Section 202(l), by contrast, calls on us to focus

specifically on controls for hazardous air pollutants emitted from motor vehicles and their fuels. 

Our mobile source emission control programs can take several forms. In some cases, the

instructions in the Clean Air Act are very specific and prescribe specific levels of control. In

these cases, our role is to promulgate these controls within fairly precise boundaries. For

example, §202(g) of the Act sets out specific emission limits for Tier 1 controls for motor

vehicles; these requirements went into effect beginning in 1994. Similarly, §211(k) of the Act

also sets out specific requirements for gasoline fuel reformulation for certain areas of the country. 

In other cases, the requirements of the Act are broader and direct us to consider controls

in certain areas but give us discretion in determining the appropriate level. For example,

§202(a)(1) of the Act directs us to “prescribe (and from time to time revise) ... standards

applicable to the emission of any air pollution from any class or classes of new motor vehicles or

new motor vehicle engines which ... cause or contribute to, air pollution which may reasonably

be anticipated to endanger public health or welfare.” Section 202(a)(3)(A)(i) specifies that those

regulations “shall contain standards which reflect the greatest degree of emission reduction

achievable through the application of technology which ... will be available for the model year to
1

 Our programs achieve VOC reductions through standards that limit HC, NMHC, or NMOG. For

gasoline vehicles, the slight technical distinctions among these ways of expressing emissions can generally be

ignored.

18

which such standards apply, giving appropriate consideration to cost, energy, and safety factors

associated with the application of such technology.” This type of broad language is echoed in

other sections of Title II, notably those concerning fuels (§211(c)(1)), nonroad engines (§213)

and toxics (§202(l)).

In addition to developing traditional regulatory programs, we have also engaged in

collaborative efforts with industry, states, and other outside parties using our authorities under

the Clean Air Act. An example is the national low-emission vehicle (NLEV) program, in which

we worked with the auto industry and the Northeast states to develop an innovative, voluntary

program to put cleaner cars on the road before they could be mandated by programs developed

under the Clean Air Act. Another example is our reformulated gasoline program, where an early

part of the regulatory process consisted of a broad “regulatory negotiation” during which many

stakeholders participated in crafting the basic elements of the ultimate program.

The remainder of this chapter briefly describes our key mobile source emission control

programs and their relationship to air toxics emission control.

2. Passenger Car Tail Pipe Emission Controls 

Since 1970, emission limits for hydrocarbons (HC), oxides of nitrogen (NOx), and carbon

monoxide (CO) from cars have been steadily declining. The history of HC control is particularly

important to today’s rule because many of the gaseous toxics from motor vehicles and their fuels

are hydrocarbons and are thus controlled through controls of HC. (HC emissions are a large

component of volatile organic compounds (VOC) emissions; the two terms are generally used

interchangeably in the context of motor vehicle emissions.)

Before our regulations, cars emitted on the order of 8 grams per mile (gpm) HC, or more. 

Our HC emission standards in the 1970s and 1980s cut these levels by more than an order of

magnitude, to a level of 0.41 gpm in 1980. In 1991, we finalized Tier 1 controls for light-duty

vehicles and light-duty trucks to be phased in from 1994 to 1996 (56 FR 25724). In 1998, we

developed an innovative, voluntary nationwide program to make new cars, called National Low

Emission Vehicles (NLEV), significantly cleaner than Tier 1 cars (63 FR 926). The NLEV

program went into effect in the Northeast states in 1999 and will go into effect in the rest of the

country in 2001. We recently finalized the Tier 2/Gasoline Sulfur control program with stringent

NOx and NMOG standards for all passenger vehicles (see Chapter 6 of this TSD for a more

detailed discussion of the Tier 2/Sulfur control program). Table I.B-1 illustrates the declining

standards through the NLEV program that have resulted in VOC and air toxic reductions from

car exhaust in the 1970s through the 1990s1

.
19

Table I.B-1 

Hydrocarbon (HC) Exhaust Emission Standards for Light-Duty Vehicles (gpm) 

Year 1970 1972 1975 1980 1994 1999

HC 2.2 3.4 1.5 0.41 0.311 0.092

1 The 1994 standard is a NMHC standard. 

2 The 1999 standard is a NMOG standard.

We also control HC emissions from cars, and thus emissions of gaseous toxics, through a

number of other standards and programs. For example, our requirements controlling carbon

monoxide emissions at cold temperatures also have an important effect in reducing HC emissions

at cold temperatures, since strategies that reduce cold CO also reduce cold HC (through such

strategies as better control of air/fuel ration and quicker catalyst “light-off”). Another example is

the Supplemental Federal Test Procedure and standards, finalized in 1996, which better captures

actual driving conditions in the test procedures for our control programs (61 FR 54852). We also

have programs that track how cars are performing in use. One program is the “on-board

diagnostics” (OBD) program, which is required for all cars and light-duty trucks beginning in

1994 (58 FR 9468). Another such program is our state-run inspection and maintenance (I/M)

program, in which the individual state programs check whether the emission control system on a

vehicle is working correctly. I/M programs are currently in place in over 150 areas (57 FR

52950).

3. Heavy-Duty Truck Tail Pipe Emission Controls

We have controlled emissions from heavy-duty engines and vehicles since 1984. As of

1998, new heavy-duty truck engines must meet standards of 4 g/bhp-hr NOx, 1.3 g/bph-hr HC,

and 0.10 g/bhp-hr PM. In a 1997 rulemaking, we finalized more stringent standards for diesel

trucks only. These standards will become effective in 2004 (62 FR 54695). We recently issued a

proposed rule to reaffirm these standards for diesel trucks for 2004, and to adopt separate

standards for gasoline trucks (64 FR 58471; see below, Table I.B-3). Table I.B-2 illustrates the

declining standards for NOx, PM and HC for heavy-duty trucks since 1984. 
20

Table I.B-2

Heavy-Duty Standards for Diesel and Gasoline1

 Engines (g/bhp-hr)

Year 1984 1988 1990 1991 1993 1994 1996 1998 2004

NOX 10.7 –> 6.0 5.0 –> 4.0 2.42

PM N/A 0.60 –> 0.25 –> 0.10 –>

HC 1.3 –> 2.42

1. Standards for Gasoline Engines are the same as for diesel, until 2004. See Table I.B-3 for gasoline engine

standards proposed for 2004.

2. This is a combined NMHC + NOx standard for heavy-duty diesel engines only. Typically, HC emissions for HD

diesel engines are in the 0.3 g/bhp-hr range. 

Table I.B-3

Heavy-Duty GasolineVehicle Standards Proposed for 2004

Gross Vehicle Weight NOX HC1

8,000-10,000 lbs 0.9 gpm 0.28 gpm

10,001-14,000 lbs 1.0 gpm 0.33 gpm

14,001 lbs and above 1.0 g/bhp-hr (combined NOX and HC) 

1. The standards for HC is in the form of NMOG standards. Manufacturers have the option to submit test

data in the form of NMHC emissions.

In our recent proposed rule for 2004 and later heavy-duty engines and vehicles, we also 

proposed improvements to the test procedure for heavy-duty diesel engines, including a “not-toexceed” (NTE) test and a steady-state certification test to ensure that emissions are met under a

wide range of operating conditions. This program would also extend the on-board refueling

vapor recovery (ORVR) program (see discussion, Section 1.B.5, below) to heavy-duty gasoline

engines weighing between 8,500 and 10,000 lbs (gross vehicle weight). Also, we propose to

extend requirements for on-board diagnostic (OBD) systems to both diesel and gasoline fueled

vehicles weighing between 8,500 and 14,000 pounds (gross vehicle weight) to help identify any

possible failure of components of the emission control system.

 Our recent proposed Heavy-Duty Engine/Diesel Sulfur (2007) program, proposed June 2,

2000, proposes stringent exhaust emission standards for heavy-duty engines and vehicles beyond

the 2004 levels starting in 2007, as well as reductions in sulfur levels from diesel fuel starting in

2006 (65 FR 35430). We describe this program in greater detail in Chapter 6 of this TSD.

4. Emission Control Programs for Buses
21

In 1993, we finalized the Urban Bus Retrofit/Rebuild Program which is intended to

reduce the ambient levels of PM in urban areas (58 FR 21359). The program is limited to buses

operating in metropolitan areas with 1980 populations of 750,000 or more, and applies only to

1993 and earlier model year buses whose engines are rebuilt or replaced after January 1, 1995. 

Approximately 40 urban areas are affected. In addition, in 1993, we finalized more stringent PM

standards that apply to new urban buses (58 FR 15781) as indicated in Table I.B-4. 

Table I.B-4

New Urban Bus Standards (g/bhp-hr)

Year 1993 1994 1996

PM 0.10 0.07 0.05

5. Evaporative Emission Controls

Evaporative and refueling emissions are a significant portion of the HC emissions

inventory for gasoline-fueled vehicles and trucks (in 1990 more than half of the VOC emissions

from light-duty vehicles were evaporative emissions). In 1971, we began testing motor vehicles

for evaporative emissions by subjecting test vehicles to typical drive and park conditions. The

test procedure measures emissions from fuel evaporation during a simulated parking experience

(diurnal emissions) and immediately following a drive (hot soak emissions). Currently, we

measure diurnal emissions over a three-day test and also a supplemental two-day test. In 1993,

we finalized revised evaporative emission test procedures which apply to light-duty and heavyduty gasoline vehicles. These procedures were fully phased-in by 1999 (58 FR 16002). We

expect the test procedure to ensure that properly functioning vehicles will effectively control

evaporative emissions for most in-use events. The 1993 rule also addressed fuel spitback during

refueling with a vehicle test to ensure that no spillage occurs when a vehicle is refueled at a rate

of up to 10 gallons (37.9 liters) per minute.

Our current evaporative emission and refueling spitback standards are shown in Table

I.B-5 (the Tier 2/Gasoline Sulfur rule further reduced evaporative emission standards, and our

proposed rule for Heavy-Duty Engines/Diesel Sulfur control also proposes to reduce evaporative

emissions standards for heavy-duty gasoline trucks, as described in Chapter 6 of the TSD).
22

Table I.B-5

Current Light Duty Vehicles, Light Duty Truck, and Heavy-Duty Gasoline Vehicle

Evaporative Hydrocarbon and Refueling Spitback Standards 

Evaporative Hydrocarbons Refueling

Spitback

Category 3 Diurnal + Hot (grams/test)

Soak (grams/test)

2 Diurnal + Hot

Soak (grams/test)

Running Loss

(grams/mile)

Light-Duty1 2 2.5 0.05 1.0 

Heavy-Duty to

14,000 lbs GVW

3.0 3.5 0.05 1.0

Heavy-Duty

above 14,000 lbs

GVW

4.0 4.5 0.05 ---

1. Note that we have different standards for light-duty trucks with fuel tanks over 30 gallons.

We have also finalized on-board refueling vapor recovery (ORVR) requirements for

light-duty gasoline vehicles (59 FR 16262), and proposed to extend ORVR to heavy-duty

gasoline vehicles between 8,500 and 10,000 lbs (gross vehicle weight) (64 FR 58471). ORVR is

a nationwide program for capturing refueling emissions by collecting vapors from the vehicle gas

tank during refueling and storing them in the vehicle. The gas tank and fill pipe are designed so

that when refueling the vehicle, fuel vapors in the gas tank travel to an activated carbon packed

canister, which adsorbs the vapor. When the engine is in operation, it draws the gasoline vapors

into the engine intake manifold to be used as fuel. Table I.B-6 indicates the phase-in periods for

ORVR for different size gasoline vehicles.

Table I.B-6

On-Board Vapor Recovery for Gasoline Vehicles Phase-In Periods and Standard

Category Phase-In Standard

Light-Duty Vehicles 1998-2000

0.2 g/gallon Light Light-Duty Trucks (to 6,000 lbs

 gross vehicle weight)

2001-2003

Heavy Light-Duty Trucks (6,000 to 8,500 lbs

gross vehicle weight)

2004-2006

Heavy-Duty Gasoline Vehicles (to 10,000 lbs

gross vehicle weight)1

2004-2006

1. Proposed October 1999 (64 FR 58471)
23

6. Fuel Control Programs

The emissions that come out of a vehicle depend greatly on the fuel that goes into it. Fuel

composition and type are critical factors in the clean vehicle equation. Since 1990, the Clean Air

Act explicitly recognizes that changes in fuels as well as in vehicle technology must play a role in

reducing air pollution from motor vehicles. 

One of the first and most successful programs to control harmful motor vehicle emissions

by changing fuel composition was the removal of lead from gasoline. The lead phase-out began

in the mid-1970s in order to enable the use of catalytic converters on cars to meet early HC

standards. This resulted in dramatic reductions in ambient lead levels and alleviated many

serious environmental and human health concerns associated with lead pollution. The Clean Air

Act prohibited the introduction of gasoline containing lead or lead additives into commerce for

use as a motor vehicle fuel after December 31, 1995. In February 1996, we finalized an action to

implement the ban on leaded gasoline (61 FR 3832). The removal of lead from gasoline has

essentially eliminated motor vehicle emissions of this highly toxic substance. The reduction and

virtual elimination of lead from gasoline has resulted in significant risk reduction to the public

and environment.

In 1990 and 1991, we promulgated regulations to reduce the volatility of gasoline (the

basic regulations were promulgated at 55 FR 23658). Like the vehicle-based evaporative and

refueling emission control programs discussed above, our gasoline volatility program has

reduced VOC emissions by reducing evaporation of gasoline.

The reformulated gasoline (RFG) program (59 FR 7716) resulted from the mandate in the

Clean Air Act requiring areas of the country with the worst ozone problems to use gasoline that

is reformulated to help improve air quality. The RFG program, which began January 1, 1995,

contains two phases. On an average basis under the Complex Model, Phase I required emissions

reductions from 1990 conventional gasoline baseline gasoline of 16.5 percent for air toxics, 

36.6 or 17.1 percent for VOC (depending on the region of the country), and 1.5 percent for NOx 

when compared with 1990 baseline gasoline (40 CFR 80.41). RFG must also contain a

minimum oxygen content of 2 percent by weight, a maximum benzene content of 1 percent by

volume, and no lead, manganese, or other heavy metals. At its inception, RFG was required in

the nine worst ozone areas, with the provision that other ozone nonattainment areas could

voluntarily opt in to the program. Currently 17 states and the District of Columbia participate in

the program, with RFG representing about 30 percent of the gasoline sold in the United States. 

Phase II of the program began January 1, 2000, and contains more stringent emissions reduction

requirements. On an average basis, air toxics must be reduced by 21.5 percent, VOC by 27.4 or

29.0 percent (depending on the region of the country), and NOx by 6.8 percent (summertime) and

1.5 percent (wintertime) from the 1990 conventional gasoline baseline (40 CFR 80.41).
24

Chapter 2: Identification of Mobile Source Air Toxics 

(MSATs)

Introduction

There are hundreds of compounds and elements that are known to be emitted from

passenger cars, on-highway trucks, and various types of nonroad equipment. Several of these

compounds may have serious effects on human health and welfare. In recognition of this fact,

Congress instructed EPA, in Section 202(l)(2) of the Act, to set emission control standards for

hazardous air pollutants from motor vehicles and their fuels. Except for benzene and

formaldehyde which are specifically mentioned in 202(l), the Act does not specify the

compounds that should be included in such a control program. Therefore, the first step in

developing a mobile source air toxics control program is to identify the compounds that should

be controlled.

This chapter describes the methodology we used to identify our proposed list of 21

mobile source air toxics (MSATs). A more detailed description of the health effects information

for these compounds is contained in Chapter 3.

A. The Methodology Used to Identify Our List of Mobile Source Air Toxics

 

EPA developed the list of MSATs by first compiling all available recent (i.e., less than 10

years old) studies which speciated the emissions from mobile sources and their fuels. The

compilation did not include speciations of emissions from alternative fueled vehicles, currently

in a very small number of vehicles, such as flexible-fueled vehicles. We then compared the list

of compounds in EPA’s Integrated Risk Information System (IRIS) database to the speciated lists

of compounds in these studies. IRIS is a database of compounds that identifies EPA’s consensus

scientific judgment on the characterization of the potential adverse health effects that may result

from a lifetime exposure to various substance. IRIS may also indicate that based on the current

data a compound has been found not to have the potential to cause adverse health effects. 

By comparing the list of compounds in IRIS to these emission speciation studies , we

generated a list of 21 compounds. An evaluation of the potential for adverse health effects

reflected in IRIS and in the ongoing agency scientific assessments of these compounds indicates

that the potential for adverse health effects from exposure to these compounds warrants inclusion

as a MSAT. EPA also compared its universe of known compounds emitted from mobile sources

against other lists or sources of information on toxic substances, and did not identify any
2 See Appendix I.

3

 EPA Speciate Database http://www.epa.gov/ttnchie1/spec/index.html

25

additional substance that we believe should be listed at this time. EPA believes this process is

fluid, and allows for re-evaluation of the MSAT list in the future, as information is learned about

additional compounds or new information is learned about the 21 compounds.

B. How we Applied the Methodology to Identify our List of Mobile Sources Air

Toxics

In the sections that follow, we will describe in more detail each step of our methodology

for identifying the list of MSATs.

1. Identifying Pollutants Emitted from Mobile Source 

In identifying a list of MSAT, EPA first compiled all available recent studies which

speciated emissions from mobile sources and their fuels. Again, this compilation did not

include speciation of emissions from alternative fueled vehicles, currently used in a very small

number of vehicles. It is difficult to get a precise picture of these emissions due to the variety and

number of databases in the literature. This is particularly true for hydrocarbon (HC) speciation

databases. Most toxic air pollutants are hydrocarbons by their chemical nature and thus will be

detected only if the HCs are chemically separated and identified (speciated). Many test programs

that characterize mobile source emissions identify only total hydrocarbons (THC) without

separating out the individual species of hydrocarbons and many use different test methods. The

issue is further complicated by the limited availability of these databases for certain vehicle

classes and engine types.

We have recent (less than ten years old) speciation profiles for emissions from light-duty

gas vehicles (LDGV), heavy-duty diesel vehicles (HDDV), heavy-duty gasoline vehicles

(HDGV), gasoline powered nonroad engines, and turbine engine aircraft.2

 Data for other

vehicle/engine types (e.g., light-duty diesel engines and nonroad diesel engines) either does not

exist or are outdated (more than 10 years old) and thus are judged not to be representative of the

emissions from vehicles on the highway today. However, it is unlikely that the lack of recent

data for these vehicle and engine types would result in the absence of compounds from the list,

since the combustion process is similar to vehicle and engine types for which we do have data.

The forty-four speciation studies listed in Appendix I at the end of this chapter all attempt

to accomplish more or less the same objective: separating and identifying the compounds that

comprise the hydrocarbon and particulate matter emissions.3

 The protocol followed in each

study is generally the same, though parts of the analysis may vary depending on the purpose of

the study. Each study generally starts by defining the fleet, or types, of vehicles or engines on

which data will be collected. This could be light-duty gas vehicles, heavy-duty diesel engines,
4 EPA IRIS Database, http://www.epa.gov/ngispgm3/iris/index.html

26

nonroad engines, or a mix of various types. The vehicles themselves can also be a mix of older

and newer models, with varying mileage. The type of fuel to be used must also be specified.

Vehicles and engines are tested on a dynamometer, which is basically mechanical

treadmill for the vehicle/engine. The test vehicle/engine is run through a set pattern of starts,

stops, idle, and acceleration, over a standard quantity of miles. The studies generally follow the

U.S. Federal Test Procedure (FTP), though other test cycles may be used. The tailpipe and

evaporative emissions are collected under a strict set of guidelines and then taken to a laboratory

for analysis. Metal and particulate emission are trapped on particulate filters and analyzed. 

Analysis of metals usually doesn’t specify the actual individual chemical form of the metal, only

reporting the total amount of the identified metal emitted. Particulate samples, in the past, have

routinely looked at particles no finer than 10 )m in diameter, whereas newer research indicates

that most particles are much smaller. Once the chemical, metal, and/or particle analysis of the

collected material is complete, the results are routinely presented as grams (or milligrams) of

chemical “x” per mile driven (g/mi).

Appendix I provides a list of the speciation studies from various types of mobile sources

and their fuels. In the next section we

describe which compounds emitted from

mobile sources and their fuels may be

considered toxic air pollutants.

2. Using IRIS to Identify

Pollutants with Potential

Adverse Health Effects

IRIS is an EPA database of scientific

information that contains the Agency

consensus scientific positions on potential

adverse health effects that may result from

lifetime exposure to various substances found

in the environment. IRIS was initially

developed for EPA staff in response to a

growing demand for consistent information

on chemical substances for use in risk

assessments, decision-making and regulatory

activities. IRIS currently provides health

effects information on over 500 specific

chemical compounds.4

IRIS contains chemical-specific

How Chemicals Are Added to the IRIS

Database

 Our Office of Research and Development

(ORD) maintains IRIS through a scientific

consensus and review process that precedes

entry of a pollutant into the IRIS database. 

This process consists of:

• An annual Federal Register announcement

of the IRIS agenda and a call for scientific

information from the public on the selected

chemical substances;

• A search of the current literature;

• Development of health assessment and draft

IRIS summaries;

• Internal EPA peer review;

• External peer review;

• Agency consensus review and management

approval within EPA;

• Preparation of final IRIS summaries and

supporting documents; and

• Entry of summaries and supporting

documents into the IRIS database.
27

summaries of qualitative and quantitative health information. IRIS information includes the

reference dose for noncancer health effects resulting from oral exposure (RfD), the reference

concentration for noncancer health effects resulting from inhalation exposure (RfC) and the

carcinogen assessment for both oral and inhalation exposure.

The RfD or RfC is an estimate (with uncertainty spanning perhaps an order of magnitude)

of a daily exposure to the human population (including sensitive subgroups) that is likely to be

without appreciable risk of deleterious effects during a lifetime. RfDs and RfCs are based on an

assumption of lifetime exposure and may not be appropriately applied to less-than-lifetime

exposure situations. RfDs and RfCs may be derived for the noncarcinogenic effects of chemicals

whether or not they are also carcinogenic. 

The carcinogenicity assessments in IRIS begin with a qualitative weight-of-evidence

judgment as to the likelihood that a chemical may be a carcinogen for humans. This judgment is

made independent of consideration of the agent's potency. Whether a quantitative assessment is

performed is dependent both on the weight-of-evidence and the suitability of the available doseresponse data. A quantitative assessment, which may include an oral slope factor and oral and/or

inhalation unit risk estimates, may then be presented. The oral slope factor is an upper-bound

estimate of the human cancer risk per mg of agent/kg body weight/day. The unit risk, which can

be calculated from the slope factor or other scientifically appropriate methodologies, is also an

upper-bound estimate in terms of either risk per )g/L drinking water, or risk per )g/cu.m air

concentration. 

Each reference dose/concentration and carcinogenicity assessment has been reviewed by a

group of EPA health scientists using consistent chemical hazard identification and dose-response

assessment methods. These methods are discussed or referenced in the IRIS Background

Documents which are specific to, and referenced, in each individual chemical profile on IRIS. It

is important to note that the information in IRIS may be revised by EPA, as appropriate, when

additional health effects data become available and new developments in assessment methods are

adopted.

It is also important to note that IRIS does not provide situational information on

individual instances of exposure. In order to evaluate potential public health risks, the summary

health hazard information in IRIS must be combined with data on specific exposure situations.

3. List of Mobile Source Air Toxics

By comparing the list of pollutants for which information is stored in IRIS to the

compounds identified in the emission speciation studies, we identified 21 compounds listed on

IRIS that have been found in the emissions from mobile sources and their fuels. This list of

MSATs is set out in Table II.B-1, below. Each of these pollutants are known, probable or

possible human carcinogens (Group A, B or C) or were considered by the Agency to pose a risk
5

 A further discussion of the potential cancer and noncancer risks, and other doseresponse information for each MSAT can be found in Chapter 3 of the TSD.

28

of adverse noncancer health effects.5 It should be noted that this list is not meant to be fixed, and

may change as new health and emissions data become available.

Table II.B-1

Proposed List of Mobile Source Air Toxics (MSATs)

Acetaldehyde Diesel Exhaust MTBE3

Acrolein Ethylbenzene Naphthalene

Arsenic Compounds1 Formaldehyde Nickel Compounds1

Benzene n–Hexane POM4

1,3-Butadiene Lead Compounds1 Styrene

Chromium Compounds1 Manganese Compounds1 Toluene

Dioxin/Furans2 Mercury Compounds* Xylene

1

 Although the different metal compounds differ in their toxicity the onroad mobile source

inventory contains emissions estimates for total metal compounds (i.e., the sum of all forms).

2

 This entry refers to two large groups of chlorinated compounds. In assessing their cancer risks,

their quantitative potencies are usually derived from that of the most toxic, 2,3,7,8-

tetrachlorodibenzodioxin.

3

 MTBE is listed due to its inhalation air toxics effects and not due to ingestion exposure 

associated with drinking water contamination.

4

 Polycyclic Organic Matter includes organic compounds with more than one benzene ring, and which

have a boiling point greater than or equal to 100 degrees centigrade. A group of seven polynuclear

aromatic hydrocarbons, which have been identified by EPA as probable human carcinogens

(benz(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, chrysene, 7,12-

dimethylbenz(a)anthracene, and indeno(1,2,3-cd)pyrene), are sometimes used as surrogates for the larger

group of POM compounds.

It is important to note that inclusion on the list is not itself a determination by EPA that

emissions of the compound in fact present a risk to public health or welfare, or that it is

appropriate to adopt controls to limit the emissions of such a compound from motor vehicles or

their fuels. The purpose of the list is more as a screening tool - it identifies those compounds

emitted from motor vehicles or their fuels, and where the available information about their

potential for adverse health or welfare effects indicates that further evaluation of emissions

controls is appropriate. In conducting any such further evaluation, pursuant to sections 202(a) or

211(c) of the Act, EPA would consider whether emissions of the compound cause or contribute

to air pollution which may reasonably be anticipated to endanger the public health or welfare. 

Such an evaluation would also consider the appropriate level of any controls, based on the

criteria established in section 202(l)(2). Inclusion of a compound on the MSAT list does not

decide these issues, but instead identifies those compounds for which such an evaluation would
6 Metals emissions are only present in mobile source exhaust and not in evaporative emissions from fuels.

29

appear to be warranted.

As described in section II.B.1, it is difficult to identify the specific form of metals being

emitted in mobile source exhaust because the databases only report the total amount of metal

compound identified. As a result, we have chosen to list the entire group of metal compounds if

any compound of the metal is listed in IRIS as potentially causing adverse human health effects

and any compound of the metal has been detected in emissions from mobile sources or their

fuels.6 For example, if we assume most chromium (Cr) emissions for mobile sources are

unidentified as to the species, we would present the emissions as total chromium and not attempt

to allocate these emissions because of the lack of accurate metal speciation information in most

cases. When we assess the range of potential health impacts associated with exposure to

chromium compounds, we consider the health effects associated with each compounds for which

we have information. For chromium, the most toxic form in IRIS is Cr+6; hence the health

impacts described for chromium compounds include these most serious effects even though it is

highly unlikely that all mobile source emissions are Cr+6. EPA believes this listing approach is a

reasonable, health-protective way to handle the uncertainty surrounding mobile source emissions

of metals. We also recognize that this is not an appropriate methodology for assessing the actual

health risks of the entire group of metal compounds emitted from mobile sources.

With regard to alternative-fueled vehicles, most of the compounds included in their

exhaust are included on our list of MSATs (e.g., formaldehyde, acetaldehyde). It should be noted

that, depending on their fuel, these vehicles may also emit unburned ethanol and methanol. 

Low level ethanol mixtures (10% ethanol and 90% gasoline) are widely used in the

United States. Higher level ethanol mixtures (e.g., 85% ethanol) are used as alternative fuel

sources in a small number of flexible fuel vehicles. However, there is a paucity of data on

potential inhalation effects of ethanol, and the compound is not listed in IRIS. Thus it has not

included on the proposed list of MSATs. 

Methanol is also a promising alternative fuel for motor vehicles, and a small number of

flexible fuel vehicles operate on a methanol mixture (e.g., 85% methanol). Inhalation of

methanol at high concentrations (greater than 1000 ppm) has caused birth defects in rats and

mice and at low levels can cause symptoms such as eye irritation, headaches, dizziness, and

nausea. Methanol is highly toxic by oral exposure routes and is listed in IRIS. 

The Agency will re-examine whether to include these two compounds on the list of

MSATs in the final technical support document, and we are requesting comment on this issue in

the preamble.

C. How Our List of MSATs Compares to Other Lists or Sources of Data on

Toxics 
7

 The Act includes 189 hazardous air pollutants, but in 1996, EPA delisted caprolactam as a HAP [61 Fed.

Reg. 30,816 (June 18, 1996)].

8

 The 112(b) list evolved from the lists of pollutants found at the time in section 313 of the Emergency

Planning and Community Right-to-Know Act, 42 U.S.C. § 11023, section 104 of the Comprehensive Environmental

Response, Compensation and Liability Act, 42 U.S.C. § 9604(i), and the National Air Toxics Information

Clearinghouse database on July 1986.

30

To address the possibility that the IRIS files do not include all the mobile source emission

chemicals with significant potential for adverse health effects, other lists of pollutants posing

potential health and/or environmental impacts were consulted.

1. Comparison to Other Lists of Toxics

In addition to IRIS, there are other lists of chemicals that have been derived in response to

potential human health or environmental hazards. IRIS was chosen as the primary source in

developing the MSATs list because it presents Agency consensus opinions, and its current

process includes both internal and external (sometimes involving EPA’s Science Advisory

Board) peer review steps. To address the possibility that the IRIS file does not include all the

mobile source emission chemicals with potential for significant adverse health effects, four other

lists of pollutants posing potential health and/or environmental impacts were consulted. The four

other lists of toxic air pollutants were: the CAA section 112(b) list of hazardous air pollutants,

the California EPA (CalEPA) list of toxic air contaminants (TAC), the Department of Health and

Human Service Agency for Toxic Substances and Disease Registry (ATSDR) list of Minimal

Risk Levels (MRLs), and the list of agents evaluated in the International Agency for Research on

Cancer (IARC) monographs series on cancer.

a. Section 112(b) of the Clean Air Act

Section 112(b) of the Clean Air Act lists 188 compounds as hazardous air pollutants

(HAPs).7

 Congress added this list as part of the 1990 Amendments to the Clean Air Act using

the lists of hazardous substances and contaminants from other federal programs and databases.8

Section 112(b)(2) authorizes EPA to add to this list pollutants that present, or may present,

through inhalation or other routes of exposure, a threat of adverse human health effects or

adverse environmental effects. To date, no additional pollutants have been added to the 112(b)

list.

b. The Agency for Toxic Substances and Disease Registry (ATSDR)

ATSDR, which is part of the U.S. Centers for Disease Control, regularly publishes Health

Guidelines Comparison Values (CVs) for many toxic substances. ATSDR describes CVs as

media-specific concentrations to be used by health assessors in selecting environmental

contaminants for further evaluation. CVs are concentrations below which it is considered

unlikely that contaminants pose a health threat. Concentrations above a CV do not necessarily
9

 CalEPA, 1997. Toxic Air Contaminant Identification List. California Environmental Protection

Agency, Air Resources Board. September 1997.

31

represent a threat, and CVs are thus not intended for use as predictors of adverse health effects or

for setting cleanup levels.

ATSDR’s chronic duration minimum risk level (MRL) is the CV that most closely

approximates EPA’s RfD and RfC. An MRL is an estimate of daily human exposure to a

substance that is likely to be without an appreciable risk of adverse effects (other than cancer)

over a specified duration of exposure. ATSDR develops MRLs for acute, intermediate, and

chronic duration exposures by the inhalation and oral routes. The concept, definition, and

derivation of MRLs are consistent with those of EPA's RfC and RfD. ATSDR publishes MRLs

as part of its toxicological profile documents for each substance (ATSDR 1998). ATSDR

toxicological profiles receive public review as well as extensive internal and external peer

review.

c. California Environmental Protection Agency (CalEPA)

CalEPA has developed dose-response assessments for many HAPs that have not been

evaluated by either EPA or ATSDR. These assessments contain information on carcinogenicity,

and health effects other than cancer resulting from chronic and acute exposure. The non-cancer

information includes available inhalation health risk guidance values developed by EPA or

CalEPA, expressed as acute or chronic reference exposure levels (RELs). CalEPA defines the

REL as a concentration level or dose at or below which no health effects are anticipated. 

Because this concept is substantially similar to EPA's non-cancer dose-response values, RELs are

useful tools for substances that EPA has not assessed. CalEPA's quantitative dose-response

information on carcinogenicity by inhalation exposure is expressed in terms of the unit risk,

defined similarly to EPA's unit risk. CalEPA’s methodology and values were subjected to an

external peer review process in 1995-1996, and although some individual values were judged in

need of improvement, the methodology was considered generally similar to that of the EPA. 

Since then Cal EPA has updated many of their assessments to further improve consistency with

the EPA and reflect current knowledge.9

The CalEPA Toxic Air Contaminant (TAC) list includes over 200 compounds that

CalEPA has determined to pose a threat to public health. The TAC list, like IRIS, receives

extensive internal and external peer review, and the supporting assessments are reviewed by an

independent Scientific Review Panel (SRP) before a compound is added to the TAC list.

d. International Agency for Research on Cancer (IARC)

 IARC was established in 1965 by the World Health Organization, to coordinate and

conduct research on the causes of human cancer and to develop scientific strategies for cancer

control. IARC performs epidemiological and laboratory research, and disseminates scientific
10 Billings, R., T. Kraus, B. Hunt, J. Mangino, R. Cook, L. Driver 1998. Development and Comparison of

1990 and 1996 Mobile sources Hazardous Air Pollutants Emissions Estimates. Presented at AWMA Conference,

“Emission Inventory: Living in a Global Environment” New Orleans, LA December 8, 1998.

11 U.S. Department of Health and Human Services. 2000. Hazardous Substances Database (HSDB, online

database), National Toxicology Information Program, National Library of Medicine, Bethesda, MD. of the U.S.

Dept of Health.

32

information through meetings, publications, courses and fellowships. As part of its mission, the

IARC assembles evidence that substances cause cancer in humans and issues judgments on the

strength of the evidence. To this end they have created a database known as the IARC

Monographs series which publishes authoritative independent assessments by international

experts of the carcinogenic risks posed to humans by a variety of agents, mixtures and exposures. 

Since its inception in 1972, IARC has reviewed more than 800 agents.

IARC's weight-of-evidence categories are Group 1 (carcinogenic in humans), Group 2A

(probably carcinogenic), Group 2B (possibly carcinogenic), Group 3 (not classifiable), and Group

4 (probably not carcinogenic). The rankings may be applied to either single chemicals or

mixtures (IARC 1998). EPA often relies on IARC weight-of-evidence determinations for

substances that EPA itself has not assessed.

e. Results of Comparison

Comparing these four lists against the emissions speciation studies, we identified two

additional compounds not included on our list of 21 MSATs -- propionaldehyde and 2,2,4-

trimethylpentane. Both the CalEPA TAC list and the CAA section 112(b) HAP list contain these

compounds.

Propionaldehyde is a highly volatile compound that is emitted into the air as a result of

combustion of wood, gasoline, and diesel fuel, as well as by certain plants and at sites where it is

produced or used as a chemical intermediate in the manufacture of propionic acid, polyvinyl and

other plastics in the synthesis of rubber chemicals and as a disinfectant and preservative. A

recent study estimates that 63 percent of the propionaldehyde in the draft 1996 National Toxics

Inventory (NTI) was attributable to on-highway motor vehicles.10 There are little data available

on the potential human health effects associated with propionaldehyde exposure. The few animal

studies available focused on acute (short-term) inhalation exposure to very high levels and

reported anesthesia (loss of consciousness) and liver damage, as well as lethal pulmonary edema

(fluid accumulation in the lung).11

The compound 2,2,4-trimethylpentane is found in both exhaust and evaporative emissions

from gasoline motor vehicles. Current inventory estimates are not available, but since it is a

component of gasoline and is not expected to have significant stationary source emission, we

presume that motor vehicles will dominate this inventory. Little data are available on potential

human health effects. The limited animal data indicate that 2,2,4-trimethylpentane is a
12 U.S. Department of Health and Human Services. 2000. Hazardous Substances Database (HSDB, online

database), National Toxicology Information Program, National Library of Medicine, Bethesda, MD. of the U.S.

Dept of Health.

33

respiratory tract irritant and central nervous system depressant.12

At this time, EPA is not including propionaldehyde or 2,2,4-trimethylpentane in the list of

MSATs because the Agency has not drawn a conclusion on the potential adverse human health

effects asiciated with exposure to these pollutants. In the preamble to the NPRM, however, EPA

is requesting comment on this decision as well as any scientific information on the potential

health effects of these pollutants.
34

Appendix I

Toxics Inventory/Speciation References

1. Auto/Oil Air Quality Improvement Research Program, 1990. Phase 1 Working Data Set

(published in electronic form). Prepared by Systems Applications International, San

Rafael, CA.

2. Ball, James C. Emission Rates and Elemental Composition of Particles Collected from

1995 Ford Vehicles Using the Urban Dynamometer Driving Schedule, the Highway Fuel

Economy Test, and the US06 Driving Cycle. Society of Automotive Engineers, SAE

paper No. 97FL-376. 1997.

3. Bass, E. A., and M. S. Newkirk, 1995. Reactivity Comparison of Exhaust Emissions

from Heavy-Duty Engines Operating on Gasoline, Diesel, and Alternative Fuels. 

Southwest Research Institute, Report No. SwRI 9856, December, 1995.

4. Billings, R., T. Kraus, B. Hunt, and J. Mangino, R. Cook, L. Driver, 1998. Development

and Comparison of 1990 and 1996 Mobile Source Hazardous Air Pollutant Emissions

Estimates. Presented at AWMA Conference “Emissions Inventory: Living in a Global

Environment,” New Orleans, LA, December 8-10, 1998.

