Toxic Weighting Factors Methodology 

 

U.S. Environmental Protection Agency 

Office of Water

Engineering and Analysis Division

1200 Pennsylvania Avenue, NW

Washington, D.C. 20460

March 2012

EPA - 820-R-12-005
i

CONTENTS

Page

1. INTRODUCTION.............................................................................................................................. 1-1

1.1 Introduction References ................................................................................................... 1-3

2. CLARIFICATION OF KEY TWF CONCEPTS AND TERMS ................................................................. 2-1

3. THE TWF EQUATION .................................................................................................................... 3-1

3.1 The TWF Equation References ........................................................................................ 3-3

4. THE AQUATIC LIFE COMPONENT OF THE TWF EQUATION .......................................................... 4-1

4.1 The Aquatic Life Component of the TWF Equation References ..................................... 4-2

5. THE HUMAN HEALTH COMPONENT OF THE TWF EQUATION....................................................... 5-1

5.1 Equation for Calculating the TWF Human Health Value for Noncarcinogens ............... 5-1

5.2 Equation for Calculating the TWF Human Health Value for Carcinogens ..................... 5-2

5.3 Data Sources for Calculating the Human Health Component of the TWF 

Equation ........................................................................................................................... 5-3

5.4 Calculating the TWF Human Health Value for Dioxins .................................................. 5-4

5.5 Variable Factors Used in the Human Health Component of the TWF Equation ............. 5-6

5.5.1 Relative Source Contribution Factors ................................................................. 5-6

5.5.2 Bioaccumulation and Bioconcentration Factors ................................................. 5-6

5.6 The Human Health Component of the TWF Equation References .................................. 5-7

6. EPA’S USE OF TWFS TO SUPPORT EFFLUENT GUIDELINES RULEMAKINGS AND PROGRAM 

PLANNING ..................................................................................................................................... 6-1

6.1 Use of TWFs in Cost-Effectiveness Analysis .................................................................. 6-1

6.2 Use of TWFs in Effluent Guidelines Program Planning .................................................. 6-3

6.3 EPA’s Use of TWFs to Support Effluent Guidelines Rulemakings and Program 

Planning References ........................................................................................................ 6-6

7. LIMITATIONS AND APPROPRIATE USES OF TOXIC WEIGHTING FACTORS .................................... 7-1

7.1 Limitations of Source Data Used to Derive TWFs .......................................................... 7-1

7.2 Limitations of Discharge Data to Which TWFs Are Applied ......................................... 7-1

7.3 Appropriate Uses of TWFs .............................................................................................. 7-3

7.4 Limitations and Appropriate Uses of Toxic Weighting Factors References ................... 7-4
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LIST OF TABLES

Page

Table 3-1. TWF for Copper ....................................................................................................................... 3-2

Table 5-1. Toxic Equivalency Factors (TEF), Bioaccumulation Factors (BAF), and TWF Values 

for Dioxin Compounds ................................................................................................................. 5-5

Table 6-1. Examples of Effluent Guidelines Cost-Effectiveness Analysis Ranked by Dollars Per 

TWPE Removed ........................................................................................................................... 6-2

Table 6-2. TWPE Ranking of Industrial Categories Based on 2008 DMR Data ....................................... 6-4

Table 6-3. TWPE Ranking of Industrial Categories Based on 2008 TRI Data ......................................... 6-5
iii

LIST OF FIGURES

Page

Figure 3-1. TWF Equation ......................................................................................................................... 3-1

Figure 5-1. Equation for Calculating the TWF Human Health Value for Noncarcinogens ....................... 5-1

Figure 5-2. Equation for Calculating the Human Health Component of TWFs for Carcinogens .............. 5-2

Figure 5-3. Hierarchy of Data Sources for Calculating the Human Health Component of TWFs ............ 5-3
1-1

1. INTRODUCTION

The purpose of this report is to explain the concept, calculation, use, and limitations of Toxic 

Weighting Factors (TWFs). The U.S. Environmental Protection Agency’s Engineering and Analysis 

Division of the Office of Water (EPA) began developing TWFs in the early 1980s to compare the relative 

cost effectiveness of treatment technology options for national industrial discharge regulations known as 

effluent guidelines. EPA expanded its application of TWFs in 2003 when it began using them in effluent 

guidelines program planning to rank industries by their total annual toxic-weighted discharges to identify 

those that may warrant additional research (U.S. EPA, 2004).

EPA has calculated TWFs for 1,064 chemicals based on information about the concentrations in 

water at which they become harmful to aquatic life, along with information about the levels in fish tissue 

at which they become harmful to humans. Information on harmful effects is used to weight the toxicity of 

chemicals relative to copper which is a common toxic pollutant in industrial waste streams. By assigning 

TWFs to toxic chemicals, EPA can add together varying amounts of different chemicals, each with 

different toxicities, to calculate the total toxic-weighted pounds in a waste stream, referred to as Toxic 

Weighted Pound Equivalents (TWPE). For example, nickel, which is less toxic than copper, has a TWF of 

0.11; whereas acrylonitrile, which is more toxic than copper, has a TWF of 2.3. A facility that annually 

discharges 50 pounds of nickel and 10 pounds of acrylonitrile (50 x 0.11 + 10 x 2.3 = 28.5 TWPE) has a 

waste stream that is almost twice as toxic as a facility that discharges 100 pounds of nickel and 2 pounds 

of acrylonitrile (100 x 0.11 + 2 x 2.3 = 15.6 TWPE). TWPE-based estimates of the toxicity of waste 

streams are solely used by EPA to better understand how treatment technologies, individual facility 

discharges, and industry discharges compare to one another, not to assess impacts to aquatic life or human 

health. 

The equations for calculating TWFs are contained in a set of Excel Worksheets known as the 

TWF Database. The TWF Database uses aquatic life values that have already been calculated for various 

purposes such as for use in water quality criteria. Human health values are derived in the TWF Database 

using information from disparate sources. EPA combines aquatic life and human health effects 

information whenever possible when calculating TWFs to help compensate for the incompleteness of data 

sources and to simplify TWF calculation and waste stream evaluation. The majority of TWFs, 58 percent 

(616), are derived solely from aquatic life values. Five percent of TWFs (54) are derived solely from 

human health values. Thirty-seven percent of TWFs (394) are derived from both aquatic life and human 

health effects information. If EPA were to separate the aquatic life and human health values during TWF 

calculation, there would be 1,010 chemicals with aquatic life-based TWFs (616 + 394 = 1,010) and 448 

chemicals with human health-based TWFs (54 + 394 = 448). The parsing of industry rankings by separate 

aquatic life and human health categories would defeat the purpose of TWFs, which is to help simplify the 
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interpretation of disparate discharge data. Moreover, there is no clear metric for decision makers to use in 

choosing between the results of separate aquatic life and human health-based industry rankings.

EPA recognizes that the approach of combining aquatic life and human health values is not 

suitable for rigorous applications, such as analyses performed for risk assessments or water quality-based 

permitting. EPA acknowledges that the TWF approach of combining aquatic life and human health values 

mixes different types of effects data, amplifies uncertainties in the underlying data, and obscures the 

relative contribution of aquatic life and human health values in the calculation of TWFs. EPA also 

recognizes that the toxic effects of many chemicals are frequently different in humans than they are in 

aquatic life. Nevertheless, EPA considers the combination of aquatic life and human health effects 

information in the calculation of TWFs to be a reasonable approach for integrating disparate toxic effects 

information from different sources for the purpose of informing regulatory decisions about the relative 

cost effectiveness of treatment technologies, and for indentifying industries that may warrant additional 

analysis.

It should be noted that although the TWF Database contains TWFs for 1,064 chemicals, only a 

small subset of chemicals with TWFs are typically assessed by the effluent guidelines program. The 

number of chemicals with TWFs regulated by effluent guidelines for any particular industrial category, 

and thus assessed in a cost effectiveness analysis, typically range anywhere from one to 20. Effluent 

guidelines program planning assessment only examines the approximately 200 chemicals with TWFs that 

are annually reported by facilities to be discharged to surface water nationwide. EPA data show that the 

total amount and toxic-weighted amount of chemicals discharged to surface water is declining over time 

as industries adopt increasingly effective treatment technologies.

