Update of Human Health

Ambient Water Quality Criteria:

Chlorophenoxy Herbicide (2,4-D)

94-75-7 

Office of Water

Office of Science and

Technology

EPA 820-R-15-028 

June 2015
EPA 820-R-15-028 

June 2015

Update of Human Health

Ambient Water Quality Criteria:

Chlorophenoxy Herbicide (2,4-D)

94-75-7 

Office of Science and Technology

Office of Water

U.S. Environmental Protection Agency

Washington, DC 20460
2,4-D 94-75-7 

Table of Contents

1 Introduction: Background and Scope of Update..................................................................... 2

2 Problem Formulation............................................................................................................... 3

3 Criteria Formulas: Analysis Plan .............................................................................................. 4

4 Exposure Factors ..................................................................................................................... 5

4.1 Body Weight.................................................................................................................. 5

4.2 Drinking Water Intake................................................................................................... 6

4.3 Fish Consumption Rate ................................................................................................. 6

4.4 Bioaccumulation Factor................................................................................................ 7

4.4.1 Approach ......................................................................................................... 7

4.4.2 Chemical-specific BAFs.................................................................................... 9

5 Hazard Identification and Dose Response............................................................................... 9

5.1 Approach....................................................................................................................... 9

5.2 Chemical-specific Toxicity Value................................................................................. 11

5.2.1 Reference Dose.............................................................................................. 11

5.2.2 Cancer Slope Factor....................................................................................... 12

6 Relative Source Contribution................................................................................................. 12

6.1 Approach..................................................................................................................... 12

6.2 Chemical-specific RSC ................................................................................................. 13

7 Criteria Derivation: Analysis.................................................................................................. 14

7.1 AWQC for Noncarcinogenic Toxicological Effects ...................................................... 15

7.2 AWQC for Carcinogenic Toxicological Effects............................................................. 15

7.3 AWQC Summary ......................................................................................................... 16

8 Criteria Characterization........................................................................................................ 16

9 Chemical Name and Synonyms............................................................................................. 18

10 References............................................................................................................................. 20

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1 Introduction: Background and Scope of Update 

EPA’s recommended ambient water quality criteria (AWQC) for human health are scientifically 

derived numeric values that EPA has determined will adequately protect human health from 

the adverse effects of pollutants in ambient water.

Section 304(a)(1) of the Clean Water Act (CWA) requires EPA to develop and publish, and from 

time to time revise, recommended criteria for the protection of water quality that accurately 

reflect the latest scientific knowledge. Water quality criteria developed under section 304(a) 

are based solely on data and scientific judgments on the relationship between pollutant 

concentrations and environmental and human health effects. Section 304(a) criteria do not 

reflect consideration of economic impacts or the technological feasibility of meeting pollutant 

concentrations in ambient water.

EPA’s recommended section 304(a) criteria provide technical information for states and 

authorized tribesa to consider and use in adopting water quality standards that ultimately 

provide the basis for assessing water body health and controlling discharges of pollutants into 

waters of the United States. Under the CWA and its implementing regulations, states and 

authorized tribes are required to adopt water quality criteria to protect the designated uses of 

waters (e.g., public water supply, aquatic life, recreational use, industrial use). EPA’s 

recommended water quality criteria do not substitute for the CWA or regulations, nor are they 

regulations themselves. Thus, EPA’s recommended criteria do not impose legally binding 

requirements. States and authorized tribes may adopt, where appropriate, other scientifically 

defensible water quality criteria that differ from these recommendations.

The water quality criteria that are the subject of this document are national AWQC

recommendations for human health issued under CWA section 304(a). Unless expressly 

indicated otherwise, all references to “criteria,” “water quality criteria,” “ambient water quality 

criteria recommendations,” or similar variants thereof are references to national AWQC

recommendations for human health.

In this 2015 update, EPA has revised the human health criteria for chlorophenoxy herbicide 

(2,4-D) to reflect the latest scientific information, including updated exposure factors (body 

weight [BW], drinking water intake [DI] rate, and fish consumption rate [FCR]), bioaccumulation 

factors (BAFs), and human health toxicity values (reference dose [RfD] multiplied by relative 

source contribution [RSC] or 10-6 divided by cancer slope factor [CSF]). The criteria continue to 

be based on EPA’s Methodology for Deriving Ambient Water Quality Criteria for the Protection 

of Human Health, which is referred to as the “2000 Methodology” in this document (USEPA 

2000a). EPA accepted written scientific views from the public on the draft updated human 

health criteria for this chemical (and 93 others) from May through August 2014.

a Throughout this document, the term states means the 50 states, the District of Columbia, the Commonwealth of 

Puerto Rico, the Virgin Islands, Guam, American Samoa, and the Commonwealth of the Northern Mariana Islands. 