5. Boekhaus, K. L., J. M. DeJovine, D. A. Paulsen, L. A. Rapp, J.S. Segal and D. J.

Townsend, 1991. Clean Fuels Report 91-03: Fleet Test Emissions Data -- EC-Premium

Emission Control Gasoline. Arco Products Co., Anaheim, California.

6. CARB, 1991. Butadiene Emission Factors. memo from K. D. Drachand to Terry

McGuire and Peter Venturini, July 17, 1991.

7. Carroll, J. N., 1991. Emission Tests of In-use Small Utility Engines: Task III Report,

Non-road Source Emission Factors Improvement. Prepared for U.S. EPA by Southwest

Research Institute. Report No. SwRI 3426-006.

8. Censullo, A. C., 1991. Development of Species Profiles for Selected Organic Emission

Sources. California Polytechnic University, report prepared for California Air Resources

Board.

9. College of Engineering - Center for Environmental Research and Technology, University

of California, 1998. Evaluation of Factor that Affect Diesel Exhaust Toxicity. Submitted

to California Air Resources Board, Contract No. 94-312.

10. Eastern Research Group, 1999. Documentation for the 1996 Base year National Toxics

Inventory for Commercial Marine Vessel and Locomotive Sources. Draft Report

prepared for Emission Factor and Inventory Group, Office of Air Quality Planning and
35

Standards, U. S. EPA, November 1, 1999.

11. Eastern Research Group, 1999. Documentation for the 1996 Base year National Toxics

Inventory for Aircraft Sources. Draft Report prepared for Emission Factor and Inventory

Group, Office of Air Quality Planning and Standards, U. S. EPA, November 1, 1999.

12. Eastern Research Group, 1999. Documentation for the 1996 Base year National Toxics

Inventory for Nonroad Sources. Draft Report prepared for Emission Factor and Inventory

Group, Office of Air Quality Planning and Standards, U. S. EPA, November 1, 1999.

13. Eastern Research Group, 1999. Documentation for the 1996 Base year National Toxics

Inventory for Onroad Sources. Draft Report prepared for Emission Factor and Inventory

Group, Office of Air Quality Planning and Standards, U. S. EPA, November 1, 1999.

14. EPA,1999. Estimation of Motor Vehicle Toxic Emissions and Exposure in Selected

Urban Areas. Prepared by Sierra Research, Inc., Radian International Corp., and Energy

& Environmental Analysis, Inc. for U. S. EPA, Office of Mobile Sources, Assessment

and Modeling Division, Ann Arbor, MI, Report No. EPA420-D-99-002, March 1999.

15. EPA, 1996. "Determining POM/PAH Emission Factors for Mobile Sources." 

Memorandum from Pam Brodowicz, Office of Mobile Sources, to Eric Ginsburg and

David Mobley, Office of Air Quality Planning and Standards, December 19, 1996.

16. EPA, 1999. 1990 Emissions Inventory of Forty Potential Section 112(k) Pollutants:

Supporting Data for EPA’s Section 112(k) Regulatory Strategy. Distributed by Office of

Air Quality Planning and Standards, U. S. EPA, May 21, 1999.

17. EPA, 1997. Memo from Rich Cook, Rich, Office of Mobile Sources, to Anne Pope,

Office of Air Quality Planning and Standards, Source Identification and Base Year 1990

Emission Inventory Guidance for Mobile Source HAPs on the OAQPS List of 40 Priority

HAPs. June 11, 1997.

18. EPA, 1998. Memo from Rich Cook, Office of Mobile Sources, to Laurel Driver and

Anne Pope, Office of Air Quality Planning and Standards, Guidance on Mobile Source

Emission Estimates in the 1996 National Toxics Inventory. June 9, 1998. 

19. EPA, 1993. Final Regulatory Impact Analysis for Reformulated Gasoline. U. S.

Environmental Protection Agency, December 13, 1993.

20. EPA,1992. Speciation for SAI Runs. April 14, 1992 memo by Chris Lindhjem, Penny

Carey, and Joe Somers.

21. EPA, 1992. 11% MTBE Exhaust Speciation. August 14, 1992 memo by Rich Cook. 
36

22. EA, 1992. Exhaust, Evaporative, and Running Loss Speciation. July 10, 1992 memo by

Chris Lindhjem, Penny Carey, and Joe Somers.

23. EPA, 1993. Motor Vehicle-Related Air Toxics Study. Office of Mobile Sources,

Emission Planning and Strategies Division, Ann Arbor, MI. Report No. EPA 420-R-93-

005. 

24. EPA, 1993. Volatile Organic Compound (VOC)/ Particulate Matter (PM) Speciation

Data System (SPECIATE), Version 1.5.

25. Cook, Rich, and Pam Brodowicz. 1998. Derivation of Mobile Source Toxic Fractions

Applied to the Gridded Houston VOC Inventory. Appendix III.C. in “Air Dispersion

Modeling of Toxic Air Pollutants in Urban Areas. Emissions, Monitoring and Analysis

Division, Office of Air Quality Planning and Standards, July, 1998.

26. EPA, 1997. "Revisions to Nonroad Toxic Emission Estimates for Five Candidate Title

III Section 112(k) Hazardous Air Pollutants: Benzene, 1,3-Butadiene, Formaldehyde,

Hexavalent Chromium, and Polycyclic Organic Matter." Memorandum from Richard

Cook, Office of Mobile Sources, to Anne Pope, Office of Air Quality Planning and

Standards, February 20, 1997.

27. EPA, 1994. Estimating Exposure to Dioxin-Like Compounds -- Volume II: Properties,

Sources, Occurrence and Background Exposures. Office of Research and Development,

Washington, DC. June, 1994 External Review Draft, Report No. EPA/600/6-88/005Cb.

28. EPA, 1993. “Piston Engine Particulate Matter Emission Factors, Toxic Emission

Fractions, and VOC to TOG Correction Factor for Aircraft.” Memorandum from Richard

Cook, Office of Mobile Sources, to Patricia Morris, Region 5, February 17, 1993.

29. Gabele, P., 1997. Exhaust Emissions from Four-Stroke Lawn Mower Engines. J. Air &

Waste. Manage. Assoc. 47:945-952.

30. Hare, C. T., and J. J. White, 1991. Toward the Environmentally Friendly Small Engine:

Fuel, Lubricant, and Emission Measurement Issues. SAE Paper No. 911222.

31. Hare, C. T., and J. N. Carroll, 1993. Speciation of Organic Emissions to Study Fuel

Dependence of Small Engine Exhaust Photochemical Reactivity. Report for Advisory

Committee for Research, Southwest Research Institute, July, 1993.

32. Sierra Research, Inc., 1998. On-Road Motor Vehicle National Toxics Exposure

Estimates. Memorandum from Philip Heirigs to Rich Cook, U.S. EPA. October 15, 1998.

33. Sierra Research, Inc., 1999. Methodologies Used to Generate Emission Estimates for
37

Five Motor Vehicle HAPs in the 1996 National Toxics Inventory. Memorandum from

Philip Heirigs to Rich Cook, July 23, 1999.

34. Sigsby, J. E., S. Tejeda, W. Ray, J. M. Lang, and J. W. Duncan, 1987. Volatile organic

compound emissions from 46 in-use passenger cars. Environ. Sci. Technol. 21:466-475.

35. Spicer, C. W., M. W. Holdren, D. L. Smith, D. P. Hughes, and M. D. Smith, 1992. 

Chemical composition of exhaust from aircraft turbine engines. Journal of Engineering

for Gas Turbines and Power 114: 111-117.

36. Spicer, C. W., M. W. Holdren, R. M. Riggin, and T. F. Lyon, 1994. Chemical

combustion and photochemical reactivity of exhaust from aircraft turbine engines. Ann.

Geophysicae 12:944-955.

37. Stump, F. D., 1997. Sun Fuels Alaska II Study. Unpublished data.

38. Stump, F. D., S. Tejada, W. Ray, D. Dropkin, F. Black, R. Snow, W. Crews, P. Siudak,

C. O. Davis, L. Baker and N. Perry, 1989. The influence of ambient temperature on

tailpipe emissions from 1984 to 1987 model year light-duty gasoline vehicles. 

Atmospheric Environment 23: 307-320.

39. Stump, F. D., K. T. Knapp, and W. D. Ray, 1996. Influence of ethanol-blended fuels on

the emissions from three pre-1985 light-duty passenger vehicles. J. Air & Waste Manage.

Assoc. 46: 1149-1161.

40. Stump, F. D., S. Tejeda, W. Ray, D. Dropkin, F. Black, R. Snow, W. Crews, P. Siudak,

C. O. Davis and P. Carter, 1990. The influence of ambient temperature on tailpipe

emissions from 1985-1987 model year light-duty gasoline vehicles -- II. Atmospheric

Environment 24A: 2105-2112.

41. Stump, F. D., K. T. Knapp, W. D. Ray, P. D. Siudak, and R. F. Snow, 1994. Influence of

oxygenated fuels on the emissions from three pre-1985 light-duty passenger vehicles. J.

Air & Waste Manage. Assoc. 44:781-786.

42. Warner-Selph, M. A. and J. DeVita, 1989. Measurements of Toxic Exhaust Emissions

from Gasoline-Powered Light-Duty Vehicles. SAE Technical Paper No. 892075.

43. Warner-Selph, M. A., and L. R. Smith, 1991. Assessment of Unregulated Emissions

from Gasoline Oxygenated Blends. Ann Arbor, Michigan: U.S. Environmental

Protection Agency, Office of Mobile Sources. Publication no. EPA

44. Wyborny, L. Methyl Tertiary Butyl Ether (MTBE) Emissions from Passenger Cars. Draft

Technical Report. U. S. Environmental Protection Agency, Office of Mobile Sources,
38

April, 1998.
13 Ligocki, M.P., and G.Z. Whitten, Atmospheric transformation of air toxics: acetaldehyde and polycyclic

organic matter, Systems Applications International, San Rafael, CA, (SYSAPP-91/113), 1991.

14 EPA 1999. Environmental Protection Agency, Integrated Risk Information System (IRIS), Office of

Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH, 1999.

39

Chapter 3: Health Effects of Mobile Source Air Toxics

Chapter 2 of this Draft Technical Support Document describes how we identified the list

of mobile source air toxics (MSATs). This chapter provides information on the 21 MSATs,

including their physical properties, uses, sources of potential exposure, and health hazards.

A . Acetaldehyde

Acetaldehyde is ubiquitous in the ambient environment. It is an intermediate product of

higher plant respiration and is formed as a product of incomplete wood combustion in fireplaces

and wood stoves, coffee roasting, burning of tobacco, vehicle exhaust fumes, and coal refining

and waste processing. Hence, many individuals are exposed to acetaldehyde by breathing

ambient air. It should be noted that residential fireplaces and wood stoves are the two highest

sources of emissions, followed by various industrial emissions. Exposure may also occur in

individuals occupationally exposed to acetaldehyde during its manufacture and use. In addition,

acetaldehyde is formed in the body from the breakdown of ethanol; this would be a source of

acetaldehyde among those who consume alcoholic beverages.

Acetaldehyde is a saturated aldehyde that is found in vehicle exhaust and is formed as a

result of incomplete combustion of both gasoline and diesel fuel. It is not a component of

evaporative emissions. Acetaldehyde comprises 0.4 to 1.0 percent of exhaust total organic gases

(TOG), depending on control technology and fuel composition. Primary acetaldehyde emissions

from mobile sources account for approximately 66 percent of the emissions in the 1996 National

Toxics Inventory.

The atmospheric chemistry of acetaldehyde is similar in many respects to that of

formaldehyde.13 Like formaldehyde, it can be both produced and destroyed by atmospheric

chemical transformation. Mobile sources contribute to ambient acetaldehyde levels both through

direct emissions of acetaldehyde and as a result of secondary formation of acetaldehyde from

VOC emissions.

Although human data on the carcinogenic potential of acetaldehyde are extremely limited,

acetaldehyde is classified as a probable human carcinogen. This classification is based on a

sufficient database of animal carcinogenicity studies.14 Specifically, increased incidences of

nasal tumors in male and female rats and laryngeal tumors in male and female hamsters have

been documented after inhalation exposure.
15 EPA 1999. Environmental Protection Agency, Integrated Risk Information System (IRIS), Office of

Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH, 1999.

16 U.S. Environmental Protection Agency (EPA). Health Assessment Document for Acetaldehyde.

EPA/600/8-86-015A. Environmental Criteria and Assessment Office, Office of Health and Environmental

Assessment, Office of Research and Development, Research Triangle Park, NC. 1987.

17 EPA 1991. Environmental Protection Agency, Integrated Risk Information System (IRIS), Office of

Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH.

40

The primary acute effects associated with exposure to acetaldehyde include irritation of

the eyes, skin, and respiratory tract. Effects on the respiratory system have been reported from

studies of animals exposed to long-term lower concentrations. Although no information is

available on the reproductive or developmental effects of acetaldehyde in humans, data from

animal studies suggest that acetaldehyde may be a potential developmental toxicant.15,16

B. Acrolein

Acrolein is an aldehyde primarily used as an intermediate in the manufacture of acrylic

acid. It can be formed from the breakdown of certain pollutants in outdoor air or from burning

tobacco or gasoline. Acrolein is found in vehicle exhaust and is formed as a result of incomplete

combustion of both gasoline and diesel fuel. It is not a component of evaporative emissions. 

Acetaldehyde comprises 0.05 to 0.4 percent of exhaust total organic gases (TOG), depending on

control technology and fuel composition. Primary acrolein emissions from mobile sources

account for approximately 38 percent of the emissions in the 1996 National Toxics Inventory. 

The atmospheric chemistry of acrolein is expected to be similar in many respects to that of

acetaldehyde and formaldehyde.

Although no information on the carcinogenic effects of acrolein in humans is available,

limited laboratory data for animals exposed by drinking water ingestion indicated an increased

incidence of adrenal cortical adenomas (non-malignant tumors of the adrenal glands adjacent to

the kidney). EPA has classified acrolein as a Group C, possible human carcinogen, based on this

limited animal data, the carcinogenic potential of an acrolein metabolite, evidence of

mutagenicity in bacteria and structural similarity to probable or known human carcinogens.

The respiratory system is the primary target in humans and animals for acrolein toxicity

resulting from inhalation exposure. Acute exposure results in upper respiratory tract irritation

and congestion, whereas chronic exposures in animals indicated an increase in cell proliferation

and in the numbers of white blood cells in the tissues lining the nasal passages. No information

is available on the reproductive or developmental effects of acrolein in humans.17

C. Arsenic Compounds

Arsenic, a naturally occurring element, is found throughout the environment. It is
18 EPA 1999. Environmental Protection Agency, Integrated Risk Information System (IRIS), Office of

Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH, 1999.

19 EPA 1999. Environmental Protection Agency, National Air Toxics Program: The Integrated Urban

Strategy Report to Congress, Office of Air Quality Planning and Standards, RTP, NC.

41

released into the air by volcanoes, the weathering of arsenic-containing minerals and ores, and by

commercial or industrial processes. For most people, food is the largest source of arsenic

exposure (about 25 to 50 μg/d), with lower amounts coming from drinking water and air. 

Among foods, some of the highest levels are in fish and shellfish; however, this arsenic exists

primarily as organic compounds, which are essentially nontoxic. Elevated levels of inorganic

arsenic may be present in soil, either from natural mineral deposits or contamination from human

activities, which may lead to dermal or ingestion exposure. Workers in metal smelters and

nearby residents may be exposed to above-average inorganic arsenic levels from arsenic released

into the air. Other sources of inorganic arsenic exposure include burning plywood treated with

an arsenic wood preservative or dermal contact with wood treated with arsenic.

Arsenic emissions from mobile sources are minimal, accounting for less than 1 percent of

the emissions in the 1996 National Toxics Inventory. It is thought that the arsenic found in the

emissions is due to impurities in either fuel additives or the fuel itself.

Inhalation exposure to inorganic arsenic has been shown to be strongly associated with

lung cancer in humans, while ingestion of inorganic arsenic in humans has been linked to a form

of skin cancer and also to bladder, liver, and lung cancer. The EPA has classified inorganic

arsenic as a Group A, human carcinogen.18

Acute (short-term) high-level inhalation exposure to arsenic dust or fumes has resulted in

gastrointestinal effects (nausea, diarrhea, abdominal pain); central and peripheral nervous system

disorders have occurred in workers acutely exposed to inorganic arsenic. Chronic (long-term)

inhalation exposure to inorganic arsenic in humans is associated with irritation of the skin and

mucous membranes. Human data suggest a relationship between inhalation exposure of women

working at or living near metal smelters and an increased risk of reproductive effects, such as

spontaneous abortions. However, as these studies evaluated smelter pollutants in general,

arsenic's role is not clear. Chronic oral exposure has resulted in gastrointestinal effects, anemia,

peripheral neuropathy, skin lesions, hyperpigmentation, and liver or kidney damage.19

D. Benzene

Benzene is an aromatic hydrocarbon which is present as a gas in both exhaust and

evaporative emissions from motor vehicles as well as from the burning of coal and oil. Benzene

comprises 3.0 to 5.0 percent of mobile source exhaust total organic gases (TOG ), which varies

depending on control technology (e.g., type of catalyst) and the levels of benzene and aromatics

in the fuel. The benzene fraction of evaporative TOG emissions is generally about one percent,
20 EPA 1998a. Environmental Protection Agency, Carcinogenic Effects of Benzene: An Update, National

Center for Environmental Assessment, Washington, DC. 1998.

21 Leukemia is a blood disease in which the white blood cells are abnormal in type or number. Leukemia

may be divided into nonlymphocytic (granulocytic) leukemias and lymphocytic leukemias. Nonlymphocytic

leukemia generally involves the types of white blood cells (leukocytes) that are involved in engulfing, killing, and

digesting bacteria and other parasites (phagocytosis) as well as releasing chemicals involved in allergic and immune

responses.

22 EPA 1993a. Motor Vehicle-Related Air Toxics Study, U.S. Environmental Protection Agency, Office

of Mobile Sources, Ann Arbor, MI, EPA Report No. EPA 420-R-93-005, April 1993.

42

but depends on control technology and fuel composition (e.g., benzene level and Reid Vapor

Pressure (RVP)). Benzene emissions from mobile sources account for approximately 76 percent

of the emissions in the 1996 National Toxics Inventory.

Benzene is also used as a solvent for fats, waxes, resins, oils, inks, paints, plastics, and

rubber; in the extraction of oils from seeds and nuts; and in photogravure printing. It is also used

as a chemical intermediate. Benzene is also used in the manufacture of detergents, explosives,

pharmaceuticals, and dyestuffs. Tobacco smoke contains benzene and accounts for nearly half

the national exposure to benzene.

The EPA has recently reconfirmed that benzene is a known (Group A) human carcinogen

by all routes of exposure20. Respiration is the major source of human exposure and at least half

of the respiratory exposure is by way of gasoline vapors and automotive emissions. Long-term

exposure to high levels of benzene in air has been shown to cause cancer of the tissues that form

white blood cells. Specifically, benzene has been linked to acute (rapid and fatal)

nonlymphocytic21 leukemia, chronic (lingering, lasting) lymphocytic leukemia and possibly

multiple myeloma (primary malignant tumors in the bone marrow), although the evidence for the

latter has decreased with more recent studies. Leukemias, lymphomas, and other tumor types

have been observed in experimental animals that have been exposed to benzene by inhalation or

oral administration. Exposure to benzene and/or its metabolites has also been linked with genetic

changes in humans and animals and increased proliferation of mouse bone marrow cells. The

occurrence of certain chromosomal changes in individuals with known exposure to benzene may

serve as a marker for those at risk for contracting leukemia.

A number of adverse noncancer health effects, blood disorders such as preleukemia and

aplastic anemia, have also been associated with low-dose, long-term exposure to benzene.22

People with long-term exposure to benzene may experience harmful effects on the blood-forming

tissues, especially the bone marrow. These effects can disrupt normal blood production and

cause a decrease in important blood components. Chronic inhalation exposure to benzene in
23 Pancytopenia is the reduction in the number of all three major types of blood cells (erythrocytes, or red

blood cells, thrombocytes, or platelets, and leukocytes, or white blood cells). 

24 Aksoy, M. 1991. Hematotoxicity, leukemogenicity and carcinogenicity of chronic exposure to benzene. 

In: Arinc, E.; Schenkman, J.B.; Hodgson, E., Eds. Molecular Aspects of Monooxygenases and Bioactivation of

Toxic Compounds. New York: Plenum Press, pp. 415-434.

25 Goldstein, B.D. 1988. Benzene toxicity. Occupational medicine. State of the Art Reviews. 3: 541-

554.

26 Rothman, N., G.L. Li, M. Dosemeci, W.E. Bechtold, G.E. Marti, Y.Z. Wang, M. Linet, L.Q. Xi, W. Lu,

M.T. Smith, N. Titenko-Holland, L.P. Zhang, W. Blot, S.N. Yin, and R.B. Hayes. 1996. Hematotoxicity among

Chinese workers heavily exposed to benzene. Am. J. Ind. Med. 29: 236-246.

27 EPA, 1985. Mutagenicity and carcinogenicity assessment of 1,3-butadiene. EPA/600/8-85/004F. U.S. 

Environmental Protection Agency, Office of Health and Environmental Assessment. Washington, DC. 

28 EPA 1998c. Environmental Protection Agency, Health Risk Assessment of 1,3-Butadiene. EPA/600/P98/001A, February 1998.

43

humans and animals results in pancytopenia,23 a condition characterized by decreased numbers of

circulating erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes

(blood platelets).24,25 Some individuals that develop pancytopenia and continue to be exposed to

benzene may develop aplastic anemia, a more severe blood disease that occurs when the bone

marrow ceases to function. The aplastic anemia can progress to AML (acute mylogenous

leukemia). The most sensitive noncancer effect observed in humans is the depression of absolute

 lymphocyte counts in the circulating blood.26

E. 1,3-Butadiene

1,3-Butadiene is formed in vehicle exhaust by the incomplete combustion of gasoline and

diesel fuel. It is not present in vehicle evaporative and refueling emissions, because it is not

present in any appreciable amount in gasoline. 1,3-Butadiene accounts for 0.4 to 1.0 percent of

total exhaust TOG, depending on control technology and fuel composition. 1,3-Butadiene

emissions from mobile sources account for approximately 60 percent of the emissions in the

1996 National Toxics Inventory.

Sources of 1,3-butadiene released into the air also include manufacturing and processing

facilities, especially oil refineries, chemical manufacturing plants, and plastic and rubber

factories. Other sources are forest fires or other combustion, and cigarette smoke.

1-3-Butadiene was classified by EPA as a Group B2 (probable human) carcinogen in

198527. This classification was based on evidence from two species of rodents and epidemiologic

data. EPA recently prepared a draft assessment that proposes that sufficient evidence exists to

propose 1,3-butadiene be a known human carcinogen28. However, the Environmental Health
29 Scientific Advisory Board. 1998. An SAB Report: Review of the Health Risk Assessment of 1,3-

Butadiene. EPA-SAB-EHC-98, August, 1998.

30 Delzell, E., N. Sathiakumar, M. Macaluso, M. Hovinga, R. Larson, F. Barbone, C. Beall, and P. Cole,

1995. A follow-up study of synthetic rubber workers. Final report prepared under contract to International Institute

of Synthetic Rubber Producers, October 2, 1995.

44

Committee of EPA’s Scientific Advisory Board (SAB), in reviewing the draft document, issued a

majority opinion that 1,3-butadiene should instead be classified as a probable human

carcinogen29. The SAB panel recommended that EPA calculate the lifetime cancer risk estimates

based on the human data from Delzell et al. 199530 and account for the highest exposure of “360

ppm-year” for 70 years. Further input recommended that EPA also take into account additional

data on health effects observed in female laboratory animals, hence indicating that females may

be a sensitive subpopulation.

1,3-Butadiene also causes a variety of reproductive and developmental effects in mice

and rats exposed to long-term, low doses of butadiene (EPA 1998c). The most sensitive effect

was reduced litter size at birth and at weaning. These effects were observed in studies in which

male mice exposed to 1,3-butadiene were mated with unexposed females. In humans, such an

effect might manifest itself as an increased risk of spontaneous abortions, miscarriages, still

births, or very early deaths.

F. Chromium Compounds

Chromium is a naturally occurring element in rocks, animals, plants, soil, and volcanic

dust and gases. Chromium occurs in the environment predominantly in one of two valence

states: trivalent chromium (Cr III), which occurs naturally and is an essential nutrient, and

hexavalent chromium (Cr VI), which, along with the less common metallic chromium (Cr 0), is

most commonly produced by industrial processes. Chromium (III) is essential to normal glucose,

protein, and fat metabolism and is thus an essential dietary element with a daily intake of 50 to

200 μg/d recommended for an adult. The body has several systems for reducing chromium (VI)

to chromium (III). This chromium (VI) detoxification leads to increased levels of chromium

(III).

Air emissions of chromium are predominantly of trivalent chromium, and in the form of

small particles or aerosols. The most important industrial sources of chromium in the

atmosphere are those related to ferrochrome production. Ore refining, chemical and refractory

processing, cement-producing plants, automobile brake lining and catalytic converters for

automobiles, leather tanneries, and chrome pigments also contribute to the atmospheric burden of

chromium. Total chromium emissions from mobile sources account for approximately 4 percent

of the emissions in the 1996 National Toxics Inventory.

Human studies have clearly established that inhaled chromium (VI) is a human
31 EPA 1999. Environmental Protection Agency, Integrated Risk Information System (IRIS), Office of

Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH, 1999.

32 EPA 1999. Environmental Protection Agency, National Air Toxics Program: The Integrated Urban

Strategy Report to Congress, Office of Air Quality Planning and Standards, RTP, NC.

33 U.S. EPA (2000). Sources of Dioxin-Like Compounds in the United States; In: Exposure and Human

Health Reassessment of 2,3,7,8-TCDD and Related compounds. Part 1: Estimating Exposure to Dioxin-like

Compounds; Volume 2. National Center for Environmental Assessment, Office of Research and Development,

U.S. Environmental Protection Agency, Washington, DC. March 2000 draft final. EPA/600/P-00/001Ab.

45

carcinogen, resulting in an increased risk of lung cancer. Animal studies have shown chromium

(VI) to cause lung tumors via inhalation exposure. The EPA has classified chromium (VI) as a

Group A, known human carcinogen.31

The respiratory tract is the major target organ for acute (short-term) and chronic

(long-term) inhalation exposures to chromium (VI). Shortness of breath, coughing, and

wheezing were reported from a case of acute exposure to chromium (VI), while perforations and

ulcerations of the septum, bronchitis, decreased pulmonary function, pneumonia, and other

respiratory effects have been noted from chronic exposure. Limited human studies suggest that

chromium (VI) inhalation exposure may be associated with complications during pregnancy and

childbirth, while animal studies have not reported reproductive effects from inhalation exposure

to chromium (VI).32

Chromium III is much less toxic than chromium (VI). The respiratory tract is also the

major target organ for chromium (III) toxicity, similar to chromium (VI) but data from animal

studies do not demonstrate that the effects observed following inhalation of chromium (VI)

particulates. Chromium III is most appropriately designated a Group D -- not classified as to its

human carcinogenicity, there are inadequate data to determine the potential carcinogenicity of

chromium III.

G. Dioxin/Furans

Dioxin comes from both natural and industrial sources, such as medical and municipal

waste incineration and paper-pulp production. Recent studies have confirmed that dioxins are

formed by and emitted from heavy-duty diesel trucks and are estimated to account for one

percent of total dioxin emissions in the dioxin inventory for the year 1995.33

In general, dioxin exposures of concern have primarily been noninhalation exposures

associated with human ingestion of certain foods, e.g., beef, pork, poultry, fish, eggs and dairy

products contaminated by dioxin. The two primary pathways for dioxin to enter the human diet 

are: air-to-plant-to-animal and water/sediment-to-fish. Vegetation receives these compound via

atmospheric transport and deposition. The compounds are retained on plant surfaces and

bioaccumulate in fatty tissues of animals that feed on the vegetation. In the aquatic food chain, 
34 IARC (1997). Polychlorinated dibenzo-para-dioxins and Polychlorinated Dibenzofurans. Volume 69,

IARC Monogram on the Evaluation of Carcinogenic Risks to Humans, International Agency for Research on

Cancer, Work Health Organization, Lyon France.

35 U.S. EPA (1994) Health Assessment Document for 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and

Related Compounds: Volume III Summary Draft Document. EPA/600/BP-92/001c.

36 U.S. EPA (1999) Health Assessment Document for Diesel Emissions: SAB Review Draft. EPA/600/8-

90/057D Office of Research and Development, Washington, D.C. The document is available electronically at

46

dioxins enter water systems via direct discharge or deposition and runoff from watersheds. Fish

accumulate dioxin through their direct contact with water, suspended particles, bottom sediments

and through the consumption of aquatic organisms. Exposure to dioxin occurs over a lifetime,

and the exposure is cumulative over a lifetime.

Based on recent human epidemiological studies from Europe and the United States,

dioxin has been linked to several cancers, including lymphomas and lung cancer, by the

International Agency For Research on Cancer (IARC) (an Office of the World Health

Organization).2

 The IARC classifies the most potent form of dioxin, 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD), as a "Group 1" carcinogen, i.e., dioxin is carcinogenic to human.34

Dioxins/furans have also linked low-grade exposure to dioxin to a wide array of other

health problems, including changes in hormone levels as well as developmental defects in babies

and children.35

H. Diesel Exhaust

Diesel exhaust includes components in the gas and particle phases. Gaseous components

of diesel exhaust include at least one organic compound known to cause cancer in humans (e.g.,

benzene) while possible or probable human carcinogens and compounds causing noncancer

effects are also present in the gas-phase (e.g., formaldehyde, acetaldehyde, 1,3-butadiene,

acrolein). The health effects of these and other gaseous compounds in diesel exhaust are

discussed elsewhere in this chapter. Three classes of compounds associated with particle-phase

diesel exhaust (e.g., polycyclic organic matter, metals, and dioxins) are also discussed here in

relation to diesel exhaust particulate matter and are also discussed under separate sections in this

chapter. Diesel exhaust is a complex mixture of carbon particles and associated organics and

inorganics, and it is not known what fraction or combination of fractions cause the health effects

(discussed below) that have been observed with exposure to diesel exhaust. 

1. Cancer Effects of Diesel Exhaust

The EPA draft Health Assessment Document for Diesel Emissions (draft Assessment) is

currently being revised based on comments received from the Clean Air Scientific Advisory

Committee (CASAC) of EPA’s Science Advisory Board.36 EPA’s draft position is that diesel
www.epa.gov/ncea/diesel.htm.

37 The EPA draft designation of diesel exhaust as a likely human carcinogen is subject to further comment

by CASAC in 2000. The draft designation of diesel exhaust as a likely human carcinogen under the 1996 Proposed

Guidelines for Carcinogen Risk Assessment is very similar to the current 1986 Guidelines for Carcinogen Risk

Assessment that designate diesel exhaust as a probable carcinogen (B1 carcinogen). The new guidelines, once

finalized, will incorporate a narrative approach to assist the risk manager in the interpretation of the carcinogen’s

mode of action, the weight of evidence, and any risk related exposure-response or protective exposure

recommendations.

47

exhaust is a likely human lung carcinogen and that this cancer hazard exists for occupational and

environmental levels of exposure.37

In evaluating the available research for the draft 1999 Assessment, EPA found that

individual epidemiological studies numbering about 30 show increased lung cancer risks

associated with diesel emissions within the study populations of 20 to 89 percent depending on

the study. Analytical results of pooling the positive study results show that on average the risks

were increased by 33 to 47 percent. Questions remain about the influence of other factors (e.g.,

effect of smoking), the quality of the individual epidemiology studies, exposure levels, and

consequently the precise magnitude of the increased risk of lung cancer. From a weight of the

evidence perspective, EPA believes that the epidemiology evidence, as well as supporting data

from certain animal and mode of action studies, support the Agency’s proposed conclusion that

exposure to diesel exhaust is likely to pose a human health hazard at occupational exposure

levels, as well as to the general public exposed to typically lower environmental levels of diesel

exhaust. 

While available evidence supports EPA's draft position that diesel exhaust is a likely

human lung carcinogen, and thus is likely to pose a cancer hazard to humans, the absence of

quantitative estimates of the lung cancer unit risk for diesel exhaust limits our ability to quantify

with confidence the actual magnitude of the cancer risk. In the draft 1999 Assessment, EPA

acknowledged these limitations and provided a discussion of the possible cancer risk consistent

with general occupational epidemiological findings of increased lung cancer risk and relative

exposure ranges in the occupational and environmental settings. 

2. Noncancer Effects of Diesel Exhaust

The noncancer effects of diesel exhaust are also of concern to the Agency. EPA believes

that chronic diesel exhaust exposure, at sufficient exposure levels, increases the hazard and risk

of an adverse consequence (including respiratory tract irritation/inflammation and changes in

lung function). The draft 1999 Assessment discussed an existing inhalation reference

concentration (RfC) for chronic effects that EPA intends to revise in the next draft 1999

Assessment in response to CASAC comments. The revised RfC will be reviewed by CASAC at

a future meeting. 
38 U.S. EPA (1999) Health Assessment Document for Diesel Emissions: SAB Review Draft. EPA/600/8-

90/057D Office of Research and Development, Washington, D.C. The document is available electronically at

www.epa.gov/ncea/diesel.htm.

48

3. Diesel Exhaust and Diesel Particulate Matter

While some gaseous components of diesel exhaust may play a role in the cancer hazard

attributed to diesel exhaust exposure, studies suggest that the particulate component plays a

substantial role in carcinogenicity and other noncancer effects. Diesel particulate matter is

mainly attributable to the incomplete combustion of fuel hydrocarbons as well as engine oil and

other fuel components such as sulfur. Primary diesel particles mainly consist of carbonaceous

material, with a small contribution from sulfuric acid and ash (trace metals). Many of these

particles exist in the atmosphere as a carbon core with a coating of organic carbon compounds, or

as sulfuric acid and ash, sulfuric acid aerosols, or sulfate particles associated with organic carbon. 

Most (80-95%) diesel particles are in the fine (<2.5 μm) and ultrafine (<0.1 μm) size range. 

Investigations show that diesel particles (the elemental carbon core plus the adsorbed

organics) induce lung cancer at high doses, and that the particles, independent of the gaseous

compounds, elicit an animal lung cancer response. The presence of non-diesel elemental carbon

particles, as well as the organic-laden diesel particles, correlate with an adverse inflammatory

effect in the respiratory system of animals. Additional evidence suggesting the importance of the

role of particulate matter in diesel exhaust includes the observation that the extractible particle

organics collectively produce cancer and adverse mutagenic toxicity in experimental test systems. 

Many of the individual organic compounds are mutagenic or carcinogenic in their own right.

Diesel PM contains small quantities of numerous mutagenic and carcinogenic

compounds. Some assessments report up to 16 organic compounds in primary and secondary

diesel exhaust with known or suspected carcinogenic activity or other toxicologically significant

effects.38 While representing a very small portion (less than one percent) of the national

emissions of metals, and a small portion of diesel particulate matter (one to five percent), we note

that several trace metals of toxicological significance are also emitted in diesel exhaust in small

amounts including chromium, manganese, mercury and nickel. In addition, small amounts of

dioxins have been measured in diesel exhaust, some of which may partition into the particle

phase, though the impact of these emissions on human health is not clear. 

Mobile sources account for almost all diesel particulate matter emissions and on-road

sources account for approximately one-third of the mobile source diesel particulate matter

emissions. Diesel PM emissions from all mobile sources account for 17% of ambient PM10

emissions in 1996, excluding the contribution from natural and miscellaneous sources.

In assessing the health impacts of diesel particulate matter, it is important to acknowledge

that diesel exhaust particulate matter is part of ambient PM2.5. While diesel particulate matter

contributes to ambient levels of PM2.5, its high content of organic compounds absorbed onto a
49

carbon core and the concentration of ultrafine particles in diesel particulate matter distinguish it

from other non-combustion sources of PM2.5. The composition of diesel PM, which is dominated

by organic matter, contrasts strongly with the typical chemical composition of ambient PM2.5

which is dominated by sulfate for aerosols measured in the eastern U.S and by nitrate,

ammonium, and organic carbon in the western U.S. Noncancer health effects associated with

exposure to diesel PM overlap with some health effects reported for ambient PM including

respiratory symptoms (cough, labored breathing, chest tightness, wheezing), and chronic

respiratory disease (cough, phlegm, chronic bronchitis and some evidence for decreases in

pulmonary function). Considerably more research has been conducted to investigate the

noncancer health effects attributable to ambient PM2.5 than those attributable to diesel particulate

matter. A qualitative comparison of adverse effects of exposure to PM2.5 and diesel exhaust

particulates shows that the respiratory system is adversely affected in both cases, though PM2.5

has a wider spectrum of adverse effects for humans. A carcinogenicity hazard for PM2.5 has not

yet been clearly shown, however. 

I. Ethylbenzene

Ethylbenzene is a colorless, aromatic hydrocarbon, that smells like gasoline. It is used

primarily in the production of styrene. It is also used as a solvent, as a constituent of asphalt and

naphtha. It is present as a gas in both gasoline and diesel exhaust and evaporative emissions

from gasoline powered vehicles. Ethylbenzene emissions from mobile sources account for

approximately 84 percent of the emissions in the 1996 National Toxics Inventory.

Ethylbenzene exposure also occurs from the use of consumer products, pesticides,

solvents, carpet glues, varnishes, paints, and tobacco smoke. Indoor air usually has a higher

average concentration of ethylbenzene (about 1 ppb) than ambient air, due to the use of

household produces such as cleaning products or paints.

Limited information is available on the carcinogenic effects of ethylbenzene in humans or

animals. Based on inadequate data from animal bioassays and human studies, EPA has classified

ethylbenzene as a Group D carcinogen, meaning it is not classifiable as to human

carcinogenicity. 

