Chapter 4 Air Toxics: Chemicals, Sources, and

Emissions Inventories

Table of Contents

4.1 Introduction ................. ................. ................. ............... 1

4.2 Air Toxics ................. ................. ................. ................ 1

4.2.1 Introduction to Air Toxics Chemical Lists ................. ................. . . 1

4.2.2 Hazardous Air Pollutants (HAPs) ................. ................. ........ 4

4.2.3 Criteria Air Pollutants ................. ................. ................. 5

4.2.4 Toxics Release Inventory (TRI) Chemicals ................. ................. . 6

4.2.5 Toxic Chemicals That Persist and Which Also May Bioaccumulate ................ 6

4.2.6 Other Chemicals ................. ................. ................. .... 11

4.3 Sources of Air Toxics ................. ................. ................. ...... 11

4.3.1 Point Sources ................. ................. ................. ...... 12

4.3.2 Nonpoint Sources ................. ................. ................. ... 13

4.3.3 On-Road and Nonroad Mobile Sources ................. ................. ... 14

4.3.4 Sources Not Included in the NEI or TRI ................. ................. . . 15

4.3.4.1 Indoor Sources ................. ................. ............... 15

4.3.4.2 Natural Sources ................. ................. ............... 18

4.3.4.3 Formation of Secondary Pollutants ................. ................. 19

4.3.4.4 Other Sources Not Included in NEI or TRI ................. ........... 20

4.4 Emissions Inventories ................. ................. ................. ...... 20

4.4.1 National Emissions Inventory (NEI) ................. ................. ..... 21

4.4.2 Toxics Release Inventory (TRI) ................. ................. ......... 25

References ................. ................. ................. ................. .... 29

a

CAS (Chemical Abstracts Service) is a division of the American Chemical Society. A CAS Registry

Number (CAS number or CASRN) is assigned in sequential order to unique, new substances identified by CAS

scientists for inclusion in the CAS REGISTRY database. Each CAS Registry Number is a unique numeric

identifier; designates only one substance; and has no chemical significance. A CAS Registry Number is a numeric

April 2004 Page 4-1

The exhaustive lists of air

toxics discussed in this

section do not include all of

the hazardous chemicals of

public health concern. Note

also that other forms of air

pollution (e.g., odors) are

not addressed in this

Reference Library.

4.1 Introduction

This chapter identifies the set of chemicals broadly, and most

commonly, considered “air toxics.” This section also describes

the general categories of air toxics sources that emit these

chemicals as well as the primary places where air toxics

emissions information (e.g., databases that contain information

on the location and nature of emissions released from various

types of sources) used in air toxics risk assessments can be

found. Section 4.2 discusses air toxics; Section 4.3 describes air

toxics sources; and Section 4.4 describes air toxics emissions

data sources.

4.2 Air Toxics

Chapter 2 of this Volume introduced Hazardous Air Pollutants (HAPs), criteria air pollutants,

Toxic Release Inventory (TRI) chemicals, and persistent, bioaccumulative, and toxic (PBT)

chemicals and discussed the relationships among these various groupings. This section will

revisit each of these groups to provide more detailed information related to the chemicals on

each of those lists. A thorough understanding of the different types of chemicals that may be of

interest for an assessment, as well as the nuances of the various ways chemicals are written into

those lists, will be important for the risk assessment team to comprehend before the assessment

begins in earnest. 

The term “air toxics” is a generic term that could conceivably encompass literally anything in the

air that poses harm to people or the environment. This Volume uses the term “air toxics” in this

general sense. Thus, while the focus of most air toxics risk assessments will be on the 188

chemicals and chemical compounds listed as HAPs in the Clean Air Act (CAA) section 112(b),

some assessment teams may wish to have a broader focus. The use of the term “air toxics” in

this general sense is meant to provide for this flexibility. Ultimately, the scope of any

assessment must clearly identify the chemicals that will be evaluated and the reason for their

inclusion or exclusion in the evaluation.

4.2.1 Introduction to Air Toxics Chemical Lists

The various lists that are the focus of this volume were all derived directly from the Clean Air

Act, the Emergency Planning and Community Right to Know Act, or a specific EPA initiative

(e.g., the PBT initiative list of chemicals). It is important to understand that there is not always

consistency among these various lists in either the naming of chemicals or the meaning of the

names. For example, as noted in Chapter 2, “glycol ethers” are defined differently for the TRI

and as HAPs (see box in Section 2.4.4).

Lists of toxic chemicals commonly provide the chemical identity by both a name and a unique

identifying number, called a Chemical Abstracts Registry Number.

(a) However, most
identifier that can contain up to nine digits, sometimes divided by hyphens into three parts. See

http://www.cas.org/faq.html for more information.

April 2004 Page 4-2

chemicals have multiple synonyms (sometime dozens). Fortunately, every unique chemical has

only one unique CAS number and one can always refer to this unique number to identify the

compound in question. For example, toluene and methylbenzene are synonyms for the same

compound (which is normally referred to as toluene). However, there is only one CAS number

for the compound: 108-88-3. No matter where one is in the world or what name is attached to a

chemical, there is unanimity of identity through the CAS numbering system. 

When there is any question about what a particular chemical name means, it is always advisable

to try to pinpoint the identity through use of the CAS number. For example, a risk assessment

team may ask for air sampling analysis for the HAP acetaldehyde (CAS number 75-07-0);

however, when they receive the analytical lab report, acetaldehyde is not reported. A quick scan

of the CAS numbers reported by the lab lists the CAS number 75-07-0 next to the name

“ethanal.” Ethanal is a synonym for acetaldehyde and, hence, has the same CAS number. 

EPA’s Handbook for Air Toxics Emission Inventory Development includes a list (Appendix C)

of synonyms and CAS numbers for HAPs that is helpful in overcoming the nomenclature

obstacle.(1) (Note, however, that there are nuances even beyond this simplistic description. For

example, some chemicals have one CAS number for their pure form and a different CAS number

for a technical grade. A knowledgeable chemist can usually identify and clarify these issues.)

Some of the entries on chemical lists are for large groups of compounds and not just one single

substance. For example, one of the HAPs is listed in the CAA as “polychlorinated biphenyls

(aroclors)” and is most commonly referred to as PCBs. This listing is not for one single

substance but, rather, for any one or a mixture of any of the 209 possible chemicals that are

themselves PCBs. As another example, the pesticide “2,4-D” is written into the list of HAPs as

“2,4-D (salts and esters).” This listing includes any possible salt of 2,4-D and any possible ester

of 2,4-D. In our earlier lead example, the lead compound listing includes any compound known

to exist in or be emitted to the environment that contains a lead molecule as part of the

compound’s molecular structure (a potentially huge number of possibilities). Another important

group of chemicals is called “POM” for polycyclic organic matter. This includes organic

compounds with more than one benzene ring, and which have a boiling point greater than or

equal to 100o C (e.g., polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene).

In reality, most risk assessments will deal with a relatively small number of chemicals because

either the sources in a given place are releasing only a limited number of chemicals or the ability

to model or monitor the numerous chemicals present is limited by the available inventories or

monitoring/analytical methods, respectively. 

In the initial stages of the assessment, risk assessors often sort the chemicals of interest into

groups that, generally, have similar physical and/or chemical properties. This is a helpful thing

to do as a way of making some educated guesses about how chemicals are likely to behave in the

environment. The groupings also help an assessment team to plan for the types of sampling and

analysis methods that will be needed, because the sampling and analytical methods tend to be

broken out along these same lines. In general, all air toxics can be broadly categorized into three

main groups, organic chemicals, inorganic chemicals, and organometallic compounds as follows:
b

The regulatory definition of VOC does not identify vapor pressure as a consideration. See 40 CFR

51.100(s).

c

“VOC” refers to volatile organic compounds that contribute to ozone formation as defined by 40 CFR

50.100(s) as ozone precursors. VOC is a subset of VOCs. VOC emissions inventory information is sometimes used

to derive estimates for specific chemicals; when this is done, the VOC number is said to have been speciated.

April 2004 Page 4-3

Organic Chemicals

Organic chemical compounds are composed of carbon in combination with other elements such

as hydrogen, oxygen, nitrogen, phosphorous, chlorine, and sulfur (not including carbonic acid or

ammonium carbonate). Organic compounds can generally be split into two different groups,

based on their propensity to evaporate. The following such groupings are commonly employed

by analytical chemistry laboratories for purposes of sample analysis. 

• Volatile Organic Compounds (VOCs). These are organic chemicals that have a high vapor

pressure and tend to have low water solubility.(b) Simply put, VOCs have a high propensity

to evaporate and remain airborne. Many VOCs are human-made chemicals that are used and

produced in the manufacture of paints, pharmaceuticals, and refrigerants, as industrial

solvents, such as trichloroethylene, or produced as by-products, such as chloroform produced

by chlorination in water treatment. VOCs are often also components of petroleum fuels (e.g.,

benzene), hydraulic fluids, paint thinners, and dry cleaning agents.(c)

A subgroup of VOCs is termed Carbonyl Compounds and includes chemicals such as

formaldehyde and acetaldehyde. While such chemicals are themselves VOCs due to their

high vapor pressure, they are often grouped as a separate class from the VOCs because of the

special sampling and analytical methods necessary to measure them in air.

• Semivolatile Organic Compounds (SVOCs). SVOCs are organic chemicals that have a

lower vapor pressure than VOCs and, thus, have a lower propensity to evaporate from the

liquid or solid form. Once airborne, they also tend to more readily condense out of the gas

phase. Examples of SVOCs include most organic pesticides (e.g., chlordane), and certain

components of petroleum, such as polycyclic aromatic hydrocarbons. Note that the

demarcation between SVOCs and VOCs is not exact. For example, the two separate air

sampling and analytical methods for VOCs and SVOCs will both usually detect naphthalene

when present, indicating that this chemical is on the lower end of the VOC scale of volatility

and on the higher end of the SVOC scale of volatility. In general, as chemicals increase in

molecular weight and/or polarity, they become more SVOC-like.