The term authorized tribe or tribe means an Indian tribe authorized for treatment in a manner similar to a state 

under CWA section 518 for the purposes of section 303(c) water quality standards. 

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It is important for states and authorized tribes to consider any new or updated section 304(a) 

recommended criteria as part of their triennial review process to ensure that state or tribal 

water quality standards reflect current science and protect applicable designated uses. These 

final 2015 updated section 304(a) human health criteria recommendations supersede EPA’s 

previous recommendations.

2 Problem Formulation

Problem formulation provides a strategic framework for water quality criteria development by 

focusing on the most relevant endpoints and increasing the transparency of the effects 

assessment. The structure of this criteria document is intended to be consistent with general 

concepts of effects assessments as described in EPA’s Framework for Human Health Risk 

Assessment to Inform Decision Making (USEPA 2014a).

In developing AWQC, EPA currently follows the assessment method outlined in its 2000 

Methodology (USEPA 2000a). The 2000 Methodology describes different approaches for 

addressing water and non-water exposure pathways to derive human health AWQC depending 

on the toxicological endpoint of concern, the toxicological effect (noncarcinogenic or 

carcinogenic), and whether toxicity is considered a linear or threshold effect. Water sources of 

exposure include both consuming drinking water and eating fish or shellfish from inland and 

nearshore waters that have been exposed to pollutants in the water body. For pollutants that 

exhibit a threshold of exposure before deleterious effects occur, as is the case for 

noncarcinogens and nonlinear carcinogens, EPA applies an RSC to account for other potential 

human exposures to the pollutant (USEPA 2000a). Other sources of exposure might include, but 

are not limited to, exposure to a particular pollutant from ocean fish or shellfish consumption 

(which is not included in the FCR), non-fish food consumption (e.g., consumption of fruits, 

vegetables, grains, meats, or poultry), dermal exposure, and inhalation exposure.

For substances for which the toxicity endpoint is carcinogenicity based on a linear low-dose 

extrapolation, only the exposures from drinking water and fish ingestion are reflected in human 

health AWQC; that is, non-water sources are not explicitly included and no RSC is applied 

(USEPA 2000a). In these situations, AWQC are derived with respect to the incremental lifetime 

cancer risk posed by the presence of a substance in water, rather than an individual’s total risk 

from all sources of exposure. The resulting criterion represents the water concentration that is 

expected to increase an individual’s lifetime risk of cancer from exposure to the particular 

pollutant by no more than one chance in one million for the general population. EPA calculates 

AWQC at a 10-6 (one in one million) cancer risk level for the general population (USEPA 2000a). 

The 2000 Methodology recommends that states set human health criteria cancer risk levels for 

the target general population at either 10-5 or 10-6 and also notes that states and authorized 

tribes can choose a more stringent risk level, such as 10-7.

For substances that are carcinogenic, EPA takes an integrated approach and considers both

cancer and noncancer effects when deriving AWQC (USEPA 2000a; USEPA 2000b). Where 

sufficient data are available, EPA derives AWQC using both carcinogenic and noncarcinogenic 

toxicity endpoints and recommends the lower value for the AWQC. The AWQC might not utilize 

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the value obtained from the cancer analysis if it is less protective than that derived from the 

noncancer endpoint. 

3 Criteria Formulas: Analysis Plan

Human health AWQC for toxic pollutants are necessary to protect any designated uses related 

to ingestion of water and ingestion of aquatic organisms. These uses can include, but are not 

limited to, recreation in and on the water, consumption of fish or shellfish (including 

consumption associated with fishing or shellfish harvesting), and protection of drinking water 

supplies.

The derivation of human health AWQC requires information about both the toxicological 

endpoints of concern for water pollutants and the pathways of human exposure to those 

pollutants. EPA considers the following two primary pathways of human exposure to pollutants 

present in a particular water body when deriving human health 304(a) AWQC: (1) direct 

ingestion of drinking water obtained from the water body and (2) consumption of fish or 

shellfish obtained from the water body.