 Noncancer, acute (short-term) exposure to ethylbenzene in humans results in respiratory

effects, such as throat irritation and chest constriction, irritation of the eyes, and neurological

effects such as dizziness. Chronic (long-term) exposure to ethylbenzene by inhalation in humans

may result in effects on the blood. Animal studies have reported effects on the blood, liver, and

kidneys from chronic inhalation exposure to ethylbenzene. No information is available on the

developmental or reproductive effects of ethylbenzene in humans, although animal studies have

reported developmental effects, including birth defects in animals exposed via inhalation.

J. Formaldehyde
39 Ligocki, M.P., G.Z. Whitten, R.R. Schulhof, M.C. Causley, and G.M. Smylie, Atmospheric

transformation of air toxics: benzene, 1,3-butadiene, and formaldehyde, Systems Applications International, San

Rafael, CA (SYSAPP-91/106), 1991.

40 EPA 1987. Environmental Protection Agency, Assessment of health risks to garment workers and

certain home residents from exposure to formaldehyde, Office of Pesticides and Toxic Substances, April 1987.

50

Formaldehyde is used mainly to produce resins used in particle board products and as an

intermediate in the synthesis of other chemicals. It also has minor uses in agriculture, as an

analytical reagent, in concrete and plaster additives, cosmetics, disinfectants, fumigants,

photography, and wood preservation. The highest levels of airborne formaldehyde have been

detected in indoor air, where it is released from various consumer products such as building

materials and home.

Formaldehyde is the most prevalent aldehyde in vehicle exhaust. It is formed from

incomplete combustion of both gasoline and diesel fuel and accounts for one to four percent of

total exhaust TOG emissions, depending on control technology and fuel composition. It is not

found in evaporative emissions. Primary formaldehyde emissions from mobile sources account

for approximately 41 percent of the emissions in the 1996 National Toxics Inventory.

Formaldehyde exhibits extremely complex atmospheric behavior39. It is present in

emissions and is also formed by the atmospheric oxidation of virtually all organic species,

including biogenic (produced by a living organism) hydrocarbons. Mobile sources contribute

both primary formaldehyde (emitted directly from motor vehicles) and secondary formaldehyde

(formed from photooxidation of other VOCs emitted from vehicles). 

EPA has classified formaldehyde as a Group B1, probable human carcinogen, based on

limited evidence for carcinogenicity in humans and sufficient evidence of carcinogenicity in

animal studies40. Epidemiological studies in occupationally exposed workers suggest that longterm inhalation of formaldehyde may be associated with tumors of the nasopharyngeal cavity

(generally the area at the back of the mouth near the nose), nasal cavity, and sinus. Studies in

experimental animals provide sufficient evidence that long-term inhalation exposure to

formaldehyde causes an increase in the incidence of squamous (epithelial) cell carcinomas

(tumors) of the nasal cavity. The distribution of nasal tumors in rats suggests that not only

regional exposure but also local tissue susceptibility may be important for the distribution of

formaldehyde-induced tumors. Research has demonstrated that formaldehyde produces

mutagenic activity in cell cultures.

Formaldehyde exposure also causes a range of noncancer health effects. At low

concentrations (0.05-2.0 ppm), irritation of the eyes (tearing of the eyes and increased blinking)

and mucous membranes are the principal effects observed in humans. At exposures of 1 to 11

ppm, other human upper respiratory effects associated with acute formaldehyde exposure include

a dry or sore throat, and a tingling sensation of the nose. Sensitive individuals may experience
41 Wilhelmsson, B. and M. Holmstrom. 1987. Positive formaldehyde PAST after prolonged formaldehyde

exposure by inhalation. The Lancet:164.

42 Burge, P.S., M.G. Harries, W.K. Lam, I.M. O'Brien, and P.A. Patchett. 1985. Occupational asthma due

to formaldehyde. Thorax 40:225-260.

43 Hendrick, D.J., R.J. Rando, D.J. Lane, and M.J. Morris. 1982. Formaldehyde asthma: Challenge

exposure levels and fate after five years. J. Occup. Med. 893-897.

44 Nordman, H., H. Keskinen, and M. Tuppurainen. 1985. Formaldehyde asthma - rare or overlooked? J.

Allergy Clin. Immunol. 75:91-99.

45 EPA 1991a. Environmental Protection Agency. Formaldehyde risk assessment update. June 11, 1991.

Office of Toxic Substances, U.S. Environmental Protection Agency, Washington, DC. External review draft, June

11, 1991.

51

these effects at lower concentrations. Forty percent of workers at formaldehyde-producing

factories reported nasal symptoms such as rhinitis (inflammation of the nasal membrane), nasal

obstruction, and nasal discharge following chronic exposure.41 In persons with bronchial asthma,

the upper respiratory irritation caused by formaldehyde can precipitate an acute asthmatic attack,

sometimes at concentrations below 5 ppm.42 Formaldehyde exposure may also cause bronchial

asthma-like symptoms in nonasthmatics.43,44 It is unclear whether asthmatics are more sensitive

than nonasthmatics to formaldehyde's effects.45

Immune stimulation may occur following formaldehyde exposure, although conclusive

evidence is not available. Also, little is known about formaldehyde's effect on the central

nervous system. Several animal inhalation studies have been conducted to assess the

developmental toxicity of formaldehyde. The only exposure-related effect noted was decreased

maternal body weight gain at the high-exposure level. No adverse effects on reproductive

outcome of the fetuses that could be attributed to treatment were noted.

K. n-Hexane

n-Hexane is a colorless volatile liquid that is insoluble in water and is highly flammable. 

Commercial grades of n-hexane are used as solvents for glues, varnishes, cements, and inks. It is

also used as a solvent in the extraction of edible fats and oils. n-Hexane is also a component of

gasoline and it is also found in the exhaust and evaporative emissions from motor vehicles. nHexane emissions from mobile sources account for approximately 43 percent of the emissions in

the 1996 National Toxics Inventory.

The most probable route of human exposure to n-hexane is by inhalation. Individuals are

most likely to be exposed to n-hexane in the workplace. Monitoring data indicate that n-hexane

is a widely occurring atmospheric pollutant as well.
52

No information is available on the carcinogenic effects of n-hexane in humans or animals.

EPA has made no determination as to the human carcinogenicity of n-hexane. 

Noncancer, acute (short-term) inhalation exposure of humans to high levels of n-hexane

causes mild central nervous system (CNS) depression and irritation of the skin and mucous

membranes. Nervous system effects include dizziness, giddiness, slight nausea, and headache in

humans. Chronic (long-term) exposure to n-hexane in air is associated with polyneuropathy in

humans, with numbness in the extremities, muscular weakness, blurred vision, headache, and

fatigue observed. Neurotoxic effects have also been exhibited in rats. Mild inflammatory and

degenerative lesions in the nasal cavity have been observed in rodents chronically exposed by

inhalation. Limited information is available on the reproductive or developmental effects of nhexane; one study reported testicular damage in rats exposed to n-hexane through inhalation. 

Birth defects have not been observed in the offspring of rats chronically exposed via inhalation in

several studies.

L. Lead Compounds

The largest source of lead in the atmosphere has been from leaded gasoline combustion. 

With the phase down of lead in gasoline, however, air lead levels have decreased considerably,

though lead is a component of racing and aviation fuels. In the 1996 National Toxics Inventory

mobile sources account for approximately 24 percent of the total inventory. This declining trend

should continue since there was a total lead phase-out from highway gasoline and its additives

that went into effect in January 1996. Other airborne sources include combustion of solid waste,

coal, and oils, emissions from iron and steel production, lead smelters, and tobacco smoke.

Exposure to lead can also occur from food and soil. Children are at particular risk to lead

exposure since they commonly put hands, toys, and other items, that may come in contact with

lead-containing dust and dirt in their mouths. Lead-based paints were commonly used for many

years and flaking paint, paint chips, and weathered paint powder may be a major source of lead

exposure, particularly for children. Lead in drinking water is due primarily to the presence of

lead in certain pipes, solder, and fixtures. Exposure to lead may also occur in the workplace,

such as lead smelting and refining industries, steel and iron factories, gasoline stations, and

battery manufacturing plants. Lead has been listed as a pollutant of concern in EPA's Great

Waters Program due to its persistence in the environment, potential to bioaccumulate, and

toxicity to humans and the environment.

Human studies are inconclusive regarding lead exposure and cancer, while animal studies

have reported an increase in kidney cancer from lead exposure by the oral route. The EPA

considers lead to be a Group B2, probable human carcinogen.

Lead is a very toxic element, causing a variety of effects at low dose levels. Brain

damage, kidney damage, and gastrointestinal distress are seen in humans receiving acute

(short-term) exposure to high levels of lead. Chronic (long-term) exposure to lead effects the
46 EPA 1994. EPA Health Effects Notebook for Hazardous Air Pollutants-Draft, EPA-452/D-95-00,

December 1994, Office of Air Quality Planning and Standards, RTP, NC. 

http://www.epa.gov/ttn/uatw/hapindex.html

53

blood, central nervous system (CNS), blood pressure, kidneys, and Vitamin D metabolism in

humans. Children are particularly sensitive to the chronic effects of lead, with slowed cognitive

development, reduced growth and other effects reported. Reproductive effects, such as decreased

sperm count in men and spontaneous abortions in women, have been associated with lead

exposure. The developing fetus is at particular risk from maternal lead exposure, with low birth

weight and slowed postnatal neurobehavioral development noted.

M. Manganese Compounds

Manganese is a naturally occurring substance found in many types of rock and soil; it is

ubiquitous in the environment and found in low levels in water, air, soil and food. Manganese

can also be released into the air by iron and steel production plants, power plants, and coke

ovens. The average manganese levels in various media are as follows: levels in drinking water

are approximately 0.004 ppm; average air levels are approximately 0.02 μg/m3

; levels in soil

range from 40 to 900 ppm; the average daily intake from food ranges from 1 to 5 mg/d. People

who work in factories where manganese metal is produced from manganese ore or where

manganese compounds are used to make steel or other products are most likely to be exposed

through inhalation to higher than normal levels of manganese.46 Manganese compounds from

mobile source comprise less than 2 percent of the 1996 National Toxics Inventory.

No studies are available regarding the carcinogenic effects of manganese in humans, and

animal studies are inadequate. The EPA has classified manganese as a Group D, not classifiable

as to carcinogenicity in humans.

Health effects in humans have been associated with both deficiencies and excess intakes

of manganese. Chronic (long-term) exposure to low levels of manganese in the diet is

considered to be nutritionally essential in humans, with a recommended daily allowance of 2 to 5

mg/d. Chronic exposure to high levels of manganese by inhalation in humans results primarily in

central nervous system (CNS) effects. Visual reaction time, hand steadiness, and eye-hand

coordination were affected in chronically-exposed workers. A syndrome named manganism may

result from chronic exposure to higher levels; manganism is characterized by feelings of

weakness and lethargy, tremors, a mask-like face, and psychological disturbances. Respiratory

effects have also been noted in workers chronically exposed by inhalation. Impotence and loss of

libido have been noted in male workers afflicted with manganism attributed to high-level

inhalation exposures to

In December, 1995, the fuel additive methylcyclopentadienyl manganese tricarbonyl

(MMT), an octane enhancer commercially labeled as HiTEC 3000, became legal to blend into

unleaded gasoline in the U.S. The approved fuel waiver for MMT allows up to 0.03125 (1/32)
47 The fuels and fuel additives testing program regulations are codified at 40 CFR part 79, subpart F. The

Alternative Tier 2 provisions appear at 40 C.F.R. § 79.58(c).

48 EPA Speciate Database http://www.epa.gov/ttnchie1/spec/index.html

54

gram per gallon manganese (60 FR 36414, July 17, 1995). Ethyl Corporation is still required to

perform health research but is able to do the research while the product is marketed. On May 19

2000, the Agency notified Ethyl Corporation of the final test program requiring emission and

health effects testing for the gasoline additive MMT, in accordance with the Alternative Tier 2

provision of the fuels and fuel additives health effects testing regulations.47 The Alternative Tier

2 health effects testing will give the general public a greater awareness of the comparative risks

associated with inhalation exposures to gasoline fuels containing MMT. The Alternative Tier 2

test requirements are within two general categories, pharmacokinetic testing of manganese

compounds and characterization of manganese emissions from vehicles utilizing fuels containing

MMT. These Alternative Tier 2 testing requirements are intended to be the first stage in a twostage Alternative Tier 2 test program. EPA intends to evaluate the results produced in the first

stage of testing, as well as any other information which may be submitted to or obtained by EPA

in the meantime, in determining the specific nature and scope of the second stage of Alternative

Tier 2 testing. Any additional Alternative Tier 2 tests proposed for fuel and additives containing

MMT in the future will be announced in a separate Federal Register notice. The docker number

for the MMT Alternative Tier 2 testing requirements is A-98-35. Section 211 allows for the

development of a Tier 3 set of tests, if necessary, to further answer questions related to these

fuels in the interest of protecting public health.

N. Mercury Compounds

Mercury exists in three forms: elemental mercury, inorganic mercury compounds

(primarily mercuric chloride), and organic mercury compounds (primarily methyl mercury). All

forms of mercury are quite toxic, and each form exhibits different health effects. Elemental

mercury is used in thermometers, barometers, and pressure-sensing devices. It is also used in

batteries, lamps, industrial processes, refining, lubrication oils, and dental amalgams. Inorganic

mercury was used in the past in laxatives, skin-lightening creams and soaps, and in latex paint. 

In 1990, EPA canceled registration for all interior paints that contained mercury. Mercury use in

exterior paint was discontinued after 1991. Methyl mercury has no industrial uses; it is formed in

the environment from the methylation of the inorganic mercurial ion.

The most recent data, for various varieties of gasoline vehicles and heavy duty diesel

vehicles, showed negligible emissions of elemental mercury and no indication that inorganic

mercury is emitted using laboratory test cycles. Recent analytical methods typically used to

measure mercury from mobile sources are not sensitive enough to measure these trace level

emissions. Thus, if data were developed using more sensitive methods, we may well have

detectable mobile source mercury emissions. Estimates of other mobile source mercury

emissions rely on an outdated database that speciates PM emissions from mobile sources.48 For
55

one category of vehicle, light duty diesel vehicles, no recently developed emissions factors were

available, so we used factors developed in the mid-1980s by South Coast Air Quality

Management District. The mobile source emissions estimate, therefore has zero emissions from

gasoline powered vehicles and heavy duty diesels, but there were emission estimates for light

duty diesels. There is no reason to believe that light duty vehicles are uniquely emitters of

mercury among all mobile sources; the more likely explanation is a measurement artifact. 

Mercury emissions from mobile sources are traditionally not speciated and are presented as total

elemental mercury emissions. Mercury compounds from mobile sources comprise less than 4

percent of the 1996 National Toxics Inventory.

A major source of exposure for elemental mercury is through inhalation in occupational

settings. Mercury has been listed as a pollutant of concern in EPA's Great Waters Program due

to its persistence in the environment, potential to bioaccumulate, and toxicity to humans and the

environment.

Human studies are inconclusive regarding the carcinogenicity of elemental mercury. The

EPA has classified elemental mercury as a Group D, not classifiable as to human carcinogenicity.

Acute (short-term) exposure to high levels of elemental mercury in humans results in

central nervous system (CNS) effects such as tremors, mood changes, and slowed sensory and

motor nerve function. High inhalation exposures can also cause kidney damage. Acute

inhalation exposure also has effects on the gastrointestinal tract and respiratory system in

humans. Chronic (long-term) inhalation exposure to elemental mercury in humans also affects

the CNS, with effects such as erethism (increased excitability), irritability, excessive shyness, and

tremors.

O. MTBE

Methyl tert-butyl ether (MTBE) is a colorless liquid that has been used in the United

States since the late-1970's as an octane-enhancing replacement for lead. Currently, MTBE’s

main use is as a fuel oxygenate as part of the Wintertime Oxygenated Fuel and Federal

reformulated gasoline (RFG) programs. MTBE emission from mobile source account for

approximately 86 percent of the total MTBE inventory in the 1996 National Toxics Inventory.

Human exposure to MTBE may occur via inhalation, dermal or by oral contact. For the

purpose of the MSATs listing in Chapter 2, only the health effects due to inhalation exposure are

considered not ingestion exposure due to drinking water contamination or dermal absorption. 

Acute (short-term) effects on humans receiving MTBE injections into the gallbladder during its

use as a medical treatment to dissolve cholesterol gallstones included nausea, vomiting, and

sleepiness have been observed; in one case renal failure was reported. No information is

available on the chronic (long-term) health effects of MTBE in humans, or on reproductive or

developmental effects. Information is also not available on the carcinogenic effects of MTBE in
49 Burleigh-Flayer.HD. Chun JS and Kintigh WJ. 1992. Methyl tertiary butyl ether: vapor inhalation

oncogenicity study in CD-1 mice. Export, PA: Bushy Run Research Center; OPTS-42098.

50 Chun JS, Burleigh-Flayer, HD and Kintigh WJ. 1992. Methyl tertiary butyl ether: vapor inhalation

oncogenicity study in Fischer 344 rats. Export, PA: Bushy Run Research Center; Report 91N0013B.

51 Belpoggi F., Soffritti M and Maltoni C. 1995. Methyl-tertiary-butyl ether (MTBE)- a gasoline additive -

causes testicular and haematopoietic cancers in rats. Toxicol Ind Health 11:119-149.

56

humans. Three animal cancer bioassys have been performed with MTBE.49,50,51 The inhalation

studies resulted in increased liver tumors in mice and increased kidney tumors in rats. EPA has

not classified MTBE with respect to potential carcinogenicity. Rodent studies have also revealed

increased liver, kidney, spleen, and adrenal weights, decreased brain weight, body weight, and

body weight gain; swollen periocular tissue; and ataxia following chronic inhalation exposure, as

well as reproductive and developmental effects.

P. Naphthalene

Naphthalene occurs as a white solid or powder that is insoluble in water. It has a strong,

mothball odor. The primary use for naphthalene is in the production of phthalic anhydride. 

Other uses include carbamate insecticides, surface active agents and resins, dye intermediates,

synthetic tanning agents, and moth repellents. Naphthalene is found in small quantities in

gasoline and diesel fuels. Naphthalene emissions have been measured in larger quantities in both

gasoline and diesel exhaust and evaporative emissions from mobile sources. Individuals may be

exposed to naphthalene through the use of mothballs. Workers may be occupationally exposed

during its manufacture and use, especially in coal-tar production, wood preserving, tanning, or

ink and dye production. Coal tar production, wood preserving, and other industries release small

amounts. Naphthalene has also been detected in tobacco smoke.

Workers occupationally exposed to vapors of naphthalene and coal tar developed

laryngeal carcinomas or neoplasms of the pylorus and cecum. Di-, tri-, and tetramethyl

naphthalene contaminants of coal tar were found to be carcinogenic when applied to the skin of

mice, but naphthalene alone was not. An increased number of lung adenomas were reported in

mice exposed by inhalation, but this was not dose-related. No carcinogenic responses were

reported in rats exposed to naphthalene in their diet and by injection. The human carcinogenic

potential of naphthalene via the oral or inhalation routes cannot be determined at this time based

on human and animal data; however, there is suggestive evidence. EPA has classified

naphthalene as a Group C, a possible human carcinogen.

Acute (short-term) exposure of humans to naphthalene by inhalation, ingestion, and

dermal contact is associated with hemolytic anemia, damage to the kidneys, and, in infants, brain

damage. Symptoms of acute exposure include headache, nausea, vomiting, diarrhea, malaise,

confusion, anemia, jaundice, convulsions, and coma. Cataracts have also been reported in

workers acutely exposed to naphthalene by inhalation and ingestion. Chronic (long-term) results
57

from rodent studies, supported by other subchronic and acute studies, identify nasal and

respiratory lesions as critical effects from chronic inhalation exposure to naphthalene.

Q. Nickel Compounds

Nickel is a natural element of the earth's crust; as a result, small amounts are found in

food, water, soil, and air. Food is the major source of nickel exposure, with an average intake for

adults estimated to be approximately 100 to 300 μg/d. Individuals also may be exposed to nickel

in occupations involved in its production, processing, and use, or through contact with everyday

items such as nickel-containing jewelry and stainless steel cooking and eating utensils, and by

smoking tobacco. Nickel is found in ambient air at very low levels as a result of releases from

oil and coal combustion, nickel metal refining, sewage sludge incineration, manufacturing

facilities, and other sources. Nickel compounds have also been identified in trace quantities in

exhaust emissions from gasoline and diesel engines. Nickel compounds from mobile sources

comprise less that nine percent of the 1996 National Toxics Inventory.

The EPA has not evaluated soluble salts of nickel as a class of compounds for potential

human carcinogenicity. Human and animal studies have reported an increased risk of lung and

nasal cancers from exposure to nickel refinery dusts and nickel subsulfide. EPA has classified

nickel refinery dust and nickel subsulfide as Group A, human carcinogens. Animal studies of

soluble nickel compounds (i.e., nickel carbonyl) have reported lung tumors. EPA has classified

nickel carbonyl as a Group B2, probable human carcinogen, and given its high instability, nickel

carbonyl exposure is extremely rare.

Nickel dermatitis, causing itching of the fingers, hands, and forearms, is the most

common effect in humans from chronic (long-term) skin contact with nickel. Respiratory effects

have also been reported in humans from inhalation exposure to nickel. No information is

available regarding the reproductive or developmental effects of nickel in humans, but animal

studies have reported reproductive and developmental effects. 

R. POM (Polycyclic Organic Matter)

Polycyclic organic matter, or POM, defines a broad class of compounds that includes the

polycyclic aromatic hydrocarbon compounds (PAHs), of which benzo[a]pyrene is a member. 

The primary source of POM is formation during combustion. A less significant formation

mechanism is the volatilization of light weight POM compounds, which occurs in the production

and use of naphthalene. Polycyclic organic compounds have been detected in ambient air from

sources including cigarette smoke, gasoline and diesel engine exhausts, asphalt road paving, coal

burning, application of coal tar, agricultural burning, residential wood burning, and hazardous

waste sites. The compounds present in POM and their relative amounts differ among different

sources (e.g, POM from diesel exhaust is chemically different than POM from wood burning). 

POM from mobile source particulate, as the sum of the seven PAHs that are probable human

carcinogens, accounts for approximately six percent of the 1996 National Toxics Inventory. 
52 EPA 1999. Environmental Protection Agency, Integrated Risk Information System (IRIS), Office of

Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH, 1999.

58

PAHs have been found in some drinking water supplies. Cooking meat or other foods at

high temperatures increases the amount of PAHs in the food. Occupational exposure to PAHs

may occur in coal tar production plants, coking plants, coal-gasification sites, smokehouses,

municipal trash incinerators, and other facilities. POM has been listed as a pollutant of concern

in EPA's Great Waters Program due to its persistence in the environment, potential to

bioaccumulate, and toxicity to humans and the environment.

Skin exposures to mixtures of carcinogenic PAHs cause skin disorders in humans and

animals. No information is available on the reproductive or developmental effects of POM in

humans, but animal studies have reported that oral exposures to benzo[a]pyrene causes

reproductive and developmental effects. Cancer is the major concern from exposure to POM.

Epidemiologic studies have reported an increase in lung cancer in humans exposed to coke oven

emissions, roofing tar emissions, and cigarette smoke; all of these mixtures contain POM

compounds. Animal studies have reported respiratory tract tumors from inhalation exposure to

benzo[a]pyrene and forestomach tumors, leukemia, and lung tumors from oral exposure to

benzo[a]pyrene. The EPA has classified seven PAHs (benzo[a]pyrene, benz[a]anthracene,

chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, dibenz[a,h]anthracene, and

indeno[1,2,3-cd]pyrene) as Group B2, probable human carcinogens.52

S. Styrene

Styrene is a colorless liquid that has a sweet smell. It is used predominately in the

production of polystyrene and resins. Styrene is also used as an intermediate in the synthesis of

materials used for ion exchange resins and to produce copolymers. Styrene is also emitted in

significant quantities in the exhaust gases of both gasoline and diesel powered engines. Styrene

from mobile sources accounts for approximately 40 percent of the 1996 National Toxics

Inventory.

 

 Indoor air is the principal route of styrene exposure for the general population, due to

building materials, consumer products, and tobacco smoke. Occupational exposure to styrene

occurs in the reinforced plastics industry and polystyrene factories.

Several epidemiologic studies suggest there may be an association between styrene

exposure and an increased risk of leukemia and lymphoma. However, the evidence is

inconclusive due to confounding factors. Animal studies have produced both negative and

positive results. The EPA is currently reviewing the potential of styrene to cause cancer, current

data is unavailable to classify styrene as a human carcinogen.

Acute (short-term) exposure to styrene results in mucous membrane and eye irritation,

and gastrointestinal effects in humans. Chronic (long-term) exposure of humans to styrene
53 EPA 1994. EPA Health Effects Notebook for Hazardous Air Pollutants-Draft, EPA-452/D-95-00,

December 1994, Office of Air Quality Planning and Standards, RTP, NC.

http://www.epa.gov/ttn/uatw/hapindex.html

59

results in effects on the central nervous system (CNS), such as headache, fatigue, weakness,

depression, peripheral neuropathy, minor effects on some kidney enzyme functions and on the

blood. Human studies are inconclusive on the reproductive and developmental effects of styrene;

several studies did not report an increase in developmental effects in women who worked in the

plastics industry, while an increased frequency of spontaneous abortions and decreased frequency

of births were reported in another study.

T. Toluene

Toluene occurs as a colorless, flammable, refractive liquid that is slightly soluble in

water. It has a sweet, pungent odor. The major use of toluene is as a mixture added to gasoline

to improve octane ratings. Toluene is also used to produce benzene and as a solvent in paints,

coatings, adhesives, inks, and cleaning agents. It is used in the production of polymers used to

make nylon, plastic soda bottles, and polyurethanes, and for pharmaceuticals, dyes, cosmetic nail

products, and the synthesis of organic chemicals. The highest concentrations of toluene usually

occur in indoor air from the use of common household products (paints, paint thinners, and

adhesives) and cigarette smoke. The deliberate inhalation of paint or glue by solvent abusers

may produce high levels of exposure to toluene, as well as to other chemicals. Toluene exposure

may also occur in the workplace, especially in occupations such as printing or painting, where

toluene is frequently used as a solvent.53

Mobile sources are the principal source of toluene to the ambient air. Toluene is found in

both gasoline and diesel fuel as well as the exhaust emissions of both types of engines. Toluene

from mobile sources accounts for approximately 74 percent of the 1996 National Toxics

Inventory. Toluene can also be released to the ambient air during the production, use, and

disposal of industrial and consumer products that contain toluene.

None of the data suggest that toluene is carcinogenic. Two epidemiological studies did

not detect a statistically significant increased risk of cancer due to inhalation exposure to toluene.

However, these studies had many confounding factors. Animal studies have been negative for

carcinogenicity. EPA has classified toluene as a Group D, not classifiable as to human

carcinogenicity.

The central nervous system (CNS) is the primary target for toluene toxicity in both

humans and animals for acute (short-term) and chronic (long-term) exposures. CNS dysfunction

(which is often reversible) and narcosis have been frequently observed in humans acutely

exposed to low or moderate levels of toluene by inhalation; symptoms include fatigue,

sleepiness, headaches, and nausea. Cardiac arrhythmia has also been reported in humans acutely

exposed to toluene. CNS depression has been reported to occur in chronic abusers exposed to
60

high levels of toluene. Symptoms include ataxia, tremors, cerebral atrophy, nystagmus

(involuntary eye movements), and impaired speech, hearing, and vision. Chronic inhalation

exposure of humans to toluene also causes irritation of the upper respiratory tract, eye irritation,

sore throats, nausea, skin conditions, dizziness, headaches, and difficulty with sleep.

Human studies have also reported developmental effects, such as CNS dysfunction,

attention deficits, and minor craniofacial and limb anomalies, in the children of pregnant women

exposed to toluene or mixed solvents by inhalation. Reproductive effects, including an

association between paternal exposure to toluene and an increased odds ratio for spontaneous

abortions but not birth defects, have also been noted. However, these studies are not conclusive

due to many confounding variables. Animal studies have shown toluene to have developmental,

but not reproductive, effects from inhalation exposure.

U. Xylene

Mixed xylenes are colorless liquids with a sweet odor. They are used in the production of

ethylbenzene, in solvents, and for paints and coatings. They are also blended into gasoline and

are also present in diesel fuel. Xylenes are emitted in the exhaust emission of both gasoline and

diesel powered engines accounting for 78 percent of the 1996 National Toxics Inventory. 

Xylenes are distributed throughout the environment; they have been detected in air, rainwater,

soils, surface water, sediments, drinking water, and aquatic organisms. Xylenes have also been

detected in indoor air; xylenes have been widely used in home use products such as paints. 

Occupational exposure to mixed xylenes may occur at workplaces where mixed xylenes are

produced and used as industrial solvents.

No information is available on the carcinogenic effects of mixed xylenes in humans, and

animal studies have reported negative results from exposure via gavage (experimentally placing

the chemical in the stomach). EPA has classified mixed xylenes as a Group D, not classifiable as

to human carcinogenicity.

Acute (short-term) inhalation exposure to mixed xylenes in humans results in irritation of

the nose and throat, gastrointestinal effects such as nausea, vomiting, and gastric irritation, mild

transient eye irritation, and neurological effects. Chronic (long-term) inhalation exposure of

humans to mixed xylenes results primarily in central nervous system (CNS) effects, such as

headache, dizziness, fatigue, tremors and uncoordination. Other effects noted include labored

breathing and impaired pulmonary function, increased heart palpitation, severe chest pain and an

abnormal EKG, and possible effects on the blood and kidney. Insufficient data are available on

the developmental or reproductive effects of mixed xylenes in humans. Animal studies have

reported developmental effects, such as an increased incidence of skeletal variations in fetuses,

and fetal resorptions via inhalation.
54 In this rulemaking the emissions inventory for diesel exhaust is looked at in terms of diesel PM, as that

is what we have measured to date. Thus, even though we are proposing to list diesel exhaust as an MSAT, all

emissions inventory and trends numbers are stated in terms of diesel PM. 

61

Chapter 4: Impacts of Motor Vehicle Emission Control

Programs on MSAT Emissions

In Chapter 2 of the TSD we identified the 21 MSATs. We now turn to an evaluation of

the impact of existing and planned controls on inventories of those air toxics by examining the

emissions inventories and estimated reductions expected to be achieved by our various mobile

source control programs. 

The data and information available on emissions of these 21 MSATs vary considerably.

While we have baseline inventory data for all of the MSATs except napthalene, we do not have

inventory projections for all of them. Therefore, we are examining the projected impacts of our

current and proposed mobile source control program by groupings of air toxics. More

specifically, we have projections of future emissions for five gaseous toxics (benzene,

formaldehyde, 1,3-butadiene, acetaldehyde, MTBE) and for diesel PM54 and we present these in

this section. However, we do not have emissions projections for the remaining gaseous toxics

(acrolein, POM, styrene, toluene, xylene, ethylbenzene, naphthalene, and n-hexane), but because

these compounds are part of VOCs, we believe it is reasonable to utilize VOC emissions

inventory projections to track the expected impact of our control programs on these other gaseous

MSATs. Finally, we also do not have emissions inventory projections for the metals on the

MSAT list (arsenic compounds, chromium compounds, mercury compounds, nickel compounds,

manganese compounds, and lead compounds) or for dioxins/furans. While metal emissions and

dioxin/furans emissions are associated with particles, and it is possible that they track PM

emissions to some extent, we do not have good data on these relationships. Therefore, we are not

presenting emission projections for these compounds in this notice.

As we describe in the following discussion, there have been and will continue to be

significant reductions in MSATs as a result of implemented, promulgated and proposed

regulations. By 2020, we project on-highway emissions of gaseous toxics such as benzene,

formaldehyde, 1,3-butadiene, and acetaldehyde, to decrease by 75 percent or more from 1990

levels as a result of our mobile source control programs up to and including our Tier 2 control

program and our recently proposed heavy-duty engine and vehicle standards and on-highway

diesel fuel sulfur control requirements (HD2007 rule). Under these current and proposed

controls we expect on-highway diesel PM emissions to be reduced by more than 90 percent by

2020, as compared with 1990 levels. 

This chapter consists of three parts. First, we describe current inventories of MSAT

emissions. Next, we describe the methodologies we used to develop our emissions inventories,

and we present our projections of how our on-highway emission control programs will reduce
55 EPA. 1999. Analysis of the Impacts of Control Programs on Motor Vehicle Toxics Emissions and

Exposure in Urban Areas and Nationwide. Prepared for U. S. EPA, Office of Transportation and Air Quality, by

Sierra Research, Inc., and Radian International Corporation/Eastern Research Group. Report No. EPA 420 –R-99-

029/030.

62

MSAT emissions in the future. Finally, we discuss limitations and uncertainties in our analyses

of MSAT emissions.

A. Baseline Inventories

In order to assess the progress we have made in controlling MSATs from our mobile

source emission control programs, we need to have a baseline from which to measure our

progress. We present this baseline information in this section. 

1. The 1999 EPA Motor Vehicle Air Toxics Study

We developed inventory estimates for several gaseous MSATs (acetaldehyde, benzene, 1-

3 butadiene, formaldehyde, MTBE) and also for diesel PM as part of the 1999 EPA Motor

Vehicle Air Toxics Study, “Analysis of the Impacts of Control Programs on Motor Vehicle Toxic

Emissions and Exposure in Urban Areas and Nationwide,”55 (hereafter referred to as the 1999

EPA Motor Vehicle Air Toxics Study, or the 1999 Study). We addressed these five MSATs and

diesel PM because we had detailed information on the emission impacts of emission control

technologies, fuel properties, and other parameters for these compounds. 

The 1999 EPA Motor Vehicle Air Toxics Study provides 1990 and 1996 estimates of

toxic emissions for these compounds. The 1990 baseline represents estimated toxics emissions

before any of the programs added by the 1990 Clean Air Act Amendment were implemented. 

The 1996 estimates reflect toxics emissions with some of the new Clean Air Act programs in

place, such as Phase 1 of the RFG program. We present emission estimates for these years in

Table IV.A-1. Note that since completion of the Study, we have updated our estimates of diesel

PM emissions; these updated estimates are presented in Table IV.A-1. It should be noted these

estimates only for on-highway vehicles. We describe how these estimates were developed in

Section IV.B, below. 
56 In this rule we report emissions in terms of short tons as opposed to metric tons. One short ton is 2,000

pounds. To convert to metric tons, multiply short tons by 0.9072. Note that all emissions and percentages in this

and subsequent tables are rounded.

57 The 1996 diesel PM estimate is based on the Tier 2 rulemaking inventories, updated to reflect the

Updated Tier 2 Emissions Inventory for light-duty diesel emissions and the proposed 2007 heavy-duty engine rule

for heavy-duty diesel emissions, and for 1990, we used estimates from EPA’s Trends Report for that year, as

described below.

58 The nonroad inventory in the 1996 NTI includes emissions data for aircraft, commercial marine vessel,

locomotives, and other nonroad engines. Note that under the Clean Air Act definition, nonroad does not include

aircraft. For convenience, in this document the term “nonroad” will generally include aircraft. It should be noted

that the NONROAD model, on which the estimates for nonroad engines are based, is still draft, and the emissions

estimates based on this model are subject to change.

63

Table IV.A-1

Annual Emission Summary for the Total U.S. for Selected Toxics 

On-Highway Vehicles Only 

(short tons56 per year)

Compound 1990 1996

1,3-butadiene 36,000 22,000

Acetaldehyde 41,000 27,000

Benzene 257,000 165,000

Formaldehyde 139,000 80,000

Diesel PM57 235,000 180,000

MTBE 55,000 65,000

2. The 1996 National Toxics Inventory

The 1996 National Toxics Inventory (NTI) prepared in connection with the Agency’s

National Air Toxic Assessment (NATA) activities, also contains emission estimates for 1,3-

butadiene, acetaldehyde, benzene, formaldehyde and MTBE. The 1996 NTI emission estimates

for these compounds differ slightly from those generated in the 1999 Study, due largely to

revisions made to the NTI based on state comments. Since diesel exhaust emissions are not

included on the list of 112(b) hazardous pollutants that is the focus of the 1996 NTI, diesel PM

estimates have not been compiled there. 

The 1996 NTI also contains 1996 emissions estimates for several other MSATs, and

includes data for nonroad58 as well as on-highway sources. We present these data in Table IV.A-
59 Naphthalene emissions are not reported in the 1996 NTI separately from 16-PAH. 

60Mass given in tons of TEQ (toxic equivalency quotient). The EPA Office of Research and Development

(ORD) has recently developed an inventory for dioxin and dioxin-like compounds using different methods than

those used in the NTI. For 1995, the EPA-ORD estimate of on-highway emissions of dioxin compounds is 0.00005

tons TEQ, comprising 1.5 percent of the national inventory in that year.

64

2. We also indicate the on-highway and nonroad percentages of the national inventories for these

MSATs, where the total national inventories are the sum of the emission from on-highway and

nonroad mobile sources, major and area stationary sources, and other sources such as forest

fires). Between the 1999 EPA Motor Vehicle Air Toxics Study and the 1996 NTI we have

inventory data for all of the 21 MSATs except naphthalene.59

Table IV.A-2

1996 On-Highway and Nonroad Emission Inventories of Proposed MSATs

1996 NTI (short tons) 

Compound

On-Highway Nonroad Mobile Sources

Tons

Percent of

Total

National

Emissions Tons

Percent of

Total

National

Emissions Tons

Percent of

Total

National

Emissions

1,3-Butadiene* 23,500 42% 9,900 18% 33,400 60%

Acetaldehyde* 28,700 29% 40,800 41% 69,500 70%

Acrolein* 5,000 16% 7,400 23% 12,400 39%

Arsenic Compounds* 0.25 0.06% 2.01 0.51% 2.26 0.57%

Benzene* 168,200 48% 98,700 28% 266,900 76%

Chromium Compounds* 14 1.2% 35 3% 49 4.2%

Dioxins/Furans*60 0.0001 0.2% N.A. N.A. 0.0001 0.2%

Ethylbenzene 80,800 47% 62,200 37% 143,000 84%

Formaldehyde* 83,000 24% 86,400 25% 169,400 49%

Lead Compounds* 19 0.8% 546 21.8% 565 22.6%

Manganese Compounds* 5.8 0.2% 35.5 1.3% 41.3 1.5%

Mercury Compounds* 0.2 0.1% 6.6 4.1% 6.8 4.2%

MTBE 65,100 47% 53,900 39% 119,000 86%

n-Hexane 63,300 26% 43,600 18% 106,600 44%
61 This list can be found in the National Air Toxics Programs: The Integrated Urban Strategy; Notice. 