This report incorporates comments from an independent peer review panel EPA convened to 

examine the TWF methodology (ERG, 2006). The majority of the panel’s suggestions concerned 

improving the clarity of the explanation of how TWFs are developed. It was also apparent from the 

panel’s comments that the TWF Methodology did not adequately discuss the limitations of TWFs and 

their specific uses by EPA. EPA prepared a detailed response report to address the peer review panel’s 

comments (U.S. EPA, 2011). Moreover, EPA has endeavored to respond to the panel’s suggestions in this 

report by more clearly explaining how TWFs are derived from toxic effects data and more precisely 

describing EPA’s specific uses of TWFs in effluent guidelines regulatory evaluation and planning. EPA

has also revised this report to more thoroughly describe the limitations of the data used to calculate 

TWFs, and the limitations of EPA’s facility discharge data to which TWFs are applied. EPA will continue 

to include a section on TWFs in the Technical Support Document accompanying effluent guidelines 

program plans summarizing any revisions to existing TWFs or development of new TWFs.
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EPA has continually developed and refined TWFs during the past three decades as new chemical 

toxicity data became available. However, given the current extensive set of chemicals for which TWFs 

have been calculated, EPA does not anticipate developing new TWFs in the foreseeable future. 

Nevertheless, EPA may decide to revise certain TWFs as new chemical toxicity data become available. 

EPA may also decide to revise this report as needed to further clarify the TWF methodology in response 

to public comments. EPA will use the peer reviewers’ comments to inform any future refinement of 

TWFs or revisions to the TWF methodology. 

The body of this report is organized into six chapters. The first four chapters discuss TWF 

terminology and the equations and data used to calculate TWFs. The chapter explaining the aquatic life 

component of TWFs is much shorter than the chapter explaining the human health component. This is 

because the TWF Database uses aquatic life values that have already been calculated by other sources, 

whereas human health values are calculated in the TWF Database and thus require more detailed 

explanation. The last two chapters summarize EPA’s use of TWFs and explain their limitations. 

1.1 Introduction References

1. ERG. 2006. Eastern Research Group, Inc. Toxic Weighting Factor Development in Support of 

CWA 304(m) Planning Process. (June). EPA-HQ-OW-2004-0032-1634. 

2. U.S. EPA. 2004. Technical Support Document for the 2004 Effluent Guidelines Program Plan. 

EPA 821R-04-014. Washington, DC. (August). EPA-HQ-OW-2003-0074-1346 through 1352. 

3. U.S. EPA. 2011. U.S. Environmental Protection Agency. Response to Peer Review Comments on 

EPA Report: Toxic Weighting Factor Development in Support of CWA 304(m) Planning Process. 

Washington, DC. EPA-HQ-OW-EPA-HQ-OW-2010-0824, DCN 07500.
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2. CLARIFICATION OF KEY TWF CONCEPTS AND TERMS

The effluent guidelines program only uses TWFs as an indicator of the relative potential of 

chemicals to cause harm prior to discharge, and never to assess harmful effects of chemicals in the 

environment. This is a critical point to understand about TWFs and, consequently it is emphasized 

throughout this report. The toxic effects of chemicals discharged to aquatic environments are highly 

variable and site specific. Adequate assessment of the toxic effects of chemicals in aquatic environments 

requires site specific information beyond the scope of TWFs.

Toxicity refers to the degree to which a substance can harm an organism. A central concept of 

toxicology is that toxic effects are variable depending on the chemical structure of a substance, the dose 

an organism receives, and an organism’s susceptibility to increasing doses. All chemicals can cause harm 

at a certain level. A chemical is considered to be toxic if a small amount of it can cause harm and 

relatively non-toxic if only large amounts of it are harmful.

The toxic properties of a waste stream begin to change as soon as it is discharged to surface 

water. The potential toxic effects of chemicals vary depending on their concentration in a waste stream, 

their dilution once they are discharged, their inherent potential for persistence or degradation in the 

environment, and the physical and chemical characteristics of the waterbody to which they are 

discharged, such as temperature and pH. After mixing with surface water, toxic chemicals may degrade 

through photochemical and microbial processes, or compartmentalize through particulate adsorption, 

sediment deposition, or volatilization. Biological alteration in the aquatic environment may decrease or 

increase the toxicity of a chemical, such as when certain anaerobic bacteria metabolize inorganic mercury 

into methyl mercury, making it biologically available. 

Aquatic life effects from toxic chemicals are generally considered to be either acute or chronic. 

Acute effects are well defined and occur relatively quickly, such as death within a couple of days. 

Chronic effects are more subtle, developing and persisting for long periods over the course of an 

organism’s life time. Chronic effects are typically measured in terms of physiological decline, such as 

reduced growth and reproduction, but they may be also measured or in terms of the lethality of a 

chemical. 

Human health effects from toxic chemicals are divided into two categories: non-threshold effects, 

such as carcinogenicity, and threshold effects, such as acute or chronic toxicity. EPA assumes that there is 

no safe level of exposure to carcinogens, and thus there is no effects threshold. The risk of developing 

cancer from a certain dose of a chemical for a given duration of exposure is therefore not an effect 

threshold, but rather represents an indicator of potential level of harm. Noncarcinogens are generally 

assumed to have an identifiable exposure threshold, both for individuals and populations, below which 
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effects are not observable. For example, EPA water quality criteria for noncarcinogens represent a type of 

toxic effects threshold because they define maximum ambient water chemical concentrations that can 

exist before harmful effects begin to occur. 

The human health component of TWFs is derived almost entirely from information concerning 

fish consumption, which is the most significant pathway of exposure to water borne toxic chemicals. 

Certain highly toxic chemicals, such as mercury, may become concentrated at harmful levels in the types 

of fish commonly eaten by humans through bioaccumulation, bioconcentration, and biomagnification. 

Bioaccumulation refers to the buildup of toxic chemicals in organic tissues from all combined routes of 

exposure, including food consumption, water ingestion, and contact with water and sediments. 

Bioaccumulation occurs when an organism absorbs a toxic chemical faster than it metabolizes or excretes 

it. The longer a toxic chemical persists in organic tissue, the greater the potential for adverse effects, even 

if environmental levels of the chemical are not very high. Bioconcentration is a related but more specific 

term, referring to uptake and accumulation of a toxic chemical from contact with water alone. 

Bioaccumulation typically accounts for significantly more chemical build up in tissues than 

bioconcentration. Biomagnification occurs when chemicals become increasingly concentrated up the 

levels of a food chain through predation. Chemical bioavailability, biota fat content, and rate of transfer 

among food chain levels can affect biomagnifications potential. Chemicals such as dioxins are 

particularly prone to biomagnification because they are highly persistent in the environment and 

accumulate in fatty tissues. 
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3. THE TWF EQUATION

EPA convened a work group in 1981 to develop TWFs for use in evaluating the costeffectiveness of treatment technology options for effluent guidelines rulemakings (U.S. EPA, 1988). The 

work group decided to simplify TWF calculation by combining information about toxic effects to aquatic 

life and human health. The equation is illustrated in Figure 3-1. The 1981 work group also decided to 

normalize the aquatic life and human health values to a standard benchmark so that different amounts of 

different chemicals could be added together to estimate the total toxicity of a waste stream. The work 

group chose the freshwater chronic aquatic life criterion for copper as the benchmark scaling factor 

because it is a common and well studied toxic chemical in industrial waste streams.

( ) AQ HH TWF + 6.56.5 



 



 =

Where:

5.6 = scaling factor (original copper freshwater chronic aquatic-life criterion of 5.6 μg/L);

AQ = Aquatic Life Value (μg/L); and

HH = Human Health Value (μg/L).

Figure 3-1. TWF Equation 

The aquatic life value is taken directly from one of several toxicity databases, depending on the 

best source of information available for each chemical. The human health value is calculated for each 

chemical using data from multiple sources of best available information depending on whether the 

chemical is a carcinogen or noncarcinogen. Data sources for TWFs are discussed in Sections 4 and 5. 

The chronic freshwater aquatic life criterion for copper, however, has been revised three times 

since it was first published in 1980 due to advances in the scientific understanding of its toxic effects. The 

original copper freshwater chronic aquatic life criterion of 5.6 micrograms per liter (μg/L) was revised to 

12.0 μg/L in 1984, then changed to 9.0 μg/L in 1998, and revised again in 2007 to a criterion based on 

site-specific water quality factors (U.S. EPA, 2007a). As shown in Figure 3-1, EPA has continued to use 

the original 5.6 μg/L value for the TWF benchmark in order to provide consistency in interpretation of 

TWF-based analyses from year to year.

The nature of the copper criterion helps to illustrate the complexity of determining the potential 

toxic effects of a chemical. Copper frequently occurs naturally in surface water because it is an abundant 

trace element in the earth’s crust. At low concentrations it is an essential micronutrient for virtually all 

plants and animals, but at higher concentrations copper is toxic to aquatic life. To better account for the 

waterbody-specific nature of copper toxicity, EPA published revised water quality criteria for copper in 

2007 (U.S. EPA, 2007a) based on a chemical speciation model, called the Biotic Ligand Model (BLM) 
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(U.S. EPA, 2007b). The BLM calculates the toxicity of copper to aquatic organisms as a function of 

certain chemical and physical characteristics of a receiving waterbody. It also simultaneously calculates

ionic concentrations of other chemicals that can either bind with copper and render it biologically 

unavailable, or compete with copper for binding sites on aquatic organisms, such as fish gills. Although 

the BLM can potentially improve the accuracy of predictions of the ecotoxicity of copper and other 

metals, its use requires significant data input, including water temperature along with concentrations of 

nine separate chemical constituents: pH, alkalinity, calcium, magnesium, sodium, sulfate, potassium, 

chloride, and dissolved organic carbon.