Inorganic Chemicals

This group includes all substances that do not contain carbon and includes a wide array of

substances such as:

• Metals (e.g., mercury, lead, and cadmium) and their various salts (e.g., mercury chloride);

• Halogens (e.g, chlorine and bromine);

• Inorganic bases (e.g., ammonia); and

• Inorganic acids (e.g., hydrogen chloride, sulfuric acid).
April 2004 Page 4-4

Organometallic Compounds

This group is comprised of compounds that are both organic and metallic in nature. The alkyl

lead compounds that were added to gasoline to enhance its properties can be used for illustration. 

“Alkyl” refers to the organic portion of a compound which is attached to the inorganic metal

lead. The result is a so-called “organometallic” material, a hybrid of both metallic and organic. 

(Note that salts, such as sodium benzoate, are usually classified as an organic chemical, rather

than an organometallic compound.)

An understanding of the general characteristics of organic chemicals, inorganic chemicals and

organometallic compounds will aid in planning a risk assessment and developing an appropriate

analysis strategy. For example, most VOCs tend to remain airborne and also do not tend to

bioaccumulate to the same extent as some of the non-volatile chemicals. Thus, if an assessment

were being planned to evaluate the impact of a source from which only VOCs were released, it

becomes less likely that a multipathway risk analysis will be necessary (since VOCs do not tend

to migrate into soil or water and do not tend to bioaccumulate as strongly in living tissue). 

In addition, the sampling and analytical methods available to test for chemicals in environmental

media are generally broken out along the same chemical groupings noted above. Thus, if one

were interested in testing for airborne chlordane (an SVOC), a VOC monitoring method would

not be used. Detailed information on available monitoring methods and the chemicals for which

they have been validated is provided in Chapter 10.

In air toxics studies, both individual substances and mixtures of substances are of interest. 

Particulate matter (PM), for example, is almost never comprised of just one substance; instead, 

PM is usually made up of numerous individual substances (sometimes in the hundreds). Both

the physical and chemical nature of a mixture will influence the fate and transport of the

chemicals in the environment as well as the potential for the mixture to cause harm. For

example, a toxic chemical adsorbed onto the surface of a relatively large particle (> 10 microns

in diameter) will usually be trapped in the upper portion of the respiratory system and either

coughed/sneezed out of the body or swallowed. The same chemical adsorbed onto a very small

particle (< 2.5 microns in diameter) has a much higher likelihood of being inhaled into the deep

lung. As we will see in later chapters, both the route of exposure (in this example, ingestion or

inhalation) as well as the toxic properties of the chemical in question are important determinants

of potential harm.

4.2.2 Hazardous Air Pollutants (HAPs)

The HAPs are a group of 188 specific chemicals and chemical compounds and are identified in

Section 112(b) of the CAA. The Agency provides additional information on the HAPs online.(2)

HAPs are pollutants known to cause or suspected of causing cancer or other serious human

health effects or ecosystem damage. They include individual organic and inorganic compounds

and pollutant groups closely related by chemical structure (e.g., arsenic compounds, cyanide

compounds, glycol ethers, polycyclic organic matter) or emission sources (e.g., coke oven

emissions). EPA may add or remove pollutants from the HAP list as new information becomes

available. A full list of the HAPs is provided in Appendix A.

When people talk about “air toxics risk assessment,” they generally mean assessments of risks

associated with one or more of the HAPs. This is largely because of the CAA listing of 188
April 2004 Page 4-5

HAPs and its requirement under Section 112(f)(2) (Residual Risk) that EPA assess the risks

associated with HAPs that remain after the application of the Maximum Achievable Control

Technology (MACT) standards (Section 112(d) of the Act). However, given that this is a

relatively short list of chemicals, many communities may want to go beyond this list when

assessing risk. It is for this reason, that assessors and other stakeholders must clearly identify

why they are conducting an “air toxics” risk assessment and what they want to include in that

assessment.

In its Integrated Urban Air Toxics Strategy, EPA identified a subset of 33 HAPs as those posing

the greatest risk in urban areas (see Section 2.2.1). These 33 HAPs were selected based on a

number of factors, including toxicity-weighted emissions, monitoring data, past air quality

modeling analysis, and a review of existing risk assessment literature.

The national-scale assessment for 1996 (see Section 2.3.2) focused on 32 of these 33 Urban

HAPs (dioxin was omitted) and also includes diesel particulate matter, which is used as a

surrogate measure of diesel exhaust. EPA recently concluded that diesel exhaust is likely to be

carcinogenic to humans by inhalation at environmental levels of exposure. Diesel exhaust is

addressed in several regulatory actions and diesel particulate matter plus diesel organic gases are

listed by EPA as a mobile source air toxic (see Section 4.3.3 below).

4.2.3 Criteria Air Pollutants

The “criteria air pollutants” are six substances regulated pursuant to Title I of the CAA, for

which “criteria documents” are developed by the Agency prior to national standard setting

decisions. There are already national ambient air quality standards (NAAQS) in place for each

of these pollutants as well as established regulatory programs and activities in place to meet

those standards. However, they are discussed here because there is some crossover between the

realm of HAPs and criteria pollutants. The more important crossover issues are discussed

below.

• Particulate matter. NAAQS have been established for particles with an aerodynamic

diameter less than or equal to 10 microns (called PM10) and particulate matter with an

aerodynamic diameter less than or equal to 2.5 microns (called PM2.5). As noted above,

however, PM can be made up of as little as one or a few, or many hundreds of individual

chemicals. In many cases (and depending on the source of the PM), any number of

specifically listed HAPs may be a part of the PM mix. It is for this reason that risk assessors

may opt to evaluate the composition of PM and to include the identified chemicals in risk

calculations. 

For example, it is possible to collect samples of PM10 for purposes of determining the types

and amounts of individual substances contained in the particles. The risks posed by those

individual chemicals may then be estimated for the inhalation route of exposure. Because

particles with diameters greater than 10 microns are not generally respirable, analysts usually

select a PM10 monitor to capture samples for risk assessment purposes rather than a total

suspended particulate (TSP) sampler, because TSP would capture larger particles that do not

penetrate very far into the respiratory tract (thus leading to an overestimate in inhalation risk

associated with the specific pollutants studied). Note that this would not be true for particlebound chemicals that exert their toxic effects on the nasal passages.
April 2004 Page 4-6

• Ozone and other criteria pollutants. Certain other criteria pollutants are not specifically

listed as HAPs, but HAPs may lead to their formation or they may lead to HAP formation. 

For example, ozone is produced by the interaction of certain VOCs, oxides of nitrogen

(called NOx), and sunlight. As noted previously, many of the HAPs are VOCs and may play

a role in ozone formation. In contrast, sulfur dioxide is a criteria pollutant that can be

transformed in the environment into sulfuric acid which, in turn, may become part of a listed

HAP (e.g., cadmium sulfate). In general, the criteria pollutants ozone, nitrogen dioxide,

sulfur dioxide, carbon monoxide are not usually considered in air toxics risk assessments.

4.2.4 Toxics Release Inventory (TRI) Chemicals

Data on TRI chemicals are reported pursuant to Section 313 of the Emergency Planning and

Community Right-To-Know Act (EPCRA) of 1986 and Section 6607 of the Pollution Prevention

Act of 1990 (PPA). EPCRA and the PPA are intended to inform communities and citizens about

chemical hazards in their areas. EPA and states are required to collect data annually on releases

(to each environmental medium) and waste management methods (e.g., recycling) of certain

toxic chemicals from industrial facilities, and to make the data available to the public in the

TRI.(3) EPCRA Section 313(d) permits EPA to list or delist chemicals based on certain criteria. 

In a 1994 rulemaking, EPA added 286 chemical categories to the TRI chemical list. The TRI

chemicals are listed in 40 CFR Section 372.65, and information about the 667 currently-listed

TRI chemicals is provided online.(4) 

The current TRI chemical list contains 582 individually listed chemicals and 30 chemical

categories (including three delimited categories containing 58 chemicals), for a total of 612

separate chemicals. If the members of the three delimited categories are counted as separate

chemicals then the total number of chemicals and chemical categories is 667 (i.e., 582 + 27 +

58). The TRI list of toxic chemicals includes most (180) of the HAPs. Similar to the HAPs, the

TRI chemicals include VOCs, SVOCs, inorganic compounds, and organometallic compounds. 

The utility of the TRI for air toxics risk assessment is two fold. First, it provides a broader

perspective of industrial emissions than the HAP list because it includes information on air

releases of many hundreds of additional chemicals. Second, accessing TRI information is

extremely quick and easy. Using the TRI Explorer search engine (http://www.epa.gov/tri/

tridata/index.htm), one may quickly identify the location of emissions sources and the identity

and quantity of chemicals released to the air. The data is also updated annually (as opposed to

the National Emissions Inventory (NEI), a nationwide inventory of emissions developed by

EPA, which is only updated triennially). However, other characteristics of the TRI data may

limit their use for risk assessments (see Section 4.4.2).

4.2.5 Toxic Chemicals That Persist and Which Also May Bioaccumulate

Some toxic compounds have the ability to persist in the environment for long periods of time

and may also have the ability to build up in the food chain to levels that are harmful to human

health and the environment. For example, releases of metals from a source may deposit out of

the air onto the ground where they remain in surface soils for long periods of time. Children

playing in the area may ingest this contaminated dirt through hand-to-mouth behaviors. The

chemicals in the dirt may also be taken up into plants through the roots and accumulate in

foraging animals. 
d

Biomagnification is the process whereby certain substances transfer up the food chain and increase in

concentration. Chemicals that biomagnify tend to accumulates to higher concentration levels with each successive

food chain level. Biomagnification is a particular concern for ecological risk assessment.