July 19, 1999, 64 Federal Register 38706-38740.

62 Aviation gasoline is used by a relatively small number of aircraft, those with piston engines, which are

generally used for personal transportation, sightseeing, crop dusting, and similar activities. 

65

Napthalene N.A. N.A. N.A. N.A. N.A. N.A.

Nickel Compounds* 10.7 0.9% 92.8 7.6% 103.5 8.5%

POM (as sum of 7 PAH)* 42.0 4% 19.3 2% 61.3 6%

Styrene 16,300 33% 3,500 7% 19,800 40%

Toluene 549,900 51% 252,200 23% 802,100 74%

Xylene 311,000 43% 258,400 36% 569,400 79%

* Indicates also on the list of urban HAPs for the Integrated Urban Air Toxics Strategy (64 Federal Register 38706,

July 19,1999). 

The above inventory data reflect certain interesting characteristics of mobile source air

toxics emissions. First, mobile sources account for the majority of the national inventory of three 

of the gaseous MSATs that are included on the urban HAP list.61 These three are 1,3-butadiene

(60 percent), acetaldehyde (70 percent), and benzene (76 percent). Mobile sources account for 

39 percent of the national inventory of acrolein and 49 percent of the national inventory of

formaldehyde, two other gaseous urban HAPs. All of these MSATs are formed as part of the

combustion process. In addition, benzene is also released through evaporative emissions from

gasoline.

Second, with regard to the other MSATs that are included on the urban HAP list, the

mobile source contribution generally is small (arsenic compounds, chromium compounds,

manganese compounds, nickel compounds, POM, and dioxins/furans). The sole exception is

lead compounds. Mobile sources contribute 23 percent of the national inventories of lead

compound emissions due primarily to nonroad sources and, more specifically, to the use of a

lead-additive package used to boost the octane of aviation gasoline.62 The mobile source

contribution to the other metals on the urban HAP list comes primarily from engine wear, from

some fuel additives, or from impurities in engine oil.

With regard to the gaseous MSATs that are not included on the urban HAP list

(ethylbenzene, MTBE, n–hexane, styrene, toluene, and xylene), mobile source contributions are

high because of the presence of these compounds in gasoline. 

In addition, mobile sources account for almost all of diesel PM emissions. As shown in

Table IV.A-1, above, we estimate that 1996 on-highway diesel PM emissions are approximately

180,000 tons. We estimate that 1996 nonroad diesel PM emissions are approximately 346,000

tons, as discussed in Chapter 8 of this Technical Support Document.
66

B. Impacts of Motor Vehicle Emission Controls on Emission Inventories

Many of the programs that we have put in place since the passage of the 1990 Clean Air

Act Amendments to achieve attainment of the National Ambient Air Quality Standards

(NAAQS) for ozone, PM and CO have also reduced MSAT emissions. For example, measures

to control hydrocarbons from motor vehicles are also effective in controlling gaseous toxics. In

addition, certain programs address air toxics directly, such as the RFG program and the gasoline

lead-phase out. We describe some of our key mobile source control programs in Chapter 1 of

this Technical Support Document. 

This section summarizes our projections of the impacts of our control programs on

emissions of MSATs in future years, and describes how we derived these projections. To provide

a framework for understanding our results, we first present an overview of our various inventory

methodologies. We then present the emissions projection estimates for the five gaseous toxics

addressed in the 1999 EPA Motor Vehicle Air Toxics Study. Next, we discuss our projected

VOC emission trends as a surrogate to reflect the emission trends of other gaseous toxics for

which we do not have specific inventory projections, including acrolein, POM, styrene, xylene,

toluene, ethylbenzene, naphthalene, and n-hexane. We conclude by discussing the trend for

diesel PM emissions.

We are not reporting inventory trends for the metals on our list of MSATs (arsenic

compounds, chromium compounds, mercury compounds, nickel compounds, manganese 

compounds, and lead compounds) or for dioxins/furans. Metals in mobile source exhaust can

come from fuel, fuel additives, engine oil, engine oil additives, or engine wear. Formation of

dioxin and furans requires a source of chlorine. Thus, while metal emissions and dioxin/furan

emissions are associated with particles, there are a number of other factors that contribute to

emission levels. While it is possible that these compounds track PM emissions to some extent,

we do not have good data on these relationships.

1. Overview of Inventory Methodologies

We analyzed emissions trends for gaseous air toxics addressed in the 1999 EPA Motor

Vehicle Air Toxics Study (benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and MTBE), for

VOC as a surrogate for the emissions trends for other gaseous air toxics, and for diesel PM. We

estimated emissions for each of these classes of air toxics for four separate years (1990, 1996,

2007, and 2020). The methods used to estimate these emissions are summarized below and

described in more detail in the following pages.

In the 1999 Study, we produced inventory estimates for various years and control

scenarios that account for the effects of fuel and vehicle technology changes for five gaseous

toxics. We used these inventories directly to estimate emissions for all four years of interest for

the Pre-Tier 2 and Tier 2 control scenarios (described below). To calculate emission inventories

for these toxics under the “Heavy-Duty 2007 Control Scenario” (described below), we relied on
63This spreadsheet model can be found in EPA Air Docket A-99-06, Item II-B-31.

64Details of this approach can be found in a memorandum by Harvey Michaels to Docket A-2000-12 titled

“Adjustment to the Tier 2 Air Quality Inventory for the Mobile Source Air Toxics Proposed Rule”.

65The analysis methodology is described in a memorandum from Meredith Weatherby, Eastern Research

Group, to Rich Cook, EPA, entitled “Estimating of 1990 VOC and TOG Emissions” in EPA Air Docket A-2000-

12. 

67

the 1999 Study and on data from a spreadsheet model developed in support of the proposed 2007

heavy-duty engine rule.63 For our final toxics rulemaking, we expect to use the emissions

inventories developed for the final 2007 heavy-duty engine rule to more accurately estimate this

rule’s effect on emissions of the five gaseous air toxics addressed in the 1999 Study. These

inventories will reflect our most recent information on emissions factors and local emissions

modeling inputs.

We did not evaluate other gaseous air toxics in our 1999 Study. However, since all of

these compounds are VOCs, we expect their emissions trends to follow the VOC emissions

trend. For 1996 and later years, we based our VOC inventories on the emissions inventories

constructed for the Tier 2 rulemaking, updated to reflect more recent information. More

specifically, we updated the Tier 2 inventories to account for changes reflected in our Updated

Tier 2 Emissions Inventory spreadsheet.64 We replaced the heavy-duty VOC inventories with

inventory estimates from the national emissions inventory spreadsheet developed and used for

our proposed 2007 heavy-duty engine rule. These heavy-duty engine emission estimates are

based on national average fleet mixes, temperatures, VMT distributions by roadway type, and

speed by roadway type. For our final toxics rulemaking, we expect to use the VOC emissions

inventories developed for the final 2007 heavy-duty engine rule, which will incorporate our most

recent information on emissions factors and local emissions modeling inputs.

For 1990, we modified the modeling methods applied in the 1999 Study to produce direct

estimates of VOC emissions that account for the effects of fuel and vehicle technology changes

as well as the information that will be used in EPA’s MOBILE6 emissions model.65 We could

not use the Tier 2 rulemaking inventories, since they do not extend to 1990. Our other alternative

was to use the 1990 VOC estimates from EPA’s Trends Report. However, such estimates would

not be comparable to the 1996 and later estimates from the Tier 2 rule, since EPA’s 1990 Trends

estimates have not been updated to reflect the data and analyses that will be used in MOBILE6. 

Diesel PM emissions for 1996 and later years were based on several sources. Light-duty

diesel PM emissions were taken from the inventory used for the Tier 2 air quality analyses,

updated to reflect the Updated Tier 2 Emissions Inventory for light-duty diesel emissions. To

estimate heavy-duty diesel PM emissions, we used the national emissions inventory spreadsheet

developed and used for our proposed 2007 heavy-duty engine rule; this spreadsheet incorporates

recent findings on heavy-duty diesel engine PM emissions that were not reflected in the 1999 Air

Toxics Study, accounts for the increasing prevalence of diesel engines in certain engine classes,
66 EPA. 2000. National Air Pollution Emission Trends, 1900-1998 (March 2000). Office of Air Quality

Planning and Standards, Research Triangle Park, NC. Report No. 454/R-00-002. 

67 EPA. 1993. Motor Vehicle-Related Air Toxics Study. Report No. EPA 420-R-93-005. 

68 Peer review comments on the 1993 study can be accessed at http://www.epa.gov/otaq/toxics.htm

68

and is capable of modeling the effects of the proposed 2007 heavy-duty engine rule.

The spreadsheet model is not capable of producing heavy-duty diesel PM estimates for

1990, so we chose to use heavy-duty diesel inventory estimates from EPA’s Trends Report66 for

that year. We believe this approach is reasonable, since we have not substantially changed our

estimates of emissions from 1990 and earlier engines since the Trends Report estimates were

developed. We also used data from the EPA’s Trends Report for light-duty diesel estimates for

1990. The 1990 diesel PM inventories developed for this proposal and for the 1999 Study are

roughly comparable; the former is 235,000 tons while the latter is 202,000 tons. The 1999 Study

results do not explicitly account for county-specific inputs, unlike Trends and the Tier 2

inventory; furthermore, we consider it likely that the 1999 Study underestimates nationwide

diesel PM emissions due to the way it extrapolates urban emissions to broader regions. 

For the final toxics rule, we expect to revise our diesel PM inventory based on the

analyses being conducted for the final 2007 heavy-duty engine rule. These analyses will include

our most current information on light- and heavy-duty diesel engine emissions factors and VMT,

and will also reflect county-by-county information on VMT distribution by vehicle class,

roadway type, and speed. 

2. 1999 EPA Motor Vehicle Air Toxics Study

Section 202(l)(1) of the Clean Air Act calls on EPA to study the need for and feasibility

of controlling toxic air pollutants associated with motor vehicles and motor vehicle fuels. We

completed the study required under Section 202(l)(1) in April 1993. The report, entitled “Motor

Vehicle-Related Air Toxics Study,” is available on our website

(http://www.epa.gov/otaq/toxics.htm).67 Specific pollutants or pollutant categories which are

discussed in this report include benzene, formaldehyde, 1,3-butadiene, acetaldehyde, diesel

particulate, gasoline particulate, gasoline vapors as well as selected metals. The study focuses on

carcinogenic risk although discussions of non-cancer effects for these and other pollutants are

also included. The study provided estimates of emissions, exposure, and risk, with projections to

the year 2010. Peer review comments on this study were received in 1994.68 Peer review

comments suggested improvements to EPA’s exposure modeling and risk assessment

methodology.

In response to these comments, EPA updated its exposure model for motor vehiclerelated air toxics. Also, since 1993, significant new information on vehicle emission rates has
69 EPA. 1999. Analysis of the Impacts of Control Programs on Motor Vehicle Toxics Emissions and

Exposure in Urban Areas and Nationwide. Prepared for U. S. EPA, Office of Transportation and Air Quality, by

Sierra Research, Inc., and Radian International Corporation/Eastern Research Group. Report No. EPA 420 –R-99-

029/030.

69

been developed, and much more is known about the impact of fuel properties on toxic emissions. 

Moreover, EPA has updated its cancer risk assessment for benzene, and has released draft risk

assessments for 1,3-butadiene and diesel exhaust emissions. Furthermore, EPA has developed

new programs, such as the NLEV and Tier 2 standards, which significantly impact projections of

toxic emissions, exposure, and risk. 

In light of new information that was developed after 1993, and in response to peer review

comments, EPA has updated estimates of emissions and exposure. The updated final emissions

and exposure assessment, “Analysis of the Impacts of Control Programs on Motor Vehicle

Toxics Emissions and Exposure in Urban Areas and Nationwide,” (the 1999 EPA Motor Vehicle

Air Toxics Study) was released in November, 1999.69

The remainder of this subsection provides additional information on how we developed

the 1999 Study, and presents emissions inventory results from that study under three control

scenarios. While we addressed diesel PM emissions in the 1999 Study, we discuss the diesel PM

emissions inventory projections in a later section, to reflect recent updates to the inventory based

on a spreadsheet model developed in support of the 2007 heavy-duty engine proposed rule.

a. Methodology for Estimating Gaseous Mobile Source Air Toxic Emission

Inventories

In the 1999 Study we estimated emissions of benzene, formaldehyde, acetaldehyde, and

1,3-butadiene using a toxic emission factor model, MOBTOX5b. This model is based on a

modified version of MOBILE5b, which estimates emissions of regulated pollutants, and

essentially applies toxic fractions to total organic gas (TOG) estimates. The TOG basic emission

rates used in this modeling incorporated available elements from MOBILE6 used to develop the

VOC inventory for the Tier 2 final rule. The modeling does not incorporate impacts of

evaporative emission standards in the Tier 2 rule. The model accounted for differences in toxic

fractions between technology groups, driving cycles, and normal versus high emitting vehicles

and engines (“high emitters”). Impacts of fuel formulations were also addressed in the modeling. 

We modeled toxic emissions for 10 urban areas and 16 geographic regions. These urban

areas and geographic regions are listed in Table IV.B-1. They were selected to encompass a

broad range of I/M programs, fuel parameters, and temperature regimes. The intent of the

selection process was to best characterize the different combinations needed to perform accurate

nationwide toxic emissions estimates. Every U. S. county in the country was then “mapped” to

one of these modeled areas or regions (i.e., the emission factor for the modeled area was also

used for the area “mapped” to it). Mapping was done based on a combination of geographic
70 Note that 1996 NTI estimates for the Northeast States were developed using VMT data supplied by

those states, rather than estimates in the Emission Trends Database.

71 Costs for Meeting 40 ppm Sulfur Content Standard for Gasoline in PADDs 1-3, via MOBILE and CD

TECH Desulfurization Processes. A Study performed for the American Petroleum Institute by Mathpro, Inc.,

February 26, 1999.

70

proximity, I/M program, and fuel control programs. Details of this process are provided in the

1999 Study. We then multiplied the resulting county level emission factors by county-level

VMT estimates from EPA’s Emission Trends Database and summed the results across all

counties to come up with nationwide emissions in tons. This approach was also used to develop

the inventory estimates in the 1996 NTI.70

Table IV.B-1

Metropolitan Areas and Regions Included in Toxic Emissions Modeling

Chicago, IL Atlanta, GA Florida

Denver, CO Western WA/ OR Northeast States – non-I/M and

non-RFG

Houston, TX Northern CA Northeast States - I/M and

non-RFG

Minneapolis, MN Southern CA Northeast States - non-I/M and

RFG

New York, NY ID/ MT/ WY Ohio Valley – non-I/M and

non-RFG

Philadelphia, PA UT/ NM/NV Ohio Valley – I/M and nonRFG

Phoenix, AZ West TX Ohio Valley – I/M and RFG

Spokane, WA ND/ SD/ NB/ IA/ KS/

Western MO

Northern MI/ WI

St. Louis, MO AR/ MS/ AL/ SC/ Northern

LA

Modeling for these areas was done on a seasonal basis. Information on fuel properties for

1990 and 1996 was obtained from surveys conducted by the National Institute for Petroleum and

Energy Research (NIPER) and the American Automobile Manufacturers Association (AAMA). 

Fuel parameters for 2007 and 2020 were projected from 1996 baseline values using information

from a February 26, 1999 report from Mathpro to the American Petroleum Institute.71 Data from
72 EPA. 1994. Regulatory Impact Analysis for the Final Rule on Reformulated and Conventional

Gasoline, February, 1994.

71

the EPA Emission Trends Database and other agency sources were used to develop appropriate

local modeling parameters for I/M programs, Stage II refueling controls, fuel RVP, average

ambient temperature, and other inputs. 

Exhaust Emissions

Analysis of speciation data from 1990 technology light-duty gasoline vehicles done for

the EPA Complex Model for Reformulated Gasoline showed that the fraction of toxic emissions

relative to TOG differs among the eight technology groups within the Complex Model as well as

between normal emitters and high emitters.72 This difference is especially significant for 1,3-

butadiene; its toxic/TOG fraction is about three times larger for high emitters than for normal

emitters. If this difference is not taken into account, the impact of I/M programs and fleet

turnover to vehicles with lower deterioration rates will be underestimated. Thus, the input format

for exhaust toxic adjustment factors in MOBTOX5b was structured to allow input of high and

normal emitter toxic emission rates for a given “target” fuel. The target fuel is simply the fuel of

concern in the modeling analysis. These toxic emission rates were then weighted to come up with

a composite toxic emission factor based on a distribution of normal and high emitters. This

distribution is not supplied directly by the MOBILE model. Instead, this distribution was

determined from the fleet average TOG emission rate on baseline fuel as determined by MOBILE

and average normal and high TOG emission rates on baseline fuel derived from the Complex

Model. Essentially, “toxic-TOG curves” were developed that plot the target fuel toxic emission

rate against the base fuel TOG emission rate.

To construct these curves, the distribution of normal and high emitters was determined in

the following manner for each model year. A TOG gram per mile emission rate for normal

emitters (TOG-N) and a TOG emission rate for high emitters (TOG-H) on baseline fuel were

input into MOBTOX5b. TOG-N from newer technology light-duty gasoline vehicles and trucks

were obtained from an unconsolidated version of the Complex Model, which provides output for

normal emitters in each of eight technology groups. The Complex Model provides estimates for

mass of exhaust VOC, which is TOG minus the mass of methane and ethane. TOG was

estimated by applying a conversion factor which accounts for the mass of these compounds. The

conversion factor was derived by analysis of weight percent emissions of methane and ethane

from available speciation data. Based on the distribution of technology groups in a given model

year, the individual TOG estimates were weighted appropriately to obtain a composite estimate

for all normal emitters. Since the unconsolidated model’s TOG-N emission rates are applicable

only to Tier 0 light duty vehicles, they had to be adjusted for Tier 1 and later vehicles. This

adjustment was performed by multiplying the unconsolidated model results by the ratio of the

emission standard for these later vehicles to the Tier 0 emission standard. TOG-H was also

obtained from the unconsolidated version of the Complex Model. TOG-H was assumed to be the

same for all Tier 0 and later high emitting vehicles.
72

For benzene, 1,3-butadiene, formaldehyde, and acetaldehyde, milligram per mile toxic

emission rates for normal and high emitters running on a given fuel formulation were also

entered into MOBTOX5b using output from the unconsolidated version of the Complex Model.

An example of the data file format is provided in Table IV.B-2. 

Table IV.B-2

Example of Data File Format for Toxic Adjustment Factors

IV MYA MYB TOG-N TOG-H BZ-N BZ-H AC-N AC-H FR-N FR-H BD-N BD-H

1 1965 1974 0.000 10.00 0.00 276.93 0.00 109.72 0.00 224.28 0.00 93.15

1 1975 1980 0.000 10.00 0.00 263.61 0.00 108.70 0.00 173.41 0.00 44.57

1 1981 1987 0.640 4.03 28.63 113.23 5.07 32.89 7.16 44.59 2.14 25.84

1 1988 1999 0.570 4.03 17.49 116.45 4.02 28.65 5.67 36.68 2.04 30.82

Notes: IV = vehicle class, MYA = initial model year, MYB = final model year, TOG-N = TOG for normal emitters

running on baseline fuel in g/mi, TOG-H = TOG for high emitters on baseline fuel in g/mi, BZ = benzene in mg/mi

for vehicles running on fuel A, AC = acetaldehyde in mg/mi on fuel A, FR = formaldehyde in mg/mi on fuel A, BD

= 1,3-butadiene in mg/mi on fuel A.

Using the information in the data file, an overall FTP (Federal test procedure) toxic

emission rate for each vehicle class in a given model year is calculated. This overall rate takes

into account the distribution of normal and high emitters by calculating the slope and intercept of

a straight line (the “toxic-TOG” curve), where the FTP toxic emission rates for a vehicle class in

a given model year are a linear function of the baseline fuel TOG emission rate:

TOXFlt, Fuel A, FTP = A + B*TOGBaseline fuel, FTP (1)

A and B are determined as follows:

A = (TOG-H*TOX-N - TOG-N*TOX-H)/(TOG-H - TOG-N) (2)

B = (TOX-H - TOX-N)/(TOG-H - TOG-N) (3)

where:

TOX-N = toxic emission rate for normal emitters derived from the Complex Model

TOX-H = toxic emission rate for high emitters derived from the Complex Model

TOG-N = total organic gas emission rate for normal emitters derived from the Complex

Model

TOX-H = total organic gas emission rate for high emitters derived from the Complex

Model

These relationships can be thought of graphically, as illustrated in Figure IV.B-1, below.
73

 Figure IV.B-1

Example Plot of Target Fuel Benzene Versus 

Baseline Fuel TOG under FTP Conditions

Hypothetical Benzene-TOG Curve

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

Baseline Fuel TOG (g/mi)

Target Fuel Benzene (mg/mi)

TOG

Normal

TOG

High

An issue related to the above methodology is whether the linear assumption is valid for baseline

TOG values above the high emitter point and below the normal emitter point. This is particularly

relevant in cases where A and B values are determined from Tier 0 vehicles (e.g., the Complex

model), but the results are applied to Tier 1 and LEV-category vehicles. For the simple example

presented above, negative benzene emissions are estimated for the target fuel when the baseline

fleet-average TOG emission rate falls below 0.295 g/mi. Thus, for fleet-average emission rates

below (and above) the normal (and high) emitter values, a different methodology was needed. In

those cases, it was assumed that the toxic emission rate was the same on a fractional basis (for

VOC emission rates below the Tier 0 normal emitter rate, for example, the toxic fraction stays

constant at the toxic fraction for Tier 0 normal emitters). In the example above, the benzene

emission rate for a baseline TOG value of 0.1 g/mi would be calculated as follows:

BZ(TOG=0.1 g/mi) = 0.1 g/mi * (16 mg/mi BZ / 0.5 g/mi TOG) = 3.2 mg/mi

This has the effect of forcing the toxic-TOG curve from the normal-emitter point back through

the origin and thus avoids negative toxic emission rate estimates for Tier 1 and LEV-category

vehicles. The same approach is used in cases where the fleet-average baseline TOG emission

rate is above the high emitter point. 

For non-light-duty vehicle classes and older technology light-duty vehicles, such as non-
73 These studies include: Auto/Oil Air Quality Improvement Research Program. Technical Bulletin No.

19: Dynomometer Study of Off-Cycle Exhaust Emissions; April, 1996; Black, F.; Tejada, S.; Gurevich, M. 

“Alternative Fuel Motor Vehicle Tailpipe and Evaporative Emissions Composition and Ozone Potential”, J. Air &

Waste Manage. Assoc. 1998, 48, 578-591; and CARB, 1998, Unpublished data.

74 1,3-Butadiene, formaldehyde, and acetaldehyde are not found in fuel and hence are not found in

nonexhaust emissions. Because their nonexhaust emissions are zero, they were not included in the portions of

MOBTOX5b used to estimate nonexhaust emissions.

74

catalyst and oxidation catalyst vehicles, adequate toxic emissions data were not available to

distinguish between emission rates of normal and high emitters. In such cases, the toxic fraction

was assumed to be constant regardless of the VOC emission level.

Next, aggressive driving corrections were applied to the FTP toxic emission rates for light

duty vehicles. These corrections were provided in an external data file and were multiplicative in

form. Several recent studies suggest that toxic fractions of TOG differ between FTP and

aggressive driving conditions.73 Thus, another adjustment to the toxic emission rates was applied

to take into account this difference in toxic fractions. This adjustment took the form of the ratio

of the toxic mass fraction over the unified cycle (FTP and off-cycle) to the toxic mass fraction

over the FTP. The adjustment was obtained from an analysis of unpublished CARB data as

described in EPA (1999d). The toxic emission rate under the unified cycle (FTP and off-cycle)

was calculated in the model as follows:

TOXUC = TOXFTP * ADJAggressive Driving * ADJTOX UC/FTP (4)

where

TOXUC = Unified Cycle toxic emission rate

TOXFTP = FTP toxic emission rate

ADJAggressive Driving = Adjustment to TOG emissions for aggressive driving

ADJTOX UC/FTP = Adjustment for difference in toxic mass fraction over the UC versus FTP

 MOBTOX5b then applies temperature, speed, humidity and load corrections.

Evaporative, Refueling, Running Loss, and Resting Loss Emissions

MOBTOX5b estimated evaporative, refueling, running loss, and resting loss toxic

emissions for benzene.74 Benzene fractions of total hydrocarbons were entered in an external

data file. Separate fractions were entered for hot soak, diurnal, refueling, running loss, and

resting loss. Toxic fractions for evaporative, refueling and running loss emissions of benzene

from gasoline vehicles were obtained from the Complex Model (EPA 1994). The Complex

Model does not estimate resting loss emissions. EPA assumed that the benzene fractions of

diurnal and resting loss emissions were the same.

Calculating Gaseous Toxic Emissions Under the Heavy-Duty 2007 Control Scenario
75 In this analysis, we adjusted the heavy-duty gasoline vehicles (HDGVs) inventory from the spreadsheet

model to reflect a 50-state inventory based on California’s share of total VMT; in our analysis, the ratio of 50-state

VMT to 49-state VMT (excluding California) was 1.133. In addition, we modeled NMHC emissions from HDGVs

in California at the same level as NMHC emissions from such vehicles in the rest of the U.S. California has its own

standards that apply to HDGVs, but available certification data on Federally-certified HDGVs suggest that these

vehicles would also meet the California standards. As a result, we did not attempt to generate unique emission

factor estimates for California-certified HDGVs.

76 The data in this scenario are the “baseline scenario” data from the 1999 Study.

75

We expect the proposed 2007 heavy-duty engine rule to further reduce gaseous toxics

emissions below the levels that were projected for the Tier 2 scenario. To estimate toxics

inventories under the Heavy-Duty 2007 control scenario (described below), we used the lightduty toxic emissions estimates for the Tier 2 scenario from the 1999 Study, and added to these

the heavy-duty toxic emissions estimates from the 1999 Study (for the Tier 2 control scenario)

adjusted by the ratio of non-methane hydrocarbons (NMHC) emissions under the “control” case

compared to NMHC emissions under the “base” case from the 2007 Heavy-Duty Inventory

Spreadsheet developed for the proposed heavy-duty 2007 rule.75 Because benzene and MTBE

have an exhaust and an evaporative component, we used the ratio of total NMHC for these

compounds; for formaldehyde, acetaldehyde, and 1,3-butadiene, we used the ratio of exhaust

NMHC.

We used this combination of sources because the 1999 Study accounts for local

conditions, but does not reflect the emissions impacts of the proposed 2007 heavy-duty engine

rule, while the HD2007 rule spreadsheet accounts for the effects of the rule, but it does not

account for local conditions. So we used the spreadsheet to calculate the percentage reduction

due to the rule, and then applied that percentage to the 1999 Study inventory, thereby accounting

for the effects of both the rule and local conditions.

b. Projected Emissions Inventories of Selected Gaseous Toxics

In this section we present the emission estimates for the five gaseous MSATs addressed

in 1999 EPA Motor Vehicle Air Toxics Study under three control scenarios (benzene,

acetaldehyde, formaldehyde, 1,3 butadiene, and MTBE). 

The first control scenario is the “Pre-Tier 2" control scenario, which reflects fuels and

emission rates assuming all on-highway emission control programs through EPA’s national lowemission vehicle (NLEV) program, the reformulated gasoline (RFG) program, and the 2004

heavy-duty diesel engine standards. This scenario does not include implementation of the Tier

2/Sulfur control program.76 The second control scenario is the “Tier 2" scenario, which includes

all the controls in the first scenario, with the addition of Tier 2 controls. The third control

scenario is the “Heavy-Duty 2007" scenario, which reflects all the controls in the second scenario

with the addition of our recently proposed 2007 heavy-duty engine controls. 
76

Pre-Tier 2 Control Scenario

Tables IV.B-3 and IV.B-4 present on-highway emission estimates from the 1999 EPA

Motor Vehicle Air Toxics Study under the Pre-Tier 2 control scenario. The results of this

analysis show that on-highway emissions of the five gaseous MSATs examined are expected to

decline by as much as 70 percent by 2020 under this scenario, as compared with 1990 levels. 

Most of this emission decrease is expected to occur between 1990 and 2007. Between 2007 and

2020, more moderate decreases are observed as VMT growth starts to overtake the fleet-average

emission reductions achieved through fleet turnover.

Table IV.B-3 

Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only

Pre-Tier 2 Control Scenario*

(thousand short tons per year)

Compound 1990 1996 2007 2020

Benzene 257 165 95 86

Acetaldehyde 41 27 15 15

Formaldehyde 139 80 37 37

1,3-Butadiene 36 22 13 13

MTBE 55 65 24 18

* Note: The Pre-Tier 2 control scenario reflects fuels and emission rates assuming all on-highway emission control

programs through EPA’s national low-emission vehicle (NLEV) program, reformulated gasoline (RFG) program,

and the 2004 heavy-duty diesel engine standards.
77 These tables reflect control scenario #7 from the 1999 Study, which reflects Tier 2 controls and also

heavy-duty gasoline vehicle controls proposed for 2004.

77

Table IV.B-4

 Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only

Pre-Tier 2 Control Scenario*

Cumulative Percent Reduction from 1990

Compound 1996 2007 2020

Benzene 36% 63% 67%

Acetaldehyde 33% 62% 63%

Formaldehyde 42% 73% 73%

1,3-Butadiene 39% 65% 65%

MTBE -18% 57% 67%

* Note: The Pre-Tier 2 control scenario reflects fuels and emission rates assuming all on-highway emission control

programs through EPA’s national low-emission vehicle (NLEV) program, reformulated gasoline (RFG) program,

and the 2004 heavy-duty diesel engine standards.

Tier 2 Control Scenario

Tables IV.B-5 and IV.B-6 present the on-highway inventories of these five gaseous toxics

under the Tier 2 control scenario, which assumes all the controls from the Pre-Tier 2 control

scenario, plus Tier 2 controls.77 While the Tier 2 rule is primarily designed for ozone and PM

control, it will also result in important toxics reductions.
78

Table IV.B-5

Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only

Tier 2 Control Scenario

(thousand short tons per year)

Compound 1990 1996 2007 2020

Benzene 257 165 86 67

Acetaldehyde 41 27 15 13 

Formaldehyde 139 80 36 33

1,3-Butadiene 36 22 11 10

MTBE 55 65 25 18

Table IV.B-6

 Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only

Tier 2 Control Scenario

Cumulative Percent Reduction from 1990

Compound 1996 2007 2020

Benzene 36% 67% 74%

Acetaldehyde 33% 64% 68%

Formaldehyde 42% 74% 76%

1,3-Butadiene 39% 69% 72%

MTBE -18% 54% 67%

Heavy-Duty 2007 Control Scenario

Tables IV.B-7 and IV.B-8 illustrate the on-highway emission estimates we project under

the Heavy-Duty 2007 control scenario, which assumes all the controls from the Tier 2 control

scenarios, plus the recently proposed heavy-duty 2007 controls.
79

 Table IV.B-7

Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only

Heavy-Duty 2007 Control Scenario

(thousand short tons per year)

Compound 1990 Emissions 1996 Emissions 2007 Emissions 2020 Emissions

Benzene 257 165 86 64

Acetaldehyde 41 27 14 7

Formaldehyde 139 80 35 17

1,3-Butadiene 36 22 11 9

MTBE 55 65 25 18

Table IV.B-8

 Annual Emissions Summary for Selected Toxics for the Total U.S.

On-Highway Vehicles Only

Heavy-Duty 2007 Control Scenario

Cumulative Percent Reduction from 1990

Compound 1996 2007 2020

Benzene 36% 67% 75%

Acetaldehyde 33% 65% 82%

Formaldehyde 42% 75% 87%

1,3-Butadiene 39% 69% 75%

MTBE -18% 54% 67%

3. VOC Emissions Inventory

With the exception of the five gaseous MSATs examined in the 1999 EPA Motor Vehicle

Air Toxics Study, we do not have detailed emissions data for the other gaseous MSATs (acrolein,

POM, styrene, xylene, toluene, ethylbenzene, naphthalene, and n-hexane). In this section, we

present the VOC emissions trend as a surrogate to understand the trend in emissions of the other

gaseous MSATs in order to estimate projected inventory impacts from our current mobile source

emission control programs. First, we describe how we developed our VOC inventory estimates,
78 This spreadsheet model can be found in EPA Air Docket A-99-06, Item II-B-31. In this analysis, we

adjusted the heavy-duty gasoline vehicles (HDGVs) inventory from the spreadsheet model to reflect a 50-state

inventory based on California’s share of total VMT; in our analysis, the ratio of 50-state VMT to 49-state VMT

(excluding California) was 1.133.

80

and then we present the results from these analyses.

a. VOC Inventory Methodology

As described in the methodology overview section, other gaseous air toxics were not

evaluated explicitly in our 1999 Study. However, since all of these compounds are VOCs, we

expect their emissions trends to follow the VOC emissions trend. For 1996 and later years, we

based our VOC inventories on the emissions inventories constructed for the Tier 2 rulemaking,

updated to reflect more recent information. Light-duty VOC emissions were calculated for the

Tier 2/Sulfur Air Quality Inventory by applying adjustment factors to county MOBILE5 runs. 

These adjustment factors were developed for all combinations of I/M (I/M and no I/M), fuel

types (RFG, conventional gasoline, geographic phase-in area), and vehicle type. This procedure

uses MOBILE5 to account for local conditions and the adjustment factors to incorporate

improved emission factors. The Tier 2/Sulfur Air Quality Inventory was generated for 1996,

2007, and 2030. 

We subsequently further improved our understanding of light-duty vehicle and truck

emissions. These improvements are reflected in the Tier 2/Sulfur Updated Inventory, a

nationwide inventory that does not fully account for local inputs. Results from this updated

inventory were also presented in the Tier 2 final rule.

For this proposal, we updated the Tier 2/Sulfur Air Quality Inventory to account for

additional improvements in our understanding of light-duty vehicle and truck emissions and to

better account for county-specific conditions. These updates were applied by multiplying

emissions by county and vehicle type in the Tier 2/Sulfur Air Quality Inventory by the ratio of

the improved emission factors to those used to generate the Tier 2/Sulfur Air Quality Inventory. 

Details of this method can be found in a memorandum by Harvey Michaels to Docket A-2000-12

titled, “Adjustments to the Tier 2 Air Quality Inventory for the Mobile Source Air Toxics

Proposed Rule.” 

We also replaced the Tier 2 heavy-duty inventory estimates with inventory estimates from

the national emissions inventory spreadsheet developed and used for our proposed 2007 heavyduty engine rule.78 These heavy-duty engine emission estimates are based on national average

fleet mixes, temperatures, VMT distributions by roadway type, and speed by roadway type. We

did not include crankcase VOC emissions in our estimates of VOC emissions since these

emissions are not a portion of the exhaust from the engine, and toxics speciation data are based

on tailpipe exhaust. As is the case for the gaseous toxics Heavy-Duty 2007 control scenario

described above, we expect to use the heavy-duty engine emissions inventories developed for the
79The analysis methodology is described in a memorandum from Meredith Weatherby, Eastern Research

Group, to Rich Cook, EPA, entitled “Estimating of 1990 VOC and TOG Emissions” in EPA Air Docket A-2000-

12. 

81

final 2007 heavy-duty engine rule for our final toxics rulemaking. These inventories will

integrate our most recent information on emissions factors with county-specific modeling inputs.

For 1990, we could not use the Tier 2 rulemaking inventories, since they do not extend to

1990. We also could not use the 1990 VOC estimates from EPA’s Trends Report, since such

estimates would not be comparable to the 1996 and later estimates from the Tier 2 rule, because

EPA’s 1990 Trends estimates have not been updated to reflect the data and analyses that will be

used in MOBILE6. For 1990, we modified the modeling methods applied in the 1999 Air Toxics

Study to produce direct estimates of VOC emissions that account for the effects of fuel and

vehicle technology changes as well as the information that will be used in EPA’s MOBILE6

emissions model.79

b. Projected VOC Emissions Inventory

The results of this analysis, presented in Table IV.B-9, show that on-highway VOC

inventories are projected to decrease by over 70 percent between 1990 and 2020 under the Tier 2

control scenario. We assume that other gaseous toxics will decrease by approximately 70 percent

as well. Most of the emission decrease is expected to occur before 2007.

Table IV.B-9

Annual VOC Emissions Summary for the Total U.S. 

On-Highway Vehicles Only

Tier 2 Control Scenario

1990 1996 2007 2020

Thousand short tons per year 7,585 4,819 2,673 2,061

Cumulative Annual Reductions

from 1990 (thousand short tons)

N.A. 2,766 4,912 5,525

Cumulative Percent Reductions

from 1990 

N.A 36% 65% 73%

Table IV.B-10 reflects our on-highway inventory estimates for VOC under the HeavyDuty 2007 control scenario, and indicates that VOC inventories from on-highway vehicles are

projected to decrease 75 percent between 1990 and 2020 under this scenario. We assume that

other gaseous toxics would decrease by approximately 75 percent as well. 
80 EPA. 2000. National Air Pollution Emission Trends, 1900-1998 (March 2000). Office of Air Quality

Planning and Standards, Research Triangle Park, NC. Report No. 454/R-00-002. 

81 The Air Toxics Study’s approach is the more appropriate one when estimating emissions, ambient

concentrations, and exposures for urban areas, which were the focus of the study.

82

Table IV.B-10

Annual VOC Emissions Summary for the Total U.S.