The equations for deriving human health AWQC for noncarcinogenic effects and carcinogenic 

effects are presented as Eqs. 1 and 2. EPA derives recommended human health AWQC based 

on the consumption of both water and aquatic organisms (Eq. 1) and based on the 

consumption of aquatic organisms alone (Eq. 2). The use of one criterion over the other 

depends on the designated use of a particular water body or water bodies (i.e., drinking water 

source and/or fishable waters). EPA recommends applying organism-only AWQC (Eq. 2) to a 

water body where the designated use includes supporting fishable uses under section 101(a) of 

the CWA but the water body is not a drinking water supply source (e.g., non-potable estuarine 

waters that support fish or shellfish for human consumption) (USEPA 2000a). 

EPA recommends including the drinking water exposure pathway for ambient surface waters 

where drinking water is a designated use for the following reasons: (1) drinking water is a 

designated use for surface waters under the CWA, and therefore criteria are needed to ensure 

that this designated use can be protected and maintained; (2) although they are rare, some 

public water supplies provide drinking water from surface water sources without treatment; 

(3) even among the majority of water supplies that do treat surface waters, existing treatments 

might not be effective for reducing levels of particular contaminants; and (4) in consideration of 

the Agency’s goals of pollution prevention, ambient waters should not be contaminated to a 

level where the burden of achieving health objectives is shifted away from those responsible 

for pollutant discharges and placed on downstream users that must bear the costs of upgraded 

or supplemental water treatment (USEPA 2000a). 

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The equations for deriving the criteria values are as follows (USEPA 2000a): 

For consumption of water and organisms:

AWQC (μg/L) = toxicity value (mg/kg-d) × BW (kg) × 1,000 (μg/mg)b (Eq. 1)

 DI (L/d) + ∑ (FCRi (kg/d) × BAFi (L/kg)) 4

i=2

For consumption of organisms only:

AWQC (μg/L) = toxicity value (mg/kg-d) × BW (kg) × 1,000 (μg/mg)c (Eq. 2)

 ∑ (FCRi (kg/d) × BAFi (L/kg)) 4

i=2

Where:

AWQC = ambient water quality criteria

toxicity value = RfD x RSC (mg/kg-d) for noncarcinogenic effects

or

10-6/CSF (kg-d/mg) for carcinogenic effectsd

RSC = relative source contribution (applicable to only noncarcinogenic and nonlinear 

low-dose extrapolation for carcinogenic effects)

BW = body weight

DI = drinking water intake

∑ 4

i=2 = summation of values for aquatic trophic levels (TLs), where the letter i stands 

for the TLs to be considered, starting with TL2 and proceeding to TL4

FCRi = fish consumption rate for aquatic TLs 2, 3, and 4

BAFi = bioaccumulation factor for aquatic TLs 2, 3, and 4

EPA rounds AWQC to the number of significant figures in the least precise parameter as 

described in the 2000 Methodology (USEPA 2000a, section 2.7.3).

4 Exposure Factors

4.1 Body Weight

EPA updated the default BW assumption to 80.0 kg based on National Health and Nutrition 

Examination Survey (NHANES) data from 1999 to 2006 as reported in Table 8.1 of EPA’s 

Exposure Factors Handbook (USEPA 2011a). The updated BW represents the mean weight for 

adults ages 21 and older. EPA’s previously recommended BW assumption for adults was 70 kg, 

which was based on the mean BW of adults from the NHANES III database (1988–1994) and a 

1989 study conducted by the National Cancer Institute (USEPA 2000a). 

b 1,000 μg/mg is used to convert the units of mass from milligrams to micrograms. c 1,000 μg/mg is used to convert the units of mass from milligrams to micrograms. d 10-6 or 1 in 1,000,000 risk level for the general population.

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4.2 Drinking Water Intake

EPA updated the default DI to 2.4 L/d, rounded from 2.414 L/d, based on NHANES data from 

2003 to 2006 as reported in EPA’s Exposure Factors Handbook (USEPA 2011a, Table 3-23). This 

rate represents the per capita estimate of combined direct and indirect community watere

ingestion at the 90th percentile for adults ages 21 and older. EPA selected the per capita rate for 

the updated DI because it represents the average daily dose estimates; that is, it includes both 

people who drank water during the survey period and those who did not, which is appropriate 

for a national-scale assessment such as CWA section 304(a) national human health criteria 

development (USEPA 2011a, section 3.2.1).