April 2004 Page 4-7

EPA’s challenge in reducing risks from this category of toxic air pollutants stems from this

ability to transfer from air, to sediments, water, land, and food; to linger for long periods of time

in the environment; and for some substances, their ability to travel long distances. Many of

these chemicals (e.g., DDT) have been banned for use in the U.S. As such, there should be no

active air emissions of these chemicals (although releases into the air are still possible, e.g., by

resuspension of previously contaminated soil). However some, such as mercury, are still in use

today. A number of lists of these persistent and bioaccumulative chemicals have been developed

through international and internal EPA efforts (see Exhibit 4-1). A number of the HAPs appear

on one or more of these lists.

Exposure to persistent and bioaccumulative air toxics through a pathway other than inhalation of

contaminated air is termed an indirect exposure pathway because contact with the chemical

occurs in a medium that is not the original medium to which the chemical was released (i.e., air). 

In contrast, a direct exposure pathway is one in which contact occurs with the chemical in the

medium to which it was originally released. When exposure of a person to a chemical (or

chemicals) occurs through more than one pathway, a multipathway analysis may be considered. 

In air toxics risk assessment, the inhalation pathway is commonly assessed (i.e., the release of a

chemical to air and human exposure through breathing that air). However, indirect exposure

pathways are usually assessed for a limited set of chemicals released to the air. EPA has

identified a preliminary set of HAPs for which indirect exposure pathway analyses should

generally be conducted for situations involving significant emissions of these chemicals in a

study area. This new list of chemicals is termed Persistent Bioaccumulative HAP Compounds

(PB-HAP Compounds) (Exhibit 4-2); however, all of the PB-HAP compounds occur on one or

more of EPA’s existing lists of PBT chemicals. The designation “PB-HAP” was developed to

distinguish this list from the existing lists of PBT chemicals (Exhibit 4-1) and specifically to

clarify that chemicals on this new list are:

• HAPs;

• Relatively persistent in the environment; and

• For some chemicals, have a strong propensity to bioaccumulate and/or biomagnify.(d)

This preliminary list of PB-HAPs was derived primarily on the basis of human health concerns. 

It does not consider direct contact by plants or inhalation by animals. Additional HAPs may be

identified as EPA gains more familiarity with ecological risk assessments for air toxics. 

Appendix D describes the process by which EPA identified the list of PB-HAPs.
April 2004 Page 4-8

Exhibit 4-1. “Lists” of Toxic Chemicals that Persist and Which Also May Bioaccumulate

LRTAP chem icals – The United States signed protocols on Persistent Organic Pollutants (POPs) and

heavy metals pursuant to the Convention on Long-Range Transboundary Air Pollution (LRTAP) in

June 1998 at a ministerial meeting in Aarhus, Denmark. Sixteen POPs and three metals are regulated

(http://www.epa.gov/oppfead1/international/lrtap2pg.htm):

• aldrin

• cadmium

• chlordane

• dieldrin

• endrin

• hexabromobiphenyl

• kepone (chlordecone)

• mirex

• toxaphene

• hexachlorobenzene

• heptachlor

• lead

• mercury

• polychlorinated biphenyls (PCBs)

• dichlorodiphenyltrichloroethane (DDT)

• lindanedioxins (polychlorinated dibenzo-p-dioxins)

• furans (polychlorinated dibenzofurans)

• hexachlorobenzene

• polycyclic aromatic hydrocarbons

PBT Chem icals – EPA has identified the following priority persistent, bioaccumulative, and toxic

(PBT) chemicals and has developed the PBT program to address the cross-media issues associated with

these chemicals (http://www.epa.gov/opptintr/pbt/):

• aldrin/dieldrin

• mercury and its compounds

• benzo(a)pyrene

• mirex

• chlordane

• octachlorostyrene

• DDT

• dichlorodiphenyldichloroethane (DDD) 

• dichlorodiphenyldichloroethylene (DDE)

• PCBs

• hexachlorobenzene

• dioxins and furans

• alkyl-lead

• toxaphene

Great Lakes Priority Substances. In keeping with the obligations of the Great Lakes Water Quality

Agreement, Canada and the United States on April 7, 1997, signed the “Great Lakes Binational Toxics

Strategy: Canada-United States Strategy for the Virtual Elimination of Persistent Toxic Substances in

the Great Lakes” (http://www.epa.gov/glnpo/p2/bns.html). This Strategy seeks percentage reductions

in targeted persistent toxic substances so as to protect and ensure the health and integrity of the Great

Lakes ecosystem. The list of “Level 1” substances is identical to EPA’s priority PBT pollutants.

Great Waters Pollutants of Concern. The 1990 Clean Air Act Amendments established research and

reporting requirements related to the deposition of hazardous air pollutants to the Great Lakes, Lake

Champlain, Chesapeake Bay, and certain other “Great Waters.” The Program has identified the

following pollutants of concern (http://www.epa.gov/airprogm/oar/oaqps/gr8water/index.html):

• cadmium and cadmium compounds

• chlordane

• DDT/DDE

• dieldrin

• hexachlorobenzene

• "-hexachlorocyclohexane

• lindane ((-hexachlorocyclohexane)

• lead and lead compounds

• mercury and mercury compounds

• PCBs

• polycyclic organic matter

• tetrachlorodibenzo-p-dioxin (dioxins)

• tetrachlorodibenzofuran (furans)

• toxaphene

• nitrogen compounds
April 2004 Page 4-9

Exhibit 4-1 (continued)

TRI PBT chem icals. EPA has published two final rules that lowered the Toxics Release Inventory

(TRI) reporting thresholds for certain persistent bioaccumulative and toxic (PBT) chemicals and added

certain other PBT chemicals to the TRI list of toxic chemicals (http://www.epa.gov/tri/lawsandregs

/pbt/pbtrule.htm). The following PBT chemicals are subject to reporting at lowered thresholds:

• dioxin and dioxin-like compounds

• lead compounds

• mercury compounds

• polycyclic aromatic compounds

• aldrin

• benzo(g,h,i)perylene

• chlordane

• heptachlor

• hexachlorobenzene

• isodrin

• lead

• mercury

• methoxychlor

• octachlorostyrene

• pendimethalin

• pentachlorobenzene

• PCBs

• tetrabromobisphenol A

• toxaphene

• trifluralin

Waste Minimization Priority Chem icals. EPA’s National Waste Minimization Partnership Program

focuses on reducing or eliminating the generation of hazardous waste containing any of 30 Waste

Minimization Priority Chemicals (WMPCs). This list replaces the list of 53 chemicals EPA identified

in 1998 (Notice of Availability: Draft RCRA Waste Minimization Persistent, Bioaccumulative and

Toxic (PBT) Chemical List, Federal Register 63(216): 60332-60343, November 9, 1998). Twenty six

of the chemicals in the current list were also in the draft list published in 1998. The remaining four

chemicals on the current list were added in response to comments and new information EPA received

from the public regarding the Agency’s methodology for selecting the 53 chemicals in the draft list 

(http://www.epa.gov/epaoswer/hazwaste/minimize/chemlist.htm).

• 1,2,4-trichlorobenzene

• 1,2,4,5-tetrachlorobenzene

• 2,4,5-trichlorophenol

• 4-bromophenyl phenyl ether

• acenaphthene

• acenaphthylene

• anthracene

• benzo(g,h,i)perylene

• dibenzofuran

• dioxins/furans

• endosulfan, alpha and endosulfan, beta

• fluorene

• heptachlor and heptachlor epoxide

• hexachlorobenzene

• hexachlorobutadiene

• hexachlorocyclohexane, gamma-

• hexachloroethane

• methoxychlor

• naphthalene

• PAH group (as defined in TRI)

• pendimethalin

• pentachlorobenzene

• pentachloronitrobenzene

• pentachlorophenol

• phenanthrene

• pyrene

• trifluralin

• cadmium and cadmium compounds

• lead and lead compounds

• mercury and mercury compounds
April 2004 Page 4-10

Exhibit 4-2. PB-HAP Compounds

PB-HAP Compound

Pollution

Prevention

Priority PBTs

Great Waters

Pollutants of

Concern

TRI PBT

Chemicals

Cadmium compounds X

Chlordane X X X

Chlorinated dibenzodioxins and furans X

(a) X X(b)

DDE X X

Heptachlor X

Hexachlorobenzene X X X

Hexachlorocyclohexane (all isomers) X

Lead compounds X

(c) X X

Mercury compounds X X X

Methoxychlor X

Polychlorinated biphenyls X X X

Polycyclic organic matter X

(d) X X(e)

Toxaphene X X X

Trifluralin X

(a) “Dioxins and furans” (“” denotes the phraseology of the source list)

(b) “Dioxin and dioxin-like compounds”

(c) Alkyl lead

(d) Benzo[a]pyrene

(e) “Polycyclic aromatic compounds” and benzo[g,h,i]perylene 
April 2004 Page 4-11

The HPV Challenge Program 

EPA, in partnership with industry and environmental

groups, recently created a voluntary chemical testing

effort, the high production volume (HPV) Challenge

Program. This program was developed to make

publicly available a complete set of baseline health and

environmental effects data on HPV chemicals (those

manufactured in, or imported into, the United States in

amounts equal to or exceeding 1 million pounds per

year). Information on HPV chemicals is available at

http://www.epa.gov/chemrtk/rtkfacts.htm.

4.2.6 Other Chemicals

The chemicals included in the various lists of

air toxics described above – HAPs, criteria

pollutants, TRI chemicals, and toxic

chemicals that persist and which also may

bioaccumulate – do not represent all of the

chemicals potentially emitted to air in a

given place. EPA is required to maintain an

inventory, known as the “Toxic Substances

Control Act (TSCA) Inventory,” of each

chemical substance which may be legally

manufactured, processed, or imported in the

U.S. The TSCA inventory currently contains

over 75,000 chemicals (see: “enforcement

programs” at http://www.epa.gov/compliance/civil/programs/tsca/). At best, we have the

capability to assess only a few hundred in detail. As noted previously, this does not imply that

risk assessments are always missing important information. To the contrary, the actual number

of chemicals used in significant amounts and released to air are relatively small compared to the

number of chemicals known. Nevertheless, it is important to keep in mind that the ability to

evaluate air toxics releases is limited by current technology, the lack of toxicity information for

all but a relatively small number of chemicals and, in some cases, costs (e.g., a single sample for

certain analytes such as dioxin can cost upwards of $1,000 per sample, making multiple

sampling events cost prohibitive).