The BLM-based copper criterion provides no national value that can be used in TWF calculations 

because it is site specific. Therefore, when calculating the TWF for copper, EPA normalizes the 1998 

copper criterion of 9.0 μg/L to the original 1980 copper criterion of 5.6 μg/L. The 1998 copper criterion is 

used because it is the most recent copper criterion prior to the BLM-based copper criterion. The copper 

human health value of 444 μg/L, derived from the reference dose contained in the 1998 EPA Region 3 

Risk-based Concentration Table and noted in Section 5, is also normalized to the original 1980 copper 

criterion of 5.6 μg/L. 

Consequently, the TWF for copper is 0.635 rather than 1.0, as illustrated in Table 3-1. 

Table 3-1. TWF for Copper 

Aquatic Life Value 

(μg/L)

Human Health Value 

(μg/L)

Application of TWF 

Scaling Factor

Toxic Weighting 

Factor

Copper 9 (1998 Criterion) 444 5.6/9.0 + 5.6/444 0.635

Although the 1981 cost effectiveness workgroup selected copper as the benchmark, any chemical 

could have been used for weighting relative chemical toxicities. Chemicals with toxicities close to the 

mean toxicity of a group of chemicals, however, would serve as the most balanced benchmark for scaling 

purposes. In the most recent version of the TWF Database, compiled in 2007, there are 258 chemicals 

with TWFs greater than that of copper and 805 chemicals with TWFs less than the TWF for copper. 

The complete list of TWFs, the values used in the aquatic life and human health components of 

TWFs, the values of the constant and variable factors used to calculate TWFs, and all data sources are 

contained in the TWF Database. The TWF Database is available in the public docket for the effluent 

guidelines planning process and can be accessed through docket number “EPA-HQ-OW-2008-0517” at 

www.regulations.gov (U.S. EPA, 2007c). The 2007 TWF Database in the docket is the most current 

version as of the date of this report. This report is also available in “EPA-HQ-OW-2008-0517.”Any 
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future revised versions of the TWF Database and reports will be posted to the effluent guidelines program 

planning docket, which is updated annually and can be found by searching www.regulations.gov. 

3.1 The TWF Equation References

1. U.S. EPA. 1988. U.S. Environmental Protection Agency. Office of Water. Ehrensberger, K., and 

R. Rico. Cost-effectiveness Analysis for Effluent Guidelines. Washington, DC. (May). 

2. U.S. EPA. 2007a. U.S. Environmental Protection Agency. Office of Water. Aquatic Life Ambient 

Freshwater Quality Criteria: Copper. EPA-822-R-07-001. Washington, DC. (February). EPAHQ-OW-2006-0771-1214. Available online at: 

http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/pollutants/copper/upload/2009_0

4_27_criteria_copper_2007_criteria-full.pdf. 

3. U.S. EPA. 2007b. U.S. Environmental Protection Agency. Office of Water. Aquatic Life Ambient 

Freshwater Quality Criteria – Copper, 2007 Revision. EPA-822-R-07-001. Washington, DC. 

(February). Available online at: 

http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/pollutants/copper/2007_index.cf

m. 

4. U.S. EPA. 2007c. U.S. Environmental Protection Agency. Office of Water. Toxic Weighting 

Factor Database. Washington, DC. (September). EPA-HQ-OW-2008-0517 DCN 04661.
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4. THE AQUATIC LIFE COMPONENT OF THE TWF EQUATION

EPA selects aquatic life effects data to calculate TWFs using a three tiered hierarchy in order of 

decreasing data quality: 

1. Freshwater chronic aquatic life criteria published by EPA as National Recommended 

Water Quality Criteria (NRWQC) (U.S. EPA, 2009);

2. Effects data based on testing of aquatic organisms; and

3. Estimated toxicity effects data derived from various nationally-recognized sources.

Based on this hierarchy, aquatic life values are available for all chemicals for which TWFs have been 

developed, with the notable exception of certain dioxin compounds as illustrated in Table 5-1. If aquatic 

life effects values are not available for a particular dioxin compound, then the copper-normalized human 

health value is used as the TWF for that chemical. Conversely, TWFs for 616 chemicals are based solely 

on aquatic life values because human health values are not available.

Aquatic life effects values from freshwater environments, only, is used in TWF calculations. 

Saltwater aquatic effects values are not used because they have been calculated for only a small set of 

chemicals due to the complexities of determining toxicity effects in marine environments.

Chronic long-term effects values rather than acute short-term effects values are used in most 

TWFs because they are more available. For a few TWFs, however, acute values are used because chronic 

values are not available. Acute effects are typically expressed as toxicity endpoints. An acute toxicity 

endpoint of LD50, for example, is the lethal dose of a chemical that kills 50 percent of test organisms in 

specified period, usually within 96 hours or less. Chronic effects are more subtle and develop and persist 

for a relatively long period of time, typically one-tenth or more of the life span of an organism. Chronic 

effects are measured in terms of physiological decline, such as reduced growth and reproduction, or in 

terms of the lethality of a chemical. 

Freshwater chronic aquatic life criteria published as NRWQC, available for 72 chemicals, are the 

highest quality source data for the aquatic life component of TWFs because they have been meticulously 

developed, extensively reviewed, and constitute national guidance for protection of the vast majority of 

aquatic freshwater communities in the United States. The Criterion Continuous Concentration (CCC) 

values from freshwater chronic aquatic life criteria are used in the aquatic life component of the TWF 

equation. The CCC is a chronic value representing the highest in-stream concentration of a toxicant to 

which organisms can be exposed indefinitely without causing unacceptable effects. Thorough discussions 

of the CCC and other elements of NRWQC are contained in the methodology for developing aquatic life 

criteria, titled Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of 

Aquatic Organisms and Their Uses (U.S. EPA, 1985). 
4-2

Aquatic organism test data, composing the second tier of source data for the aquatic life 

component of TWFs, are available for only 46 chemicals. The results of tests on three types of indicator 

organisms are used in TWFs: fish, macroinvertebrates, and algae. Chronic effects values from organism 

tests indicate concentrations above which long-term effects are likely to occur. Acute toxicity test results, 

such as those for short-term lethal concentrations, are used when chronic toxicity information is not 

available for a chemical. Organism test data were obtained from two complementary EPA databases: the 

Ecotoxicity Database (ECOTOX) (U.S. EPA, 2011a) and the Assessment Tools for Evaluation of Risk 

Database (ASTER) (U.S. EPA, 2011b).

Estimates of chronic toxicity, taken from several sources, are used as TWF aquatic life values 

when NRWQC or organism test data are not available. The primary source of chronic toxicity estimates is 

EPA’s Quantitative Structure Activity Relationship (QSAR) model, which derives statistical relationships 

between chemical properties and biological effects. QSAR toxicity estimates are maintained in ASTER. 

The TWF Database contains a complete list of other less frequently cited sources of toxicity estimates.

4.1 The Aquatic Life Component of the TWF Equation References

1. U.S. EPA. 1985. U.S. Environmental Protection Agency. Office of Research and Development. 

Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of Aquatic 

Organisms and Their Uses. Washington, DC. Available online at: 

http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/upload/85guidelines.pdf. 

2. U.S. EPA. 2009. U.S. Environmental Protection Agency. Office of Water. National 

Recommended Water Quality Criteria. Washington, DC. Available online at: 

http://water.epa.gov/scitech/swguidance/standards/current/upload/nrwqc-2009.pdf. 

3. U.S. EPA. 2011a. U.S. Environmental Protection Agency. Office of Research and Development. 

ECOTOX Database Website. Washington, DC. Available online at: http://cfpub.epa.gov/ecotox. 

4. U.S, EPA. 2011b. U.S. Environmental Protection Agency. Office of Research and Development. 

ASsessment Tools for the Evaluation of Risk (ASTER) Website. Washington, DC. Available 

online at: http://www.epa.gov/med/Prods_Pubs/aster.htm.
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5. THE HUMAN HEALTH COMPONENT OF THE TWF EQUATION

The derivation of the human health component of the TWF equation is more complex than that of 

the aquatic life component, which is simply an effects value taken from a nationally-recognized source. 