On-Highway Vehicles Only

Heavy-Duty 2007 Control Scenario

1990 1996 2007 2020

Thousand short tons per year 7,585 4,819 2,662 1,838

Cumulative Annual Reductions

from 1990 (thousand short tons)

N.A. 2,765 4,924 5,748

Cumulative Percent Reductions

from 1990 

N.A. 36% 65% 76%

4. Diesel PM Inventory 

This section describes how we derived diesel PM estimates for this rule, and then

presents those estimates.

a. Diesel PM Inventory Methodology

As described in the methodology overview section, our diesel PM emissions estimates are

based on several sources. For 1990, we used the diesel PM emissions estimates from EPA’s

Emissions Inventory Trends Report.80 These estimates account for county-specific inputs in a

more reliable way than our 1999 Study.81

For 1996 and later years, light-duty diesel PM emissions were taken from our Tier 2 Air

Quality Analysis Inventory. These estimates were based on the PART5 model, which is similar

in structure and function to the MOBILE series of models. It calculates exhaust and non-exhaust

(e.g., road dust) particulate emissions for each vehicle class included in the MOBILE models. A

particle size cut-off of 10 μm was specified in the model inputs since essentially all exhaust PM

from diesel engines is smaller than 10 μm. We have determined that our Tier 2 Air Quality

Analysis Inventory is a more appropriate source for national light-duty diesel PM emission

estimates than the 1999 Study since it better accounts for county-specific conditions. 

To estimate heavy-duty exhaust diesel PM emissions for 1996 and later years, we used
82 This spreadsheet model can be found in EPA Air Docket A-99-06, Item II-B-31.

83

the national emissions inventory spreadsheet developed and used for our proposed 2007 heavyduty engine rule.82 This spreadsheet incorporates recent findings on heavy-duty diesel engine PM

emissions that were not reflected in the 1999 Motor Vehicle Air Toxics Study, accounts for the

increasing prevalence of diesel engines in certain engine classes, and is capable of modeling the

effects of the proposed 2007 heavy-duty engine rule. We did not include crankcase PM

emissions in our estimates of diesel PM, since they are not part of diesel exhaust. The

spreadsheet model is not capable of producing heavy-duty diesel PM estimates for 1990.

For the final toxics rule, we expect to revise our diesel PM inventory based on the

analyses being conducted for the final 2007 heavy-duty engine rule. These analyses will include

our most current information on light- and heavy-duty diesel engine emissions factors and VMT,

and will also reflect county-by-county information on VMT distribution by vehicle class,

roadway type, and speed.

b. Projected Diesel PM Emissions Inventory

Our diesel PM inventory estimates for the Tier 2 control scenario are presented below in

Table IV.B-11. Diesel PM emissions are expected to decline by over 60 percent under the Tier 2

control scenario, as compared with 1990 levels. These emissions are expected to decline even

more, by almost 95 percent as compared with 1990 levels, under the Heavy-Duty 2007 control

scenario, as shown in Table IV.B-12.

Table IV.B-11

 Annual Diesel PM Emissions Summary for the Total U.S.

On-Highway Vehicles Only

Tier 2 Control Scenario

1990 1996 2007 2020

Thousands of short tons per year 235 180 93 88

Cumulative Annual Reduction

from 1990 (thousand short tons)

N.A 55 142 147

Cumulative Percent Reduction

from 1990 

N.A. 23% 60% 63%
84

Table IV.B.12

 Annual Diesel PM Emissions Summary for the Total U.S.

On-Highway Vehicles Only

Heavy-Duty 2007 Control Scenario

1990 1996 2007 2020

Thousands of short tons per year 235 180 82 15

Cumulative Annual Reduction

from 1990 (thousand short tons)

N.A. 55 153 220

Cumulative Percent Reduction

from 1990 

N.A. 23% 65% 94%

C. Limitations and Uncertainties in the Analyses

While the 1999 EPA Motor Vehicle Air Toxics Study and our modeling analyses for

VOC and diesel PM emissions for this rule are the most recent information we have to project

MSAT emissions inventories for future years, we are aware of limitations and uncertainties in

these sources. In this section, we describe these limitations and uncertainties. 

1. 1999 EPA Motor Vehicle Air Toxics Study

There are a number of limitations and uncertainties associated with the emissions

modeling in the analyses in the 1999 EPA Motor Vehicle Air Toxics Study. First, the VMT

distribution by vehicle type we used in the 1999 Study differ from what we plan to use in

MOBILE6, and what we used to develop criteria pollutant inventory estimates in our recent 2007

heavy-duty proposed rule. Table IV.C-1 below presents the two sets of VMT fractions. The

VMT fractions that are planned for MOBILE6 will result in a greater proportion of total fleet

VMT allocated to heavy-duty vehicles and a correspondingly smaller proportion of fleet VMT

allocated to light-duty gasoline vehicles. Table IV.C-2 presents toxic emission factors for 1996,

2007, and 2020, under the Tier 2 control scenario. These data suggest that the VMT shift from

gasoline- to diesel-powered vehicles should have little impact on fleet average emissions of

benzene and 1,3-butadiene, but will cause fleet average emissions of acetaldehyde and

formaldehyde to increase by about 10 percent. It should be noted however, that diesels emit

lower amounts of VOC than gasoline engines; since VOCs react in the atmosphere to form

aldehydes, the reduction in VOC emissions should result in reduced levels of secondary

aldehydes.
85

Table IV.C-1

VMT Fractions used in the 1999 EPA Motor Vehicle Air Toxics Study 

Compared with proposed fractions for MOBILE6

VMT Fractions

Year Source LDGV LDGT1 LDGT2 HDGV LDDV LDDT HDDV MC

1996 Toxics

Assessment 0.556 0.269 0.089 0.021 0.003 0.002 0.055 0.005

1996 Planned

MOBILE6 0.565 0.233 0.080 0.035 0.003 0.002 0.077 0.005

2007 Toxics

Assessment 0.395 0.383 0.127 0.023 0.000 0.002 0.065 0.005

2007 Planned

MOBILE6 0.388 0.361 0.124 0.036 0.000 0.001 0.085 0.005

2020 Toxics

Assessment 0.300 0.448 0.146 0.025 0.000 0.005 0.071 0.005

2020 Planned

MOBILE6 0.279 0.440 0.152 0.036 0.000 0.001 0.088 0.004

Notes: LDGV = light-duty gasoline vehicles, LDGT1 = light-duty gasoline truck 1 (up to 6,000 pounds gross

vehicle weight, LDGT2 = light-duty gasoline truck 2 (6,001 to 8,500 pounds gross vehicle weight), HDGV = heavyduty gasoline vehicle, LDDV = light-duty diesel vehicle, LDDT = light-duty diesel truck, HDDV = heavy-duty

diesel vehicle, MC = motorcycle.
86

Table IV.C-2

U.S. Annual average toxic emission factors (mg/mi) from 

the 1999 EPA Motor Vehicle Air Toxics Study 

Emission Factors (mg/mi)

Year Pollutant LDGV LDGT1 LDGT2 HDGV LDDV LDDT HDDV MC

1996 Benzene 53.94 65.13 90.40 161.28 16.67 25.81 17.52 88.81

Acetaldehyde 6.05 7.87 12.37 32.74 10.25 15.83 48.06 18.84

Formaldehyde 16.26 22.59 37.53 154.83 32.18 49.66 130.49 68.60

1,3-Butadiene 6.32 7.77 13.13 29.85 7.50 11.58 10.18 26.27

2007* Benzene 20.46 25.15 39.28 54.82 4.45 7.78 9.38 74.41

Acetaldehyde 1.96 2.51 4.18 9.10 2.74 4.78 25.73 17.99

Formaldehyde 4.03 5.35 9.08 32.54 8.58 15.02 69.85 59.87

1,3-Butadiene 2.31 3.07 4.68 5.29 2.00 3.50 5.45 23.30

2020* Benzene 13.07 15.18 20.75 24.98 2.22 2.37 8.25 74.48

Acetaldehyde 1.18 1.37 1.91 3.33 1.36 1.45 22.61 18.01

Formaldehyde 2.47 2.90 3.90 10.39 4.28 4.57 61.39 59.73

1,3-Butadiene 1.69 2.04 2.79 1.43 1.00 1.06 4.79 23.37

* 2007 and 2020 estimates reflect the Tier 2 control scenario

Notes: LDGV = light-duty gasoline vehicles, LDGT1 = light-duty gasoline truck 1 (up to 6,000 pounds gross

vehicle weight, LDGT2 = light-duty gasoline truck 2 (6,001 to 8,500 pounds gross vehicle weight), HDGV = heavyduty gasoline vehicle, LDDV = light-duty diesel vehicle, LDDT = light-duty diesel truck, HDDV = heavy-duty

diesel vehicle, MC = motorcycle.

Second, in estimating toxic emissions in tons per year, VMT estimates from the Emission

Trends Database were used. These data represent EPA’s best estimate of VMT, but in some

instances, States may have more accurate VMT data. 

Third, our emission modeling also assumed an average vehicle speed of 19.6 miles per

hour. Actual vehicle speeds will tend to vary considerably within and between areas. Since the

relationship between emissions and speed distribution is complex, actual toxics emissions are

likely to differ from the toxics emissions estimates based on a 19.6 mph vehicle speed.

Fourth, there are significant uncertainties in projecting emissions for future years under

various control scenarios. For instance, the future year VMT mix among vehicle classes was

based on sales projections. Actual sales may differ from these projections. Our projections of

fuel properties in future years are also subject to considerable uncertainty.

Fifth, our emissions estimates are sensitive to assumptions about compliance margins, inuse deterioration, and toxics fractions. We explored some of these parameters in the 1999 EPA
87

Motor Vehicle Air Toxics Study.

2. VOC Inventory Modeling

Our 1990 VOC emissions estimate was derived using the 1999 Study methodology, hence

it has the same uncertainties and limitations as those described above.

Our light-duty vehicle VOC emissions estimates for 1996 and later years have several

notable sources of uncertainty. First, it treats California the same as the rest of the country,

which results in an overestimate of VOC emissions in 1996 and 2007. Second, our light-duty

VOC estimates do not account for the changes in VMT distribution between light-duty and

heavy-duty described in the previous section, which results in an approximately three percent

overestimate in light-duty vehicle VOC emissions. Finally, our light-duty vehicle VOC estimates

do not account for county-specific inputs directly, nor are they based on explicit calculations of

projected emissions in 2020. Instead, we attempted to adjust the county-specific inventories

generated for the Tier 2 Air Quality Analysis to account for more recent information about lightduty vehicle emissions for 1996, 2007, and 2030; we then used an interpolation method to

generate inventory estimates for 2020. Both the adjustment approach and the interpolation

method introduce uncertainty in our results.

Our heavy-duty VOC estimates for 1996 and later years are based on the spreadsheet

developed for the 2007 heavy-duty engine proposed rule. This spreadsheet is designed to

generate national emissions estimates that do not fully account for local inputs. It is based on

national average fleet mixes, temperatures, VMT distributions by roadway type, and speed by

roadway type. It also does not account for the differences between Federal and California

emission standards for heavy-duty gasoline vehicles or between Federal and California diesel

fuel sulfur standards. While we believe these differences are small, we expect to account for

them more explicitly and completely in our final rule.

3. Diesel PM Inventory Modeling

Our 1990 diesel PM inventory is taken from EPA’s Emission Inventory Trends Report. 

This estimate may not fully account for the shift from gasoline to diesel engines in mediumheavy-duty vehicles because it is based on 1987 data regarding diesel engine use. It should be

noted that the 1999 EPA Motor Vehicle Air Toxics Study analysis has the same limitation while

also not reflecting local inputs as fully.

For 1996 and later years, we based our light-duty vehicle diesel PM estimates on the Tier

2 Air Quality Analysis Inventory. This inventory does not account for the changes in VMT

distribution between light-duty and heavy-duty vehicels, which results in an approximately three

percent overestimate in light-duty vehicle diesel PM emissions. Furthermore, our light-duty

vehicle diesel PM emissions estimates are based on the PART5 model, which assumes that PM

emissions do not increase over time due to engine wear or use. However, PM emissions from
83 DeLucchi, M. A. 1999. Analysis of Particulate Matter Emission Factors in the PART5 Model.

Submitted to EPA.

88

diesel engines may well increase over time as a result of deterioration. We are currently

evaluating diesel PM emission estimates for PART6.

Our heavy-duty vehicle diesel PM estimates for 1996 and later years are based on the

spreadsheet developed for the 2007 heavy-duty engine proposed rule. This spreadsheet is

designed to generate national emission estimates and incorporates recent findings on heavy-duty

vehicle diesel PM emissions that were not reflected in the 1999 EPA Motor Vehicle Air Toxics

Study, accounts for the increasing prevalence of diesel engines in certain engine classes, and is

capable of modeling the effects of the proposed 2007 heavy-duty engine rule. It is based on

national average fleet mixes, temperatures, VMT distributions by roadway type, and speed by

roadway type. It also does not account for the differences between Federal and California diesel

fuel sulfur standards. While we believe these differences are small, we expect to account for

them more explicitly and completely in our final rule. 

And while our spreadsheet model does incorporate some degree of PM emissions

deterioration over time, real-world diesel PM emissions from diesel engines may increase over

time to a considerably greater extent. Also, there could be more idling and hard acceleration in

the real world than in the current heavy-duty vehicle test cycle used to determine compliance

with engine emission standards; this could also result in an underestimate of real world PM

emissions. One reviewer83 suggested that PART5 may underestimate heavy-duty diesel vehicle

emissions by up to 50 percent. EPA is currently evaluating diesel PM emission estimates for

PART6.

For the final toxics rule, we expect to revise our diesel PM inventory based on the

analyses being conducted for the 2007 heavy-duty engine rule. These analyses will include our

most current information on light- and heavy-duty diesel engine emissions factors and VMT, and

will also reflect county-by-county information on VMT distribution by vehicle class, roadway

type, and speed. 
89

Chapter 5: Mobile Source Air Toxic Ambient

Concentrations and Exposures

The purpose of this chapter is to review what we know about ambient concentrations and

exposures associated with emissions of mobile source air toxics. First, we will review

monitoring and modeled data on ambient concentrations of five of the 21 mobile source air

toxics. These compounds are benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and diesel

PM. We will then review results of an on-highway vehicle inhalation exposure assessment

prepared by EPA. The exposure estimates for gaseous air toxics are compared to estimates of the

on-highway vehicle contribution to modeled ambient concentrations. As exposure estimates via

a different approach are not yet available for comparison, the NATA national scale assessment

modeled ambient concentration estimates (which will be used to prepare exposure estimates later

summer) are used to evaluate the reasonableness of the exposure estimates. We will also discuss

what we know about inhalation exposures in various micro-environments. As discussed in

Chapter 4, we have the most reliable inventory data for these five compounds, and hence the

most accurate modeled mobile source estimates of ambient concentrations and exposure. 

Because of uncertainties associated with assessing ambient concentrations and exposures,

particularly for micro-environments and mobile source “hotspots,” we have developed an action

plan to further investigate these issues. The action plan is described in the preamble.

A. Survey of Data Ambient Concentrations of Mobile Source Air Toxics.

Monitor data for air toxics are somewhat limited, but are very useful for evaluating the

reasonableness of modeled ambient concentrations and bounding exposure estimates. Monitor

data can also be used to identify the locations where concentrations are highest. It should be

noted, however, that duration of exposure as well as concentration level influence the potential

for chronic health risks. Thus, if individuals spend only a short period of time at a location

where high monitored values of a pollutant have been found, there may not be a big impact on

overall exposure or risk.

 1. Ambient Monitoring

In this section, analyses of monitor data for benzene, 1,3-butadiene, formaldehyde,

acetaldehyde, and diesel PM from EPA and State and local programs are reviewed.

a. EPA Monitoring Data

This section summarizes monitored air toxics concentration data from the EPA
84 The AIRS Air Quality System contains measurements of ambient concentrations of air pollutants and

associated meteorological data. The data is collected by thousands of monitoring stations operated by EPA, national,

state and local agencies. EPA uses this data to assess the overall status of the nation's air quality and to prepare

reports to Congress as mandated by the Clean Air Act. EPA also uses the data to identify areas where

improvements in air quality are needed.

90

Aerometric Information Retrieval System (AIRS), Air Quality System.84 Using data where yearround measurements were available, we calculated 1996 mean ambient concentrations

nationwide for benzene, acetaldehyde, formaldehyde, and 1,3-butadiene. These data are

presented in Table V.A-1. Also presented are the number of monitor sites, standard deviations

and the concentrations at monitor sites in the 95th percentile. The 95th percentile concentrations

for these compounds are about twice the level of the mean concentrations. 

Data are especially limited for aldehydes, where only 26 sites have complete data in 1996. 

Because the monitored ambient concentration results have been compiled using data from a

number of different sources, often using different collection and chemical analysis methods, there

is a great deal of variability in the numbers. In addition, differences in criteria used to select the

sites of monitors may also result in discrepancies in the significance of different measurements. 

For example, many monitors are placed at sites where readings are expected to be high. 

Conversely, other monitors might be sited away from areas of highest concentration (for instance

to measure concentrations in residential areas). These differences make it difficult to interpret

these results. EPA is working with State and local air agencies to develop a monitoring network

that will develop estimates of ambient concentrations that are representative of regional area

concentrations throughout the U.S.

Air toxics trends were recently analyzed by EPA using ambient monitor data, and results

were summarized in the recently released 1998 Air Quality Trends Report.1 Between 1993 and

1998, ambient concentrations of benzene decreased by 37% (Figure V.A-1). It is likely that this

decrease is largely attributable to penetration of new highway vehicles compliant with tighter

VOC standards into the existing fleet, and use of reformulated gasoline. Data for 1,3-Butadiene

were also analyzed, but a consistent downward trend was not observed. 
91

Table V.A-1

Monitored 1996 ambient concentration estimates nationwide from AIRS

Compound No. of

sites

Mean

Conc.

(μg/m3) 

Standard

deviation

95th percentile

conc. (μg/m3) 

Benzene 119 1.90 0.91 3.7

1,3-Butadiene 51 0.79 2.5 1.6

Formaldehyde 26 2.90 1.2 5.4

Acetaldehyde 26 1.89 0.81 3.5

Figure V.A.-1 

National trend in annual average benzene concentrations in metropolitan areas, 1993–1998

(Source:1998 Air Quality Trends Report).

(Lines on box plots show highest value, 75th percentile value, average, 25th percentile value, and

lowest value)
92

b. State and Local Monitoring Data

Table V.A-2 presents ambient monitor concentration estimates from studies conducted in

the South Coast Air Basin and the State of Minnesota, with highway and nonroad concentrations

estimated using inventory apportionment.2, 3

 It should be noted that there is considerable

uncertainty in apportioning concentrations of aldehydes in this way, because much of the ambient

concentration of aldehydes is formed secondarily from other precursors. The South Coast

represents an area with a toxic emissions inventory dominated by mobile sources. According to

American Association of Automobile Manufacturers’ fuel survey data, Minnesota has high levels

of benzene in gasoline (average 1.73% in 1996, compared to a nationwide average of 1.1% for

non-RFG areas). As a result, we might expect that ambient benzene concentrations are higher in

Minnesota than in other States, but the average monitor values for benzene in Minnesota are not

significantly higher than the nationwide average based on AIRS in Table V.A-1. 

Table V.A-2

Monitored average ambient concentration estimates (μg/m3

), and estimated highway

and nonroad contributions, in the South Coast Air Basin and in Minnesota

South Coast Air District (1998-9) Minnesota (1996)

Compound All

Sources

Highway Nonroad All

Sources

Highway Nonroad

Benzene 3.53 2.46 0.73 1.69 0.48 0.78

1,3-Butadiene 0.79 0.53 0.21 N.A. N.A. N.A.

Formaldehyde 4.82 2.28 2.21 1.64 0.65 0.43

Acetaldehyde 3.17 1.52 1.57 N.A. N.A. N.A.

Sources: 1. South Coast Air Quality Management District. 1999. Multiple Air Toxics Exposure

Study in the South Coast Air Basin – MATES-II. 2. Minnesota Pollution Control Agency. 1999. 

MPCA Staff Paper on Air Toxics.

 

c. Diesel PM Monitoring

We do not have a way of actually measuring diesel PM. However, there are two indirect

methods of estimating diesel PM concentrations based on monitor data. First, monitoring data

on elemental carbon concentrations can be used as a surrogate to determine ambient diesel PM

concentrations. Elemental carbon is a major component of diesel exhaust, contributing to
93

approximately 60%-80% of diesel particulate mass, depending on engine technology, fuel type,

duty cycle, lube oil consumption, and state of engine maintenance.4 5 6 7 In most areas, diesel

engine emissions are major contributors to elemental carbon, with other potential sources

including gasoline exhaust, combustion of coal, oil, or wood, charbroiling, cigarette smoke, and

road dust. Because of the large portion of elemental carbon in diesel particulate matter, and the

fact that diesel exhaust is one of the major contributors to elemental carbon in most areas,

ambient diesel PM concentrations can be bounded using elemental carbon measurements. The

Agency’s draft Health Assessment Document for Diesel Emissions8 presents one approach for

calculating ambient diesel particulate matter concentrations using elemental carbon

measurements. In the absence of a more sophisticated modeling analysis, using elemental carbon

as a surrogate for diesel PM is a useful approach where elemental carbon concentrations are

available.

The second approach for monitoring diesel PM uses the chemical mass balance (CMB)

model in conjunction with ambient PM measurements to estimate ambient diesel PM

concentrations. Inputs to the CMB model include particulate matter measurements made at the

receptor site as well as measurements made of each of the source types suspected to affect the

site. Because of the co-emission of diesel and gasoline particulate matter in time and space,

chemical molecular species that provide markers for separation of these sources have been

identified. Recent advances in chemical analytical techniques have facilitated the development

of sophisticated molecular source profiles, including detailed speciation of organic compounds,

which allow the apportionment of particulate matter to gasoline and diesel sources with increased

certainty. Older studies that made use of only elemental carbon source profiles have been

published and are summarized here, but are subject to more uncertainty. It should be noted that

since receptor modeling is based on the application of source profiles to ambient measurements,

this estimate of diesel particulate matter concentrations includes the contribution from onhighway and nonroad sources of diesel PM. In addition, this model accounts for primary

emissions of diesel PM only; the contribution of secondary aerosols is not included.

Ambient diesel PM concentration estimates using these two approaches are summarized

in Table V.A-3.
94

Table V.A-3

Ambient Diesel Particulate Matter Concentrations from Receptor Modeling

and Elemental Carbon Measurements

Location Year of Sampling Diesel PM10

& PM2.5

)g/m3

(mean)

Diesel PM

% of

Total PM

Source of Data

West LA, CA

Pasadena, CA

Rubidoux, CA

Downtown LA, CA9

1982, annual 4.4 13% Source-Receptor

1982, annual 5.3 19% Modeling

1982, annual 5.4 13%

1982, annual 11.6 36%

Phoenix area, AZ10 Winter,1989-90 4-22* †

Phoenix, AZ11 1994-95, annual 0-5.3 (2.4) 0-27%

California, 15 Air Basins12 1988-92, annual 0.2-3.6* †

Manhattan, NY13 3 days, Spring, 1993 13.2-46.7* 31-68%

Welby, CO

Brighton, CO14

60 days, Winter, 1996-97 0-7.3 (1.7) 0-26%

60 days, Winter, 1996-97 0-3.4 (1.2) 0-38%

Boston, MA

Rochester, NY 

Quabbin, MA

Reading, MA

Brockport, NY15

1995, annual 0.7-1.7 (1.1) 3-15% Elemental Carbon

Measurement 1995, annual 0.4-0.8 (0.5) 2-9%

1995, annual 0.2-0.6 (0.4) 1-6%

1995, annual 0.4-1.3 (0.6) 2-7%

1995, annual 0.2-0.5 (0.3) 1-5%

Washington, DC16 1992-1995, annual 1.3-1.8 (1.6) 6-10%

South Coast Air Basin1 1995-1996, annual 2.4-4.7‡ †

*PM10 † Not Available

‡ The Multiple Air Toxics Exposure Study in the South Coast Air Basin reported values for maximum

monthly elemental carbon concentrations across a ten-site network. 

2. Modeled Ambient Concentrations

In this section, data on modeled ambient concentrations of air toxics are reviewed. 

Sources of data include the Agency’s Cumulative Exposure Project and the National Air Toxics

Assessment.

a. Cumulative Exposure Project

In 1998, EPA’s Office of Policy, Planning, and Evaluation released results of a modeling

study that estimated outdoor concentrations of hazardous air pollutants.17 This analysis was done

as part of the Cumulative Exposure Project (CEP). The study estimated 1990 annual average

outdoor concentrations of 148 toxics, nationally, and by census tract. In this study, county level
95

emissions were allocated to individual census tracts using spatial surrogates (for example,

roadway miles were used a surrogate for gasoline highway motor vehicle emissions). Emissions

were also allocated temporally into three-hour time blocks. The emission estimates were

obtained from an inventory developed specifically for the CEP. Since the time this inventory

was developed, higher quality and more recent emissions data have become available. 

Concentrations were estimated from these inventory data using a dispersion model known

as ASPEN (Assessment System for Population Exposure Nationwide). The modeling estimated

concentrations attributable to major, area, and mobile sources. In order to model a large number

of pollutants nationwide, ASPEN makes a number of simplifying assumptions. For instance,

where specific latitude and longitude coordinates were not available, facilities were randomly

located within a county. Moreover, concentration estimates at the census tract level were

estimated using modeling assumptions to allocate emissions from the county level, and the model

is very sensitive to the assumptions used. In addition, dispersion of emissions from non-point

sources (e.g. on-highway and nonroad vehicles) was treated simplistically. For resident tracts

that have radii greater than 0.3 km, non-point source ambient concentrations are estimated on the

basis of five pseudo point sources. The average concentration for the census tract is determined

by spatially averaging the ambient concentrations associated with the receptors defined for the

five pseudo sources which fall within the bounds of the tract. For resident tracts with radii less

than 0.3 km, ambient concentrations are set equal to zero. Other limitations include: terrain

impacts on dispersion were not included; no long range transport was included; and reliance on

long term climate summary data.

Thus, the results are most meaningfully interpreted when viewed over large geographic

areas (i.e., at the national or State level). Comparison of modeled concentrations to monitored

concentrations indicate that the model is more likely to underestimate monitored values than to

overestimate them. 

Table V.A-4 presents the CEP’s estimated nationwide average concentrations of benzene,

1,3-butadiene, formaldehyde, and acetaldehyde from all sources, as well as the contribution

attributable to mobile sources, separated into on-highway and nonroad. The mobile source

concentration estimates were allocated to on-highway and nonroad sources based on the onhighway and nonroad shares of the nationwide CEP inventory. Allocation of aldehydes was

based on direct emissions. There is considerable uncertainty in apportioning concentrations of

aldehydes in this way, because much of the ambient concentration of aldehydes is formed

secondarily from other precursors.
85 The nationwide numbers are for 48 States plus the Virgin Islands and Puerto Rico.

96

Table V.A-4

Average estimated nationwide concentrations of selected air toxics in 1990 

from the Cumulative Exposure Project (μg/m3

)

Compound Conc.

(μg/m3

)

All sources

Mobile

Contribution to

Ambient Conc.

On-Highway

Contribution to

Ambient Conc.

(% of avg. conc.)

Nonroad

Contribution to

Ambient Conc.

(% of avg. conc.)

Benzene 2.10 1.1 0.87 (41) 0.23 (11)

1,3-Butadiene 0.15 0.11 0.08 (53) 0.03 (20)

Formaldehyde 1.50 0.76 0.50 (33) 0.26 (17)

Acetaldehyde 0.72 0.44 0.29 (40) 0.15 (21)

Source: Systems Applications International. 1998. Modeling Cumulative Outdoor

Concentrations of Hazardous Air Pollutants. Report No. SYSAPP 98-96/33, Prepared for U. S.

EPA, Office of Policy, Planning and Evaluation, February, 1998.

b. Preliminary National Air Toxics Assessment Results for Mobile Sources

As part of its National Air Toxics Assessment (NATA) activities, EPA is currently

conducting a national-scale air toxics assessment using the ASPEN dispersion model, in

conjunction with the 1996 National Toxics Inventory, to estimate ambient concentrations of 33

air toxics identified in the UATS, plus diesel PM. The NATA national scale assessment will

report average ambient concentrations nationally and at the county level, as well as average

concentrations for counties at or above the 95th percentile. Since this national scale assessment,

like the CEP analysis, uses ASPEN, it has similar limitations. Again, the results are most

meaningfully interpreted when viewed over large geographic areas. The NATA national scale

analysis will also apportion the contribution to ambient concentrations between major, area,

nonroad mobile, and on-highway sources. Draft results will be released later this year, in

conjunction with a review by the Scientific Advisory Board. Estimates of the mobile source

contributions have been developed in advance of the estimates for stationary sources.18 Table

V.A-5 presents draft mean and median nationwide gaseous toxic ambient concentrations

attributable to on-highway and nonroad mobile sources.85 Estimates are provided for benzene,

1,3-butadiene, formaldehyde, and acetaldehyde. Both mean and median values are reported,

because high outlying values may bias the means. The estimates take into account

photochemical reactivity. Concentrations for other mobile source toxics estimated using ASPEN

can be found in Appendix 1. Diesel PM estimates are being revised to account for inventory
97

changes and will be included in the release for EPA Science Advisory Board review. 

Table V.A-5

Draft average estimates of mobile source contributions to nationwide concentrations

of selected air toxics in 1996 from the NATA national scale assessment

Compound On-Highway Contribution to

Ambient Conc. (μg/m3

)

Nonroad Contribution to

Ambient Conc. (μg/m3)

Mean Median Mean Median

Benzene 0.55 0.45 0.24 0.16

1,3-Butadiene 0.05 0.04 0.02 0.01

Formaldehyde 0.38 0.29 0.48 0.20

Acetaldehyde 0.40 0.32 0.27 0.12

c. Diesel PM Estimates

Two dispersion model studies reporting diesel PM have been conducted in Southern

California. Results are summarized in Table V.A-6. Secondary formation of particulate matter

accounted for 27% to 67% of the total particulate matter associated with diesel engines.19 20 

Dispersion modeling conducted in Southern California reported that the on-highway contribution

to the reported diesel PM levels ranged from 63% to 89%. 
98

Table V.A-6

Annual average diesel particulate matter concentrations predicted

from dispersion modeling

Location Year of Sampling Diesel PM2.5

)g/m3

(mean)

Diesel

PM2.5 % of

Total PM2.5

Azusa, CA

Pasadena, CA

Anaheim, CA

Long Beach, CA

Downtown LA, CA

Lennox, CA

West LA, CA21

1982, annual 1.4** 5%

1982, annual 2.0** 7%

1982, annual 2.7** 12%

1982, annual 3.5** 13%

1982, annual 3.5** 11%

1982, annual 3.8** 13%

1982, annual 3.8** 16%

Claremont, CA22 18-19 Aug 1987 2.4** 8% 

Long Beach, CA

Fullerton, CA

Riverside, CA23

24 Sept 96 1.9 8% 

24 Sept 96 2.4 9%

25 Sept 96 4.4 12%

+Value in parenthesis includes secondary diesel PM (nitrate, ammonium, sulfate and hydrocarbons) due to

atmospheric reactions of primary diesel emissions of NOx, SO2 and hydrocarbons.

**On-highway diesel vehicles only

B. Modeled Inhalation Exposures

As part of the National Air Toxics Assessment (NATA) national scale assessment, 1996

inhalation exposure estimates are being developed to assess the exposure concentrations

attributable to on-highway and nonroad mobile sources. As mentioned previously, this effort

uses a dispersion model, ASPEN, to model ambient concentrations of air toxics at the county

level. These data will then be used as input into version 4 of the Hazardous Air Pollutant

Exposure Model (HAPEM4), which is currently under development. Although this model is not

designed to estimate individual exposures, it can provide exposure distribution estimates for the

general population as well as for various subpopulations of interest (e.g., children aged 0-17

years).

Exposure estimates for diesel PM from on-highway and nonroad sources were recently

modeled by CARB using the California Population Indoor Exposure Model (CPIEM). Results

from this model are presented in Table V.B-1 below and described in more detail in CARB’s

“Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant Appendix III Part A:

Exposure Assessment”.24

Other than these two efforts, the only available estimates of inhalation exposure to motor
99

vehicle related air toxics were developed using the Hazardous Air Pollutant Exposure Model for

Mobile Sources, version 3 (HAPEM-MS3).25, 26, 27 This model uses CO as a tracer for air toxics

exposure, rather than using modeled ambient concentrations of toxics as inputs like HAPEM4. 

Also, this version of the model is designed to address exposure attributable to on-highway

vehicle emissions, whereas HAPEM4 addresses exposures attributable to all source categories. 

Methods used to develop exposure estimates using this model are discussed below.

Table V.B-1

California annual average diesel PM exposure estimates for all

mobile sources from the California Population Indoor Exposure Model

Year California Exposure Estimates (On-Highway & Nonroad) in

)g/m3

1990 California Annual Average 1.5

2007 Projected California Annual Average 1.3

2020 Projected California Annual Average 1.2

Source: CARB. Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant

Appendix III Part A: Exposure Assessment. 

1. Methodology for Modeling Inhalation Exposures to Benzene, Formaldehyde,

Acetaldehyde, 1,3-Butadiene and Diesel PM: HAPEM-MS3

Estimates of exposure for gaseous air toxics using a previous version of the HAPEM-MS

model were published in the 1993 Motor Vehicle-Related Air Toxics Study.28 Based on peer

review comments, a number of improvements were made to the model, resulting in HAPEMMS3. These improvements include:

• 1980 census and CO monitoring data were replaced with 1990 data for base-year

modeling;

• 32 additional micro-environments were added to the 5 micro-environments in the original

model (20 of these micro-environments were indoors);

• Activity data from three cities were used, rather than data from one city as in the original

model; and

• More detail on seasonal and regional exposures was retained.

Exposure modeling projections considering currently planned controls are presented in

the next section. This section focuses on methods used to develop these estimates, limitations

and uncertainties, and an evaluation of the reasonableness of the baseline estimates for 1990 and

1996, based on a comparison to other data sources.

a. Modeling Approach 
100

Exposure modeling was done for 1990. Data from 10 urban areas were used. These areas

were Atlanta, GA, Chicago, IL, Denver, CO, Houston, TX, Minneapolis, MN, New York, NY,

Philadelphia, PA, Phoenix, AZ, Spokane, WA, and St. Louis, MO. As mentioned previously,

HAPEM-MS3 uses CO as a tracer for toxics. Thus, these areas were selected because a large

percentage of the population lived within reasonable proximity to CO monitors, and also to

represent good geographic coverage of the U.S. Since most ambient CO comes from cars and

light trucks, we believe CO exposure is a reasonable surrogate for exposure to other motor

vehicle emissions, including toxics emissions. The HAPEM model links human activity patterns

with ambient CO concentration to arrive at average exposure estimates for 22 different

demographic groups (e.g., outdoor workers, children 0 to 17, working men 18 to 44, women 65+)

and for the total population. The model simulates the movement of individuals between home

and work and through a number of different micro-environments. The CO concentration in each

micro-environment is determined by multiplying ambient concentration by a microenvironmental factor derived from regression analysis of ambient and personal monitor data. 

Each micro-environmental factor has a multiplicative term, which represents ambient exposure,

and an additive term, which represents exposure to emissions originating within microenvironments. These factors were derived by IT Corporation using paired ambient and personal

exposure monitor measurements from CO studies in Denver and Washington.29, 30 In our

modeling, we set the additive term to zero, to eliminate non-ambient sources of CO, such as gas

stoves. The multiplicative term has a component that represents penetration from the ambient air

into the micro-environment, and a factor that represents the proximity of the micro-environment

to monitors.

With the 1990 CO exposure estimates generated by the HAPEM-MS3 model for each

urban area, EPA determined the fraction of exposure attributable to on-highway vehicle

emissions. This calculation was accomplished by scaling the exposure estimates (which reflect

exposure to total ambient CO) by the fraction of the 1990 CO emissions inventory from onhighway motor vehicles, determined from the EPA Emission Trends database.31, 32 This scaling

takes into account the contribution of background CO to the inventory. Nationwide urban CO

exposure attributable to on-highway vehicle emissions was estimated by first calculating a

population-weighted average CO exposure for the ten modeled areas. This number was adjusted

by applying a ratio of population-weighted annual average CO ambient concentrations for urban

areas in the entire country versus average ambient CO concentration for the modeled areas. To

estimate rural exposure, the urban estimate was scaled downward using rough estimates of urban

versus rural exposure from the 1993 Motor Vehicle-Related Air Toxics Study (EPA 1993a). The

scaling factor was 0.56, based on an average CO concentration in rural areas of 470 μg/m3

 and a

concentration in urban areas of 842 μg/m3

.

Modeled CO exposure attributable to on-highway vehicle emissions for 1990 was divided

by 1990 CO grams per mile emission estimates to create a conversion factor. The conversion

factor was applied to modeled toxic emission estimates (in grams per mile) to determine

exposure to on-highway vehicle toxic emissions, as shown in Equation 6:
101

TOXExposure(μg/m3) = [COExposure(μg/m3)/COEF(g/mi)]1990 × TOXEF(g/mi) (6)

where:

TOXExposure(μg/m3) = exposure to on-highway vehicle toxic emissions

COExposure(μg/m3) = exposure to on-highway vehicle CO emissions

COEF(g/mi) = CO emission factor

TOXEF(g/mi) = toxic emission factor

 The exposure estimates for calendar years 1996, 2007, and 2020 were adjusted for VMT

growth relative to 1990. We also included in the model various assumptions regarding

transformation of the toxics. For example, benzene was treated as inert, but 1,3-butadiene

exposure was adjusted to account for its atmospheric transformation into other species. The

multiplicative factors used to adjust for this transformation were 0.44 for summer, 0.70 for spring

and fall, and 0.96 for winter.33 These factors account for the difference in reactivity between

relatively inert CO, which is being used as the tracer for toxics exposure, and the more reactive

1,3-butadiene. In contrast, estimated exposures to formaldehyde and acetaldehyde were based on

direct emissions. For these pollutants, removal of direct emissions in the afternoon was assumed

to be offset by secondary formation. Limitations and problems with this assumption are

discussed in the following section. Annual average exposure estimates to the gaseous air toxics

for the entire population in 1990 and 1996 are presented in Tables V.B-2 and V.B-3. Annual

average inhalation exposure to diesel PM was estimated to be 0.82 μg/m3 in 1990 and 0.73 μg/m3

in 1996. Estimates were also developed for outdoor workers, and children 0 - 17 years of age. 