4.3 Sources of Air Toxics

Many anthropogenic and natural activities are sources of air pollutants. Examples of human

activities that result in the release of air toxics include:

• Fuel combustion activities in power plants, factories, automobiles, and homes;

• Biomass burning and other agricultural activities;

• Use of consumer products, such as pesticides and cleaning agents;

• Commercial activities, such as dry cleaning; and

• Industrial activities, such as petroleum refining, chemical manufacture, and metal plating.

Sources of air toxics can be categorized in various ways – whether they occur indoors or out,

whether they are stationary or mobile, by the amount of chemicals they release, or by other

approaches. For the purposes of this discussion, air toxics have been placed into several major

groupings that track EPA’s programs and emissions inventories. Note that some differences in

terminology exist between the CAA and the NEI (Exhibit 4-3).

• Point sources;

• Nonpoint sources;

• On-road mobile sources;

• Nonroad mobile sources;

• Indoor sources;

• Natural sources; and

• Exempt sources.
April 2004 Page 4-12

The first four categories are groupings of emission sources of HAPs and criteria air pollutants in

EPA’s National Emissions Inventory (NEI). The NEI is a nationwide inventory of emissions

that has been developed by EPA with input from numerous state, local, and tribal (S/L/T) air

agencies. The NEI is discussed in more detail as a source of quantitative emissions release data

in Section 4.4.1 below. For detailed information on NEI, refer to EPA’s main NEI web page.(5)

NEI summaries were posted in October 2003.

Exhibit 4-3. Terminology Related to Groupings of Source Types

Source Type How Defined in CAA How Reported in NEI

Point source – Major Point source – Major Point source

Point source – Area Point source – Area Point source if location coordinates reported

Area source if coordinates not reported

Nonpoint source Nonpoint source Area

Mobile source – On-road Mobile source – On-road Modeled

Mobile source – Nonroad Mobile source – Nonroad Modeled or estimated

Indoor Not defined Not reported

Natural Not defined Not reported

Exempt Not defined Not reported

4.3.1 Point Sources

Point sources of air toxics are stationary sources (i.e., sources that remain in one place) that can

be located on a map. A large facility that houses an industrial process is an example of a point

source – the facility and its emission release points (e.g., stacks, vents, fugitive emissions from

valves) are stationary, and the emission rates of air toxics can be characterized, either through

direct measurements, such as stack monitoring, or indirect methods, such as engineering

estimates based on throughput, process information, and other data. The CAA divides point

sources into two main categories primarily on the basis of annual emissions rates:

• Major sources are defined in Section 112(a)(1) as “any source or group of stationary sources

located within a contiguous area and under common control that emits or has the potential to

emit, considering controls, in the aggregate, 10 tons per year (tpy) or more of any hazardous

air pollutant or 25 tpy or more of any combination of hazardous air pollutants.” 

• Area sources are defined in Section 112(a)(2) as “any stationary source of hazardous air

pollutants that is not a major source. For purposes of this section, the term ‘area source’

shall not include motor vehicles or nonroad vehicles subject to regulation under Title II.” 

Examples of area sources include dry cleaners, gas stations, chrome electroplaters, and print

shops. Though emissions from individual area sources may be relatively insignificant in

human health terms, collectively their emissions can be quite significant, particularly where

large numbers of sources are located in heavily populated areas. Note that sources that are

classified as “area sources” pursuant to the CAA may be reported in the NEI as “point

sources” if they can be located on a map.
April 2004 Page 4-13

Many sources of HAPs are subject to National Emission Standards for Hazardous Air

Pollutants (NESHAPs) pursuant to Section 112 of the CAA. This Section of the CAA directs

EPA to issue regulations listing categories and subcategories (commonly referred to collectively

as source categories) of major sources and area sources of HAPs and to develop standards for

each listed category and subcategory.(6) EPA periodically updates the list of source categories

(see Appendix E).(7)

Physical Forms of Emissions

Gas Emissions that are distinguished from solid and liquid states 

Fume Tiny particles trapped in vapor in a gas stream

Mist Liquid particles measuring 40 to 500 micrometers that are formed by

condensation of vapor

Particulate Matter (and

Aerosols)

Fine liquid or solid particles

Air pollutants can be found in all three physical phases: solid, liquid, or gaseous. The distinct

chemical and physical attributes of each phase contribute to the pollutant’s transport and fate. For

example, as reported in the Mercury Study Report to Congress,

(8) elemental mercury vapor is not

thought to be susceptible to any major process of direct deposition to the earth’s surface due to its

relatively high vapor pressure and low water solubility. Therefore, it is carried by the wind and

subsequently dispersed throughout the atmosphere. However, divalent mercury, in either vapor or

particulate phase, is thought to be subject to much faster atmospheric removal, and is expected to be

deposited near its source. For further details on fate and transport analysis, see Chapter 8.

As noted in Chapter 2, EPA regulates stationary sources in a two-phase process. First, EPA

issues technology-based MACT standards that require sources to meet specific emissions limits. 

The emission limits are typically expressed as maximum emission rates, or minimum percent

emission reductions, for specific pollutants from specific processes. In the second phase, EPA

applies a risk-based approach to assess how well MACT emissions limits reduce health and

environmental risks. Based on these residual risk assessments, EPA may implement additional

standards to address any significant remaining, or residual, health or environmental risks (see

Chapter 2 for a more detailed discussion of the MACT and residual risk programs).

Area sources may be subject to either MACT or Generally Available Control Technology

(GACT) standards. GACT standards are generally less stringent than MACT standards. Area

sources subject to MACT standards include Commercial Sterilizers using Ethylene Oxide,

Chromium Electroplaters and Anodizers, Halogenated Solvents Users, and Asbestos Processors.

4.3.2 Nonpoint Sources

The term nonpoint source refers to smaller and more diffuse sources within a relatively small

geographic area. In the context of EPA’s NEI, nonpoint sources of air toxics are stationary

sources for which emissions estimates are provided as an aggregate amount of emissions for all

similar sources within a specific local geographic area, such as counties or cities, rather than on a

facility- or source-specific basis. Emission estimates for nonpoint sources are generated using

“top-down” methods, when detailed information at the local level is lacking. Instead, the total

emissions over a large geographic area (e.g., n tons in the northeastern states) are allocated to the
April 2004 Page 4-14

local level (e.g., x percent is assigned to locality 1, y percent is assigned to locality 2, and so on). 

Note that for the purposes of this discussion, the nonpoint source category includes only

stationary sources and does not include mobile sources.

Source-specific information may be available for some (but not all) of the specific facilities

within a certain nonpoint source type. Area sources may be reported as either point or nonpoint

sources in the NEI. If a state or local agency reports an area source emission as a point source,

then the NEI retains the area source emission as a point source. The NEI does not aggregate

point area sources as nonpoint sources, and EPA has taken steps to avoid “double-counting”

of emissions in the point and nonpoint source inventories. 

To compile nonpoint estimates for a category, the EPA first estimates county level emissions for

nonpoint source categories. Then EPA replaces nonpoint EPA generated estimates with state

and local agency and tribal estimates. If a state or local agency or tribe includes point source

estimates for an EPA generated nonpoint source category, EPA removes the nonpoint estimate

that it had generated and the point source inventory contains the S/L/T estimate. For example, in

the Denver area, the State of Colorado inventories dry cleaners and service stations as point

sources. The NEI contains point sources estimates for these two categories in the six county area

of Denver and the NEI does not contain nonpoint estimates for these two categories. Dry

cleaners and service station emissions are contained in the NEI nonpoint inventory for the other

fifty counties on Colorado. 

A variety of sources are categorized as nonpoint sources in the NEI, including some small

industrial/commercial processes (e.g., small dry cleaning facilities, hospital sterilization

facilities, and dental offices). Additional nonpoint sources that contribute to air pollution are

agricultural activities, residential trash and yard-waste burning, wood stoves and fireplaces,

releases from spills and other accidents, and volatilization and resuspension of pollutants from

contaminated sites. Examples of agricultural activities contributing to air pollution are biomass

burning (e.g., for land clearing) and the application of fertilizers and pesticides. The open

burning of forests (including wildfires) are also categorized as nonpoint sources. (Note that

forest fires are generally considered for the purposes of the NEI to be an anthropogenic source of

air toxics because they are assumed to be directly or indirectly, for purposes of the NEI, caused

by man.)

Some nonpoint sources emit HAPs and are subject to NESHAPs pursuant to Section 112 of the

CAA (see Section 4.3.1 above for more information on NESHAPs). These nonpoint sources are

area sources in that they emit less than 10 tpy of a single air toxic or less than 25 tpy of a mixture

of air toxics. For example, facilities that perform perchloroethylene dry cleaning belong to a

source category that is subject to NESHAPs.

4.3.3 On-Road and Nonroad Mobile Sources

Mobile sources pollute the air with fuel combustion products and evaporated fuel. These sources

contribute greatly to air pollution nationwide and are the primary cause of air pollution in many

urban areas. Section 202(l) of the CAA gives EPA the authority to regulate air toxics from

motor vehicles. Based on 1996 National Toxics Inventory data (the NTI is the former name of

the air toxics portion of the current NEI), mobile sources contributed 2.3 million tpy or about

half of all air toxics emissions in the U.S. Mobile sources emit hundreds of air pollutants – for

example, exhaust and evaporative emissions from mobile sources contain more than 700
April 2004 Page 4-15

compounds. EPA’s Final Rule, Control of Emissions of Hazardous Air Pollutants from Mobile

Sources, commonly known as the “Mobile Source Air Toxics” (MSAT) rule,(9) identified 21

compounds as HAPs emitted by mobile sources (see Chapter 2). All of these compounds except

diesel particulate matter and diesel exhaust organic gases (DPM + DEOG) are included on the

CAA Section 112 HAPs list. Although some mobile source air toxics are TRI chemicals, mobile

sources are not generally subject to TRI reporting. Other mobile source regulations address

emissions of criteria pollutants and their precursors, including carbon monoxide (CO), nitrogen

dioxide (NO2), particulate matter (PM), volatile organic compounds (VOCs), and sulfur dioxide

(SO2). These criteria air pollutant control programs for mobile sources have and will continue to

result in substantial reduction of HAP releases.