The human health values are derived from separate equations to estimate effects thresholds for 

noncarcinogens and non-threshold cancer risk-based factors for carcinogens. These equations are based 

on EPA’s Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health: 

Technical Support Document Volume 1: Risk Assessment (U.S. EPA, 2000). The noncarcinogen and 

carcinogen human health values are based on assumptions about fish consumption, which is the most 

common pathway for human exposure to toxic chemicals. Both values represent maximum possible toxic 

chemical concentrations in fish, based on the best available toxicological science, that are considered to 

minimize the long-term exposure risk to the general population. 

5.1 Equation for Calculating the TWF Human Health Value for Noncarcinogens

If a chemical is not considered to have the potential to cause cancer in humans, based on EPA’s 

Guidelines for Carcinogen Risk Assessment (U.S. EPA, 2005), the human health effects value is derived 

using a threshold concentration for noncancer adverse effects, which is referred to as the oral reference 

dose (RfD). More specifically, the RfD is the amount of a noncarcinogen to which a person may be 

exposed each day for a lifetime of 70 years without suffering harmful effects. The RfD includes sensitive 

subgroups. Usually doses less than the RfD are unlikely to be associated with adverse health effects. 

However, RfD uncertainty may span up to an order of magnitude. The equation for calculating the TWF 

human health value for noncarcinogens, shown in Figure 5-1, uses the same adult body weight (70 kg), 

and fish consumption rate (17.5 grams per day), that are used in the calculation of water quality criteria 

(U.S. EPA, 2000). It should be noted that, in accordance with the NRWQC methodology, a different fish 

consumption rate of 18.7 grams per day is used for calculating the TWF human health value for mercury. 

TWF Human Health Noncarcinogenic Value (

μg

L ) =

Reference Dose �

mg

kg

day� × Relative Soure Contribution (%) × Bodyweight (kg) × μg to mg Conversion

Daily Fish Consumption Estimate � kg

day� × Bioaccumulation or Bioconcentration Factor � Lkg�

= 

RfD �

mg

kg

day� × RSC (%) × 70 kg × 1000 μg

mg

0.0175 � kkkk

dddddd � × BAF or BCF� Lkg�

a In accordance with the NRWQC methodology, a different fish consumption rate of 18.7 grams per day is used for calculating 

the TWF human health value for mercury.

Figure 5-1. Equation for Calculating the TWF Human Health Value for Noncarcinogens 
5-2

5.2 Equation for Calculating the TWF Human Health Value for Carcinogens

EPA does not calculate human health effects thresholds for carcinogens because the Agency 

assumes that there is no safe level of exposure. Instead, EPA estimates the lifetime risk of developing 

cancer using a cancer potency factor based on a linear relationship between dose and response. The 

cancer potency factor represents an upper-bound estimate of the lifetime probability that an individual 

will develop cancer from a specific life time dose of a carcinogen. Lifetime cancer risk is expressed in 

terms of the likelihood that one person in an equally exposed population of a specific size would develop 

cancer if exposed continuously 24 hours per day to a specific concentration of a carcinogen throughout a 

lifetime of 70 years. 

TWF Human Health Carcinogenic Value (

μg

L ) =

Cancer Risk Level

Cancer Potency Factor �

kkkk ∙dddddd

mm �

 × Relative Soure Contribution (%) × Bodyweight (kg) × μg to mg Conversion

Daily Fish Consumption Estimate � kg

day� × Bioaccumulation or Bioconcentration Factor � Lkg�

= 10−5

Cancer Potency Factor �

kkkk∙dddddd

mm �

 × RSC (%) × 70 kg × 1000 μg

mg

0.0175 � kkkk

dddddd� × BAF or BCF�

L

kg�

a In accordance with the NRWQC methodology, a different fish consumption rate of 18.7 grams per day is used for calculating 

the TWF human health value for mercury. 

Figure 5-2. Equation for Calculating the Human Health Component of TWFs for 

Carcinogens

The level of cancer risk that is of concern is a matter of personal, community, and regulatory 

judgment. In general, EPA considers a lifetime risk of developing cancer below one in a million (10-6) to 

be so small as to be negligible, and cancer risk of more than one in ten thousand (10-4) to be sufficiently 

large to warrant action, such as the remediation of a Superfund site. The need for action to address a 

lifetime cancer risk between 10-6 and 10-4 is evaluated on a case-by-case basis.

When TWFs were first developed in 1981, EPA decided to use a cancer level of 10-5 for 

calculating the human health value of TWFs for carcinogens to be consistent with the 10-5 cancer risk 

level used in the calculation of NRWQC. However, a more protective risk level of 10-6 began to be used 

in the calculation of water quality criteria when the NRWQC methodology was revised in 2000 (U.S. 

EPA, 2000). In order to maintain data consistency for comparing TWF-based analytical results from year 

to year, EPA decided to continue using 10-5 in TWF calculations as shown in Figure 5-2. 
5-3

The equation for calculating the TWF human health value for carcinogens uses the same adult 

body weight (70 kg), and fish consumption rate (17.5 grams per day), that are used in the TWF human 

health value for noncarcinogens. 

5.3 Data Sources for Calculating the Human Health Component of the TWF Equation

Since there is no single comprehensive source of toxicity information, EPA draws data from 

multiple sources to calculate TWF human health values. As illustrated in Figure 5-3, RfD and cancer 

potency factors are taken from a hierarchy of sources based on criteria outlined in Section 2.4.6 of the 

Technical Support Document for Water Quality-Based Toxics Control (U.S. EPA, 1991). 

Tier I

• EPA National Recommended Water Quality Criteria, published after 2000 (EPA, 2007) 

• EPA Integrated Risk Information System (IRIS) Oral RfDs or Oral Cancer Potency Factors (EPA, 2011a) 

Tier II

• EPA’s Health Effects Assessment Summary Tables (HEAST)

• EPA’s Region III Risk-Based Concentration Table (RCB)

• California Environmental Protection Agency health assessments and toxicity criteria (CalEPA)

Tier III 

• Criteria from EPA National Recommended Water Quality Criteria Documents , prior to 2000

• Agency for Toxic Substances and Disease Registry (ATSDR) profiles

Figure 5-3. Hierarchy of Data Sources for Calculating the Human Health Component of 

TWFs

EPA’s Integrated Risk Information System (IRIS) and NRWQC published after 2000 are 

preferred sources of RfDs and cancer potency factors for calculating TWF human health values. IRIS 

contains quantitative and qualitative information on effects that may result from exposure to 

environmental contaminants (U.S. EPA, 2011a). IRIS information is used to support risk assessments, 

policy development, and regulatory activities. IRIS has varying levels of data for most of the chemicals 

typically assessed by the effluent guidelines program. NRWQC are developed through rulemaking 

process.

If RfDs or cancer potency factors are not available in IRIS or NRWQC developed after 2000, 

then other sources are checked beginning with the Health Effects Assessment Summary Tables (HEAST). 

The HEAST tables are compiled from toxicity studies by EPA’s National Center for Environmental 

Assessment (NCEA) for analysis of Superfund and hazardous waste sites. If RfDs or cancer potency 

factors are not available in HEAST, then EPA Region 3’s Risk-based Concentration Table (RBC) is 

checked. The RBC is similar to HEAST and also used to support analysis of Superfund and hazardous 
5-4

waste sites EPA, (U.S. EPA, 2011b). If RfDs or cancer potency factors are not available in HEAST or 

RBC, California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment 

(CalEPA) health assessments and toxicity criteria are checked. If none of these data sources have RfDs or 

cancer potency factors for a chemical, then NRWQC developed prior to 2000 are checked. If still no data 

are available, then EPA searches the Agency for Toxic Substances and Disease Registry (ATSDR) 

toxicological profiles (ASTDR, 2011). 

If a chemical has both noncarcinogenic and carcinogenic effects, and if both the RfD and cancer 

potency factor are from the same tier in the data hierarchy, then the TWF is calculated using the cancer 

potency factor. For example, if IRIS contains both an RfD and cancer slope factor for a chemical, then the 

TWF is calculated using the cancer slope factor. If, however, the RfD and cancer potency factor for a 

chemical come from sources on different tiers in the data hierarchy, then the value from the higher tier 

data source is used. For example, if an RfD for a chemical is available from IRIS but the cancer potency 

factor is only available from the EPA Region 3 RBC table, then the TWF is calculated using the RfD.

5.4 Calculating the TWF Human Health Value for Dioxins

The calculation of the TWF human health value for dioxin compounds differs slightly from that 

of other carcinogens. The term dioxin refers to a set of chlorinated compounds that share similar chemical 

structures and toxic properties. Dioxin compounds, which are among the chemicals in the TWF Database 

with the highest TWF values, have widely ranging toxicities and potential to increase the risk of cancer. 