EPA’s updated DI of 2.4 L/d is consistent with the 2000 Methodology. In that document, EPA 

recommended a default DI of 2 L/d, which represented the per capita community water 

ingestion rate at the 86th percentile for adults surveyed in the U.S. Department of Agriculture’s 

1994–1996 Continuing Survey of Food Intake by Individuals (CSFII) analysis (USEPA 2000a, 

section 4.3.2.1).

4.3 Fish Consumption Rate

The updated FCR for the general adult population is 22.0 g/d, or 0.0220 kg/d (USEPA 2014b, 

Table 9a). This FCR represents the 90th percentile per capita consumption rate of fish from 

inland and nearshore waters for U.S. adults ages 21 years and older based on NHANES data 

from 2003–2010. The 95 percent confidence interval (CI) of the 90th percentile per capita FCR is 

19.1 g/d and 25.4 g/d. This updated FCR replaces EPA’s previously recommended default FCR of 

17.5 g/d, which represented an estimate of the 90th percentile per capita consumption rate of 

fish from inland and nearshore waters for U.S. adults ages 21 years and older. That default FCR 

was based on USDA’s CSFII 1994–1996 data (USEPA 2002a). 

As recommended in the 2000 Methodology, EPA updated the AWQC to reflect trophic level- 

(TL-) specific FCRs to better represent human dietary consumption of fish. An organism’s 

trophic position in the aquatic food web can have an important effect on the magnitude of 

bioaccumulation of certain chemicals. The TL-specific FCRs are numbered 2, 3, and 4, and they 

account for different categories of fish and shellfish species based on their position in the 

aquatic food web: TL2 accounts for benthic filter feeders; TL3 accounts for forage fish; and TL4 

accounts for predatory fish (USEPA 2000a). 

EPA used the following TL-specific FCRs to derive the updated AWQC: TL2 = 7.6 g/d

(0.0076 kg/d) (95 percent CI [6.4, 9.1] g/d); TL3 = 8.6 g/d (0.0086 kg/d) (95 percent CI [7.2, 

10.2] g/d); and TL4 = 5.1 g/d (0.0051 kg/d) (95 percent CI [4.0, 6.4] g/d). Each TL-specific FCR 

represents the 90th percentile per capita consumption rate of fish and shellfish from inland and 

nearshore waters from that particular TL for U.S. adults ages 21 years and older (USEPA 2014b, 

e Community water includes direct and indirect use of tap water for household uses and excludes bottled water 

and other sources (USEPA 2011a, section 3.3.1.2). Direct ingestion is defined as direct consumption of water as a 

beverage, while indirect ingestion includes water added during food preparation (e.g., cooking, rehydration of 

beverages) but not water intrinsic to purchased foods (USEPA 2011a, section 3.1).

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Tables 16a, 17a, and 18a). The sum of these three TL-specific FCRs is 21.3 g/d, which is within 

the 95 percent CI of the overall FCR of 22.0 g/d. EPA recommends using the TL-specific FCRs

when deriving AWQC; however, the overall FCR rate (22.0 g/d) may be used if a simplified 

approach is preferred.

4.4 Bioaccumulation Factor

4.4.1 Approach

Several attributes of the bioaccumulation process are important to understand when deriving 

national BAFs for use in developing national recommended section 304(a) AWQC. First, the 

term bioaccumulation refers to the uptake and retention of a chemical by an aquatic organism 

from all surrounding media, such as water, food, and sediment. The term bioconcentration

refers to the uptake and retention of a chemical by an aquatic organism from water only. For 

some chemicals (particularly those that are highly persistent and hydrophobic), the magnitude 

of bioaccumulation by aquatic organisms can be substantially greater than the magnitude of 

bioconcentration. Thus, an assessment of bioconcentration alone might underestimate the 

extent of accumulation in aquatic biota for those chemicals. Accordingly, the EPA guidelines 

presented in the 2000 Methodology emphasize using, when possible, measured or estimated 

BAFs, which account for chemical accumulation in aquatic organisms from all potential 

exposure routes (USEPA 2000a). 

EPA estimated BAFs for this updated AWQC using EPA’s 2000 Methodology (USEPA 2000a) and 

its Technical Support Document, Volume 2: Development of National Bioaccumulation Factors

(Technical Support Document, Volume 2) (USEPA 2003a). Specifically, these documents provide 

a framework for identifying alternative procedures to derive national TL-specific BAFs for a 

chemical based on the chemical’s properties (e.g., ionization and hydrophobicity), metabolism, 

and biomagnification potential (USEPA 2000a; USEPA 2003a). 