Mobile sources include a wide variety of vehicles, engines, and equipment that generate air

pollution and that move, or can be moved, from place to place. In the NEI, EPA divides mobile

sources into two broad categories. On-road mobile sources include motorized vehicles that are

normally operated on public roadways for transportation of passengers or freight. This includes

passenger cars, motorcycles, minivans, sport-utility vehicles, light-duty trucks, heavy-duty

trucks, and buses. Nonroad mobile sources, (sometimes also called “off-road”) include aircraft,

commercial marine vessels (CMVs), locomotives, and other nonroad engines and equipment. 

The other nonroad engines and equipment included in NEI comprise a diverse list of portable

equipment, such as lawn and garden equipment; construction equipment; engines used in

recreational activities; and portable industrial, commercial, and agricultural engines.

EPA’s National Air Pollutant Trends Report, 1900–1998(10) indicates that about 60 percent of

mobile source air toxics emissions in the U.S. are from on-road sources, and 40 percent of

mobile source air toxics emissions are from nonroad sources. The emissions distribution

between on- and off-road sources emitting criteria pollutants depends on the chemical. CO

comprises the majority of criteria pollutants emitted, with over 100 million tons per year emitted

in the U.S. Releases of CO are primarily the result of mobile sources – like HAPs, these

emissions are split approximately 60/40 between on-road and off-road sources. (The use of CO

as a monitoring surrogate for mobile source emissions is discussed in Section 4.4.1.)

Within the two broader categories of mobile sources, EPA further distinguishes on-road and

nonroad sources by size, weight, use, horsepower and/or fuel type. For example, categories of

on-road vehicles include light-duty gasoline vehicles (i.e., passenger cars), light-duty gasoline

trucks, heavy-duty gasoline vehicles, and diesel vehicles. Examples of nonroad sources include

nonroad gasoline engines and vehicles, (e.g., recreational off-road vehicles, construction

equipment, lawn and garden equipment, and recreational marine vessels that use gasoline),

nonroad diesel engines and vehicles (including the vehicles and equipment listed above, except

those that use diesel fuel), aircraft, non-recreational marine vessels, and locomotives. An

additional category covers all nonroad sources that use liquified petroleum gas or compressed

natural gas.

4.3.4 Sources Not Included in the NEI or TRI

In addition to the four primary categories used in compiling the NEI, five other sources of air

toxics which are not captured by either the NEI or TRI are described below: Indoor sources,

natural sources, secondary formation of air toxics, exempt sources, and international transport.
April 2004 Page 4-16

4.3.4.1 Indoor Sources

Indoor pollution sources that release gases or particles into the air are the primary cause of

indoor air quality problems in homes (Exhibit 4-4). Inadequate ventilation can increase indoor

pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources

and by not carrying indoor air pollutants out of the home. High temperature and humidity levels

indoors can increase the uptake of some pollutants, thereby magnifying negative health effects.

There are many sources of indoor air pollution in any home. These include combustion sources

such as oil, gas, kerosene, coal, wood, and tobacco products; building materials and furnishings

as diverse as deteriorated, asbestos-containing insulation, wet or damp carpet, and cabinetry or

furniture made of certain pressed wood products; products for household cleaning and

maintenance, personal care, or hobbies; central heating and cooling systems and humidification

devices; and outdoor sources such as radon, pesticides, and outdoor air pollution. 

The relative importance of any single source depends on how much of a given pollutant it emits

and how hazardous those emissions are. In some cases, factors such as how old the source is and

whether it is properly maintained are significant. For example, an improperly adjusted gas stove

can emit significantly more carbon monoxide than one that is properly adjusted.

Some sources, such as building materials, furnishings, and household products like air

fresheners, release pollutants more or less continuously. Other sources, related to activities

carried out in the home, release pollutants intermittently. These include smoking, the use of

unvented or malfunctioning stoves, furnaces, or space heaters, the use of solvents in cleaning and

hobby activities, the use of paint strippers in redecorating activities, and the use of cleaning

products and pesticides in housekeeping. High pollutant concentrations can remain in the air for

long periods after some of these activities.

In addition to the same indoor air problems as single-family homes, apartments can have indoor

air problems similar to those in offices, which are caused by sources such as contaminated

ventilation systems, improperly placed outdoor air intakes, or maintenance activities.

One particularly important indoor air toxics problem actually results from an outdoor natural

source. In fact, radon gas, a HAP, is one of the leading causes of lung cancer in the U.S. The

most common source of indoor radon is uranium in the soil or rock on which homes are built

(thus, a natural source becomes an indoor air quality problem). As uranium naturally breaks

down, it releases radon as a colorless, odorless, radioactive gas. Radon gas enters homes

through dirt floors, cracks in concrete walls and floors, floor drains, and sumps. When radon

becomes trapped in buildings and indoor concentrations build up, exposure to radon becomes a

concern.

Sometimes radon enters the home through well water. In a small number of homes, the building

materials can give off radon, too. However, building materials alone rarely cause radon levels of

concern (see http://www.epa.gov/radon/risk_assessment.html for more information on radon

risks). Exhibit 4-5 shows EPA’s map of radon zones in the U.S.
April 2004 Page 4-17

Exhibit 4-4. Major Indoor Air Pollutants and their Sources

Major Indoor Air Pollutants Sources

Radon (Rn) Earth and rock beneath home; well water; building materials

Environmental Tobacco Smoke

(includes carbon monoxide,

nitrogen dioxide, and respirable

particles)

Cigarette, pipe, and cigar smoking

Biologicals (e.g., pollen, mold,

animal dander, and fungi)

Wet or moist walls, ceilings, carpets, and furniture; poorly

maintained humidifiers, dehumidifiers, and air conditioners;

bedding; household pets

Carbon Monoxide Unvented kerosene and gas space heaters; leaking chimneys and

furnaces; back-drafting from furnaces, gas water heaters,

woodstoves, and fireplaces; gas stoves. Automobile exhaust from

attached garages

Nitrogen Dioxide (NO2) Kerosene heaters, unvented gas stoves and heaters. Environmental

tobacco smoke

Volatile Organic Compounds

(such as xylene) 

Paints, paint strippers, and other solvents; wood preservatives;

aerosol sprays; cleansers and disinfectants; moth repellents and air

fresheners; stored fuels and automotive products; hobby supplies;

dry-cleaned clothing

Respirable Particles Fireplaces, wood stoves, and kerosene heaters. Environmental

tobacco smoke

Formaldehyde Pressed wood products (hardwood plywood wall paneling, particle

board, fiberboard) and furniture made with these pressed wood

products. Urea-formaldehyde foam insulation (UFFI). Combustion

sources and environmental tobacco smoke. Durable press drapes,

other textiles, and glues 

Pesticides Products used to kill household pests (insecticides, termiticides,

and disinfectants). Also, products used on lawns and gardens that

drift or are tracked inside the house

Asbestos Deteriorating, damaged, or disturbed insulation, fireproofing,

acoustical materials, and floor tiles 

Lead Lead-based paint, contaminated soil, dust, and drinking water

Source: U.S. Environmental Protection Agency and the United States Consumer Product Safety

Commission. 1995. Office of Radiation and Indoor Air (6604J) EPA/402/K/93/007, April 1995. 

Available at: http://www.epa.gov/iaq/pubs/insidest.html.
April 2004 Page 4-18

Exhibit 4-5. EPA Map of Radon Zones

• Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L (pico

curies per liter) 

• Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L

• Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L

4.3.4.2 Natural Sources

Natural processes are significant sources of some air pollutants, including VOCs, NOx, O3, PM

and other pollutants (Exhibit 4-6). Examples of natural sources of air pollutants that are not

covered by the four main categories described above include natural processes occurring in

vegetation and soils (e.g., emissions from trees), in marine ecosystems, as a result of geological

activity in the form of geysers or volcanoes, as a result of meteorological activity such as

lightning, and from fauna, such as ruminants and termites. Sources associated with biological

activity are called biogenic sources. 

Natural pollutants contribute significantly to air pollution. For example, biogenic emission

estimates for the United States were 28.2 million tons of VOC and 1.53 million tons of NOx in

1997.(10)
April 2004 Page 4-19

Exhibit 4-6. Categories of Natural Sources

Category Examples of Emissions Sources

Geological

• Sulphuric, hydrofluoric and hydrochloric acids

• Radon

• Nitrogen oxides

• Volcanic gases

• Radioactive decay of rock

• Soils, lightning

Biogenic

• Ammonia

• Methane

• VOCs

• Animals wastes

• Animal wastes, plant decay

• Vegetation

Marine • Dimethyl sulfide, ammonia, chlorides, sulfates,

alkyl halides, nitrous oxides

• Sea spray released by

breaking waves

Source: International Fertilizer Industry Association. 2001. Food and Agriculture Organization of the 

 United Nations. Global estimates of gaseous emissions of NH3, NO and N2O from 

 agricultural land. ISBN 92-5-104698-1. Available at: 

www.fao.org/DOCREP/004/Y2780E/y2780e01.htm.

4.3.4.3 Formation of Secondary Pollutants

Some air pollutants, in addition to being directly emitted to the atmosphere by identifiable

sources, are generated in the atmosphere by the chemical transformation of precursor compounds

(a process called secondary formation). For example, under some meteorological conditions,

up to 90 percent of ambient formaldehyde originates from secondary formation from a variety of

precursor compounds in the presence of light (i.e., via a photochemical reaction). Some of the

precursor compounds include isoprene (an organic compound released from trees), isobutene,

and propene. The secondary formation of pollutants like formaldehyde and acetaldehyde is a

complex process but can be estimated by some photochemical models (e.g., UAM-Tox, a special

version of the Urban Airshed Model (UAM)). Other available models also address secondary

formation but in a much more limited way (see Chapter 9 for a more detailed discussion of air

models).