Dioxin compounds include seven polychlorinated dibenzodioxins (PCDDs) and 10 polychlorinated 

dibenzofurans (PCDFs). There are TWF values for all of these dioxin compounds in the TWF Database. 

PCDDs and PCDFs are unintentional byproducts of most forms of combustion and several 

industrial chemical processes. Although release of dioxins to the environment has dropped sharply during 

the past two decades – by 90 percent or more – they are still a concern because of their long persistence in 

the environment and high toxicity. Reservoirs of 2,3,7,8-tetrachlorodibenzo-p-dioxin, for instance, which 

has the second highest TWF after radium-228, can persist for decades in soils and sediments before 

eventually entering the human food chain. Dioxin compounds, moreover, especially PCDDs, have high 

octanol to water partition coefficient values, which means that they easily bioaccumulate in the fatty 

tissues of fish and biomagnify up the food chain. 

Because dioxin compounds have widely ranging toxicities and typically occur together in 

mixtures, the World Health Organization has developed Toxic Equivalency Factors (TEF) to assess their 

toxic potency relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin. As shown in Table 5-1, the cancer potency 

factor of each dioxin compound is adjusted by a TEF relative to the cancer potency factor of 2,3,7,8-

tetrachlorodibenzo-p-dioxin (U.S. EPA, 1985). 
5-5

Table 5-1. Toxic Equivalency Factors (TEF), Bioaccumulation Factors (BAF), and TWF Values for 

Dioxin Compounds

Dioxin TEF

TEFAdjusted 

Cancer 

Potency 

Factor BAF

TWF 

Aquatic 

Life Value 

(μg/L) a 

TWF Human 

Health Value 

(μg/L) a TWF

Tetrachlorodibenzo-p-dioxin, 2,3,7,8- 1 160,000 31,160 5,600,000 697,984,000 703,584,000

Pentachlorodibenzo-p-dioxin, 1,2,3,7,8- 1 160,000 30,934 NA 692,928,000 692,928,000

Pentachlorodibenzofuran, 2,3,4,7,8- 0.5 80,000 49,760 NA 557,312,000 557,312,000

Hexachlorodibenzofuran, 2,3,4,6,7,8- 0.1 16,000 22,859 NA 51,204,160 51,204,160

Hexachlorodibenzofuran, 1,2,3,7,8,9- 0.1 16,000 21,120 NA 47,308,800 47,308,800

Tetrachlorodibenzofuran, 2,3,7,8- 0.1 16,000 19,549 29,474 43,790,080 43,819,554

Hexachlorodibenzo-p-dioxin, 

1,2,3,4,7,8- 0.1 16,000 10,490 NA 23,498,240 23,498,240

Hexachlorodibenzofuran, 1,2,3,6,7,8- 0.1 16,000 6,299 NA 14,109,440 14,109,440

Hexachlorodibenzo-p-dioxin, 

1,2,3,7,8,9- 0.1 16,000 4,730 NA 10,595,840 10,595,840

Hexachlorodibenzo-p-dioxin, 

1,2,3,6,7,8- 0.1 16,000 4,266 NA 9,556,480 9,556,480

Hexachlorodibenzofuran, 1,2,3,4,7,8- 0.1 16,000 2,571 NA 7,632,640 5,760,000

Pentachlorodibenzofuran, 1,2,3,7,8- 0.05 8,000 6,815 NA 5,760,000 7,632,640

Heptachlorodibenzofuran, 1,2,3,4,7,8,9- 0.01 1,600 13,545 NA 3,033,984 3,033,984

Heptachlorodibenzo-p-dioxin, 

1,2,3,4,6,7,8- 0.01 1,600 1,835 NA 411,136 411,136

Heptachlorodibenzofuran, 1,2,3,4,6,7,8- 0.01 1,600 383 NA 85,760 85,760

Octachlorodibenzofuran 0.0001 16 590 5,600 986 2,021

Octachlorodibenzo-p-dioxin 0.0001 16 440 700 1,321 6,586

Methylmercury 1,225,029 NA 23,494 23,494

Source: Toxic Equivalency Factors (TEFs) for PCBs, PCDDs, PCDFs for Humans and Wildlife (Van den Berg, 

1998). 

a - The aquatic life and human health component values in this table have been normalized to copper. There are no 

TWF aquatic life values for most dioxins. 

b - Methymercury data are shown for comparison. Currently there is no cancer potency factor for methylmercury, 

nor is there an aquatic life value. 

Given the high toxicity of dioxins and their high potential for biomagnification in aquatic food 

chains, it is worth noting that only a small number of facilities discharge small amounts of dioxins to 

surface water. In 2008, the most recent year for which discharge monitoring data were available for this 

report, only six facilities submitted DMRs showing the discharge of dioxin and dioxin-like compounds. 

Four facilities in the pulp and paper point source category reported a total annual discharge of 0.000208 

pounds of dioxins (145,000 TWPE). Two facilities in the chlorine and chlorinated hydrocarbons point 

source category reported a total annual discharge of 0.000619 pounds of dioxins (436,000 TWPE) (U.S. 

EPA, 2010).
5-6

5.5 Variable Factors Used in the Human Health Component of the TWF Equation

Two variable factors representing sources of exposure and aquatic food chain bioaccumulation 

are used in the human health component of the TWF equation to adjust the otherwise constant factors 

representing RfD, cancer potency factor, body weight, and fish consumption rate: 1) relative source 

contribution factors and, 2) bioaccumulation and bioconcentration factors. 

5.5.1 Relative Source Contribution Factors

The relative source contribution (RSC) factor is used in the calculation of the human health 

component of TWFs to apportion exposure pathways of a chemical among drinking water, fish 

consumption, skin contact, or inhalation. The RSC is applied to both the RfD for noncarcinogens and the 

cancer potency factor for carcinogens. The RSC approach is documented in the Methodology for Deriving 

Ambient Water Quality Criteria for the Protection of Human Health (U.S. EPA, 2000). EPA, however, 

has published RSCs for only 28 chemicals. The RSCs were calculated as part of the development of 

NRWQC. For TWF human health calculations for all other chemicals, fish consumption is assumed to be 

the only exposure pathway, and thus an RSC of 100 percent is applied. Consequently, the drinking water 

exposure pathway is assumed to be zero percent for most TWFs. EPA considers this assumption to be 

reasonable for the purpose of screening-level analysis, considering that most Americans drink treated 

water, or tested well water, and thus generally are not exposed to unsafe levels of toxic chemicals with

TWFs through drinking water. 

5.5.2 Bioaccumulation and Bioconcentration Factors 

Toxic chemicals have a range of physical characteristics which determine how readily they 

partition from an aqueous phase into an organic phase in the surface water environment. In fish, 

chemicals typically partition into the organic phase in the gills. An aquatic bioaccumulation factor (BAF)

is the ratio of a toxic chemical in an organism to its concentration in the ambient water, considering 

exposure from all potential pathways. An aquatic bioconcentration factor (BCF) is the ratio of a toxic 

chemical in an organism to its concentration in the ambient water, considering direct contact with the 

water as the only exposure pathway. Bioaccumulation typically accounts for significantly more chemical 

build up in tissues than bioconcentration, especially for highly toxic and highly persistent chemicals like 

dioxin compounds. National-level BAFs are not available, however, because they are complex to develop. 

BCFs, on the other hand, are readily available, and EPA uses them to calculate most TWFs with 

exception of the 17 dioxin compounds and methylmercury as shown in Table 5-1. 

EPA estimates BAFs for dioxins because of their extremely high potential to bioaccumulate. EPA 

derives BAFs for dioxins by combining estimates of the percentage of fish tissue that is fat with estimates 

of the tendency of a dioxin to be absorbed in fat. These estimates were developed by the Great Lakes 
5-7

Water Quality Initiative (GLWQI) (U.S. EPA, 1995). EPA applied the GLWQI estimates to equations for 

BAF calculation published in EPA’s Methodology for Deriving Ambient Water Quality Criteria for the 

Protection of Human Health (U.S. EPA, 2000). 

EPA estimates the BAF for methylmercury differently than it does for dioxins. Unlike other 

NRWQC, the water quality criterion for methymercury is based on fish tissue concentration rather than on 

ambient water concentration. For the purpose of calculating methymercury TWFs, EPA converts the fish 

tissue criterion of 0.3 milligrams methylmercury per kilogram in fish tissue to a national BAF estimate of 

1,225,029 using EPA published methodology (EPA, 2001).

The TWF Database contains the complete list of sources for BCFs used in TWF calculations. 

Most BCFs come from the following sources:

• “Estimating Bioconcentration Potential from Octanol/Water Partition Coefficients” 

(Veith and Kosian, 1983);

• Assessment Tools for Evaluation of Risk (ASTER) Database (U.S. EPA, 1999); 

• “Evaluation of Waterborne Exposure Pathways to Paragraph 4(c) Pollutants” (Little, 

1983); and 

• Handbook of Chemical Property Estimation Methods – Environmental Behavior of 

Organic Compounds (Lyman et al., 1981). 