EPA’s approach for developing national BAFs represents the long-term average 

bioaccumulation potential of a pollutant in aquatic organisms that are commonly consumed by 

humans across the United States. National BAFs are not intended to reflect fluctuations in 

bioaccumulation over short periods (e.g., a few days) because human health AWQC are 

generally designed to protect humans from long-term (lifetime) exposures to waterborne 

chemicals (USEPA 2003a). 

EPA followed the approach described in Figure 3-1 of the Technical Support Document, Volume 

2 (USEPA 2003a). EPA used peer-reviewed, publicly available information to classify each 

chemical using this framework to derive the most appropriate BAFs according to EPA’s 2000 

Methodology (USEPA 2000a). The framework provides six alternatives, or procedures, resulting 

in up to four possible methods for each chemical, based on the chemical’s properties. These 

four methods follow:

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• BAF Method. This method uses measured BAFs derived from data obtained from field 

studies. Field-measured BAFs were normalized by adjusting for the water-dissolved 

portions of the chemical and the lipid fraction of fish tissue for each species, as well as 

the fraction of the total concentration of chemical in water that is freely dissolved. EPA 

averaged multiple field BAFs using a geometric mean of the normalized BAFs by species 

and TL; then EPA further averaged the BAFs across species to compute TL baseline BAFs. 

The national-level BAF adjusts the TL baseline BAFs by national default values for lipid 

content, dissolved and particulate organic carbon content, and the n-octanol-water 

partition coefficient (Kow). EPA chose the recommended 50th percentile dissolved and 

particulate organic carbon content for the national-level default values, as described in 

section 6.3 of the Technical Support Document, Volume 2 (USEPA 2003a). 

• BSAF Method. This method uses biota-sediment accumulation factors (BSAFs) to 

estimate BAFs. EPA did not use measured BSAFs to calculate national BAFs because the 

two major compilations of these data—EPA’s Biota-Sediment Accumulation Factor Data 

Set, Version 1.0 (USEPA 2015a), and the U.S. Army Corps of Engineers’ BSAF database 

(USACE 2015)—have not been peer-reviewed.

• BCF Method. This method uses BAFs estimated from laboratory-measured 

bioconcentration factors (BCFs) with or without adjustment by a food chain multiplier. 

Similar to field BAFs, laboratory-measured BCFs are normalized with the lipid fraction 

and the fraction of the total concentration of chemical in water that is freely dissolved, 

then multiplied by the food chain multiplier where applicable. Multiple values are 

averaged using a geometric mean across species and then across TL to compute 

baseline BAFs. The national-level BAF adjusts the TL baseline BAFs by national default 

values for lipid content, dissolved and particulate organic carbon content, and the Kow. 

EPA chose the recommended 50th percentile dissolved and particulate organic carbon 

content for the national-level default values, as described in section 6.3 of the Technical 

Support Document, Volume 2 (USEPA 2003a). 

• Kow Method. This method predicts BAFs based on a chemical’s Kow, with or without 

adjustment using a food chain multiplier, as described in section 5.4 of the Technical 

Support Document, Volume 2 (USEPA 2003a). 

Following the decision framework presented in Figure 3-1 of the Technical Support Document, 

Volume 2 (USEPA 2003a), EPA selected one of the six procedures to develop a national-level 

BAF for this chemical. For a given procedure, EPA selected the method that provided BAF 

estimates for all three TLs (TL2–TL4) in the following priority:

1. BAF estimates using the BAF method (i.e., based on field-measured BAFs) if possible.

2. BAF estimates using the BCF method if (a) the BAF method did not produce estimates 

for all three TLs and (b) the BCF method produced national-level BAF estimates for all 

three TLs.

3. BAF estimates using the Kow method if (a) Procedure 1 or 3 was applicable (see Figure 

3-1 of the Technical Support Document, Volume 2 [USEPA 2003a]) and (b) the BAF and 

BCF methods did not produce BAF estimates for all three TLs.