Exposure among outdoor workers was higher than for the entire population, and among children

it was slightly lower.

As discussed in Chapter 4, diesel PM emission estimates for 1996, 2007, and 2020

developed for the 2007 heavy-duty engine proposed rule are higher than estimates in the EPA

1999 Study. Thus, exposure estimates for these years were adjusted upward to account for the

discrepancy.

b. Limitations and Uncertainties

Use of the HAPEM-MS3 model to estimate exposure to toxic emissions from motor

vehicles introduces several notable sources of uncertainty. First, the model may underestimate

CO exposures of the maximally exposed population. Although a validation study of HAPEMMS3 has not been done, such an analysis has been done for the pNEM/CO Model (NAAQS

Exposure Model for CO), which uses an approach similar to that used in HAPEM-MS3 as well

as much of the same data.34 Generally speaking, pNEM/CO’s estimates of CO exposures for the

population in the 5th percentile were overestimated by about 33%, and those in the 98th

percentile were underestimated by about 30%. This result suggests that pNEM/CO

underestimates CO exposures of the maximally exposed population. These results would likely

also hold for HAPEM-MS3 estimates of toxics exposure as well, and suggest that the model is
102

probably best suited for estimating average exposures.

Second, the data used to derive micro-environmental factors are limited. As described

earlier, the data are obtained from only two cities. Thus, the regression equations used to derive

the micro-environmental factors are subject to substantial error. Moreover, activity data are very

limited for some demographic groups. For instance, there was very little activity data for African

Americans or Hispanic Americans in the database. 

Third, because we set the additive terms of the micro-environmental factors to zero, the

HAPEM-MS3 results do not account for exposures to emissions originating within microenvironments. For instance, the model does not account for indoor exposure to evaporative

benzene emissions from vehicles parked in attached garages. The potential impact of these

additional sources of emissions within micro-environments is discussed in Section V.C.

Fourth, the modeling done in this assessment assumes that the on-highway fleet

emissions ratio of CO to diesel PM can be used as an adjustment factor to convert estimated CO

personal exposures to diesel PM exposures. However, most CO emitted from on-highway

vehicles is emitted by gasoline vehicles, while most on-highway diesel PM is emitted from

heavy-duty diesel engines. Even though gasoline- and diesel-fueled on-highway vehicles travel

the same roadways, temporal and spatial patterns for diesel vehicle operation are different than

gasoline-fueled vehicles. This could result in underestimates of diesel PM exposure, for instance,

in areas where the proportion of heavy-duty diesel vehicle traffic is significantly greater than

average relative to light-duty gasoline vehicle traffic. Conversely, overestimates of commuting

exposures could occur where there is very little heavy-duty diesel vehicle traffic.

Similarly, the model also does not take into account the fact that spatial and temporal

allocation of benzene evaporative emissions are different than CO emissions. However, in

modern technology vehicles, with evaporative emission controls, benzene emissions are

dominated by the exhaust component. The modeling approach also assumes that emissions of

toxics vary linearly with CO as a function of ambient temperature. Also, although we know that

emissions of CO as well as the toxic compounds modeled are all higher at lower temperatures,

we do not know if the relationship is linear. Moreover, the assumption that exposure increases

proportionally to VMT does not account for urban spreading or building of new roadways within

an urban area.

Finally, as mentioned previously, we assumed that estimated exposure to formaldehyde

and acetaldehyde was based on direct emissions. As will be discussed in Section V.C.4 below,

we believe this assumption results in an underestimate of acetaldehyde exposure by about a

factor of three. Thus, HAPEM-MS3 based estimates of acetaldehyde exposure must be adjusted

to account for this underestimate. 

c. Planned Improvements to HAPEM
86 Comparisons to monitor data were not made, due to the difficulty in estimating mobile source

contributions with accuracy.

103

As mentioned previously, a new version of HAPEM is being developed for use in the

National Air Toxics Assessment. This new version will have a number of major modifications

and improvements. The model has been revised to accept monitored or modeled toxics

concentrations directly as input, rather than using CO as a surrogate for toxics. Thus the mobile

source acronym (-MS) was dropped. The model also now incorporates a new time-activity

database derived from the CHAD (Comprehensive Human Activity Database). Draft results of

modeling with HAPEM4 for 1996 should be available later in 2000.

2. Comparison of Exposure Modeling Results to Modeled Ambient

Concentrations

In this section, we compared HAPEM-MS3 exposure modeling results to modeled

ambient concentrations from the Cumulative Exposure Project and NATA national scale

assessment.86 Average ambient concentrations do not represent average inhalation exposures

because they do not take into account human activity patterns, intrusion of ambient air toxics into

specific micro-environments, or emissions of air toxics from micro-environmental sources. 

Nonetheless, we expect them to be within the same order of magnitude, and in the absence of

other exposure estimates, these data represent the best surrogate source of information with

which to evaluate the reasonableness of HAPEM-MS3 results.

A number of other limitations and uncertainties make it difficult to directly compare

modeled ambient concentration estimates to exposure estimates. First, uncertainties result from

the surrogates (e.g., roadway miles and population density) used to allocate emissions from the

county to census tract level for use in dispersion modeling. Also, the ASPEN dispersion model

does not include a terrain component, and relies on long-term climate summary data. 

We first compared HAPEM-MS3 average 1990 nationwide exposure estimates for onhighway vehicles for the entire U.S. population to 1990 CEP ambient concentration estimates for

on-highway vehicles. Results are presented in Table V.B-2. These results indicate close

correspondence except for acetaldehyde, where CEP results are about 70% higher. 

We also compared HAPEM-MS3's 1996 average nationwide estimates of gaseous toxic

exposure from on-highway vehicle emissions to estimates of the on-highway vehicle contribution

to ambient concentrations from the draft NATA national scale analysis. Results are presented in

Table V.B-3. Overall, average modeled ambient concentrations from the national scale analysis

are within the same order of magnitude of the average HAPEM-MS3 exposure results. 

Agreement is fairly close for benzene,1,3-butadiene, and formaldehyde, while HAPEMMS3 estimates for acetaldehyde are low compared to ambient concentration estimates from the

NATA national scale analysis. HAPEM-MS3 exposure estimates for formaldehyde and
104

acetaldehyde do not account for photochemistry, and removal of primary emissions are assumed

to be offset by secondary formation. ASPEN, on the other hand, accounts for aldehyde

photochemistry. It assumes that about 68% of formaldehyde is primary but only about 20% of

acetaldehyde is assumed to be primary. Since most ambient acetaldehyde is secondary, the

HAPEM-MS3 exposure estimate based on direct emissions will underestimate acetaldehyde

exposure. Thus, we have adjusted HAPEM-MS3-based estimates of acetaldehyde exposure by a

factor of 3 to be consistent with NATA modeled ambient concentrations. Subsequent estimates

of acetaldehyde exposure will include this adjustment. 

Until the NATA HAPEM4 results are available, however, it is difficult to make a strong

inference on whether NATA exposures are likely to be very close to HAPEM-MS3 exposures. 

This will depend on the micro-environmental factors selected and the activity data from the

CHAD. Nonetheless, given the limitations inherent in making comparisons, the results available

at this time suggest that the HAPEM-MS3 approach provides reasonable estimates of inhalation

exposure with the exception of acetaldehyde, which we are adjusting to account for the model’s

inherent limitations. 

Table V.B-2

Comparison of 1990 average exposure attributable to on-highway vehicle emissions

(HAPEM-MS3) to 1990 ambient concentration estimates

attributable to on-highway vehicle emissions (CEP)

Compound HAPEM-MS3 Based

Exposure (μg/m3

)

CEP Ambient Conc. (μg/m3)

Benzene 1.07 0.87

1,3-Butadiene 0.11 0.08

Formaldehyde 0.57 0.50

Acetaldehyde 0.17* 0.29

*Unadjusted estimate based on direct emissions – adjusted level which includes secondary

formation is 0.51 μg/m3

.
105

Table V.B-3

Comparison of 1996 annual average exposures attributable to on-highway vehicles

(HAPEM-MS3) and the on-highway vehicle portion of 1996

 modeled ambient concentrations (National Scale Assessment)

Compound HAPEM-MS3 OnHighway Vehicle

Exposure (μg/m3)

NATA On-Highway

Vehicle Mean Ambient

Concentration (μg/m3)

Benzene 0.68 0.55

1,3-Butadiene 0.07 0.05

Formaldehyde 0.34 0.38

Acetaldehyde 0.12* 0.40

*Unadjusted estimate based on direct emissions – adjusted level which includes secondary

formation is 0.36 μg/m3

.

3. Variance in Exposures

a. Distribution of Exposures

HAPEM-MS3 reports average annual and seasonal CO exposures by demographic group,

as well as the distribution of exposures around the mean. These distributions for CO exposure

can be used to estimate distributions of toxics exposure. As mentioned previously, we believe

that HAPEM-MS3 overestimates low-end exposures and underestimates high-end exposures (by

about 30% for the 98th percentile). Figure V.B-1 presents the on-highway vehicle portion of both

average annual and the 95th percentile annual benzene exposures for the general population in

New York City. Exposures for the 95th percentile of the entire population are about twice the

average. Results are presented for children 0 to 17 and outdoor workers as well. The distribution

of exposures for children and outdoor workers are not as broad as the distribution for the entire

population. This could be due to the smaller database for these demographic groups as well as

less variability in activity patterns.

Differences in the amount of time individuals spend in various micro-environments

contributes significantly to the overall distribution in annual exposures. For instance, individuals

spending the greatest percentage of time in high-exposure micro-environments have annual CO

exposures about 30% higher than average. Conversely, individuals spending the greatest amount

of time in low-exposure micro-environments have annual CO exposures about 5% lower than

average.
106

Benzene Exposures Attributable to OnHighway Vehicles, New York City, 1996

0.00

0.50

1.00

1.50

2.00

All

persons

Children,

0 to 17

Outdoor

workers

Demographic Group

Exposure (ug/m3)

1996 Average

Benzene Exposure

1996 95th

Percentile

Benzene Exposure

b. Variance among demographic groups

We have analyzed average inhalation exposures for three demographic groups -- the

overall population, outdoor workers, and children 0 to 17. Since inhalation exposures are lower

indoors than outdoors, exposures for outdoor workers are somewhat higher than the general

population. Exposures for children are similar to the general population, although slightly lower

since children spend a little more time indoors than most other demographic groups. Nationwide

average inhalation exposures for the three demographic groups in 1996 are presented in Table

V.B-4. 

Figure V.B-1

Average and 95th percentile benzene exposures

(attributable to on-highway vehicles) in New York City, 1996
107

Table V.B-4

Highway vehicle portion of nationwide average inhalation exposures to benzene,

1,3-butadiene, formaldehyde, acetaldehyde, and diesel PM for three

demographic groups in 1996, based on HAPEM-MS3

Pollutant Overall Population

Exposure (μg/m3)

Outdoor Worker

Exposure (μg/m3)

Children’s Exposure

(μg/m3)

Benzene 0.68 0.79 0.65

1,3-Butadiene 0.07 0.08 0.07

Formaldehyde 0.34 0.39 0.32

Acetaldehyde 0.36 0.39 0.33

Diesel PM 0.73 0.85 0.70

c. Geographic variation

HAPEM-MS3 modeling results indicate that average inhalation exposures vary

significantly between geographic locations. Toxics exposures are impacted by ambient

temperatures, local fuel properties, age of the in-use fleet, I/M programs, traffic density,

demographics, and many other factors. To illustrate, Table V.B-5 presents the on-highway

portion of the1996 average annual benzene inhalation exposure estimates for the 10 areas

modeled in the EPA 1999 Study, as well as the estimates for urban areas and rural areas

nationwide. Among the urban areas modeled, Phoenix had the highest level of annual onhighway vehicle contribution to benzene exposure in 1996. Phoenix had a high level of CO

exposure attributable to highway vehicles in 1996 (484 μg/m3

) combined with an average fuel

benzene level of 1.07% in summer and 1.40% in winter (based on AAMA fuel surveys). 

Average benzene exposure in Phoenix is expected to drop substantially by 2007 due to the

adoption of California reformulated gasoline and as a result of more stringent Federal emission

standards and fleet turnover. Minnesota also has high benzene exposure levels relative to other

modeled areas in 1996; this is due to significantly higher than average fuel benzene levels of

1.81% in summer and 1.65% in winter (based on AAMA fuel survey data). 

Not surprisingly, individuals in rural areas, which have lower population and traffic

density than urban areas, are expected to experience lower benzene-related exposures than

individuals in urban areas (Table V.B-5). Moreover, data from the 14 cities modeled in Glen and

Shadwick (1998) demonstrate that average CO levels increase proportionally with population

density (Table V.B-6). HAPEM-MS3 toxics exposure estimates will follow the same trend,

since CO is used as a surrogate for toxics.
108

Table V.B-5

 On-Highway vehicle portion of 1996 benzene exposure estimates

for 10 urban areas, and urban and rural areas nationwide,

 based on HAPEM-MS3 exposure modeling

Urban Area Average Highway Vehicle

Benzene Concentration

(μg/m3

) in 1996 

Atlanta 0.84

Chicago 0.48

Denver 0.71

Houston 0.53

Minneapolis 1.11

New York 0.79

Philadelphia 0.51

Phoenix 1.26

Spokane 0.91

St. Louis 0.47

Urban Area Average 0.74

Rural Area Average 0.43
109

Table V.B-6

Annual Average Ambient CO Levels as a Function

of Population Density, 14 Cities

Population Density at

Monitor (residents per square

mile)

Average CO Levels, 1990

(ppm)

< 300 1.10

300 - 1,000 1.01

1,000 - 2,000 1.19

2,000 - 5,000 1.35

5,000 - 10,000 1.41

> 10,000 1.97

Overall Average 1.32

d. Seasonal variation

Average nationwide exposures to on-highway vehicle air toxics are much higher in winter

than in summer (Table V.B-7). This is primarily due to higher cold start emissions in winter.

Table V.B-7

Seasonal Average Nationwide Exposures (μg/m3

) Attributable to On-Highway Vehicle

Emissions, for the General Population, 1996

Pollutant Winter Spring Summer Fall

Benzene 0.87 0.62 0.53 0.71

Acetaldehyde 0.47 0.30 0.25 0.36

Formaldehyde 0.45 0.30 0.26 0.34

1,3-Butadiene 0.12 0.06 0.03 0.07

Diesel PM 0.85 0.71 0.63 0.73
110

4. Impact of Current On-Highway Vehicle Control Programs on Toxics

Exposure

Projected emissions reductions resulting from programs currently in place will result in

proportional reductions in inhalation exposure levels attributable to on-highway vehicles. Figure

V.B-2 presents exposure estimates for benzene, formaldehyde, acetaldehyde,1,3-butadiene, and

diesel PM in 1990, 1996, 2007, and 2020. These estimates assume implementation of Phase II

reformulated gasoline, the National Low Emission Vehicle (NLEV) program, Tier 2 emissions

standards with 30 ppm sulfur gasoline, and the 2004 heavy-duty standards. With current

controls, by 2020 we expect that inhalation exposure to benzene attributable to on-highway

vehicles will decrease from 1990 levels by 74 percent, exposure to formaldehyde by 75 percent,

exposure to acetaldehyde by 65 percent, exposure to 1,3-butadiene by 73 percent, and exposure

to diesel PM by 55 percent. Standards recently proposed for heavy-duty engines in 2007 will

result in additional exposure reductions, as shown in Figure V.B-2. With these standards, diesel

PM exposure will be reduced by 93% from 1990 levels. 

5. Sensitivity Analyses

 Below is a discussion of the key sources of uncertainty and variability in the models and

data affecting exposure. The Agency has conducted some sensitivity analyses to address these

issues. However, additional aspects of sensitivity and uncertainty must be explored before we

are ready to release results. 

1) Representativeness of, and Variability in, Measured Ambient Carbon Monoxide Levels.

Toxics exposure estimates are based on measured ambient CO concentrations averaged

over 10 selected urban areas. Important limitations of these data relate to variations in

CO levels both within and among urban areas, and to the spatial variations in CO levels

relative to exposure locations for particular potential highly exposed or sensitive groups

(e.g., children, the elderly, ethnic minorities).

2) Estimates of Present and Future HAPs Emissions. The MOBTOX5b and PART5

models include data sets related to vehicle and fleet characteristics, VMT accumulation,

climatic conditions, and fuel characteristics. These input data relate to the estimated

characteristics of both “current” (1990) and “future” (1996, 2007, and 2020) vehicle

fleets. 

3) Appropriateness of CO as a Proxy for Estimating Toxics Exposures. Aside from the

uncertainties associated with the ambient CO measurements themselves, the inhalation

exposure estimates assume that ambient CO concentrations are an appropriate starting

point for estimating toxics exposures. This assumption is only valid if (1) the average

contributions of mobile sources to ambient CO and toxics exposures levels are similar

across exposure areas, and (2) degradation or secondary generation of toxics due to

photochemical and other reactions can be neglected or accurately modeled. Also,
111

0

0.2

0.4

0.6

0.8

1

1.2

Benzene

Formaldehyde

Acetaldehyde1,3-Butadiene

Diesel PM

Pollutant

National Annual Average 

Exposure (ug/m3)

1990

1996

2007

2020

2020 -- HD 2007

Standards

HAPEM-MS3 accounts only for exposures to ambient air toxics. Micro-environmental

sources of exposure are not accounted for. If the generation patterns or atmospheric fate

and transport characteristics of the toxics differ significantly from those of CO, or if there

are significant sources of toxics emissions that are not addressed in the current exposure

assessment methodology, the toxics exposure estimates may underestimate true exposure.

Figure V.B-2

Exposure levels for four gaseous toxics and diesel PM under currently

planned controls and with 2007 Standards for Heavy Duty Engines (ug/m3

)

4) Micro-environmental Factors and Time-Activity Patterns. The revised ME factors and

time use patterns supporting EPA’s exposure assessment have been criticized as

substantially underestimating exposures to certain potentially sensitive populations and

highly exposed demographic groups, and failing to capture important inter-individual

variability in behavior and exposure patterns within demographic groups.35

5) Methods Used to Extrapolate National Urban and Rural Exposures. To estimate urban

exposures, we developed CO and HAPs emission estimates for urban counties, and scaled

them to the estimated average exposure for the 10 modeled urban areas. National average
112

rural exposures were calculated by assuming that rural HAPs exposures were, on average,

a constant fraction of the average urban exposures, scaled to the estimated CO and HAPs

emissions in the individual counties. Both of these procedures have some uncertainties.

C. Exposures in Micro-environments

Exposures to air toxics result not only from intrusion of ambient air into microenvironments, but also from emissions of air toxics originating within micro-environments. If

the contribution to long-term exposure from sources within micro-environments is large,

estimates of these exposures to ambient concentrations could significantly underestimate the

actual risk from exposure to motor vehicle air toxics. Among the most significant microenvironments where such exposures might occur are in vehicles, at service stations during

refueling, inside homes with attached garages, and near roadways. Unfortunately, measurements

of toxics concentrations within specific micro-environments is very limited, and much of it is

dated. This section briefly summarizes what we know about micro-environmental exposures for

diesel PM, benzene, 1,3-butadiene, formaldehyde, and acetaldehyde. It should be noted that the

information presented here addresses only level of exposure, not the amount of exposure.

1. Diesel PM

Micro-environmental levels of diesel PM can be estimated using elemental carbon as a

surrogate. This approach provides some estimates of diesel PM in micro-environments such as

in-vehicle concentrations (2.8-36.6 )g/m3

), near roadways with diesel traffic (diesel PM

concentrations are calculated to be 0.7-7.5 )g/m3

 higher than background), and in schools (0.9-

5.5 )g/m3).

Recently, elemental carbon measurements were reported for enclosed vehicles driving on

Los Angeles roadways.36 Applying a ratio of diesel PM mass to elemental carbon mass, diesel

PM concentrations in the vehicle ranged from approximately 2.8 )g/m3 to 36.6 )g/m3 depending

on the type of vehicle being followed; higher concentrations were observed when the vehicle

followed heavy-duty diesel vehicles. CARB also collected elemental carbon near the Long

Beach Freeway for four days in May, 1993, and for three days in December, 1993.37 Using

emission estimates from their EMFAC7G model, and elemental/organic carbon composition

profiles for diesel and gasoline exhaust, tire wear and road dust, CARB estimated the

contribution of the freeway traffic to diesel PM concentrations. For the two days of sampling in

December 1993, diesel exhaust from vehicles on the nearby freeway were estimated to contribute

average concentrations ranging from 0.7 )g/m3

 to 4.0 )g/m3 excess diesel PM above background

concentrations with a maximum 24 hour measurement of 7.5 )g/m3

.

In a study designed to investigate relationships between diesel exhaust exposure and

respiratory health of children in the Netherlands, elemental carbon measurements were collected

in 24 schools located from 47 to 377 meters from a freeway.38 Thirty-two samples were collected

inside schools and 46 samples were collected outside the schools. Preliminary estimates of

elemental carbon concentrations indoors and outdoors ranged from 1 )g/m3

 to 6 )g/m3, with an
113

average between 3 and 4 )g/m3

. These results correspond to diesel PM concentrations ranging

from 0.9 )g/m3

 to 5.5 )g/m3 with a mean of approximately 2.7-3.7 )g/m3. Total PM2.5

concentrations inside the schools averaged 23.0 )g/m3 while PM2.5 outside was only slightly

higher (24.8 )g/m3

), suggesting extensive intrusion of outdoor air into the school environment. 

2. Benzene

The information contained in Table V.C-1 summarizes data from several studies that

have measured micro-environmental exposures to benzene. These studies are the EPA's Total

Exposure Assessment Methodology (TEAM) Study,39 Commuter's Exposure to Volatile Organic

Compounds, Ozone, Carbon Monoxide, and Nitrogen Dioxide,40 In-Vehicle Air Toxics

Characterization Study in the South Coast Air Basin,41 Air Toxics Micro-environment Exposure

and Monitoring Study,42 a 1998 California EPA study of in-vehicle concentrations (California

EPA, 1998), a 2000 study of commuter exposures in Detroit, MI,43 and a 1993 NIOSH study of

concentrations at service stations.44

The TEAM Study was planned in 1979 and completed in 1985. The goals of this study

were: 1) to develop methods to measure individual total exposure (exposure through air, food

and water) and resulting body burden to toxic and carcinogenic chemicals, and 2) to apply these

methods with a probability-based sampling framework to estimate the exposures and body

burdens of urban populations in several U.S. cities. This was achieved through the use of small

personal samplers, a specially designed spirometer (used to measure the chemicals in exhaled

breath), and a survey designed to insure the inclusion of potentially highly exposed groups.

The study, Commuter's Exposure to Volatile Organic Compounds, Ozone, Carbon

Monoxide, and Nitrogen Dioxide (Chan et al., 1989), focused on the driver's exposure to VOC's

in the Raleigh, NC area. The primary objective of this study was to measure driver's exposure to

all possible VOC and some combustion gases during one rush-hour driving period (18 sampling

days, two trips per day). Factors that could influence drivers’ exposure, such as different

roadways, car models, vehicle ventilation modes and times of driving were also tested. Car

exterior samples were also collected from the exterior of the moving vehicles by setting sampling

probes on the middle of the car roof. Another objective was to find the relationships between

fixed-site measurements and drivers' exposure (one fixed-site monitor matched per trip). Lastly,

the pedestrian's exposure to VOC in urban walking was evaluated with six walking samples.

The study by the South Coast Air Quality Management District (SCAQMD), In-Vehicle

Air Toxics Characterization Study in the South Coast Air Basin (Shikiya et al., 1989), was

conducted to refine the assessment of health risk due to exposure to toxic air pollutants. This

study examined the relative contribution of in-vehicle exposure to airborne toxics to an

individual's total exposure by measuring concentrations within vehicle interiors during home-towork commutes. Other objectives of this study were to develop statistical and concentration

measurement methods for a vehicular survey and to identify measures which might reduce

commuters' exposure to toxic air pollutants. Vehicles of home-to-work commuters from a non-
114

industrial park were sampled for in-vehicle concentrations of 14 toxic air pollutants, carbon

monoxide, and lead.
115

Table V.C-1

Micro-environmental concentrations of benzene (μg/m3)

Scenarios In-Vehicle Service Station Parking Garage O

Mean Max. Mean Max. Mean Max. Mea

TEAM Study (EPA, 1987b) -- 40-60a -- 3000b -- -- -- 

Raleigh, NC Studyc

(Chan et al., 1989)

10.9 42.8 -- -- -- -- – 

SCAQMD Studyd(Shikiya et al., 1989)

42.5 267.1 -- -- -- -- -- 

SCAQMD Studye(Wilson et al., 1991) -- -- -- 288 -- 67.1 -- Los Angeles, CA(Cal. EPA, 1998)

10-22 -- -- -- -- -- -- 

Sacramento, CA(Cal. EPA, 1998)

3-15 -- -- -- -- -- -- 

Detroit, MIf(Batterman, et. al., 2000)

5.3 --

NIOSH(Hartle, 1993)

 -- -- 195 -- -- -- -- 

aMaximum benzene concentrations could not be reliably determined because exposures were averaged over a 12-hour period; however

of 3 to 4 times normal exposures were calculated.

bThis concentration was estimated, rather than measured directly.cA one-hour measurement was taken for each experimental trip.dThe estimated sampling time period was 1.5 hours/round-trip.eThe measurements from this study are five-minute levels.fMeasurements taken from interiors of urban buses.
116

The second study by SCAQMD, Air Toxics Micro-environment Exposure and

Monitoring Study (Wilson et al., 1991), attempted to monitor exposures to motor vehicle

emissions in micro-environments other than in-vehicle. The study randomly sampled 100 selfservice filling stations and took samples at 10 parking garages and 10 offices nears the garages in

Los Angeles, Orange, Riverside, and San Bernadino Counties of Southern California. The study

took five-minute samples of 13 motor vehicle air pollutants in each micro-environment and in

the ambient environment. 

The 1998 California EPA study characterized the concentration of several pollutants

inside vehicles during commutes in Sacramento and Los Angeles. A variety of scenarios were

assessed, based on such variables as roadway type, traffic congestion, ventilation setting, and

vehicle type. 

Another recent study conducted in Detroit characterized concentrations of benzene and a

number of other compounds inside urban buses and compared them to ambient samples collected

on the outside of passenger cars, and to ambient monitor values. The study found that

concentrations inside buses were representative of concentrations in the ambient air collected

along bus routes. The concentrations inside buses were three to five times higher than

concentrations at fixed site monitors in Detroit.

The 1993 NIOSH study assessed benzene and MTBE concentrations and service station

attendant exposures at service stations with and without Stage II vapor recovery. The study

found that Stage II vapor recovery did not significantly reduce exposure to benzene during

refueling. However, the efficiency of Stage II vapor recovery has improved over the years. 

NESCAUM has suggested that Stage II vapor recovery systems are greater than 90% effective at

capturing MTBE and benzene vapors during refueling.45 These systems would therefore be

expected to reduce exposure beyond that shown in this initial exposure assessment.

In general, these micro-environmental exposures are short in duration, and thus are of

greater relevance to potential short-term risks rather than potential chronic risks. One microenvironmental source of exposure which could be more significant is inside homes with attached

garages. Results of sensitivity analyses on HAPEM-MS3 enable us to estimate how significant

exposures from this micro-environmental source might be.46 HAPEM-MS3 was run with

alternative sets of micro-environmental factors. The only factors that differed were those for

residential garages and homes with attached garages. The second set of factors were designed to

account for evaporative benzene emissions in these micro-environments and were used to adjust

CO concentrations upward. However, these factors were developed using data collected before

vehicles had evaporative emission controls (circa 1980).47 Since MOBILE data indicate that in

conventional fuel areas with no I/M, evaporative emissions have declined 60% between 1980 and

1990, we scaled the CO concentrations to account for the reduction. The result is average

exposure concentrations that are 90% higher than the estimates in Figure V.B-2. Such an

estimate assumes all evaporative benzene emissions originating in attached garages are from

vehicles, with none from gasoline cans, lawnmowers, snowblowers, solvents, and so on. Thus,
117

this estimate of 90% higher exposure to motor vehicle benzene emissions should be viewed as an

upper bound.

3. Acetaldehyde

The only data on micro-environmental exposures to acetaldehyde from motor vehicles are

from the In-Vehicle Air Toxics Characterization Study in the South Coast Air Basin (Shikiya et

al., 1989), which focused on the driver's exposure to VOC's in the southern California area. The

in-vehicle exposure level of acetaldehyde was determined in this study to have a mean of 13.7

μg/m3

 and a maximum measured level of 66.7 μg/m3.

4. Formaldehyde

The information contained in Table V.C-2 is excerpted from three studies that have

measured micro-environment exposures to formaldehyde. These two studies are the In-Vehicle

Air Toxics Characterization Study in the South Coast Air Basin (Shikiya et al., 1989), Air Toxics

Micro-environment Exposure and Monitoring Study (Wilson et al., 1991), and the 1998

California EPA Study of in-vehicle concentrations (California EPA, 1998).

Maximum micro-environment exposure levels of formaldehyde related to motor vehicles

were determined in these studies to range from 4.9 μg/m3

 for exhaust exposure at a service

station to 41.8 μg/m3 for exhaust exposure at a parking garage.

5. 1,3-Butadiene

There are very few data on micro-environmental exposures to 1,3-butadiene. Some invehicle measurements were taken as part of the Commuter's Exposure to Volatile Organic

Compounds, Ozone, Carbon Monoxide, and Nitrogen Dioxide (Chan et al., 1989). The invehicle exposure level of 1,3-butadiene was determined in this study to have a mean of 3.0 μg/m3

and a maximum measured level of 17.2 μg/m3. Exterior to the vehicle, the mean was determined

to also be 3.0 μg/m3 with a maximum level of 6.9 μg/m3. 
118

Table V.C-2

Micro-environmental exposure to formaldehyde (μg/m3

)

Scenarios In-Vehicle Service

Station

Parking

Garage

Office

Building

Mean Max

.

Mean Max. Mean Max. Mean Max.

SCAQMD Studya

(Shikiya et al.,

1989)

15.4 35.4 -- -- -- -- -- --

SCAQMD Studyb

(Wilson et al.,

1991)

-- -- -- 4.9 -- 41.8 -- 44.2

Los Angeles, CA

(Cal. EPA, 1998)

<MQLc

to 22

Sacramento, CA

(Cal. EPA, 1998)

5-14

a

The estimated sampling time period was 1.5 hours/round-trip.

b

The measurements from this study are five-minute levels.

c

<MQL - below method quantification limit
119

Appendix 1: Mean and Median Highway and Nonroad Contributions to Nationwide

Concentrations of Mobile Source Air Toxics in 1996, from ASPEN Modeling Using the

1996 NTI

Pollutant

Highway Contribution to

Ambient Conc. (μg/m3)

Nonroad Contribution to

Ambient Conc. (μg/m3)

Mean Median Mean Median

1,3-Butadiene 0.05 0.04 0.02 0.01

Acetaldehyde 0.40 0.32 0.27 0.12

Acrolein 0.05 0.04 0.04 0.02

Arsenic Compounds 6.5E-07 4.2E-07 7.7E-06 9.1E-08

Benzene 0.55 0.45 0.24 0.16

Chromium Compounds 4.2E-05 2.8E-05 1.4E-04 4.5E-05

Dioxins/Furans 2.9E-10 not estimated 2.0E-10 not estimated

Ethylbenzene 0.32 0.23 0.11 0.07

Formaldehyde 0.38 0.29 0.48 0.20

Lead Compounds 5.9E-05 3.9E-05 3.4E-03 4.1E-04

Manganese

Compounds

1.7E-05 1.1E-05 1.4E-04 5.1E-05

MTBE 0.44 0.041 0.29 0.001

n-Hexane 0.24 0.18 0.08 0.05

Nickel Compounds 3.3E-05 2.2E-05 3.2E-04 3.3E-05

POM (as sum of 7-

PAH)

1.2E-04 8.8E-05 2.6E-05 1.7E-05

Styrene 0.04 0.03 0.005 0.003

Toluene 2.18 1.55 0.43 0.29

Xylenes 1.20 0.85 0.40 0.26

1

Nationwide median values are much lower than mean values because a limited number of areas

in the country use MTBE in non-premium gasoline as part of the Federal Reformulated Gasoline

Program, California Reformulated Gasoline Program, or Winter Oxygenated Gasoline Program.
120

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26. Glen, G. and Shadwick, D., “Final Technical Report on the Analysis of Carbon Monoxide

Exposure for Fourteen Cities Using HAPEM-MS3,” Prepared by Mantech Environmental

Technology, Inc. for the U.S. Environmental Protection Agency, March 1998.

27. Glen, G. and Shadwick, D. 1999. HAPEM-MS3 Exposure Modeling data for Atlanta. 

Prepared by Mantech Environmental Technology, Inc. for the U.S. Environmental

Protection Agency. 

28. EPA. 1993. Motor Vehicle-Related Air Toxics Study. Office of Mobile Sources, Ann

Arbor, MI. Report No. EPA 420-R-93-005.

29. Johnson, T., M. McCoy and J. E. Capel. 1993. Enhancements to the Hazardous Air

Pollutant Exposure Model (HAPEM) as Applied to Mobile Source Pollutants. Prepared

for U. S. EPA, Office of Air Quality Planning and Standards and Office of Research and

Development by International Technology Corporation, September, 1993.

30. Palma, T., M. Riley, and J. Capel. 1996. Development and Evaluation of Enhancements

to the Hazardous Air Pollutant Exposure Model (HAPEM-MS3). Prepared for U. S.

EPA, Office of Air Quality Planning and Standards and Office of Research and

Development by International Technology Corporation, September, 1996.

31. E. H. Pechan and Associates, Inc. 1997. Determination of Annual Average CO

Inventories and the Mobile Source Contribution in Selected Areas Using the 1990

OAQPS Trends Database. Prepared for U. S. EPA, Office of Mobile Sources,

Assessment and Modeling Division, September, 1997. 

32. E. H. Pechan and Associates, Inc. 1999. CO Inventories and Mobile Source Contribution

for Atlanta. Prepared for U. S. EPA, Office of Mobile Sources, Assessment and

Modeling Division.
123

33. Systems Applications International. 1994. Projected Emission Trends and Exposure

Issues for 1,3-Butadiene. Prepared for the American Automobile Manufacturers

Association, March, 1994.

34. Law, P. L., P. J. Lioy, M. P. Zelenka, A. H. Huber, and T. R. McCurdy. 1997. Evaluation

of a probabilistic exposure model applied to carbon monoxide (pNEM/CO) using Denver

personal exposure monitoring data. Journal Air and Waste Manage. Assoc. 47:491-500.

35. Northeast States for Coordinated Air Use Management. Peer review of U.S. EPA’s Draft

Estimation of Motor Vehicle Emissions and Exposure in Selected Urban Areas.

36. California Environmental Protection Agency (1998b) Measuring Concentrations of

Selected Air Pollutants Inside California Vehicles. Final Report.

37. California Environmental Protection Agency (1998a) Report to the Air Resources Board

on the Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant. Appendix

III, Part A: Exposure Assessment. April 1998. 

38. Brunekreef, B. (1999) Environmental Diesel Exhaust Exposure and Respiratory Health of

Children in the Netherlands. Presented at the Health Effects Institute Diesel Workshop:

Building a Research Strategy to Improve Risk Assessment, Stone Mountain, GA, March

7-9, 1999.

39. EPA. 1987. The Total Exposure Assessment Methodology (TEAM) Study: Summary

and Analysis: Volume I. Office of Research and Development, Washington, D.C. June

1987. EPA Report No. EPA/600/6-87/002a.

40. Chan, C.C., H. Ozkaynak, J.D. Spengler, L. Sheldon, W. Nelson, and L. Wallace. 1989. 

Commuter's exposure to volatile organic compounds, ozone, carbon monoxide, and

nitrogen dioxide. Prepared for the Air and Waste Management Association. AWMA

Paper 89-34A.4.

41. Shikiya, D.C., C.S. Liu, M.I. Kahn, J. Juarros, and W. Barcikowski. 1989. In-vehicle air

toxics characterization study in the south coast air basin. South Coast Air Quality

Management District, El Monte, CA. May, 1989.

42. Wilson, A. et al. 1991. Air toxics micro-environment exposure and monitoring study. 

South Coast Air Quality Management District, El Monte, CA. 

43. Batterman, S. A., J. Braun, C. Peng, P. Warner, and E. Butrym. 2000. Exposure to

Urban Air Toxics During Commuting: A Field Study in the Motor City. Air & Waste

Management Association 93rd Annual Meeting, Salt Lake City, UT, June 19-22, 2000.

44. Hartle, R. Exposure to Methyl tert-Butyl Ether and Benzene among Service Station

Attendants and operators. Environmental Health Perspectives, Supplements: 101
124

(Suppl. 6): 23-26, 1993.

45. NESCAUM. 1999. RFG/MTBE Findings and Recommendations. August, 1999.

46. Glen, G. and D. Shadwick. 1998. Sensitivity Analysis Report: Analysis of Carbon

Monoxide Exposure for Fourteen Cities Using HAPEM-MS3. Prepared for National

Exposure Research Laboratory, U.S. EPA by Mantech Environmental Technology, Inc.

47. Palma, T., M. Riley, and J. E. Capel. 1996. Development and Evaluation of

Enhancements to the Hazardous Air Pollutant Exposure Model (HAPEM-MS3). 