5.6 The Human Health Component of the TWF Equation References

1. ASTDR. 2011. Agency for Toxic Substances and Disease Registry. ASTDR Website. Available 

online at: http://www.atsdr.cdc.gov/. 

2. Little, Arthur D. 1983. “Evaluation of Waterborne Exposure Pathways to Paragraph 4(c) 

Pollutants.” Draft Report. (April 28). 

3. Lyman WJ, WF Reehl, and DH Rosenblatt. 1981. Handbook of Chemical Property Estimation 

Methods - Environmental Behavior of Organic Compounds. New York, NY: McGraw-Hill Book 

Company. 

4. U.S. EPA. 1985. U.S. Environmental Protection Agency. Office of Health and Environmental 

Assessment. Health Assessment Document for Polychlorinated Dibenzo-p-dioxins. Washington, 

DC. EPA-600-8-84-814F. 

5. U.S. EPA. 1991. U.S. Environmental Protection Agency. Office of Water. Technical Support 

Document for Water Quality-Based Toxics Control. Washington, DC. (March). EPA-HQ-OW2004-0032-0935 

6. U.S. EPA. 1995. U.S. Environmental Protection Agency. Great Lakes Water Quality Initiative 

Technical Support Document for the Procedure to Determine Bioaccumulation Factors. EPA820-B-95-005. (March).

7. U.S. EPA. 1999. U.S. Environmental Protection Agency. Assessment Tools for Evaluation of Risk 

(ASTER) Database. Duluth, MN: U.S. EPA, Environmental Research Laboratory.
5-8

8. U.S. EPA. 2000. U.S. Environmental Protection Agency. Office of Water. Methodology for 

Deriving Ambient Water Quality Criteria for the Protection of Human Health: Technical Support 

Document Volume 1: Risk Assessment. EPA-822-B-00-005. Washington, DC. (October).

9. U.S. EPA. 2001. U.S. Environmental Protection Agency. 66 FR 1344. Water Quality Criteria: 

Notice of Availability of Water Quality Criterion for the Protection of Human Health: 

Methylmercury, (January 8).

9. U.S. EPA. 2005. U.S. Environmental Protection Agency. Guidelines for Carcinogen Risk 

Assessment. EPA/630/P-03/001F. Washington, DC. (March). Available online at: 

http://www.epa.gov/raf/publications/pdfs/CANCER_GUIDELINES_FINAL_3-25-05.pdf. 

10. U.S. EPA. 2007. U.S. Environmental Protection Agency. Water Quality Standards Handbook: 

 Second Edition. EPA-823-B-94-005. Washington, DC. (June). Available online at: 

 http://water.epa.gov/scitech/swguidance/standards/handbook/ 

11. U.S. EPA. 2010. U.S. Environmental Protection Agency. Technical Support Document for the 

 2010 Effluent Guidelines Program Plan. EPA 820R-10-021. Washington, DC. (December). EPAHQ-OW-2008-0517 DCN 07320. 

12. U.S. EPA. 2011a. U.S. Environmental Protection Agency. Integrated Risk Information System 

(IRIS) Website. Available online at: http://www.epa.gov/IRIS/. 

13. U.S. EPA. 2011b. U.S. Environmental Protection Agency. Mid-Atlantic Risk Assessment – 

Human Health Risk Assessment Website. Available online at: 

http://www.epa.gov/reg3hwmd/risk/human/index.htm. 

14. Van den Berg, et al., 1998. Toxic Equivalency Factors (TEFs) for PCBs, PCDDs, PCDFs for 

 Humans and Wildlife. Environ. Health Perspect. 106:775-792. 

15. Veith, GD and P. Kosian. 1983. “Estimating Bioconcentration Potential from Octanol/Water 

Partition Coefficients.” In D. Mackay et al. (Eds.) “Physical behavior of PCBs in the Great 

 Lakes”. Ann Arbor Scie. Publ. Ann Arbor, MI:269-282.
6-1

6. EPA’S USE OF TWFS TO SUPPORT EFFLUENT GUIDELINES RULEMAKINGS AND PROGRAM 

PLANNING

EPA’s effluent guidelines program uses TWFs for two specific purposes. Since the early 1980s, 

EPA has used TWFs to help inform regulatory decision making about the relative cost effectiveness of

technology options to treat effluent containing toxic chemicals. In 2003, EPA began using TWFs in 

effluent guidelines program planning to rank industries by their total annual toxic-weighted discharges to 

identify major sources nationwide that may warrant further analysis.

EPA has issued effluent guidelines for 56 industrial categories encompassing 35,000 to 45,000 

facilities that discharge directly to waters of the United States. The effluent guidelines program has also 

issued pretreatment standards that apply to approximately 12,000 facilities that discharge to publically 

owned treatment works (POTWs). Each year, effluent guidelines and pretreatment standards prevent the 

discharge of more than 1.2 billion pounds of toxic pollutants.

EPA does not use TWFs in water quality-based permitting or risk assessment because they do not 

provide the information on site-specific water chemistry, environmental alteration of chemicals, exposure 

pathways, and organism susceptibility required to adequately set permit limits or assess risk. Moreover, 

EPA only uses TWFs to help inform decision making, and never as a sole source of information on which 

to base policy. 

6.1 Use of TWFs in Cost-Effectiveness Analysis

As explained in the introduction of this report, TWFs are used to calculate Toxic Weighted Pound 

Equivalents (TWPEs), which are employed in cost-effectiveness analysis. Cost effectiveness is defined as 

the incremental annual cost, expressed in 1981 constant dollars, per incremental toxic weighted pound of 

pollutant removed by a treatment technology (U.S. EPA, 1988). As shown in Table 6-1, the use of 

constant 1981 dollar values allows effluent guidelines developed and revised at different times to be 

compared with one another. 

Cost-effectiveness analysis is used to help evaluate a set of treatment technologies to identify the 

one that has the lowest cost per pound of toxic pollutants from a waste stream. It should be noted that 

there is no absolute scale for determining whether or not a treatment technology option is cost effective. 

The cost effectiveness of treatment technologies are considered to be high or low relative to one another.

Although not required by the Clean Water Act, cost effectiveness analysis is a useful tool for 

effluent guidelines regulatory analysis. It is, however, just one of many factors considered during 

evaluation of treatment technology options. Moreover, cost-effectiveness analysis is not always used in 

effluent guidelines regulatory development. It is only used to help assess regulatory options for industries 
6-2

that discharge toxic chemicals classified by the Clean Water Act (CWA) as “Priority Pollutants,” which 

are 126 chemicals specified in the Act, and for substances the CWA classifies as “Non-conventional 

Pollutants,” such as chlorides, iron, and ammonia. Substances the CWA classifies as “Conventional 

Pollutants,” such as oil and grease, biochemical oxygen demand, and total suspended solids are not 

included in cost-effectiveness analysis because they are addressed in a different analogous analysis that 

pertains to all pollutants, not just toxic chemicals.

Table 6-1. Examples of Effluent Guidelines Cost-Effectiveness Analysis Ranked by Dollars Per 

TWPE Removed 

Industry

Estimated 

TWPE 

Discharged 

Prior to Rule 

(thousands)

Estimated TWPE 

Discharged after 

Implementation of 

Rule (thousands)

Estimated TWPE 

Removed by Rule 

(thousands)

Cost-Effectiveness 

of Rule (1981 

Dollars per TWPE 

removed)

Inorganic Chemicals I 32,503 1,290 31,213 <1

Battery Manufacturing 4,126 5 4,121 2

Nonferrous Metals 

Manufacturing I 

6,653 313 6,340 4

Organic Chemicals 54,225 9,735 44,490 5

Inorganic Chemicals II 605 27 578 6

Nonferrous Metals 

Manufacturing II 

1,004 12 992 6

Porcelain Enameling 1,086 63 1,023 6

Centralized Waste 

Treatment 

3,372 1,267 2,105 5-7

Canmaking 12 0 12 10

Metal Finishing 3,305 3,268 37 12

Pesticides 2,461 371 2,090 14

Copper Forming 70 8 62 27

Iron & Steel 1,053 853 200 27

Pulp & Paper 61,713 2,628 59,085 39

Coil Coating 2,289 9 2,280 49

Metal Products & 

Machinery

140 70 70 50

Nonferrous Metals Forming 34 2 32 69

Foundries 2,308 39 2,269 84

Pharmaceuticals 89,790 471 89,320 47-96

Aluminum Forming 1,340 90 1,250 121

Electronics I 9 3 6 404

Source: Adapted from Cost-Effectiveness Analysis of Effluent Limitation Guidelines and Standards for the 

Centralized Waste Treatment Industry (U.S. EPA, 2000). 