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In cases where the procedure called for the BAF method but there were fewer than three TL

estimates and the Kow method did not apply (i.e., Procedures 2, 4, 5, and 6), EPA used the BAF 

method estimate for the reported TLs by averaging the estimates using a geometric mean when 

there were two BAFs and using the single estimate when only one was available. EPA did not 

mix values from the BAF and BCF methods. If the BAF method did not have sufficient reliable 

data for any TLs, EPA used the BCF method estimates in the same manner. If none of the four 

methods provided sufficient data, or if none were appropriate for the procedure, EPA used the 

BCF from the previously recommended 2002/2003 criteria (USEPA 2002b; USEPA 2003b). 

EPA primarily used field-measured BAFs and laboratory-measured BCFs available from peerreviewed, publicly available databases (Arnot and Gobas 2006; Environment Canada 2006) to 

develop national BAFs. If field-measured BAFs and laboratory-measured BCFs were not 

available from those sources, EPA selected Kow values from peer-reviewed sources (i.e., Agency 

for Toxic Substances and Disease Registry [ATSDR] preferentially, followed by U.S. Department 

of Health and Human Services’ Hazardous Substances Data Bank) for use in calculating national 

BAFs using the Kow method described in EPA’s Technical Support Document, Volume 2 (USEPA 

2003a). For those chemicals for which the Kow method was not applicable based on the

Technical Support Document, Volume 2 (USEPA 2003a), EPA performed open literature 

searches of peer-reviewed journal articles to find field-measured BAFs or laboratory-measured 

BCFs.

4.4.2 Chemical-specific BAFs

EPA selected a national BAF value of 13 L/kg for 2,4-D. EPA followed the framework for 

selection of methods for deriving national BAFs in Figure 3-1 of the Technical Support 

Document, Volume 2 (USEPA 2003a) to select a procedure for estimating national BAFs for 

2,4-D. Based on the characteristics of this chemical, EPA selected Procedure 5 for deriving a 

national BAF value. 2,4-D has the following characteristics:

• Ionic organic chemical, with ionization not negligible (USDHHS 2012)

• Biomagnification unlikely (USDHHS 2012)

EPA was not able to locate peer-reviewed, field-measured BAFs or lab-measured BCFs for all 

three TLs (2, 3, and 4). Therefore, EPA used the BCF method estimate for the reported TLs by 

calculating the geometric mean of the TL2 and TL3 BCF values available for 2,4-D (Wang et al. 

1994) to derive the national BAF value of 13 L/kg for this chemical.

5 Hazard Identification and Dose Response

5.1 Approach

EPA considered all available toxicity values for both noncarcinogenic and carcinogenic 

toxicological effects to develop this updated AWQC for 2,4-D. As described in the 2000 

Methodology (USEPA 2000a), where data are available EPA derives AWQC for both 

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noncarcinogenic and carcinogenic effects and recommends the more protective value for the 

AWQC. (See section 7, Criteria Derivation: Analysis.)

For noncarcinogenic toxicological effects, EPA uses a chronic-duration oral RfD to derive human 

health AWQC. An RfD is an estimate (with uncertainty spanning perhaps an order of magnitude) 

of a daily oral exposure of the human population to a substance that is likely to be without an 

appreciable risk of deleterious effects during a lifetime. An RfD is typically derived from a 

laboratory animal dosing study in which a no-observed-adverse-effect level (NOAEL), lowestobserved-adverse-effect level (LOAEL), or benchmark dose can be obtained. Uncertainty factors 

are applied to reflect the limitations of the data (USEPA 2000a).

For carcinogenic toxicological effects, EPA uses an oral CSF to derive human health AWQC. The 

oral CSF is an upper bound, approximating a 95 percent confidence limit, on the increased 

cancer risk from a lifetime oral exposure to a stressor.

For this update, EPA conducted a systematic search of eight peer-reviewed, publicly available 

sources to obtain the toxicity value (RfD or CSF) for use in developing AWQC. EPA’s primary 

source of toxicity values for developing human health AWQC is its Integrated Risk Information 

System (IRIS) program (USEPA 2015b). EPA also systematically searched for toxicological 

assessments from the following EPA program offices, other national and international 

programs, and state programs:

• EPA, Office of Pesticide Programs (USEPA 2015c)

• EPA, Office of Pollution Prevention and Toxics (USEPA 2015d)

• EPA, Office of Water (USEPA 2015e)

• EPA, Office of Solid Waste and Emergency Response (USEPA 2015f)

• U.S. Department of Health and Human Services, Agency for Toxic Substances and

Disease Registry (ATSDR 2015)