Prepared for U.S. EPA by International Technology Corporation, September, 1996. 
125

Chapter 6: Motor Vehicle-Based Controls of Mobile Source

Air Toxics

Introduction

The Chapters 4 and 5 discussed the reductions in toxics emissions and exposure that have

already resulted from the Agency’s mobile source Volatile Organic Compound (VOC) and diesel

particulate matter (PM)) control programs. This chapter presents the rationale for our proposed

determination that it is appropriate that additional motor vehicle-based controls (beyond those

already adopted or proposed) not be proposed at this time under §202(l)(2). This is based on

considerations of the technical feasibility and cost of proposing further controls at this time. The

first section presents an overview of vehicle-based emission control technologies and their role in

reducing air toxics. The second section reviews the Agency’s most recent actions to further

reduce VOC and PM emissions from on-highway vehicles and engines. 

A. Vehicle-Based Technologies that Control Air Toxics

To better understand the nature of mobile source air toxics and their control it is helpful

to categorize them into three groups: gaseous organic toxics, diesel PM, and metals. For each

group, the following sections present an overview of these toxics and the impact of emission

control technology on these toxics. It is well documented that the Agency’s effort to control

criteria pollutants and their precursors through motor vehicle based controls has dramatically

reduced VOC emissions. As is discussed below and in Chapter 4, these VOC controls have

contributed to large reductions in gaseous mobile source air toxics (MSATs). Similarly, other

EPA controls have had major impacts on the non-gaseous MSATs as well. 

1. Gaseous Organic Toxics

Fourteen of the 21 proposed MSATs are gaseous organics. These 14 gaseous toxics can

be further categorized depending on whether they are fuel components or combustion products. 

Those that are gasoline fuel components are found in both evaporative and exhaust emissions,

while the non-fuel components that are combustion products are found only in exhaust emissions

(see Table IV-A.1).
126

Table VI.A-1

Gaseous MSATs

Gaseous Organic

Toxics

Fuel Component Exhaust Component

Acetaldehyde YES

1,3-Butadiene YES

Formaldehyde YES

Acrolein YES

Dioxin/Furans YES

POM YES YES

Styrene YES

Benzene YES YES

MTBE YES YES

Ethylbenzene YES YES

n-Hexane YES YES

Naphthalene YES YES

Toluene YES YES

Xylene YES YES

Vehicle-based controls are effective in reducing all 14 of the gaseous air toxics. The toxics that

are fuel components result in vehicle exhaust emissions and evaporative emissions (depending on

the volatility of the compound), and can be reduced through exhaust and evaporative emission

control as well by controlling the fuel composition directly. For those compounds that are

formed during the combustion process, control strategies rely on exhaust emission control

technology and changes to fuel composition.

a. Exhaust Controls

This section describes that control of exhaust VOC emissions in general. The Agency is

not aware of any exhaust emission controls that work only on the gaseous toxics listed in Table

VI.A-1. All of the control technologies of which we are aware control other VOCs as well. It is

worth noting that since almost all of the compounds listed in Table VI.A-1 contain either oxygen

functional groups or unsaturated carbon bonds, the VOC controls that are described here are
87 Siegl, W.O., et al., A Comparison of Conversion Efficiencies of Individual Hydrocarbon Species

Across Pd- and Pt-Based Catalysts as a Function of Fuel-Air Ratio, SAE 982549.

88 EPA Speciate Database http://www.epa.gov/ttnchie1/spec/index.html

127

generally slightly more effective at oxidizing these toxics than many other exhaust VOCs.87

Table VI.A-2 shows typical exhaust fractions of MSATs for a gasoline-fueled vehicle operating

with a typical fuel. (Note: actual in-use emissions can vary significantly for different vehicles

and different fuels.) As is true for this typical case, MSATs can comprise one-quarter of all

exhaust VOC emissions from a gasoline-fueled vehicle.

Table VI.A-2

Gaseous MSATs in Typical Gasoline-Fueled Vehicle Exhaust

Toxic Exhaust Fraction of VOC88

Acetaldehyde 0.5 %

1,3-Butadiene 0.5 %

Formaldehyde 1 %

Benzene 4 %

Toluene 10 %

Other Gaseous MSATs 9 %

VOCs are the result of incomplete combustion occurring in a vehicle’s engine. Some

VOC emissions are unburned fuel and engine oil, some are combustion byproducts from

partially-burned fuel and engine oil. This is true for both gasoline-fueled vehicles and diesel

vehicles. To reduce both types of VOC emissions from gasoline-fueled vehicles, manufacturers

have designed their engines to achieve virtually complete combustion and have installed catalytic

converters in the exhaust system. As is discussed later in this section a similar approach can be

used with diesel engines. In order for these controls to work well for gasoline-fueled vehicles, it

is necessary to maintain the mixture of air and fuel at a nearly stoichiometric ratio (that is, just

enough air to completely burn the fuel). Poor air-fuel mixture can result in significantly higher

emissions of incompletely combusted fuel. Current generation highway vehicles are able to

maintain stoichiometry by using closed-loop electronic feedback control of the fuel systems. As

part of these systems, technologies have been developed to closely meter the amount of fuel

entering the combustion chamber to promote complete combustion. Sequential multi-point fuel

injection delivers a more precise amount of fuel to each cylinder independently and at the

appropriate time increasing engine efficiency and fuel economy. Electronic throttle control

offers a faster response to engine operational changes than mechanical throttle control can

achieve, but it is currently considered expensive and only used on some higher-price vehicles. 

The greatest gains in fuel control can be made through engine calibrations, the algorithms
89 McDonald, J., L. Jones, Demonstration of Tier 2 Emission Levels for Heavy Light-Duty Trucks, SAE

2000-01-1957. 

90 Burch, S.D., and J.P. Biel, SULEV and "Off-Cycle" Emissions Benefits of a Vacuum-Insulated

Catalytic Convert, SAE 1999-01-0461.

91 Laing, P.M., Development of an Alternator-Powered Electrically-Heated Catalyst System, SAE 941042.

128

contained in the powertrain control module (PCM) software that control the operation of various

engine and emission control components/systems. As microprocessor speed becomes faster, it is

possible to perform quicker calculations and to increase response times for controlling engine

parameters such as fuel rate and spark timing. Other advances in engine design have also been

used to reduce engine-out emissions of VOCs, including: the reduction of crevice volumes in the

combustion chamber to prevent trapping of unburned fuel; "fast burn" combustion chamber

designs that promote swirl and flame propagation, and multiple valves with variable-valve timing

to reduce pumping losses and improve efficiency. Improvements in the overall efficiency of the

vehicle can also reduce emissions by reducing the amount of fuel that is consumed. These

technologies are discussed in more detail in the RIA for the Tier 2 FRM. 

As noted above, manufacturers are using aftertreatment control devices to oxidize VOCs

emitted by the engine. The primary approach is to use a three-way catalyst (TWC) that

simultaneously controls VOCs, CO, and NOx. New three-way catalysts are so effective that

once a TWC reaches its operating temperature, VOC emissions are virtually undetectable (0.01

gpm or less).89 Manufacturers are now working to improve the durability of the TWC and to

reduce light-off time (that is, the amount of time necessary after starting the engine before the

catalyst reaches its operating temperature and is effectively controlling VOCs and other

pollutants). EPA expects that manufacturers will be able to design their catalyst systems so that

they light off within less than thirty seconds of engine starting. Other potential exhaust

aftertreatment systems that could further reduce cold-start emissions are thermally insulated

catalysts, electrically heated catalysts, and HC adsorbers (or traps). Each of these technologies,

which are discussed below, offer the potential for VOC reductions in the future. However, there

are technological, implementation, and cost issues that still need to be addressed.

Thermally insulated catalysts maintain sufficiently high catalyst temperatures by

surrounding the catalyst with an insulating vacuum. Prototypes of this technology have

demonstrated the ability to store heat for more than 12 hours.90 Since ordinary catalysts typically

cool down below their light-off temperature in less than one hour, this technology could reduce

in-use emissions for vehicles that have multiple cold-starts in a single day. However, this

technology would have less impact on emissions from vehicles that have only one or two coldstarts per day. 

Electrically-heated catalysts reduce cold-start emissions by applying an electric current to

the catalyst before the engine is started to get the catalyst up to its operating temperature more

quickly.91 These systems require a modified catalyst, as well as an upgraded battery and charging
129

system. These can greatly reduce cold-start emissions, but could require the driver to wait until

the catalyst is heated before the engine would start to achieve optimum performance. 

Hydrocarbon adsorbers are designed to trap VOCs while the catalyst is cold and unable to

sufficiently convert them. They accomplish this by utilizing an adsorbing material which holds

onto the VOC molecules. Once the catalyst is warmed up, the trapped VOCs are automatically

released from the adsorption material and are converted by the fully functioning downstream

three-way catalyst. There are three principal methods for incorporating an adsorber into the

exhaust system. The first is to coat the adsorber directly on the catalyst substrate. The advantage

is that there are no changes to the exhaust system required, but the desorption process cannot be

easily controlled and usually occurs before the catalyst has reached light-off temperature. The

second method locates the adsorber in another exhaust pipe parallel with the main exhaust pipe,

but in front of the catalyst and includes a series of valves that route the exhaust through the

adsorber in the first few seconds after cold start, switching exhaust flow through the catalyst

thereafter. Under this system, mechanisms to purge the adsorber are also required. The third

method places the trap at the end of the exhaust system, in another exhaust pipe parallel to the

muffler, because of the low thermal tolerance of adsorber material. Again a purging mechanism

is required to purge the adsorbed VOCs back into the catalyst, but adsorber overheating is

avoided. One manufacturer who incorporates a zeolite hydrocarbon adsorber in its California

SULEV vehicle found that an electrically heated catalyst was necessary after the adsorber

because the zeolite acts as a heat sink and nearly negates the cold start advantage of the adsorber. 

This approach has been demonstrated to effectively reduce cold start emissions. 

Historically, control of VOC emissions from diesel engines has relied primarily on

technologies that improve combustion. Because diesel engines are designed to operate with very

high air/fuel ratios (that is, with excess oxygen), they have inherently lower VOC emissions than

gasoline-fueled engines. Nevertheless, since combustion is not always complete, diesels do have

significant VOC emissions. Recent efforts to lower PM emissions have lead to much more

complete combustion for diesel engines being produced today. This has resulted in VOC

emissions from diesel engines being about 0.2-0.3 g/bhp-hr. It is possible to achieve even lower

VOC emission levels by incorporating an oxidation catalyst in the exhaust. Since these catalysts

are generally used to control PM, they are discussed in more detail in that section.

b. Evaporative Controls

Evaporative emissions occur when fuel evaporates and is vented to the atmosphere. They

can occur during refueling, while the vehicle is operating, or while it is parked. This section

describes the control of evaporative VOC emissions from on-highway gasoline vehicles. Diesel

vehicles do not have significant evaporative emissions because diesel fuel has such a high boiling

point (over 300°F). Table VI.A-3 shows typical evaporative fractions of MSATs for a gasolinefueled vehicle operating with a typical fuel. MSATs can comprise one-tenth of all evaporative

VOC emissions from a gasoline-fueled vehicle.
92 EPA Speciate Database http://www.epa.gov/ttnchie1/spec/index.html

130

Table VI.A-3

Gaseous MSATs in Typical Gasoline-Fueled Evaporative Emissions

Toxic Evaporative Fraction of VOC92

Acetaldehyde 0 %

1,3-Butadiene 0 %

Formaldehyde 0 %

Benzene 2 %

Toluene 4 %

Other MSATs 5 %

In general, evaporative emission control is accomplished by sealing the fuel system, and

forcing all vented vapors to go through a charcoal canister, which adsorbs any fuel vapors

present. If the canister is sufficiently large, evaporative emissions from venting can be virtually

eliminated for normal operating conditions. The canister is occasionally flushed with fresh air

which "purges" the fuel vapors from the canister. These purged gases are then routed into the

engine air intake system so that they can be combusted in the engine. Since using the purge

gases for intake air instead of fresh air changes the air/fuel ratio, it must be coordinated with the

fueling control to ensure that the engine does not run rich. However, with modern electronic

controls, this is relatively straightforward. 

Other sources of evaporative emissions are fuel vapor venting, fuel spit-back, and postfill drip during refueling; fuel permeation through the fuel tank and fuel lines; and leaking

connections. The refueling emissions can be controlled by optimizing the nozzle-vehicle

interface, including the fill neck design, and capturing the vapors with a charcoal cannister. 

These systems are called onboard refueling vapor recovery (ORVR) systems. For design

efficiency, ORVR systems are integrated with the evaporative systems. The refueling vapors are

vented to the same cannister that is used for other evaporative emissions and are purged into the

engine along with the other vapors. The primary physical difference between an evaporative

control system and an ORVR system is the fillneck seal. In most cases, the fillneck seal is

achieved using a liquid seal. The liquid seal can be as simple as incorporating a "J-tube" into the

fillneck, much like the trap in a sink drain. ORVR systems also include anti-spitback valves to

control fuel spillage during refueling. 

Permeation and leaks can be greatly reduced by reducing the number of hoses, fittings

and connections, and by using less permeable hoses and lower loss fittings and connections. 

Fluoropolymer materials can be added as liners to hose and component materials to yield large

reductions in permeability over such conventional materials as monowall nylon. In addition,
93 McClure, B'T', et al., The Influence of an Oxidation Catalytic Converter and Fuel Composition on the

Chemical and Biological Characteristics of Diesel Exhaust Emissions, SAE 920854.

94 Pataky, G.M., et al., Effects of an Oxidation Catalytic Converter on Regulated and Unregulated Diesel

Emissions, SAE 940243.

131

fluoropolymer materials can greatly reduce the adverse impact of alcohols in gasoline on

permeability of evaporative components, hoses and seals. Manufacturers are also beginning to

incorporate “returnless” fuel injection systems. These systems use more precise fuel pumping

and metering to eliminate the return of heated fuel from the injectors, which is a significant

source of fuel tank heat and vapor generation. The elimination of return lines also reduces the

total length of hose on the vehicle and also reduces the number of fittings and connections which

can leak.

The test procedures and requirements associated with EPA's evaporative and refueling

emission standards essentially require manufacturers to design for zero emission levels. 

Upgrades to evaporative emission requirements and expansion of the refueling standard coverage

as part of recent and proposed EPA rules (see Section B) have gone even further towards the goal

of eliminating gasoline evaporation as a potential source of MSATs. More information on

evaporative control technologies is contained in the RIAs for the 1994 "Refueling Emissions

Regulations for Light-duty Vehicles and Trucks and Heavy-duty Vehicles" and the recent Tier 2

Rulmaking.

2. Diesel Exhaust

Diesel PM consists of three primary constituents: unburned elemental carbon particles

(or "soot"), which make up the largest portion of the total PM; the soluble organic fraction

(SOF), which consists of unburned hydrocarbons that have condensed into liquid droplets or

have condensed onto unburned carbon particles; and sulfates, which result from oxidation of

fuel-borne sulfur in the engine's exhaust. Diesel engines have made great progress in lowering

engine out emissions from uncontrolled levels between 0.8 and 1.0 g/bhp-hr PM to 0.1 g/bhp-hr

PM for current engines. These reductions came initially with improvements to combustion and

fuel systems. Several exhaust aftertreatment devices have also been developed to control PM. 

They generally fall into two categories: diesel oxidation catalysts (DOCs) and particulate filters

(or traps). DOCs have been shown to be durable in-use, but they control only a relatively small

fraction of the total PM mass (mostly the soluble fraction, which is typically less than 30 percent

of the total). Nevertheless, DOCs have been shown to significantly reduce the emissions of toxic

organics from diesel engines.93, 94

PM traps work by passing the exhaust through a ceramic or metallic filter to collect the

PM. The collected PM must then be burned off the filter before the filter becomes plugged. This

burning off of collected PM is referred to as "regeneration," and can occur either: on a periodic

basis by using base metal catalysts or an active regeneration system such as an electrical heater, a
95 Allansson, et al, European Experience of High Mileage Durability of Continuously Regenerating Diesel

Particulate Filter Technology. SAE 2000-01-0480.

96 Kleeman, M.J., Schauer, J.J., Cass, G. R., 2000, Size and Composition Distribution of Fine Particulate

Matter Emitted From Motor Vehicles, Environmental Science and Technology, Vol. 34, No. 7.

132

fuel burner, or a microwave heater; or, on a continuous basis by using precious metal catalysts. 

Uncatalyzed diesel particulate traps demonstrated high PM trapping efficiencies many years ago,

but the regeneration characteristics were not dependable. As a result, some systems employed

electrical heaters or fuel burners to improve upon regeneration, but these complicated the system

design and still did not provide the durability and dependability required for HD diesel

applications. 

Catalyzed diesel particulate traps have the potential to provide the same reductions in

diesel PM emissions and provide the durability and dependability required for diesel

applications. They have lower average backpressure than other traps and they need no extra

burners or heaters. Most importantly, however, they are highly efficient at trapping all forms of

diesel PM and are reliably regenerated under normal operating conditions typical of a diesel

engine. These catalyzed PM traps are able to provide in excess of 90 percent control of diesel

PM. More than one aftertreatment manufacturer is developing these precious metal catalyzed,

passively regenerating PM traps. In field trials, they have demonstrated highly efficient PM

control and promising durability. A recent publication documents results from a sample of these

field test engines after years of use in real world applications. The sampled filters had on

average four years of use covering more than 225,000 miles in applications ranging from city

buses to garbage trucks to intercity trains. Yet when tested on the US Heavy-Duty Federal Test

Procedure (HD FTP), they demonstrated PM reductions in excess of 90 percent.95 It should be

noted, however, these catalyzed traps work well only with diesel fuel with very low sulfur

content.

Modern catalyzed PM traps have been shown to be very effective at reducing not only

PM mass, but overall number of particles emitted. Hawker, et. al., found that a modern catalyzed

PM trap reduced the particle count by over 95 percent, including ultrafine particles (< 50 nm), at

most tested operating conditions.96 Particles smaller than 1,000 nanometers (nm) comprise more

than 90 percent of PM mass. Of these particles, approximately half of the mass is from particles

smaller than 100 to 200 nm. PM traps have very high particle capture efficiencies. Smaller

particles (<200 to 300 nm) are captured primarily by diffusional deposition to surfaces within the

trap walls. Capture efficiency of primary PM by diffusion actually increases for decreasing

particle size. Larger particles are captured primarily by inertial impaction on surfaces due to the

tortuous path of the exhaust gases as they pass through the porous trap walls. Capture

efficiencies for the elemental carbon fraction (soot) of diesel PM nearing 100 percent are

possible with PM traps, with the only remaining PM downstream of the trap being sulfate and a

small amount of organic material. 

3. Metals
133

Mobile source toxics included compounds of six metals: arsenic, chromium, lead,

manganese, mercury, and nickel. The source of these toxic emissions are trace amounts of these

compounds in engine oil or fuel that come from additives, impurities, and products of engine

wear. From a vehicle or engine perspective, the primary methods of reducing emission of toxic

metals would be to reduce engine wear and oil consumption. (Metal products of engine wear

collect in the engine oil, and can be emitted if the oil is burned in the cylinder or otherwise enters

the exhaust stream.) Manufacturers already have a strong incentive to reduce both engine wear

and oil consumption due to consumer demand. Moreover, manufacturers must limit oil

consumption to very low levels in order to comply with the existing PM standards. 

It is also worth noting that some engines are equipped with devices that intentionally

introduce small amounts of used crankcase oil into the fuel system as a means of disposing of the

used oil and capturing its energy content. The effects of using this type of system are essentially

the same as the effects of ordinary oil consumption. To the extent that there are toxic metals in

the oil, they would be emitted in the exhaust (generally associated with the PM emissions). We

currently require that these systems be certified to ensure that they do not cause noncompliance

with our standards. The very low PM standards that have been proposed for 2007 may

effectively preclude the use of these devices.

B. Emission Control Requirements

The previous section described the emission control technologies that are currently being

used as well as others that are projected to be used in the near future. This section describes that

regulatory requirements that will force the introduction of these projected technologies in future

model years, and the impacts that these requirements will have on toxic emissions.

1. Tier 2 Standards for Light-Duty Vehicles

On February 10, 2000, EPA published new "Tier 2" emissions standards for all passenger

vehicles, including sport utility vehicles(SUVs), minivans, vans and pick-up trucks. The new

standards will ensure that exhaust VOC emissions be reduced to less than 0.1 g/mi on average

over the fleet, and that evaporative emissions be reduced by at least 50 percent. ORVR

requirements were also extended to medium-duty passenger vehicles. By 2020, these standards

will reduce VOC emissions from light-duty vehicles by more than 25 percent of the projected

baseline inventory. (See Chapter 4 for a more detailed discussion of the impact of the Tier 2

FRM on VOC inventories.) To achieve these reductions, manufacturers will need to incorporate

nearly all available emission controls, including: larger and improved close-coupled catalysts,

optimized spark timing and fuel control, improved exhaust systems, and improved evaporative

controls. However, the Tier 2 standards are projected to be feasible without using HC traps or

electrically heated catalysts. In that rulemaking, EPA determined that those technologies were

not likely to be cost-effective.

The Tier 2 FRM also included the first federal formaldehyde emission standards for light-
134

duty vehicles. However, it is actually the VOC controls that are expected to provide the toxic

emission reduction. According to the Tier 2 RIA, these controls will reduce benzene emissions

by more 20,000 tons per year, acetaldehyde by 2,000 tons per year, formaldehyde by 4,000 tons

per year, and 1,3-butadiene by more than 2,000 tons per year. Although not calculated for that

rule, the standards will also significantly reduce emission of the other gaseous toxics since it will

require reductions of exhaust and evaporative VOC emissions in general.

2. Heavy-Duty Engines and Vehicles

EPA recently proposed to significantly reduce VOC emissions from heavy-duty vehicles

in two separate proposals (64 FR 58472, October 29, 1999; and 65 Federal Register 35430, June

2, 2000). In the first proposal, we proposed new standards for 2004 and later heavy-duty

gasoline vehicles that are projected to reduce exhaust VOC emissions by two-thirds or more. To

comply with these proposed 2004 standards, manufacturers are expected to optimize existing

emission controls, but are not expected to need to use the more sophisticated controls projected

for Tier 2 vehicles. We also proposed to require all complete vehicles under 10,000 pounds

GVWR to comply with the ORVR requirements.

The other proposal includes new emission standards that would begin to take effect in

2007, and would apply to all heavy-duty highway engines and vehicles. These proposed

standards will require the use of high-efficiency catalytic aftertreatment devices as well as

advanced engine technologies. For diesel engines, manufacturers are expected to incorporate

catalyzed PM traps that could virtually eliminate both organic PM (elemental carbon and SOF)

and VOC emissions from diesel engines during normal operation. However, the engines will

still emit some sulfate PM, and can also emit some VOCs during start-up operation where the

catalysts are below minimum functional temperature. Fortunately, since diesels are used mostly

in commercial applications, almost all operation will occur with a warm catalyst. For gasoline

engines manufacturers are expected to incorporate the technologies similar to those that will be

used to comply with the Tier 2 light-duty standards. This will include improved fuel injection,

fast electronic throttle controls, reduction of crevice volumes, "fast burn" combustion chamber,

and improved three-way catalysts. When combined, these new heavy-duty standards are

projected to result in a 83,000-ton reduction in PM emissions and a 230,000-ton reduction in

VOC emissions in 2020.

C. Potential for Further Reductions

Given the technology-forcing nature of the recently finalized Tier 2 emission standards

for light-duty vehicles and the recently proposed emission standards for heavy-duty vehicles and

engines, it is not feasible that manufacturers would be able to further reduce toxic emissions

significantly at this time. For both gasoline and diesel vehicles, these standards will result in

near-zero exhaust VOC emissions for all operation other than engine starting. Since the only

significant exhaust emissions will occur within the first minute after engine starting, further

reductions would require manufacturers develop technologies specifically for this very short

window. While start-up controls such as HC traps and electrically-heated catalysts exist, the
97 Analysis of Cold-Start Emission Controls, Memo to Docket.

98 58 FR 9467, February 19, 1993.

135

issues of technological feasibility and cost are significant enough that EPA does not believe it

would be appropriate at this time to propose more stringent standards based on these

technologies.97 For diesels, carbonaceous PM emissions are expected to be near-zero for all

operation, including engine starting. The only PM that is expected to occur in significant

amounts will be in the form of sulfate, which cannot be reduced by vehicle-based controls.

Similarly, for evaporative emissions, there remains little that a manufacturer would be

able to do to reduce emissions further. They are already required to design their Tier 2 vehicles

to have essentially zero evaporative emissions, and must also account for the effect of alcohol in

the fuel on fuel line permeability and in-use performance. For an in-use Tier 2 vehicle, the only

evaporative emissions that are expected to occur would be the result of an occasional leaking

connection in the fuel or evaporative control system, or abnormal vehicle operation (e.g. vehicles

parked for several days without being driven). While manufacturers are expected to minimize

the number of fuel connections used in the vehicle and to use designs that are not prone to

leaking, some leaks are still likely to occur in use. Thus, we believe that these remaining sources

of in-use evaporative emissions cannot be readily addressed by the manufacturer with current

technology.

Since metal emissions are primarily the result of the combination of engine wear and oil

consumption, manufacturers already have a very strong incentive to minimize metal emissions. 

Those metal emissions that are not a result of engine wear and oil consumption are caused by

contaminants or additives in the fuel, and are thus beyond a manufacturer's control. An EPA

standard for metals set at a level that is feasible for manufacturers would only enforce existing

engine designs. It would not be likely to achieve any real reductions. Such a standard would not

justify the administrative and testing burden that it would cause. As discussed earlier, EPA is

addressing in the 2007 heavy-duty rulemaking the issue of blending used oil into diesel fuel.

The primary vehicle-based opportunity to reduce toxic emissions is in the area of in-use

operation. With the new standards that are coming into effect, manufacturers will be designing

and building their vehicles and engines to have very low toxic emissions. However, this does not

guarantee that all of these vehicles and engines will have low toxic emissions in use. 

Malmaintenance of and/or tampering with the emission controls could result in increased toxic

emissions. EPA is continually working to improve in-use maintenance and enforce the

tampering prohibition. To address the malmaintenance issue, EPA has established onboard

diagnostic (OBD) requirements for manufacturers.98 These OBD provisions require that vehicle

manufacturers install dashboard indicators that alert drivers to the need for emission-related

maintenance, and electronic monitors that store codes in the vehicle's computer to assist

mechanics in the diagnosis and repair of the malfunction. To address both the malmaintenance

and tampering issues, EPA is working with states to develop and optimize inspection and
136

maintenance (I/M) programs that monitor the emission performance of in-use vehicles. 

Historically, these programs have relied on tailpipe testing to identify high-emitting vehicles. 

However, these programs have begun to rely more on the OBD systems to identify the highemitting vehicles, as well as the cause of the emission problem. 
99 Petroleum Supply Annual 1998, Energy Information Administration, June 1999. Table 2.

137

Chapter 7: Fuel Controls

This Chapter contains background information and analyses supporting our proposed

benzene control program. We first summarize benzene levels in gasoline produced by the U.S.

oil industry, the benzene reduction technologies expected to be used by the industry to maintain

or reduce gasoline benzene levels, and the means through which we expect the industry to

comply with our proposed anti-backsliding requirement. We then describe the costs and

emission benefits of our proposed program, and end with a short discussion of the more stringent

controls for fuel benzene content that we have chosen not to propose in this rulemaking.

A. Industry and Product Characterization

1. Description of entities subject to the proposed benzene standards

Our proposed anti-backsliding program will apply to every domestic refinery which

produces gasoline and every importer or foreign refiner of gasoline to the U.S. We are not

proposing any new standards applicable to parties who buy, sell, and/or trade gasoline

downstream of refineries. This Section will provide a summary of current gasoline operations

for refiners and importers.

a. Refiners

Approximately 146 domestic refineries collectively produced approximately 2.9 billion

barrels of gasoline in 1998, or approximately 7.9 million barrels per calendar day on average99

(however, not all refineries produce gasoline). Refineries are often identified by the Petroleum

Administration for Defense District (PADD) in which they reside. PADDs are shown in Figure

7.A-1, while the number of refineries in each PADD, along with gasoline production and

consumption in that PADD, are shown in Table VII.A-1. Although a refinery may be located in

a particular PADD, the gasoline produced at that refinery may not necessarily be distributed in

that same PADD. For example, most of the gasoline consumed in PADD 1 is produced by

PADD 3 refineries.
100 Petroleum Supply Annual 1998, Volume 1, Energy Information Administration, June 1999. 

101 Petroleum Marketing Annual 1998, Energy Information Administration, May 1999.

138

Figure VII.A-1

Location of PADDs in the Contiguous U.S.

IV

V I

II

III

Table VII.A-1 

Refinery Count and 1998 Gasoline Volumes by PADD

PADD

Number of

operating

refineries as

of 1/1/2000 

Gasoline

production

(million

barrels)100

Gasoline

consumption

(million

barrels)101

I 14 354 1092

II 30 673 886

III 59 1270 418

IV 15 94 96

V 28 490 521
102 Petroleum Supply Annual 1998, Volume 1, Energy Information Administration, June 1999. 

139

b. Gasoline importers

In 1998, approximately 113 million barrels of gasoline were imported into the U.S. 102

Gasoline is imported into each coast, as well as into Alaska and Hawaii. Gasoline imports by

PADD are shown in Table VII.A-2.

Table VII.A-2 

1998 Imported Gasoline

Millions of barrels

PADD I

PADD II

PADD III

PADD IV

PADD V

104

1

6

0.2

2

2. Gasoline benzene level variations

This Section presents the average benzene levels for refineries across the U.S. Brief

explanations are given for why the benzene levels vary from one refinery to another.

a. Refinery impacts

The 1998 average benzene levels for gasoline are shown in Table VII.A-3 on a PADDby-PADD basis.
103 Based on 1998 batch reports collected by EPA under the RFG and anti-dumping programs

140

Table VII.A-3

PADD-Average Refinery Benzene Levels for Gasoline in 1998103

CG benzene

concentration

RFG benzene

concentration

PADD I

PADD II

PADD III

PADD IV

PADD V Excl. Caa

 0.87 vol%

1.37

0.91

1.59

2.15

 0.56 vol%

0.91

0.65

0.92

n/a a Excluding gasoline consumed in California

The values shown in Table VII.A-3 represent the average benzene levels for all refineries

in each PADD. However, fuel benzene levels can also vary significantly from refinery to

refinery. There are a number of reasons for these variations, including:

1) Type(s) of crude being processed

2) Refinery size and configuration

3) Other products produced

4) Proximity to petrochemical markets for extracted benzene

5) Available capital funds to install benzene reduction equipment

6) Whether a refinery produces CG, RFG, or both

b. Reformulated Gasoline Production Impacts

The Clean Air Act Amendments of 1990 mandated that the benzene content of

reformulated gasoline (RFG) not exceed 1.0 volume percent. EPA’s RFG rules include this

requirement, and also provide refiners and importers the option of meeting this requirement on

an annual average basis. Under the averaging program for RFG, a refinery or importer is in

compliance with the requirements of the program if the benzene content of their gasoline

annually averages 0.95 volume percent or less. This averaging standard also includes a pergallon standard of 1.3 volume percent to ensure that benzene levels in RFG are not dramatically

different from one area to another. The more stringent averaging standard was intended to

recapture the margin of safety that gasoline suppliers were expected to build into their fuels to

comply with the per-gallon standards. Thus, because of the reformulated gasoline program, RFG

benzene levels, on average, would be expected to be less than 1.0 volume percent. In fact, RFG

benzene levels in 1998 averaged 0.65 volume percent, significantly less than required by the

RFG program. EPA believes that this "overcompliance" with respect to the RFG benzene

requirement is due to a number of factors, including:
104 "Analysis of the impacts of control programs on motor vehicle toxics emissions and exposure in urban areas

and nationwide," U.S. EPA report number EPA420-R-99-030, November 1999

141

1) Benzene extraction for the petrochemical industry

2) Reduction in overall aromatics due to use of oxygenates for octane

3) Dilution with oxygenates

Absent regulatory changes affecting toxic emissions and/or oxygenates, or reduction in

the petrochemical demand for benzene, EPA expects that this average level of overcompliance

would likely continue. Note that the overcompliance is only a national average, and not all

gasoline overcomplies with the current fuel benzene requirements. For instance, in 1998

approximately seven percent of RFG contained benzene at levels higher than the 0.95 volume

percent averaging standard (this does not necessarily represent a breech of the standard, since

individual batches can be above the standard so long as the annual average is at or below the

standard).

The anti-dumping program applicable to conventional gasoline (CG) was instituted to

ensure that refiners could not move the dirtiest components of gasoline from RFG to CG. It

requires that CG produced after 1994 be no dirtier than it was in 1990 on a refinery-by-refinery

basis in terms of exhaust toxics and NOx emissions. The anti-dumping program does not include

any direct controls on the benzene content of CG. However, in 1998 the CG benzene content

averaged 1.1 volume percent while the average 1990 fuel benzene content of those refiners or

importers with individual baselines is approximately 1.3 volume percent. Thus, there is

significant overcompliance on CG benzene content. We believe that this overcompliance is due

to many of the same factors given above for reformulated gasoline as most refiners in

overcompliance for RFG benzene are also in overcompliance for CG benzene. 

B. Benzene Emissions and Inventory

As described in more detail in the preamble, we have determined that an anti-backsliding

program focusing on gasoline benzene control is the most appropriate program to propose at this

time. In this section we present a brief discussion of the benzene inventories by motor vehicle

category, as well as the emissions. On the basis of these inventory and emission estimates, it is

clear that gasoline benzene control provides an optimum means for lowering toxics emissions

through fuel changes.

According to a study conducted by Sierra Research, Inc.104, the vast majority of benzene

emissions from on-highway vehicles is produced by gasoline-powered vehicles. See Table

VII.B-1. In 2007, benzene emissions from diesel-powered vehicles are expected to be only onefortieth that coming from gasoline-powered vehicles. This substantial difference is due in part to

the fact that the fraction of total hydrocarbon emissions which is benzene is significantly lower

for diesel vehicles than it is for gasoline vehicles. 
142

Table VII.B-1

On-highway Baseline Benzene 

Inventories from Sierra Report (50-state Tons)

Gasoline Diesel

1990 252,136 4625

1996 162,117 2905

2007a 83,907 2152

2020a 64,894 2609 a Assumes that Tier 2 vehicle standards and 30 ppm 

gasoline sulfur standards have gone into effect. 

Implications of the proposed standards for MY2007 

heavy-duty engines have not been included.

From this table it is clear that current ambient benzene concentrations are more a function

of gasoline than of diesel. However, benzene and other MSATs are produced by diesel vehicles,

and so do represent a potential alternative avenue for control. At this time we do not have

sufficient data to correlate fuel properties with individual toxic compounds from diesel vehicles. 

As a result, it is unclear how refiners would change diesel fuel to lower toxics emissions.

Benzene emissions from gasoline-powered vehicles are an ideal focus of any toxics

control program. Of the toxic compounds emitted by gasoline-powered vehicles, benzene clearly

constitutes the largest single fraction on a mass basis. This fact is illustrated both by the baseline

emissions used in the RFG program as well as the toxics inventories estimated by Sierra

research. These values are summarized in Table VII.B-2.
143

Table VII.B-2

Relative Importance of Toxics Emissionsa

 from Gasoline-powered Vehicles

Summer baseline emissions for

Phase II RFG program

Sierra Research 50-State inventories

for gasoline vehicles in 2007

mg/mi % of total Tons % of total

Benzene 59.04 69 % 83,907 68 %

Acetaldehyde 4.44 5 % 8,849 7 %

Formaldehyde 9.70 11 % 20,161 16 %

1,3-Butadiene 9.38 11 % 10,049 8 %

POM 3.04 4 % n/a n/a

a Other toxic compounds may also be present in vehicle emissions. However, those compounds are emitted at much

lower levels than those listed in this table.

C. Technological Feasibility of the Proposed Program

1. Requirements for refiners and importers

The proposed program would require that, beginning in 2002 the benzene content of

gasoline produced at a domestic refinery or imported cannot exceed the average benzene content

of the gasoline produced at that refinery or importer from January 1, 1998 through December 31,

1999. In effect, the proposal maintains current gasoline performance with respect to benzene

content, on an individual refinery or importer basis. Because we are not requiring benzene

controls more stringent than current performance, compliance should not require extra effort by

refiners, barring unexpected circumstances. 

We are proposing that compliance with the proposed benzene requirement be met

separately for RFG and CG. EPA believes this is the most appropriate methodology for benzene

accounting, as it would ensure that RFG and CG areas are in general receiving the same quality

of gasoline, with respect to benzene, as they did during the 1998-1999 timeframe. We do not

expect significant changes between 1998-1999 and 2000-2001 (immediately prior to the program

start) in terms of gasoline benzene levels despite the fact that the Phase II RFG program began

on January 1, 2000. Phase II RFG will require more stringent emission performance standards

that could include lower concentrations of butanes and pentanes and might tend to lower octane

levels. However, we believe that these potential changes will have only negligible effects on fuel

benzene levels, and that as a result the 1998-1999 baselines will remain representative of current

benzene levels.
105 Higgins, Terry, Background Paper on Benzene Control in Gasoline, National Petroleum Refiners

Association. March 23, 2000.

144

2. Requirements for those without complete 1998-1999 benzene data

Certain regulated parties did not produce or import gasoline into the U.S. during some or

all of 1998-1999 , and thus do not have an appropriate benzene baseline as required under the

proposal. EPA is proposing the following criteria for determining the baseline benzene for a

regulated entity for use in our proposed benzene control program:

1) For parties which produced or imported for more than one year during 1998-1999 .

EPA is proposing that refineries and importers must establish an individual benzene

baseline if it produced gasoline for at least 12 consecutive months during 1998-1999 . 

All relevant and valid data from 1998-1999 must be included in the baseline

determination.

2) For parties which produced or imported for less than one year during 1998-1999 .

EPA is proposing that refiners or importers who produced or imported gasoline for less

than 12 consecutive months during 1998-1999 would be assigned the 1998-1999 industry

averages for CG as their benzene baseline. The industry average baseline will be

determined and announced by EPA as part of this rulemaking.