Data used in conducting cost-effectiveness analyses may come from a variety of sources. Most 

typically, however, data are compiled through industry surveys and effluent sampling and analysis 

specifically designed for individual effluent guidelines rulemakings. The number of chemicals assessed in 
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a cost effectiveness-analysis may range from only a few to more than 100 depending on the nature of an 

industry’s waste streams. For the centralized waste treatment industry, for example, cost effectiveness 

was assessed for technologies to treat a set of 146 chemicals.

6.2 Use of TWFs in Effluent Guidelines Program Planning

CWA Section 304(b) requires EPA to annually review industrial categories for which effluent 

guidelines have been promulgated to determine if any revisions are warranted, and to identify possible 

new industrial categories for regulation. Section 304(m) requires EPA to publish a biennial plan 

announcing the schedule for the annual reviews, as well as the schedule for completing any effluent 

guidelines revisions or new regulations identified during the annual reviews. EPA has published a draft 

program plan every odd-numbered year since 1989 and a final program plan every even-numbered year

since 1990. 

EPA began using TWPEs in 2003 to rank industrial categories by their total annual discharge 

amounts to support development of the 2004 effluent guidelines program plan (U.S. EPA, 2004). The use 

of TWPEs as an indicator of the relative potential for industrial waste streams to cause harm prior to 

discharge assists EPA in setting priorities for effluent guidelines rulemakings so as to achieve the greatest 

environmental and health benefits from toxic discharge reduction. It is important to note that TWPEbased industry ranking is just one of four factors considered in effluent guidelines program planning, in 

addition to consideration of public comments. EPA uses the following three additional factors in its 

annual review of industrial categories:

1. The performance and cost of applicable and demonstrated wastewater treatment 

technologies, process changes, or pollution prevention alternatives that could effectively 

reduce the pollutants in the industrial category’s wastewater and, consequently, reduce 

the potential harm to human health or the environment associated with pollutant 

discharges;

2. The affordability or economic achievability of the identified wastewater treatment 

technologies, process changes, or pollution prevention measures; and 

3. Opportunities to eliminate inefficiencies or impediments to pollution prevention or 

technological innovation or opportunities to promote innovative approaches. This factor 

might also prompt EPA during an annual review to decide against identifying an existing 

set of effluent guidelines or pretreatment standards for revision where the pollutant 

source is already efficiently and effectively controlled by other regulatory or 

nonregulatory programs.

EPA calculates two TWPE estimates for each industrial category based on data from individual 

facility discharge monitoring reports (DMRs) and EPA’s Toxics Release Inventory (TRI). Facilities are 

required to submit DMRs to EPA or state permitting agencies as a condition of their National Pollutant 
6-4

Discharge Elimination System (NPDES) water discharge permits. DMRs are stored in the Permit 

Compliance System (PCS) and the Integrated Compliance Information System for the National Pollutant 

Discharge Elimination System (ICIS-NPDES). EPA began replacing PCS with ICIS-NPDES in 2006. 

Until the transition is complete, EPA retrieves certain data from both databases.

TRI is the common name for Section 313 of the Emergency Planning and Community Right-toKnow Act (EPCRA) (U.S. EPA, 2009a). Facilities above certain reporting thresholds, such as size and 

amount of toxic chemicals discharged, must annually report estimates of discharges of certain toxic 

chemicals listed under EPCRA. Unlike DMRs, TRI reports are primarily based on estimates rather than 

on sampling and analysis. TRI reports, however, include estimates of toxic chemical waste streams sent to 

POTWs, which DMRs do not. TRI also includes estimates of some pollutant discharges that are not 

currently regulated (i.e., pollutants not in DMRs). 

When facilities discharge indirectly, POTWs remove varying amounts of toxic chemicals from 

industrial waste streams, depending to the type of treatment process they use, before discharging the 

treated effluent to surface waters. This type of discharge is referred to as an indirect discharge. For the 

purpose of effluent guidelines program planning, EPA primarily uses TRI data to help estimate the 

amount of indirect discharges from industrial sources. EPA only uses TRI data to support effluent 

guidelines program planning, not for cost-effectiveness analysis.

Table 6-2. TWPE Ranking of Industrial Categories Based on 2008 DMR Dataa

Rank Point Source Category

Number of 

Dischargers Total Annual Pounds Total TWPE

1 Nonferrous Metals 

Manufacturing 68 1,760,000,000 955,000

2 Fertilizer Manufacturing 54 160,000,000 818,000

3 Petroleum Refining 727 7,740,000,000 618,000

4 Organic Chemicals, Plastics 

And Synthetic Fibers b 545 4,740,000,000 512,000

5 Pulp, Paper And Paperboard 267 2,350,000,000 510,000

6 Ore Mining And Dressing 117 558,000,000 339,000

7 Textile Mills 106 39,500,000 247,000

8 Waste Combustors 306 3,970,000,000 245,000

9 Inorganic Chemicals 

Manufacturing b 206 1,790,000,000 228,000

10 Miscellaneous Foods And 

Beverages 152 110,000,000 193,000

Source: Technical Support Document for the Preliminary 2010 Effluent Guidelines Program Plan (U.S. EPA, 

2009b). a Excludes categories currently or recently under review for a potential rulemaking or rulemaking revision.

b Excludes facilities manufacturing chlorine or chlorinated hydrocarbons, as these discharges are currently 

undergoing review for possible rulemaking revision. 
6-5

Table 6-2 shows the ten industrial categories with the highest current TWPE discharges according 

the 2008 DMR data analyzed for the 2010 effluent guidelines program plan. Although DMR and TRI data 

show that EPA regulatory programs are reducing discharges of toxic chemicals over time, discharge 

amounts of individual toxic chemicals vary from year to year, depending largely on economic conditions. 

The heaviest industries, such as metal manufacturing, petroleum refining, and ore mining, produce the 

highest toxic discharges, and thus tend to be among the highest ranked industries from year to year. 

Table 6-3 shows the ten industrial categories with the highest TWPE discharges according the 

2008 TRI data analyzed for the 2010 effluent guidelines program plan. The discrepancies between Table 

6-2 and Table 6-3 are the result of differing DMR and TRI reporting requirements. TRI contains 

discharge data for fewer facilities due to reporting thresholds that only require larger facilities to submit 

data. TRI also reflects a lower estimate of the pounds of pollutant discharged, for the same reason.

Table 6-3. TWPE Ranking of Industrial Categories Based on 2008 TRI Data a

Rank Point Source Category

Number of 

Dischargers

Total Annual 

Pounds

Total 

TWPE

1 Pulp, Paper And Paperboard 250 19,300,000 523,000

2 Petroleum Refining 298 25,700,000 410,000

3 Organic Chemicals, Plastics And Synthetic Fibers b 679 27,600,000 137,000

4 Ore Mining And Dressing 34 491,000 109,000

5 Plastics Molding And Forming 123 2,200,000 74,700

6 Inorganic Chemicals Manufacturing b 162 8,510,000 71,300

7 Nonferrous Metals Manufacturing 122 4,190,000 38,700

8 Pesticide Chemicals 73 2,090,000 35,500

9 Carbon Black Manufacturing 7 279 27,600

10 Timber Products Processing 122 48,400 27,300

Source: Technical Support Document for the Preliminary 2010 Effluent Guidelines Program Plan (U.S. EPA, 

2009b). a Excludes categories currently or recently under review for a potential rulemaking or rulemaking revision.

b Excludes facilities manufacturing chlorine or chlorinated hydrocarbons, as these discharges are currently 

undergoing review for possible rulemaking revision. 

For the purpose of effluent guidelines program planning analysis, EPA adds the TWPEs 

calculated from TRI and DMR data to produce a total TWPE value for each industrial category. EPA 

takes this approach because the combination of DMR and TRI data helps to overcome certain limitations 

in each data source and provides a more comprehensive portrait of total toxic discharges. A thorough 

discussion of the effluent guidelines program, the effluent guidelines planning process, use of TWFs and 

TWPEs in industry ranking, and the complete list of industry rankings is contained in the Technical 

Support Document for the Preliminary 2010 Effluent Guidelines Program Plan (U.S. EPA, 2009b). 
6-6

6.3 EPA’s Use of TWFs to Support Effluent Guidelines Rulemakings and Program Planning 

References

1. U.S. EPA. 1988. U.S. Environmental Protection Agency. Office of Water. Ehrensberger, K., and 

R. Rico. Cost-effectiveness Analysis for Effluent Guidelines. Office of Water. Washington, DC. 

(May). 