The NEI and other emission inventories generally do not include estimates of pollutants formed

through secondary formation – only the initially emitted species are included. Because the

formation of secondary pollutants depends on the meteorological conditions and the presence or

absence of other compounds and/or light, a model that incorporates chemical transformation

algorithms is required to estimate how much secondary product is formed from precursor

compounds once they enter the atmosphere. EPA has in some instances developed estimates of

secondarily formed chemicals to better inform the assessment of exposure of people to toxic air

pollutants. For example, for the 1996 NATA, National-scale Air Toxics Assessment, risk

characterization exercise, EPA developed a special inventory of precursor compounds to

supplement the NTI, which was used in conjunction with the Assessment System for Population

Exposure Nationwide (ASPEN) model to calculate ambient concentrations (see

http://www.epa.gov/ttn/atw/nata/). Formation of secondary pollutants is discussed in greater

detail in Chapter 8.
April 2004 Page 4-20

International Transport of Air Toxics

As noted earlier in Chapter 2 and Section 4.2.5,

certain air toxics may be transported over long

distances, sometimes across international borders. 

International sources may be an important

contributor to local pollutant levels in some study

areas.

4.3.4.4 Other Sources Not Included in NEI or TRI

Many air toxics sources, usually relatively small ones, may not be covered or are exempt from

various emissions control, reporting, and other requirements, and in some cases the number or

stringency of requirements is tiered according to source size or other criteria. For example, air

pollution regulations for municipal waste combustors (MWCs) promulgated pursuant to Section

129 of the CAA include separate rules for large MWCs (i.e., with capacities greater than 250

tons per day) and small MWCs (i.e., with capacities between 35 and 250 tons per day). 

However, there are no rules for MWCs with capacities less than 35 tons per day.

Other miscellaneous sources of air pollution

(e.g., agricultural and residential burning) are

controlled primarily by other S/L/T

requirements. However, EPA conducts

research, provides information, and pursues

other non-regulatory means of addressing

some of these pollution sources. For

example, EPA, in conjunction with the

Consumer Product Safety Commission and

the American Lung Association, has

published a guide for reducing pollution from residential wood combustion, including design

information for less-polluting stoves and fireplaces.(11) Some local areas have ordinances that

require new fireplace and wood stove installations to comply with the certification program, and

others have ordinances that prohibit the use of a wood stove or fireplace on days that are

conducive to the concentration of wood smoke emissions.

Ultimately, there is no single comprehensive source of information on all sources of air toxics in

a given area. The NEI and TRI are good places to start an investigation of what is being released

in a study area, but as noted above, in any given place, there are probably a number of air toxics

sources that are not accounted for in these inventories. Nonregulated sources, natural sources,

and material moving into a study area from distant sources all have an impact on overall air

quality. Assessors need to clearly understand what these limitations are as they move into the

planning and scoping stage of the risk assessment (see Chapter 6). (A description of how EPA

addressed background concentrations for the NATA national-scale assessment is provided at

http://www.epa.gov/ttn/atw/nata/natsa2.html.)

4.4 Emissions Inventories

As mentioned previously, information on releases of air toxics is primarily compiled and

maintained in emissions inventories. The primary emission inventory for HAPs and criteria

pollutants is EPA’s NEI. EPA’s TRI is a second inventory that has some utility for planning

and scoping an air toxics risk assessment, but is of limited use for risk assessment because of the

nature of the way the data are reported. In addition to the NEI and the TRI, S/L/T air agency

permit files and, in some instances, S/L/T inventories that have been developed, but not

submitted to the NEI, can also provide information on the location, identity, magnitude, and

source characteristics of air toxics releases. 

The best inventory data are collected near the ground, literally at the source. For example, an

urban scale study might opt to do a “drive by” or “windshield” verification of the number and
April 2004 Page 4-21

location of dry cleaners and gas stations in the study area rather than rely on an aggregate

county-level estimate. Ultimately, the needs of the assessment (e.g., screening level or more

refined) will determine the level of accuracy needed in the emissions inventory. This section

will describe the NEI and the TRI. Other potential sources of air toxics data are described in

Chapter 7. The process of developing an emissions inventory is also described in Chapter 7.

4.4.1 National Emissions Inventory (NEI)

EPA’s Office of Air and Radiation compiles and maintains the National Emissions Inventory

(NEI) that includes quantitative data on anthropogenic emissions of criteria pollutants and HAPs

and characteristics of the sources of these air toxics.(5) It includes point, non-point, and mobile

sources for all 50 states, Washington, D.C., and U.S. territories.

Previously, emissions of criteria pollutants and HAPs were tracked separately by EPA in

databases that preceded the NEI. Criteria pollutant emissions data for 1985 through 1998 are

available in the National Emission Trends (NET) database. Hazardous air pollutant (HAP)

emissions data are available for 1993 and 1996 in the National Toxics Inventory (NTI) database. 

For 1999 (the most recent year for which data are available), criteria and HAP emissions data

have been prepared separately but in a more integrated fashion. The final version of both the

criteria and HAP inventories (for 1999) are available at http://www.epa.gov/ttn/chief/net/

1999inventory.html. Note that the data collection and processing requirements for this

undertaking are significant. As such, EPA plans to update the NEI every three years. 

The NEI inventories are developed by EPA’s Emission Factors and Inventories Group with input

from S/L/T agencies, industry, and a number of EPA offices. In some cases, if a S/L/T agency

does not submit data, EPA may use data from an earlier year and “grow” the emissions (e.g., for

criteria stationary sources) or use only data available from other sources (e.g., HAP collected by

EPA as part of the development of emission standards, or data submitted by sources under the

Toxics Release Inventory program). Separate inventory documentation files have been prepared

for each part of NEI (i.e., for criteria pollutants and HAPs, and for point, nonpoint, and mobile

sources). These detailed documentation files are available online for criteria pollutants(12) and

HAPs.(13) The reader should refer to these documentation files for detailed information on NEI. 

Summaries of data sources for the components of the current version of NEI are also provided

below.

An important fact to keep in mind about the NEI is that it includes data on HAPs from both

small and large stationary sources and both on- and off-road mobile sources. Equally important,

it is much more likely to include the data necessary for modeling (although many of the data

fields needed for modeling are not “mandatory,” and thus states are not required to provide this

information to the NEI). Information such as stack height, emission rate, and temperature are

critical to developing reasonably accurate estimates of human exposure in the areas surrounding

a source. It is for this reason that the NEI can be of more use than other databases, for example,

for getting a better handle on realistic exposure and risk estimates in an actual study.

NEI for HAPs – Point Sources. For the NEI for HAP emissions from point sources, S/L/T

agencies are asked to supply HAP emission inventory data to EPA. If they do not provide HAP

emission inventory data to EPA, then EPA prepares default emission inventory data (this has

been done for the 1993, 1996, and 1999 inventory years). As discussed previously, EPA uses a

variety of methods to develop data and fill in gaps, where necessary (for point sources of HAPs,
April 2004 Page 4-22

EPA uses S/L/T data, EPA estimates for MACT and source categories, and TRI data; all the TRI

facilities are in the NEI). This is one reason why the NEI provides, for some sources, data that

may not accurately reflect actual emissions in any given place. Depending on a study’s specific

data quality objectives, closer inspection and verification of emissions estimates may be

necessary.

The target area for the NEI includes every state and territory in the United States and every

county within a State. There are no boundary limitations pertaining to traditional criteria

pollutant nonattainment areas or to designated urban areas. If a facility was included in a S/L/T

database, it is included in the NEI regardless of where in the state it was located. The pollutants

inventoried included all 188 HAPs identified in Section 112(b) of the CAA. Some S/L/T

agencies collect information on more than just these HAPs, but only the 188 are included in the

HAP NEI. In addition to numerous specific chemical species and compounds, the list of 188

HAPs includes several compound groups (e.g., individual metals and their compounds,

polycyclic organic matter [POM], and glycol ethers); the NEI includes emission estimates for the

individual compounds within these groups wherever possible. Appendix F lists all of the

specific pollutants and compound groups included in the 1999 NEI along with their Chemical

Abstract Services (CAS) numbers (for individual compounds).

NEI for Criteria Pollutants – Point Sources. For the NEI for criteria emissions from point

sources, EPA solicits point source data from S/L/T governments. EPA uses S/L/T point source

data preferentially, except for NOx and SO2 emissions from utilities. For utilities, EPA uses NOx

and SO2 emissions that facilities report to the Emissions Tracking System/Continuous Emissions

Monitoring (ETS/CEM) Scorecard database. Some other criteria pollutant emissions data in the

most recent version of NEI have been supplemented by EPA based on submissions to other

emissions databases. In addition, emissions of ammonia (NH3) (which is not a criteria pollutant,

but is a precursor for PM) have been added to NEI based on reports submitted by S/L/T offices,

TRI data, and (for locations where reports were not submitted) also based on EPA estimation

methods.

Nonpoint Sources (Both HAPs and Criteria Pollutants). Much of the nonpoint source data in

NEI for HAPs was initially compiled as a national-level inventory. National-level emission

estimates are spatially allocated to the county-level using a number of allocation factors, such as

population and employment within certain industries. For example, aggregate amounts of dry

cleaner emissions for a county might be estimated from the number of people living within a

county. For HAPs, EPA uses MACT data and S/L/T data, where available.

When S/L/T- or locality-specific emissions data are available, those data are substituted for data

that had been allocated from national emission estimates. EPA prepares emissions for several

area source categories for the NEI each year using the most current activity and emission factor

data available. Emissions for other area source categories for which methodologies were not

prepared in a given year are extrapolated (and assumed to increase some percentage each year)

from the most recent S/L/T inventory submitted previously to EPA. For example, if an

inventory was submitted in the past 3 years to EPA for the 1996 base year, the 1999 NEI

emissions are extrapolated from the 1996 inventory. In some cases, criteria air emissions may

also be extrapolated from other inventories (e.g., the 1985 National Air Pollutant Assessment

Program inventories). A more detailed discussion of emissions estimation routines for source

categories with national-level emission estimates are described in the documents referenced

above.
eVolatile Organic Compound means any organic compound which participates in atmospheric

photochemical reactions; or which is measured by a reference method, an equivalent method, an alternative method,

or which is determined by procedures specified under any subpart (40 CFR Part 60).