3. Refinery technologies for controlling gasoline benzene

Under today’s proposal, a refinery must produce gasoline with an annual average fuel

benzene level no greater than the refinery’s 1998-1999 average. This Section presents a

summary of how refineries control the benzene content of their gasoline. These technologies can

be used to maintain benzene levels at the 1998-1999 baselines, or could be used to lower fuel

benzene levels further in the event the we promulgate more stringent benzene standards in the

future. Other means of controlling gasoline benzene levels may be available in 2004, and will be

considered during our evaluation of the appropriateness of additional future benzene controls.

a. General description

Gasoline is a mixture of blended streams from various units in the refinery. The refinery

unit which contributes the most benzene to gasoline is the reformer. The product from the

reformer, reformate, is a highly aromatic, high octane gasoline-blending stream. Reformate

constitutes approximately 30 percent of gasoline volume and typically contains 3 to 8 percent

benzene. Thus, the reformer contributes 70 to 85 percent of the benzene in the gasoline pool105 . 

As a result, most methods for controlling gasoline benzene either affect reformer operation or

address reformate benzene, as discussed below.

Some refiners may be able to make adjustments to their reformer in order to maintain
145

current benzene levels in their gasoline pool. Refinery adjustments to the reformer may include

changing out the catalyst, less throughput, and/or running the reformer less severely. Two types

of reformers are most prevalent in refineries: semi-regenerative and continuous regeneration. 

Continuous regeneration reformers operate at less pressure than semi-regenerative reformers,

which means less dealkylation of heavy aromatics to benzene. Therefore, another reformer

option would be to change from a semi-regenerative reformer to a continuous regeneration

reformer because a continuous regeneration reformer generally produces less benzene than a

semi-regenerative reformer. Another option available to a refinery in order to maintain current

benzene levels in their gasoline would be to blend oxygenates in their gasoline. Since

oxygenates do not contain benzene, it may be used as a diluent to maintain or reduce benzene

levels in gasoline. Also, oxygenates are high octane blending components which enable a

refinery to run their reformer less severely and use less feed to reformer. 

The primary methods of reducing the benzene content of gasoline are pre-fractionation

and post-fractionation. Pre- and post- refer to the timing of fractionation, or stream separation,

relative to the reformer, that is, either before or after the reformer, respectively. There are several

processes of pre-fractionation and post-fractionation, which are explained below. Refiners may

use one method or a combination of methods to achieve their desired gasoline benzene content,

depending on refinery configuration and crude oil mix.

There are two pre-fractionation options with different processes. The first option

involves the use of a naphtha splitter which splits the naphtha stream coming from the crude

tower into a light naphtha stream and a heavy naphtha stream. The light naphtha stream, which

are C-5 and C-6 compounds, may be blended directly to gasoline if the light naphtha benzene

content is low and octane is not needed. The naphtha splitter removes and diverts part of this

stream which contains some of the chemical components that would have formed benzene in the

reformer (at some refiners this split is made in the crude tower). The heavy naphtha stream,

which are C-7 and heavier compounds, are sent to the reformer. Since this process removes

some of these "benzene precursors" from the feed to the reformer, the benzene content of the

reformate, and thus of gasoline, is reduced. The second option of pre-fractionation involves the

use of the naphtha splitter technology described previously, and in addition, uses an

isomerization unit which rearranges the structure of the benzene precursors and converts the light

naphtha stream into a higher octane stream. 

Benzene reduction strategies which occur after the reformer is called post-fractionation. 

Post-fractionation is the fractionation of the reformate stream after the reformer. There are three

post-fractionation options that a refinery may choose, but each post-fractionation option has

different processes.

The first option of post-fractionation is isomerization of the benzene rich portion of the

reformate stream. In this process, benzene is saturated to cyclohexane using hydrogen. This

strategy recovers the octane of the stream by isomerizing the C5/C6 paraffins to higher octane

iso-paraffins. One variation of this type of technology involves a combination of both prefractionation light naphtha stream and post-fractionation of the reformate product, where both
106 Meyers, Robert A, Handbook of Petroleum Refining Processes, second edition,

McGraw-Hill, Boston (1997).

146

resulting splits are routed to the C5/C6 isomerization unit. This option would be chosen by a

refinery if they needed more higher octane blending components. Benzene saturation in an

isomerization unit normally requires a separate reactor specifically designed for saturating

benzene.

The second option of post-fractionation is hydrogen saturation of the benzene rich stream

from the reformate product. In the case of benzene, the process of hydrogen saturation converts

benzene to cyclohexane. This process can require a significant amount of hydrogen, and

hydrogen is an expensive commodity in a refinery. Further, cyclohexane has a lower octane than

benzene. If hydrogen saturation is used as a stand alone unit, then it will most likely have an

impact on the octane of the refiner's gasoline. However, the octane may be increased by further

processing the saturation product in an isomerization unit. This additional step may further

increase the cost of benzene reduction. As a result of these issues, a refinery may choose not to

pursue this route unless it already has hydrogen saturation capacity or if hydrogen is readily

available. 

Finally, extraction is used to separate and collect chemical grade benzene for the

petrochemical market. It requires the use of a reformate splitter to separate out a benzene-rich

stream from the reformate. This benzene-rich stream is then sent to an aromatic extraction

complex which extracts the benzene or converts it into other petrochemicals, such as xylenes. 

The capital cost of a benzene extraction unit is quite high, approximately $12.5 million for a

10,400 barrel per day unit106. Thus a refinery wants to ensure that such a unit will be run to the

fullest extent possible so as to maximize the return on investment. Additionally, benzene has a

very high freezing point (i.e., around 40 degrees F) which requires it to be shipped in heated

barges or heated railway cars. This method of transportation costs approximately three times

more than transportation costs of other petrochemicals. Further, stationary source benzene

emissions from a refinery are heavily regulated, so refiners must invest significant capital at the

refinery to control stationary source benzene emissions. Therefore, in order to make benzene

extraction economically feasible for a refinery, the refinery must be located near a petrochemical

market. Most petrochemical markets are on the East or Gulf coasts. Approximately twenty-five

refiners (outside of California) have benzene extraction units.

b. Impacts of refinery size and location

In general, larger refineries and multi-refinery refiners have more options for reducing

benzene than smaller refiners. In general, larger refineries and multi-refinery refiners have more

technological options for reducing benzene. For example, of the estimated 26 refineries with

benzene extraction units, 20 have crude capacities greater than 140,000 barrels per calendar day,
107 Oil & Gas Journal, December 20, 1999

108 Based on an analysis from Honeywell Hi-Spec Solutions, based on 1999 NPRA Petrochemical Survey for

benzene. Contact: Robert Harvin, Honeywell Hi-Spec Solutions, 713-953-3400.

147

and all but one belong to multi-refinery refiners107.

On average, refiners in PADDs I, II and III have more complex refineries with more

options for stream processing, than refiners in PADDs IV and V outside of California. 

Additionally, most of the petrochemical benzene markets are in PADDs I, II and III, thus

providing a readily accessible market for chemical benzene. Thus, in general, refiners in PADDs

I, II and III would be expected to have less difficulty dealing with any production issues which

would affect their compliance with the proposed program.

D. Costs and Benefits of the Proposed Program

We have referred to our proposed program as an "anti-backsliding" program because it is

designed to ensure that fuel benzene levels rise no higher than they were in 1998-1999 . 

Refineries will not be required to reduce their benzene levels below the average levels they

achieved in 1998-1999 . In addition, we believe that, because our proposed program applies

annual average standards to each refinery, each refinery will be able to meet their 1998-1999

baseline without the need for a compliance margin to account for blending tolerances,

measurement uncertainty, and contamination. As a result, we do not expect that our proposed

standards will result in reductions in fuel benzene levels in comparison to 1998-1999 levels, and

refiners should therefore incur negligible costs, if any, for compliance with these proposed

standards. 

No reduction in fuel benzene levels also means that our proposed standards will produce

no emission benefits in comparison to 1998-1999 levels. However, benzene emissions

associated with fuel benzene content could go down over time for another reason. Currently,

much of the overcompliance on gasoline benzene levels arise from the demand for chemical

benzene which rose approximately 30 percent between 1990 and 1998108. This benzene was used

in the production of ethylbenzene, styrene, cumene, and other compounds. The trends for

chemical benzene demand show continued increases through 1998, and future forecasts from

both Dewitt & Company, Inc. and Honeywell Hi-Spec Solutions include continued increases for

the next several years. This forecast is shown in Figure 7.D-1.
148

Figure VII.D-1

Chemical Benzene Demand in the U.S. (Honeywell Hi-Spec Solutions forecast)

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

1700

1800

1900

2000

2100

2200

2300

2400

Million gallons

Based on this forecast, then, it is possible that existing extraction capacity could be expanded in

the coming years to take advantage of the expected growth in the chemical benzene market,

which would lower benzene levels in gasoline even further than their 1998-1999 levels. 

This proposed anti-backsliding program does not include a credit trading program. However,

EPA is seeking comment on the need for and viability of a credit trading program such as

outlined below. While the agency believes it has provided sufficient flexibility with the proposed

deficit carryover program, we are seeking comment on this credit trading approach as an

alternative, or additional, means of providing compliance flexibility.

The current Reformulated Gasoline Rules provide a credit program that allows the

transfer of benzene credits by refiners, importers, and blenders (see 40 CFR 80.67). In this

program, benzene credits can be generated from a baseline average of 0.95 vol% benzene. This

program will remain in place. Refiners that currently rely on this program, if any, will continue

to be able to use it in meeting the basic RFG requirements in 40 C.F.R. subpart D.

This credit generation and transfer approach could also be incorporated in the proposed

anti-backsliding benzene standard. Refiners could generate credits by reducing the average

benzene in their product below the anti-backsliding baseline. Under such a trading program,

compliance could be achieved through a transfer of benzene credits provided that (1) the credits
149

are generated in the same averaging period as they are used; (2) the credit transfer takes place not

later than 15 working days following the end of the averaging period in which the benzene

credits were generated; (3) the credits were properly created; and (4) the credits are transferred

directly from the refiner, importer, or blender that created the credits to the refiner, importer,

blender that used the credits to achieve compliance (i.e., no brokering of credits). In addition,

based on the fact that RFG and CG would have separate baselines, EPA believes it would be

inappropriate to allow credit trading between the RFG and conventional gasoline pools.

E. More Stringent Control Programs

There are a wide variety of programs more stringent than the anti-backsliding program we

are proposing today which may yield additional toxics emissions reductions beyond the

reductions we expect under existing programs. These include further fuel benzene controls,

controls on other fuel properties which affect toxics emissions, or emission performance

standards directed at one or multiple toxic compounds. Because we do not have sufficient

information to adequately evaluate the costs and benefits of such additional programs at this

time, we are not proposing any such an additional program in today's action. Instead, we will

conduct further evaluations of the costs and benefits of more stringent toxics controls through

fuels in our Technical Analysis Plan over the next few years. 
109 “Nonroad Engine and Vehicle Study - Report and Appendices,” EPA-21A-201, November 1991

(available in Air Docket A-96-40).

110 HC stands for hydrocarbon. HC and VOC are very similar and are generally used interchangeably.

111 59 FR 31306, June 17, 1994.

150

Chapter 8: Nonroad Mobile Source Air Toxics

In this chapter we first describe our current nonroad engine emission control programs

and then present our estimates of the impacts of these programs on future air toxics inventories. 

We are looking at nonroad MSATs emissions separately from motor vehicle emissions of

MSATs primarily because our understanding of nonroad MSAT emissions is much more limited

than that of motor vehicle MSAT emissions. Therefore, this chapter ends with a discussion of

the significant uncertainty and data gaps that exist with respect to toxics emissions from nonroad

engines. We will need to fill these gaps in our data before we can assess the need for, and

appropriateness of, programs intended to further reduce nonroad MSATs.

A. Overview of Current Nonroad Engine Emission Control Programs

The 1990 Clean Air Act Amendments specifically directed us to study the contribution of

nonroad engines to urban air pollution, and regulate them if warranted. "Nonroad" is a term that

covers a diverse collection of engines, equipment, and vehicles. Also referred to as "off-road" or

"off-highway," the nonroad category includes outdoor power equipment, recreational equipment,

farm equipment, construction equipment, lawn and garden equipment, and marine vessels.

Though dealt with separately in the Clean Air Act, locomotives and aircraft can also be

considered categories of nonroad engines. Except for aircraft, we did not regulate emissions from

nonroad engines prior to the mid-1990s.

In 1991, we released a study documenting emission levels across a broad spectrum of

nonroad equipment that were higher than expected.109 The study showed that emissions from

nonroad engines are a significant source of oxides of nitrogen (NOx), volatile organic compound

(VOC), and particulate matter (PM) emissions. In some areas of the country, emissions from

nonroad engines represent a third of the total mobile source NOx and VOC inventory and over

two-thirds of the mobile source PM inventory. Based on the results of this study, referred to as

NEVES, we determined that emissions of NOx, HC,110 and CO from nonroad engines and

equipment contribute significantly to ozone and CO concentrations in more than one

nonattainment area.111 Thus, we initiated regulatory programs for several categories of nonroad

engines as required by section 213(a)(3) of the Clean Air Act.

In addition to the determination of significance for NOx, HC, and CO emissions just

discussed, we made a determination, under section 213(a)(4) of the Clean Air Act, that smoke

and PM emissions from nonroad engines and equipment significantly contribute to air pollution
112 59 FR 31306, June 17, 1994.

151

that may be reasonably anticipated to endanger public health or welfare.112 Under this

determination we are authorized to establish smoke and PM emission standards for nonroad

engines and equipment. While we have established smoke and PM regulations for many

categories of nonroad engines and equipment, our efforts to date have been more focused on

achieving NOx reductions from diesel engines, and HC and CO reductions from gasoline

engines.

The broad category of nonroad equipment encompasses a large variety of equipment

types, from hand-held lawn and garden equipment to locomotives and large marine vessels. The

engines used in nonroad equipment also vary dramatically, from very small two-stroke sparkignited (SI) gasoline engines to very large two-, and four-stroke diesel engines. Many of these

engines are designed and manufactured specifically for their nonroad applications. Others are

adaptations of on-highway engine designs, or even other nonroad engines. For example, most

land-based nonroad diesel engines are based on on-highway engines, with modifications as

necessary for nonroad application. Likewise, most small and medium size diesel marine engines

are modified land-based nonroad engines.

Even though many nonroad engines are derived from on-highway engines, the

technologies applied to on-highway engines to reduce emissions are often not readily

transferrable to nonroad engines, or are transferrable to different nonroad applications in different

degrees. The physical limitations of nonroad equipment, as well as different operating

environments and duty cycles, sometimes limit the application of on-highway emission reduction

technologies. For example, charge air cooling is widely used as a NOx reduction technique for

large diesel truck engines. With the aftercooler mounted on the front of the truck, the ram air

available as a truck travels down the road can afford a large degree of cooling. However, with

land-based nonroad equipment the available cooling tends to be significantly lower, both because

of the physical limitations of the equipment in terms of mounting the aftercooler, as well as the

typically slower speeds at which such equipment tends to operate. Conversely, there is a large

amount of cooling available in marine applications through the use of the surrounding water to

cool the charge air.

Due to the variety of nonroad engine and equipment types and sizes, combustion

processes, uses, and potential for emissions reductions, we have placed nonroad engines into

several categories for regulatory purposes. These categories include land-based diesel engines

(e.g., farm and construction equipment), small land-based spark-ignition (SI) engines (e.g., lawn

and garden equipment), large land-based SI engines (e.g., forklifts, airport ground service

equipment), marine engines (including diesel and SI, propulsion and auxiliary, commercial and

recreational), locomotives, aircraft, and recreational vehicles (e.g., large land-based SI engines

used in off-road motorcycles, all-terrain vehicles and snowmobiles). Summaries of our current

or anticipated programs for these nonroad categories follow. The information presented for these

programs is, in many cases, taken directly from the preambles and supporting documents of the
113 59 FR 31306, June 17, 1994.

114The Mine Safety and Health Administration is responsible for setting requirements for

underground mining equipment. Locomotives and marine vessels are covered by separate EPA

programs.

115 63 FR 56968, October 23, 1998.

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final rules. Following the discussion of the specific nonroad control programs, we present a

general overview of the nonroad fuels issue.

1. Land-Based Nonroad Diesel Engines

Nonroad diesel (also referred to as compression-ignition) engines dominate the large

nonroad engine market and comprise approximately 25 percent of the current mobile source NOx

emissions inventory and 40 percent of the current mobile source PM emissions inventory.

Examples of applications falling into this category include agricultural equipment such as

tractors, construction equipment such as backhoes, material handling equipment such as heavy

forklifts, and utility equipment such as generators and pumps.

Under our regulations, diesel engines greater than 50 horsepower (hp) must comply with

Tier 1 emissions standards that are being phased in between 1996 and 2000, depending on the

size of the engine.113 Under the Tier 1 standards, we project that NOx emissions from new diesel

nonroad equipment will be reduced by over 30 percent from uncontrolled levels. The Tier 1

standards do not apply to engines used in underground mining equipment, locomotives, and

marine vessels.114

In August 1998, we adopted more stringent emission standards for NOx, HC, and PM for

new nonroad diesel engines, to be phased in over several years beginning in 1999.115 Engines

used in underground mining equipment, locomotives, and marine engines over 50 hp are not

included. This comprehensive new program includes the first set of standards for nonroad diesel

engines less than 50 hp. Standards for these small engines will be phased in from 1999 to 2000. 

The rule also phases in more stringent Tier 2 standards for all engine sizes from 2001 to 2006,

and yet more stringent Tier 3 standards for engines over 50 hp from 2006 to 2008. Finally, the

new program includes a voluntary program to encourage the production of advanced, very-low

emitting engines. Under these new standards, we project that emissions from new nonroad diesel

equipment will be further reduced by 60 percent for NOx and 40 percent for PM compared to the

emission levels of engines meeting the Tier 1 standards. We are currently working on the

Nonroad Tier 3 technology review, which will include proposal and adoption of appropriate Tier

3 standards for PM.

2. Small Land-Based SI Engines
116 60 FR 34582, July 3, 1995.

117 64 FR 15208, March 30, 1999 and 65 FR 24267, April 25, 2000.

153

Small spark-ignition (SI) engines (e.g., engines operating on gasoline, natural gas,

propane, or methanol) at or below 25 hp comprise about 9 percent of the mobile source VOC

inventory. These small engines are used primarily in lawn and garden equipment, such as

lawn-mowers, string trimmers, edgers, chain saws, commercial turf equipment, and lawn and

garden tractors.

Under Phase 1 of our nonroad small SI engine regulations, new small SI engines must

comply with emission standards for HC, CO, and NOx beginning in 1997. 116 The Phase 1

standards apply to all SI engines at or below 25 hp, except for those used in aircraft, marine

vessels, and recreational equipment. We expect that these Phase 1 standards will result in a 32

percent reduction in HC emissions from small SI engines (approximately 340,000 tons from

uncontrolled levels). 

We finalized Phase 2 nonroad small SI engine regulations in March 1999 for

nonhandheld engines, and in March 2000 for handheld engines.117 The Phase 2 programs include

more stringent emission levels and new provisions to ensure low in-use emissions. We expect

the Phase 2 program for nonhandheld engines to achieve approximately 350,000 tons of HC +

NOx emission reductions, and the program for handheld engines to achieve approximately

230,00 tons of reduction in HC + NOx emissions. These reductions represent reductions in

HC+NOX beyond the Phase 1 levels of 60 percent for nonhandheld engines and 70 percent for

handheld engines.

3. Large Land-Based Spark-Ignition Engines

We do not currently have emission standards in place for spark-ignition engines above 25

hp used in commercial applications. These engines are used in a variety of industrial equipment,

including forklifts, airport ground-service equipment, generators, and compressors. We are

currently developing an emission control program for these engines.

4. Marine Engines

Like land-based nonroad engines, marine engines serve a wide variety of applications.

The smallest marine engines, virtually all of which use gasoline, are used in recreational

outboards and personal watercraft. Small gasoline or diesel marine engines provide auxiliary

power on many vessels. Larger marine engines provide propulsion for both recreational and

commercial applications. Recreational sterndrive and inboard engines tend to be gasoline,

though diesel engines are making inroads into that market. Commercial engines, virtually all

diesel, power vessels such as tugs, ferries, and crew/supply boats. These engines also provide

auxiliary power on larger vessels. The largest marine diesel engines, sometimes exceeding
118 61 FR 52088, October 4, 1996.

119 64 FR 73300, December 29, 1999.

154

60,000 hp, propel ocean-going vessels. We group engines under three control programs

reflecting their application and, to some extent, the fuel they use.

a. Gasoline Outboards and Personal Watercraft Marine Engines 

Gasoline outboards and personal watercraft contribute about 5 percent of the national

mobile source VOC inventory. However, in areas with large boat populations, the contribution

of these recreational marine engines may exceed 10 percent of the regional HC inventory. These

engines typically employ 2-stroke technology, which has changed very little over the last 50

years. Regulations to control exhaust emissions from new outboards and personal watercraft

went into effect beginning with the 1998 model year.118 The emission controls for these engines

involve increasingly stringent standards over the course of a nine-year phase-in period beginning

in model year 1998. By the end of the phase-in, each manufacturer must meet an emission

standard, on a corporate-average basis, that represents a 75-percent reduction (on the order of

500,000 tons) in HC compared to unregulated levels. The gradually decreasing emission

standard allows manufacturers to determine the best approach to achieving the targeted

reductions over time. Manufacturers are able to phase in the types of control technologies in the

most sensible way, while minimizing the cost impact to the consumer. 

b. Commercial Diesel Marine Engines 

Commercial diesel marine engines contribute about 8 percent of the national mobile

source NOx inventory, and about 1 percent of the national mobile source PM inventory. In areas

with large commercial ports or near busy shipping lanes, the contribution of diesel marine

engines to the local NOx and PM inventory may be much higher. We published regulations for

the control of exhaust emissions from new marine diesel engines in December 1999.119 The

emission limits, which vary depending on the size of the engine, are similar to emission limits for

corresponding land-based nonroad or locomotive engines. These limits apply beginning with

engines manufactured in 2004, and will result in 13-percent VOC and 26 percent diesel PM

reductions from uncontrolled levels. The emission limits for very large commercial marine

diesel engines are the same as those contained in Annex VI of the International Convention on

the Prevention of Pollution from Ships (MARPOL). Consistent with MARPOL Annex VI, these

limits will apply to engines installed on ships constructed on or after January 1, 2000.

c. Recreational Sterndrive and Inboard Engines 

Recreational sterndrive and inboard engines can be either gasoline or diesel engines.

While their contribution to national mobile VOC and NOx levels is smaller than the other two

marine engine categories, their emissions are expected to increase due to the growing number of
120 63 FR 18978, April 16, 1998.

155

recreational vessels. We did not finalize emission limits for gasoline sterndrive and inboard

engines as part of the 1996 marine rule. Likewise, we did not propose limits for recreational

diesel engines in the commercial diesel engine rule. We do not currently have emission

regulations in place for this category of marine engine, but have begun developing them.

5. Locomotives 

Locomotives are estimated to contribute about 9 percent of the nationwide mobile source

NOx emissions inventory. These engines are generally larger and last longer than any land-based

nonroad diesel engines. In April 1998, we published emission standards for NOx, HC, CO, PM,

and smoke for locomotives.120 The new standards are ultimately expected to reduce NOx

emissions by two-thirds, while HC and PM emissions from these engines will be decreased by 50

percent.

A unique feature of the locomotive program is that it includes emission standards for

remanufactured engines, including all those that were originally built since 1973. Regulation of

the remanufacturing process is critical because locomotives are generally remanufactured 5 to 10

times during their total service lives, which is typically 40 years or more.

Three separate sets of emission standards have been adopted, with applicability of the

standards dependent on the date a locomotive is first manufactured. The first set of standards

(Tier 0) applies to locomotives and locomotive engines originally manufactured from 1973

through 2001, any time they are manufactured or remanufactured. The second set of standards

(Tier 1) applies to locomotives and locomotive engines originally manufactured from 2002

through 2004. These locomotives and locomotive engines will be required to meet the Tier 1

standards at the time of original manufacture and at each subsequent remanufacture. The final

set of standards (Tier 2) applies to locomotives and locomotive engines originally manufactured

in 2005 and later. Tier 2 locomotives and locomotive engines will be required to meet the

applicable standards at the time of original manufacture and at each subsequent remanufacture.

Electric locomotives, historic steam-powered locomotives, and locomotives originally

manufactured before 1973 do not contribute significantly to the emissions problem and, thus, are

not subject to the locomotive regulations.

While the Tier 0 and Tier 1 regulations are primarily intended to reduce NOx emissions,

the Tier 2 regulations will result in 50 percent reductions in VOC and diesel PM, as well as

additional NOx reductions beyond the Tier 0 and Tier 1 regulations. As a result, almost half of

the NOx reductions we ultimately expect will be achieved by 2005. In contrast, the VOC and

diesel PM reductions are achieved more slowly, due to the very slow fleet turnover. By 2040 we

expect VOC reductions of about 18,000 tons per year and diesel PM reductions of about 12,000

tons per year. However, only about one third of these VOC and diesel PM reductions will be

realized by 2010.
121 International Civil Aviation Organization (ICAO) Annex 16, Volume II, Environmental Protection,

Aircraft Engine Emissions.

156

6. Aircraft

Aircraft emissions comprise less than 2 percent of the mobile source NOx emissions

inventory, but they are significant contributors to the NOx inventory in some cities. In addition,

commercial aircraft emissions are a fast growing segment of the transportation emissions

inventory. Aircraft emissions are potentially important contributors to global climate change and

may also contribute to the depletion of the stratospheric ozone layer.

Emission standards for gas turbine engines that power civil aircraft have been in place for

about 20 years. Such engines are used in virtually all commercial aircraft, including both

passenger and freight airlines. The standards do not apply to military or general aviation aircraft.

Controls on engine smoke and prohibitions on fuel venting were instituted in 1974 and have been

revised several times since then. Beginning in 1984, limits were placed on the amount of

unburned HC gas turbine engines can emit per landing and takeoff cycle.

In April 1997, we adopted the existing International Civil Aviation Organization (ICAO)

NOx and CO emission standards for gas turbine engines.121 ICAO, a specialized agency of the

United Nations, is the most appropriate forum for first establishing commercial aircraft engine

emission standards due to the international nature of the aviation industry.

None of the actions just discussed have resulted in significant emissions reductions, but

rather have largely served to prevent increases in aircraft emissions. We are also exploring other

ways to reduce the environmental effects associated with air travel throughout the nation. We are

working with the Federal Aviation Administration (FAA) to encourage continuing progress in

reducing emissions from airport ground service equipment and aircraft auxiliary power units. 

We sponsored compilation of technical data and emission inventory methods, which the FAA

will use to develop an Advisory Circular for airlines and airport authorities interested in reducing

emissions from these sources.

7. Recreational Vehicles

We do not have standards in place for large land-based SI engines used in recreational

vehicles, such as off-road motorcycles, all-terrain vehicles, and snowmobiles. However, we are

currently developing emission regulations for recreational vehicles.

8. Fuels

In addition to the above engine technology-based emission control programs, fuel

controls will also reduce emissions of air toxics from nonroad engines. For example, gasoline

formulation (the removal of lead, limits on gasoline volatility and reformulated gasoline) will
157

reduce nonroad MSATs, because most gasoline-fueled nonroad vehicles are fueled with the same

gasoline used in on-highway motor vehicles. An exception to this is lead in aviation gasoline. 

Aviation gasoline is a high octane fuel used in a relatively small number of aircraft (those with

piston engines). Such aircraft are generally used for personal transportation, sightseeing, crop

dusting, and similar activities. 

As just discussed, most of our fuel controls aimed at gasoline cover both on-highway and

nonroad vehicle fuel. The same is not true for diesel fuel. We have regulations in place which

have dramatically reduced the sulfur levels in on-highway diesel fuel, and are currently

considering further reductions in on-highway diesel fuel sulfur levels. These controls do not

apply to nonroad diesel fuel, and prior to these controls there was no distinction between onhighway and nonroad diesel fuel. We are considering the control of sulfur in nonroad diesel fuel

as part of our Tier 3 technology review. This would allow more effective diesel PM control

technologies such as catalysts to be applied to nonroad engines and vehicles.

B. Impacts of Nonroad Control Programs on Air Toxics

As a whole, our nonroad programs significantly reduce the impact of nonroad equipment

on the nation’s air quality. As with motor vehicle controls, while we’ve focused our controls on

achieving reductions in criteria pollutants (NOx, HC, and PM), our control programs have also

been effective in reducing emissions of air toxics.

As is the case with motor vehicle emissions, we expect nonroad emissions of gaseous

toxics to decrease over the next 20 years under our current control programs. By 2020, we

estimate that benzene emissions will decrease by 31 percent (over 31,000 tons) and formaldehyde

emissions will decrease by 49 percent (over 38,000 tons), as compared with 1990 levels. 

However, nonroad emissions of diesel PM are not decreasing dramatically. We estimate that by

2020, nonroad engine will emit more that 310,000 tons per year of diesel PM emissions, as

compared with 346,000 tons in 1996, a 10-percent decrease. Our land-based nonroad Tier 3

technology review will examine nonroad engine diesel PM emissions. As part of our Technical

Analysis Plan, described in the preamble to the proposed regulation, we will also assess the need

for and appropriateness of controls for nonroad sources of MSATs.

1. Nonroad MSAT Baseline Inventories

We previously presented the 1996 baseline inventories for several key nonroad MSATs in

Table IV.A-2. This nonroad MSAT data was taken from the 1996 National Toxics Inventory

(NTI). In general, the data shows that nonroad vehicles tend to be significant contributors of

those MSATs for which motor vehicles are also significant contributors. Nonroad vehicles

contribute as much as 39 percent of the national inventory of some MSATs, such as acetaldehyde

and MTBE, and contribute significantly to the national inventories of several others, including

1,3-butadiene, acrolein, benzene, formaldehyde, lead compounds, n-hexane, toluene and xylene.
122 The draft NONROAD model is a model we are developing which is used to project emissions

inventories from nonroad mobile sources. Because this is a draft model and subject to future revisions, the

inventories derived from the draft NONROAD model and presented here are subject to change. The version of the

NONROAD model that was used in this analysis is the one which we also used in support of our recently proposed

2007 heavy-duty vehicle and diesel fuel control proposal (65 FR 35429, June 2, 2000).

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2. Emission Reductions from Current Programs

The programs summarized in Section A of this chapter are expected to result in

reductions of national inventories of the MSATs. This section summarizes our estimates of

nonroad MSAT inventories into the future, based on the nonroad emission control programs we

currently have in place. The discussion in this section consists of three parts. First, we discuss

the inventories of four MSATs: benzene, formaldehyde, acetaldehyde and 1,3-butadiene. 

Second, we discuss nonroad VOC emissions inventories as a surrogate for the other nonroad

gaseous MSATs. Finally, we discuss the trend of nonroad diesel PM emissions. We focused on

these pollutants for nonroad mobile sources primarily to allow comparisons with the on-highway

analyses presented in earlier chapters. The inventories presented here are based only on

regulations which we have completed. As previously discussed, we are developing the first

national regulations applicable to recreational vehicles and recreational marine sterndrive and

inboard engines. We are also developing additional Tier 3 emission regulations applicable to

land-based nonroad CI engines. These regulations, when completed, will result in additional

VOC (and, thus, gaseous MSAT) reductions. This rulemaking action will also include proposal

and adoption of appropriate Tier 3 standards for PM.

We are not reporting inventory trends for the metals on our list of MSATs (arsenic

compounds, chromium compounds, mercury compounds, nickel compounds, manganese 

compounds, and lead compounds) or for dioxin/furans. Metals in mobile source exhaust can

come from fuel, fuel additives, engine oil, engine oil additives, or engine wear. Formation of

dioxin and furans requires a source of chlorine. Thus, while metal emissions and dioxins/furans

emissions are associated with particles, there are a number of other factors that contribute to

emission levels. While it is possible that these compounds track PM emissions to some extent,

we do not have good data on these relationships.

a. MSATs

Table VIII..B-1 shows our estimates of four nonroad gaseous MSAT emissions. These

estimates were based on the 1996 inventories contained in the 1996 NTI study. The 1990

estimates were derived by applying nationwide VOC totals and toxic fractions from the draft

NONROAD model to the 1996 NTI numbers.122 The 2007 and 2020 estimates were derived

from the draft NONROAD model, also with the toxic fractions applied to the VOC results. The

draft NONROAD model does not include locomotives, commercial diesel marine engines, or

aircraft. We do not have enough information to estimate the inventories of the four gaseous

MSATs for these three nonroad vehicle categories. Thus, they are not included in the 1990, 2007
159

and 2020 estimates. For consistency’s sake, we have excluded these categories from the 1996

NTI numbers as well. Thus, the 1996 estimates shown here differ slightly from those shown in

Table IV.A-2. However, these three nonroad categories only represent about three percent of the

total nonroad VOC. Using VOC as a surrogate for gaseous toxics, as discussed in the next

section, we conclude that the exclusion of locomotives, commercial diesel marine engines, and

aircraft from our estimates of gaseous MSATs does not have a significant impact on those

estimates.

Table VIII.B-1

Annual Toxics Emissions Summary for Selected Air Pollutants for the Total U.S.

Nonroad Mobile Sources from 1990 to 2020

(thousand short tons per year)

Compound 1990 Emissions 1996 Emissions 2007 Emissions 2020 Emissions

Benzene 100.2 98.7 75.4 69

Acetaldehyde 37.7 40.8 26.3 20

Formaldehyde 79.2 86.4 53.8 40.7

1,3-Butadiene 9.4 9.9 8.8 7.8

Table VIII.B-2 summarizes the percent reductions in 2007 and 2020 from 1990 and 1996

levels represented by the inventories in Table VIII.B-1. This table shows that the reductions

expected from our existing nonroad control programs are significant, although not as substantial

as the reductions of these pollutants for on-highway motor vehicles presented in Chapter 4.

Table VIII.B-2

Summary of Percent Emission Reductions in 2007 and 2020 for Selected Air Pollutants for

the Total U.S. from 1990 or 1996

Nonroad Mobile Sources

Reduction in 2007 Reduction in 2020

Compound From 1990 From 1996 From 1990 From 1996

Benzene 25% 24% 31% 30%

Acetaldehyde 30% 36% 47% 51%

Formaldehyde 32% 38% 49% 53%

1,3-Butadiene 7% 11% 18% 21%

b. VOCs
123 “National Air Pollutant Emission Trends, 1900-1996,” EPA-454/R-97-011, December, 1997.

160

With the exception of the four MSATs shown in Table VIII.B-1, we do not have detailed

emissions data from nonroad mobile sources for the other gaseous MSATs. Therefore, to

estimate projected inventory impacts from our current nonroad mobile source emission control

programs, we use VOC inventories. We believe this is appropriate because the gaseous MSATs

are constituents of total VOC emissions. By using VOC emissions as a surrogate, we are

assuming that MSAT emissions track VOC reductions. In reality, however, some gaseous

MSATs may not decrease at the same rate as VOCs overall. Without having more detailed

emission data for each of the MSATs, however, we are unable to offer any insights on how those

rates may differ. This is one of the issues we intend to address as part of the Technical Analysis

Plan described in the preamble to the proposed regulation.

Our VOC emission inventories were developed using the draft NONROAD model. 

Because the draft NONROAD model does not include locomotives, commercial marine diesel

engines, or aircraft we supplemented the draft NONROAD model inventories with locomotive

and diesel marine inventories developed in support of our regulations for those categories, and

with aircraft emission inventories from the National Air Pollutant Emissions Trends, 1900-1996

report.123 The results of this analysis shows that VOC inventories are projected to decrease

approximately 44 percent between 1996 and 2020 due to existing nonroad mobile source

emission control programs. This analysis shows that our existing nonroad emission control

programs will also result in significant gaseous MSAT reductions (assuming, as previously

discussed, that gaseous MSATs emissions track VOC reductions).

Table VIII.B-3

Annual VOC Emissions Summary for the Total U.S.

Nonroad Mobile Sources

Year 1996 2007 2020

Million short tons per year 3.6 2.2 2.0

Cumulative Percent Reduction *** 39% 44%

c. Diesel PM

We estimated the nonroad diesel PM inventories using the draft NONROAD model. 

Because the draft NONROAD model does not include locomotives, commercial marine diesel

engines, or aircraft we supplemented the draft NONROAD model inventories with locomotive

and diesel marine inventories developed in support of our regulations for those categories, and

with aircraft emission inventories from the National Air Pollutant Emissions Trends, 1900-1996

report. Table VIII.B-4 shows our estimates of nonroad diesel PM emissions inventories. As can

be seen, we expect nonroad diesel PM emissions to begin to drop with the implementation of
161

some of our nonroad regulations. However, in the absence of additional controls, we expect that

nonroad diesel PM emission inventories will begin to increase due to expected growth in the

populations of nonroad vehicles and equipment.

Table VIII.B-4

Annual Diesel PM Emissions Summary for the Total U.S.

Nonroad Mobile Sources

Year 1996 2007 2020

Thousand short tons per year 345,800 282,800 310,800

Cumulative percent reduction

from 1996

*** 18% 10%

C. Data Gaps and Uncertainties

There are significant gaps in data on MSAT emissions from nonroad engines. These data

gaps contribute to a less developed understanding of nonroad MSAT inventories compared to our

understanding of on-highway vehicle MSAT emissions. The largest single data gap is in the area

of emission factors. While we have basic emission factors for VOC and PM for most of the

nonroad categories, we have very little VOC speciation data for the given categories that would

allow us to use VOC as a surrogate to estimate emissions of specific MSATs. Given the large

variety of nonroad engine sizes, types and uses, as well as the likelihood that this variety will

result in some differences in VOC composition, it is important that we obtain or develop

speciated VOC data specific to each nonroad category in order to more accurately project

nonroad MSAT inventories. These gaps, too, must be filled in order to accurately assess the need

for, and the most appropriate direction of, any future MSAT control program targeted specifically

at nonroad mobile sources. Our Technical Analysis Plan, described in the preamble to our

proposed rule, contains a strategy to obtain and evaluate this data so we can evaluate the

feasibility of, and need for, additional nonroad engine controls in the future.