2. U.S. EPA. 2000. U.S. Environmental Protection Agency. Office of Water. Cost-Effectiveness 

Analysis of Effluent Limitation Guidelines and Standards for the Centralized Waste Treatment 

Industry. Washington, DC. (December). Available online at: 

http://water.epa.gov/scitech/wastetech/guide/treatment/upload/2006_12_28_guide_cwt_final_effe

ctive.pdf. 

3. U.S. EPA. 2004. Technical Support Document for the 2004 Effluent Guidelines Program Plan. 

EPA 821R-04-014. Washington, DC. (August). EPA-HQ-OW-2003-0074-1346 through 1352. 

4. U.S. EPA. 2009a. U.S. Environmental Protection Agency. 2009 Toxics Release Inventory 

National Analysis Overview. Washington, DC. (Unknown). Available online at:

http://www.epa.gov/tri/tridata/tri09/nationalanalysis/overview/2009TRINAOverviewfinal.pdf. 

5. U.S. EPA. 2009b. Technical Support Document for the Preliminary 2010 Effluent Guidelines 

Program Plan. EPA 821R-09-006. Washington, DC. (October). EPA-HQ-OW-2008-0517-0515. 
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7. LIMITATIONS AND APPROPRIATE USES OF TOXIC WEIGHTING FACTORS

Appropriate uses of TWFs are constrained by the limitations of the data used to derive them, and 

by limitations in the discharge data to which TWFs are applied. This section discusses the appropriate 

uses of TWFs given the data limitations.

7.1 Limitations of Source Data Used to Derive TWFs

Due to the lack of a single comprehensive source of toxicity information for the range of 

chemicals assessed by the effluent guidelines program, EPA must combine disparate sources of effects 

information in the calculation of TWFs. The data used to derive TWFs were compiled for different 

purposes at different times. Effects information used in the aquatic life component of TWFs is compiled 

primarily from three different sources, each based on different underlying analytical methodologies. 

Similarly, RfDs and cancer potency factors used to derive the human health component of TWFs are 

compiled from a hierarchy of multiple sources based on data quality and availability. Inconsistent source 

data is the primary reason why TWFs are not suitable for more rigorous applications, such as analyses to 

support water-quality based permitting or risk assessment.

In addition, complete data are lacking for the aquatic life and human health components of the 

majority of chemicals for which TWFs have been calculated. Only 37 percent of TWFs (349 of 1,064) are 

based on both aquatic life and human health effects information. The majority of TWFs, 62 percent (661), 

are based solely on aquatic life values. The remaining five percent of TWFs (54) are derived solely from 

human health values. 

7.2 Limitations of Discharge Data to Which TWFs Are Applied

There are usually no data limitations issues when conducting cost-effectiveness analyses because 

the data are typically compiled through industry surveys and effluent sampling specifically designed for 

individual effluent guidelines rulemakings. For the screening-level analysis, however, TWF-based 

analysis of PCS/ICIS-NPDES data, must be caveated to note the limitations of these two databases, which 

are the only sources of national level facility discharge data available to EPA for effluent guidelines 

program planning analysis. 

One of the biggest limitations of PCS/ICIS-NPDES is that for many industries, annual discharge 

amounts can only be calculated for facilities classified as major dischargers, which are typically the 

largest facilities with the most toxic discharges. Facilities with NPDES permits are not required to report 

the actual quantities of chemicals they discharge. Instead, they must only report the discharge flow rate, 

such as gallons per day, and the concentration of chemicals in the waste stream. EPA combines the flow 

and concentration information to produce annual discharge load estimates expressed as pounds discharged 
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per year. Facilities classified as minor dischargers, however, are not required to report their discharge 

flow. Some states report both flow and concentration information for minor dischargers, others do not. In 

addition, there is considerable inconsistency in how state permitting authorities classify facilities as either 

major or minor. Moreover, the majority of facilities are classified as minor dischargers in some industries 

with significant toxic discharges, such as most coal and ore mining facilities. State permitting authorities 

are required to consider the following six factors when classifying facilities as major or minor (U.S. EPA, 

2010): 

1. Toxic pollutant potential; 

2. Discharge flow to stream flow ratio; 

3. Conventional pollutant loading; 

4. Public health impact; 

5. Water quality factors; and 

6. Proximity to coastal waters. 

In addition to incompleteness of data reported to PCS/ICIS-NPDES, there are several other

important limitations to the database (U.S. EPA, 2009). 

• PCS/ICIS-NPDES contain data only for the pollutants facilities are required to monitor as 

stipulated in their NPDES permits. Facilities are not required to monitor or report all of 

the pollutants they actually discharge.

• PCS/ICIS-NPDES include very limited data characterizing indirect discharges from 

facilities to POTWs. 

• Many of the pollutant parameters reported to PCS/ICIS-NPDES are not individual 

chemical compounds. For example, “oil and grease” is a common industrial parameter 

reported to PCS/ICIS-NPDES that can contain a wide variety of industrial chemicals. 

Moreover, many elemental chemicals have multiple ionic species with differing toxic 

properties. Levels of these multiple ionic species are frequently reported as a single total 

value. For example, the various ionic forms of mercury may be reported together as “total 

mercury” levels in a waste stream. 

• Most reported flow rates do not indicate the type of wastewater. Thus total flow rates of 

industrial process wastewater may be diluted by stormwater or noncontact cooling water. 

• Facilities are identified by SIC code, not by effluent guidelines point source category. For 

some SIC codes, it may be difficult or impossible to identify the actual industry that is the 

source of the reported wastewater discharge.

A primary advantage of TRI is that, unlike PCS/ICIS-NPDES, facilities are required to report 

annual discharge amounts of chemicals rather than separate flow and concentration values. However, a 

disadvantage of TRI is that facilities are only required to report estimated discharge amounts rather than 
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actual sampled discharge amounts. There are several additional notable limitations of the data reported to 

TRI (U.S. EPA, 2009). 

• Information in TRI does not represent total national discharges because not all facilities 

are required to report to TRI. Small business with less than ten employees and facilities 

that do not meet discharge amount reporting thresholds are not required to report to TRI. 

Thus the completeness of discharge information varies from industry to industry. 

• The list of chemicals that facilities are required to report to TRI is not all- inclusive and 

has varied over time.

• TRI requirements encourage facilities to report some compounds as present at one-half 

the detection level if a facility suspects that the compound has the potential to be present, 

even if analysis of discharge samples shows the compound is below its detection level. 

Many companies adopt this conservative reporting approach. For facilities with high 

discharge flows, this can result in large estimates of pounds discharged with no sampling 

data to determine that the compound was actually present. 

• As with PCS/ICIS-NPDES, certain chemicals are reported as a class, such as polycyclic 

aromatic compounds, dioxin compounds, and metal compounds. Because the individual 

compounds in the class may have widely varying toxicities, actual toxicities of waste 

streams can be difficult to estimate accurately.

• Facilities are identified by NAICS code, not by effluent guidelines point source category. 

For some NAICS codes, it may be difficult or impossible to identify the point source 

category that is the actual source of the toxic wastewater discharge.

• Only facilities in certain NAICS codes are required to report, so some sources of toxic 

chemical discharges are not included in TRI. 

7.3 Appropriate Uses of TWFs

In spite of the limitations of the TWF source data and the discharge data to which they are 

applied, EPA considers TWFs appropriate for use in effluent guidelines planning analysis to identify 

industries or facilities that may warrant additional research. Moreover, TWPE-based ranking of industries 

is just one of four factors that EPA considers when setting priorities for effluent guidelines program. 

EPA also considers TWFs appropriate for use in the calculation of cost-effectiveness values 

because such values only serve as indicators of the relative cost effectiveness of treatment technology 

options and not as absolute metrics. Moreover, as with TWPE-based industry ranking in effluent 

guidelines program planning, cost-effectiveness analysis is not used as the sole basis for decision making

about technology options, but rather as one of many factors that EPA considers.

EPA’s effluent guidelines program cautions other organizations that may consider using TWFs 

for applications other than the specific ones for which they were designed. Other potential users of TWFs 

should carefully evaluate TWF source data limitations, the limitations of EPA national discharge 
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databases, and the uncertainties created by the TWF approach of combining aquatic life and human health 

effects values before deciding to use TWFs.

7.4 Limitations and Appropriate Uses of Toxic Weighting Factors References

1. U.S. EPA. 2009. U.S. Environmental Protection Agency. Office of Water. Technical Support 

Document for the Preliminary 2010 Effluent Guidelines Program Plan. EPA 821-R-09-006. 

Washington, DC. (October). EPA-HQ-OW-2008-0517-0515. 

2. U.S. EPA. 2010. U.S. Environmental Protection Agency. Office of Water. NPDES Permit Writers 

Manual. Washington, DC. EPA-833-K-10-001. (September). Available online at: 

http://www.epa.gov/npdes/pubs/pwm_2010.pdf.