.

April 2004 Page 4-23

EPA uses an emissions estimation model known as the Biogenic Emissions Inventory System

(BEIS) to predict emissions of VOC and NOx from forests, crop lands, and fertilized lands. 

Emission rates are dependent on several meteorological factors. VOC emissions are dependent

on temperature and sunlight, and NOx emissions from fertilized soils are dependent on

temperature and soil moisture. The BEIS model is used to predict emissions that are included in

the NEI inventory for criteria pollutants. (Keep in mind that VOCs, as a group, are inventoried,

but not speciated, to help evaluate an area’s potential for ozone production. Non-speciated VOC

data are of limited use for performing air toxics risk assessments.)(e)

On-road Mobile Sources. In the final Version (V 3.0) of the 1999 NEI, EPA used the most

recent version of the MOBILE6 (Version 6.2) model to calculate emission factors for criteria

pollutants and 36 HAPs. On-road emissions inventories for CO, NOx , VOC, PM10, PM2.5, SO2,

NH3, and the 36 HAPs are calculated by multiplying an appropriate emission factor in grams

emitted per mile by the corresponding vehicle miles traveled (VMT) in millions of miles, and

then converting the product to units of tons of emissions. Emission estimates include

calculations by month, county, road type, and vehicle type, with VOC broken down by exhaust

and evaporative emissions and PM10 and PM2.5 broken down by exhaust, brake wear, and tire

wear emissions. The MOBILE6 model used is the publicly available version from EPA’s Office

of Transportation Air Quality’s (OTAQ) Website (http://www.epa.gov/otaq/m6.htm). This

model incorporates both MOBILE6.0, which is used to estimate emission factors of VOC, CO,

and NOx , and MOBILE6.1, which is used to calculate emission factors of PM10, PM2.5, SO2 and

NH3 and MOBTOX, which is used to calculate certain HAPs. The particulate and SO2 emission

factors were previously calculated using EPA’s PART5 model.

Nonroad Mobile Sources. To develop this component of the NEI, data were compiled on

criteria and HAP emissions data for aircraft, commercial marine vessels, and locomotives. HAP

emissions for other nonroad engines operating in the United States were estimated using the

latest nonroad model. S/L/T data are used when provided. In this effort, national emission

estimates were often developed for each of the above types of nonroad sources and allocated to

counties based on available Geographic Information System (GIS) data. For some pollutants

associated with the nonroad category, county-level (instead of national) data were used to

estimate emissions. The methodologies used to estimate emissions and the procedures used to

spatially allocate them to the county level vary by source category and pollutant. For some

pollutants and categories, the NONROAD model was utilized to estimate emissions (see

http://www.epa.gov/otaq/nonrdmdl.htm).

Concurrent with the development of the national emission estimates, S/L/T agencies developed

and provided to EPA emissions inventory data for their areas based on local knowledge and

activity information. These S/L/T agency data replaced the national emission estimates when

the pollutant, source type, and emission type matched with the national estimates. Submitted

S/L/T data that did not match the nationally-derived data were retained along with the national

estimates. S/L/T data were used as provided and not adjusted to better match the national data. 

Some S/L/T inventories did not provide estimates for all of the pollutants included in the
April 2004 Page 4-24

nationally-derived emission estimates; in these cases, the submitted S/L/T data were used and

the national estimates were included only for the missing pollutants.

Although the current NEI data files represent a valuable source of emissions data, there are

numerous uncertainties associated with the current versions of these inventories that should be

considered when using the data in a risk assessment. Sources of uncertainty include the

following: 

• The emission data included in NEI are of variable, and in some cases, undocumented

derivation. Many of the emission estimates were submitted by the sources of the emissions

to S/L/T air agencies, and then to EPA, without full explanations of how the emissions were

estimated.

• Not all sources are accounted for. In some S/L/T data sets, very small sources have been

reported, while in others only the largest sources in certain types of industry are included.

• Not all S/L/T agencies have submitted data. Specifically, for the NEI for criteria pollutants,

35 out of 50 states submitted data to EPA for the 1999 version of NEI. For the other states,

EPA extrapolated the affected portions of the inventory from an earlier year. This omits

sources that came online in the target year and erroneously include sources that have shut

down. For more information on which S/L/T governments submitted data and for which

states the inventory is extrapolated, the user can refer to the documentation for the respective

inventory sector and the respective pollutant type (see website addresses above). Some of

the states for which 1999 data were not available when these inventory versions were

compiled have now provided EPA with their data, and EPA is working to incorporate this

data into the next versions. For HAPs, 46 states have participated in the development of the

1999 NEI (with some revisions from states still under way).

• Duplicate facilities may be present, but most of the duplicates have been removed. Facility

identification (ID) codes are a potential source of confusion. The NEI Unique Facility ID is

the ID for the entire facility, while the state IDs are usually for individual processes;

therefore an NEI Unique Facility ID can have multiple state facility IDs.

• The primary source of uncertainty associated with the inventory is the methodology used to

generate the emission estimates. The emission estimation methodology is often poorly

documented in the NEI Input Format - this data field is not mandatory. Data in the 1999 NEI

for HAPs are made using different estimation methods. Future versions of the NEI will

include a data quality rating to each emissions record, which should help characterize the

quality of the emissions estimate. 

Emissions data in the NEI are submitted to EPA according to the NEI Input Format (NIF)

Shell (see http://www.epa.gov/ttn/chief/nif/index.html). This format consists of data fields

grouped into tables that provide the basic structure of NEI. The NIF shell consists of eight tables

for point sources, five tables for area sources, three tables for mobile sources, and two tables for

biogenic sources. EPA has developed data element descriptions and data element validation

rules to enforce mandatory data fields and relationships between the various tables and records

of the NIF. As the NEI has evolved (and continues to be improved and developed), the NIF shell

has evolved as well. Version 3.0 of the NIF shell was released in May 2003 and updated in

November 2003.
April 2004 Page 4-25

In June 2002, EPA promulgated the final Consolidated Emissions Reporting Rule,(14) which

simplifies and consolidates emission inventory reporting requirements (for criteria air pollutants

only) to a single location within the Code of Federal Regulations (CFR), establishes new

reporting requirements related to PM2.5 and regional haze, and establishes new requirements for

the statewide reporting of area source and mobile source emissions. Many state and local

agencies asked EPA to take this action to consolidate reporting requirements; improve reporting

efficiency; provide flexibility for data gathering and reporting; and better explain to program

managers and the public the need for a consistent inventory program. Consolidated reporting

should increase the efficiency of the emission inventory program and provide more consistent

and uniform data.

In conjunction with the NIF shell, EPA has developed an automated software program to help

NIF users perform quality assurance/quality control (QA/QC) checks on their files to ensure

correct format specification. This software, available for download from the NIF shell web

page, separates QA/QC checks into format and content. Format checks ensure that the submitted

information includes the minimum data elements required for Emission Factor and Inventory

Group (EFIG) to accept the submitted data. Content checks are provided for the user as a way to

highlight possible errors in the submitted data. The latest version of the software allows the user

to choose whether to perform QA/QC checks on the data for format, the minimum standards

required to put the data in the database, or the more resource intensive content or reasonableness

checks. When checking for content, the format is also checked as the format must be correct in

order for content checks to be performed at all.

4.4.2 Toxics Release Inventory (TRI)

The Toxics Release Inventory (TRI) is a publicly available EPA database that contains

information about releases and other waste management activities reported annually by certain

covered industry groups as well as federal facilities for over 650 toxic chemicals (see

http://www.epa.gov/tri/). This inventory was established under the Emergency Planning and

Community Right-to-Know Act of 1986 (EPCRA) and expanded by the Pollution Prevention

Act of 1990. 

TRI reporting is required only for facilities that meet all of the following three criteria: 

• They have ten or more full-time employees or the equivalent (i.e., a total of 20,000 hours or

greater; see 40 CFR 372.3);

• They are included in specified industrial sectors (see Exhibit 4-7); and

• They exceed any one reporting threshold for manufacturing, processing, or otherwise using a

TRI chemical (see Exhibit 4-8).

If a facility meets these criteria, then it must report releases to environmental media as well as

waste management data. In 2001 (the latest year for which data are publicly available), air

emissions of toxic chemicals totaled 1.7 billion pounds (over a quarter of all releases of TRI

chemicals to the environment).
April 2004 Page 4-26

Exhibit 4-7. Standard Industrial Classification (SIC) Codes in TRI Reporting

Original Industries

SIC Code Industry Group SIC Code Industry Group

20 Food 30 Rubber and Plastics

21 Tobacco 31 Leather

22 Textiles 32 Stone, Clay, and Glass

23 Apparel 33 Primary Metal

24 Lumber and Wood 34 Fabricated Metals

25 Furniture 35 Machinery (excluding electrical)

26 Paper 36 Electrical and Electronic Equipment

27 Printing and Publishing 37 Transportation Equipment

28 Chemicals 38 Instruments

29 Petroleum and Coal 39 Miscellaneous Manufacturing

New Industries Reporting to TRI as of the 1998 Reporting Year

SIC Code Industry Group

10 Metal mining (except for SIC codes 1011,1081, and 1094)

12 Coal mining (except for 1241 and extraction activities) 

4911, 4931,

and 4939

Limited to electrical utilities that combust coal and/or oil for distribution in

commerce (SIC codes 4911, 4931, and 4939)

4953 Limited to hazardous waste treatment and disposal facilities regulated under the

Resource Conservation and Recovery Act (RCRA) Subtitle C

5169 Chemicals and allied products wholesale distributors 

5171 Petroleum bulk plants and terminals

7389 Solvent recovery services primarily engaged on a contract or fee basis

Source: U.S. Environmental Protection Agency. 2004. Toxic Release Inventory (TRI) Program. 

 Standard Industrial Classification (SIC) Codes in TRI Reporting. Updated March 2, 2004. 

 Available at: http://www.epa.gov/tri/report/siccode.htm. (Last accessed April 2004.)
April 2004 Page 4-27

Exhibit 4-8. Thresholds for Reporting to the TRI

EPCRA Section 313 non-PBT chemicals (Section 372.25). A facility meeting the SIC code (or

Federal facility) and employee criteria must file a TRI report for a non-PBT Section 313 chemical if

the facility:

• Manufactured (including imported) more than 25,000 pounds per year; or

• Processed more than 25,000 pounds per year; or

• Otherwise used more than 10,000 pounds per year.

EPCRA Section 313 PBT chem icals (40 CFR372.28). If a facility manufactures, processes, or

otherwise uses any chemicals that are listed as persistent, bioaccumulative, and toxic (PBT), the

threshold quantity is one of the following (per Section 313 chemical or category per year):

Type of Chemical

Reporting Threshold by Activity

Manufacture Process Otherwise Used

Highly persistent and bioaccumulative

compounds 10 pounds 10 pounds 10 pounds

Dioxin and dioxin-like compounds 0.1 grams 0.1 grams 0.1 grams

Other persistent and bioaccumulative

compounds (lead and lead compounds) 100 pounds 100 pounds 100 pounds

Activity thresholds are calculated independently of each other based on cumulative quantities per

Section 313 chemical over the reporting year.

Current list of Section 313 PBT Chemicals

• aldrin

• benzo(g,h,i)perylene

• chlordane

• dioxin and dioxin-like compounds

• heptachlor

• hexachlorobenzene

• isodrin 

• lead

• lead compounds

• mercury

• mercury compounds

• methoxychlor

• octachlorostyrene

• pendimethalin

• pentachlorobenzene

• polychlorinated biphenyl

• polycyclic aromatic compounds

• tetrabromobisphenol A

• toxaphene

• trifluralin
April 2004 Page 4-28

Note in Exhibit 4-7 that additional industries have been added to the TRI over time. Thus, some

industries were not required to report in the past and, as such, no records will exist for these

facilities in the historical TRI files. The list of covered chemicals has also grown over time. 

Thus, the ability to track trends for more recently added industries and chemicals is more limited

than for industries and chemicals that have been covered throughout the history of the TRI.

Industrial sectors subject to TRI reporting are identified by Standard Industrial Classification

(SIC) codes. SIC codes are numerical codes developed by the U.S. government as a means of

consistently classifying the primary business of business establishments. A full list of the

industry groups that are required to report can be found at http://www.epa.gov/tri/report/

siccode.htm.

Although most of the existing emissions data in the TRI system are organized according to SIC

codes, EPA has proposed regulations that would result in the use of the North American Industry

Classification System (NAICS) rather than SIC codes (see http://www.census.gov/epcd/www/

naics.html). Rather than classifying industries on the basis of several different economic

concepts (as the SIC structure does), NAICS classifies establishments according to similarities in

the processes used to produce goods and services. The TRI program issued a proposed rule to

implement the NAICS system classification on March 21, 2003. (See 66 Fed. Reg. 13872.) It is

expected that the use of NAICS in the TRI system will allow EPA to more accurately

characterize the current state of the national economy (including new and emerging industries

not adequately covered by SIC codes). The existing SIC structure will not be updated in the

future because the Office of Management and Budget has adopted NAICS as the United States’

new industry classification system. In addition, using NAICS for TRI reporting purposes will

enable more efficient database integration and will promote public access to commonly defined

data from disparate sources. This change will not affect the universe of facilities that is currently

required to report to TRI.

EPCRA requires only that facilities report their releases of the listed chemicals. There are no

additional control or mitigation actions required. The information collected through the TRI

program is made public, however, and pressure from local citizen groups has been an incentive

to many industries to reduce the quantity of pollutants they release.

While the TRI data have utility for the scoping out of an air toxics risk assessment project, they

have several limitations that assessors must understand. Importantly for risk assessors, the TRI

program requires only that one single annual value representing total releases to the air

(segregated only by stack releases and fugitive releases) be reported by the individual affected

facilities. So while annual average emissions may be useful in screening-level assessments for

chronic exposures, it may be difficult to assess acute noncancer hazard associated with shortterm, peak emission levels. Source-specific information within the facility is not routinely

reported through the TRI. Likewise, no information is reported on release parameters critical to

air dispersion modeling (e.g., location of release on the facility property, release rates, stack

height, stack diameter, release temperature). (See Chapter 9, for more information on modeling

parameters used in air quality and exposure modeling.)

As discussed in Section 4.2, the list of TRI pollutants is organized differently than the list of

HAPs in CAA Section 112, causing some complications in interpreting emission data. It is

difficult to correctly relate some of the SIC codes (under which TRI emissions are grouped) to

specific air emission processes. Because quantities are only reported if a statutory threshold is
April 2004 Page 4-29

1. United States Environmental Protection Agency. 1998. Handbook for Air Toxics Emission

Inventory Development, Volume 1: Stationary Sources (Appendix C). Office of Air Quality

Planning and Standards, Research Triangle Park, NC, November 1998. EPA/454/B98/002.

Available at: http://www.epa.gov/ttn/chief/eidocs/airtoxic.pdf.

2. U.S. Environmental Protection Agency. 2003. Technology Transfer Network

Air Toxics Website. The Original List of Hazardous Air Pollutants. Updated January 9,

2004. Available at: http://www.epa.gov/ttn/atw/188polls.html. (Last accessed March 2004.)

3. U.S. Environmental Protection Agency. 2001. The Emergency Planning and Community

Right-to-Know Act: Section 313 Release and Other Waste Management Reporting

Requirements. Washington, D.C. EPA/260/K01/001. Available at:

http://www.epa.gov/tri/guide_docs/2001/brochure2000.pdf.

4. U.S. Environmental Protection Agency. 2001. Emergency Planning and Community Rightto-Know Section 313; List of Toxic Chemicals. Office of Environmental Information.

Washington, DC, March 2001. EPA/ 260/B01/001. Available at:

http://www.epa.gov/tri/chemical/chemlist2001.pdf. 

5. U.S. Environmental Protection Agency. 2004. Technology Transfer Network Clearinghouse

for Inventories & Emission Factors. National Emissions Inventories for the U.S. Updated

February 4, 2004. Available at: http://www.epa.gov/ttn/chief/net/. (Last accessed March

2004.)

6. U.S. Environmental Protection Agency. 1992. Notice Initial Source Category List. Federal

Register 57:31576, July 16, 1992.

7. U.S. Environmental Protection Agency. 2002. National Emission Standards for Hazardous

Air Pollutants: Revision of Source Category List Under Section 112 of the Clean Air Act.

Federal Register 67:6521, February 12, 2002.

met, a facility may report emissions for one year but not the next, even though the facility is still

in operation. Similarly, individual pollutants may not be reported consistently from year to year

due to the thresholds that apply to individual pollutants (e.g., a facility may report releases of 10

pollutants one year and releases of only five pollutants the next year because the others dropped

below the reporting threshold).

Furthermore, for some facilities, it is possible that, for a variety of releases, the data included for

a facility’s emissions in the TRI do not match the same data reported to the NEI, indicating a

potential problem with either or both data sets. The risk assessor should apply care and

discretion when using TRI information to estimate exposures and risk from individual facilities. 

Ultimately, the TRI provides information about the location, identity, and amount of air toxics

emissions in a community. However, due to the nature of the way the data are developed and

reported, TRI data should generally be considered a source of limited information about a

facility and should not be used in risk assessments involving modeling (as noted above, S/L/T

and NEI data are more likely to be useful for modeling). For robust analysis, it should generally

be considered a starting point, not an end.

References
April 2004 Page 4-30

8. U.S. Environmental Protection Agency. 1997. Mercury Study Report to Congress. Office of

Air Quality Planning and Standards. Washington D.C. EPA/452/R97/0003. Available at:

http://www.epa.gov/ttn/atw/112nmerc/mercury.html. 

9. U.S. Environmental Protection Agency. 2001. Control of Emissions of Hazardous Air

Pollutants From Mobile Sources. Final Rule. Federal Register 66:17230, March 29, 2001.

Available at: http://www.epa.gov/fedrgstr/EPA-AIR/2001/March/Day-29/a37.htm.

10. U.S. Environmental Protection Agency. 2000. National Air Pollutant Emission Trends:

1900-1998. Office of Air Quality Planning and Standards, Research Triangle Park, NC. 

EPA/454/R00/002. Available at: (http://www.epa.gov/ttn/chief/trends/trends98/index.html).

11. U.S. Environmental Protection Agency, Consumer Product Safety Commission (CPSC), and

American Lung Association. 1998. What You Should Know About Combustion Appliances

and Indoor Air Pollution. CPSC Document #452. Available at: http://www.cpsc.gov/cpscpub

/pubs/452.html. (Last accessed March 2004.)

12. U.S. Environmental Protection Agency. 2004. Technology Transfer Network

Air Toxics Website. Area Source Standards. Updated January 15, 2004. Available at:

ftp://ftp.epa.gov/EmisInventory/finalnei99ver2/criteria/documentation/. (Last accessed

March 2004.)

13. U.S. Environmental Protection Agency. 2003. Documentation for the Final 1999 Nonpoint

Area Source National Emission Inventory for Hazardous Air Pollutants (Version 3). 

Emissions, Monitoring and Analysis Division, Research Triangle Park, NC, August 2003. 

Available at: ftp://ftp.epa.gov/EmisInventory/finalnei99ver3/haps/documentation/.

14. U.S. Environmental Protection Agency. 2002. Consolidated Emissions Reporting. Final

Rule. Federal Register 67:39602, June 10, 2002. Available at:

http://www.epa.gov/ttn/chief/cerr/index.html.