• Health Canada (HC 2015)

• California Environmental Protection Agency, Office of Environmental Health Hazard 

Assessment (CalEPA 2014)

After identifying and documenting all available toxicity values, EPA followed a systematic 

process to select the toxicity values used to derive the AWQC for noncarcinogenic and 

carcinogenic effects. EPA selected IRIS toxicity values to derive the updated AWQC if any of the 

following conditions were met:

1. EPA’s IRIS toxicological assessment was the only available source of a toxicity value.

2. EPA’s IRIS toxicological assessment was the most current source of a toxicity value.

3. EPA’s IRIS program was reassessing the chemical in question and had published the 

draft Toxicological Review for public review and comment, discussion at a public 

meeting, and subsequent expert peer review.

f

f Equivalent to Step 4 in the July 2013 EPA Process for Developing IRIS Health Assessments. Available online at 

http://www.epa.gov/iris/process.htm.

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4. The toxicity value from a more current toxicological assessment from a source other 

than EPA IRIS was based on the same principal study and was numerically the same as 

an older EPA IRIS toxicity value.

5. A more current toxicological assessment from a source other than EPA IRIS was available,

but it did not include the relevant toxicity value (chronic-duration oral RfD or CSF).

6. A more current toxicological assessment from a source other than EPA IRIS was 

available, but it did not introduce new science (e.g., the toxicity value was not based on 

a newer principal study) or use a more current modeling approach compared to an older 

EPA IRIS toxicological assessment.

EPA selected the toxicity value from a peer-reviewed, publicly available source other than EPA 

IRIS to derive the updated AWQC if any of the following conditions were met:

1. The chemical is currently used as a pesticide, and EPA Office of Pesticide Programs had a 

toxicity value that was used in pesticide registration decision-making.

2. A toxicological assessment from a source other than EPA IRIS was the only available 

source of a toxicity value.

3. A more current toxicological assessment from a source other than EPA IRIS introduced 

new science (e.g., the toxicity value was based on a newer principal study) or used a 

more current modeling approach compared to an older EPA IRIS toxicological 

assessment.

5.2 Chemical-specific Toxicity Value

5.2.1 Reference Dose

EPA selected an RfD of 2.1 × 10–1 mg/kg-d (0.21 mg/kg-d) for 2,4-D, a current-use pesticide, 

based on EPA Office of Pesticide Programs (OPP) registration documents (USEPA 2013a; USEPA 

2014c). EPA OPP identified a study by Marty et al. (2010) as the critical study and kidney toxicity 

and decreased BW as the critical effects in rats exposed orally to 2,4-D. The extended 1-

generation reproduction toxicity study had a NOAEL of 21 mg/kg-d. At the study LOAEL of 55.6 

mg/kg-d (males) and 46.7 mg/kg-d (females), kidney toxicity—manifested as increased kidney 

weights and increased incidence of degeneration of the proximal convoluted tubules—was 

observed and decreased BW in pups was observed throughout lactation (USEPA 2013a). In 

deriving the RfD, EPA OPP applied a composite uncertainty factor of 100 to account for 

interspecies extrapolation (10) and intraspecies variation (10) (USEPA 2013a).

EPA identified one other RfD source through the systematic search described in section 5: a 

1986 EPA IRIS assessment (USEPA 1986a). Based on the selection process described in section 

5, the 2013 OPP RfD is preferred for use in AWQC development at this time. The OPP RfD was 

selected to derive the updated AWQC because this chemical is a current-use pesticide.

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5.2.2 Cancer Slope Factor

Under the 1986 EPA Guidelines for Carcinogen Risk Assessment (USEPA 1986b), 2,4-D is 

classified as Group D, “not classifiable as to human carcinogenicity” (USEPA 2013a; USEPA 

2014c). 2,4-D has not undergone a complete evaluation and determination under EPA’s IRIS 

program for evidence of human carcinogenic potential (USEPA 1990). 

EPA identified no CSF source through the systematic search described in section 5. 

6 Relative Source Contribution

6.1 Approach

The RSC component of the AWQC calculation allows a percentage of the RfD’s exposure to be 

attributed to the consumption of ambient water and fish and shellfish from inland and 

nearshore waters when there are other potential exposure sources. The RSC describes the 

portion of the RfD available for AWQC-related sources (USEPA 2000a); the remainder of the RfD 

is allocated to other sources of the pollutant. The rationale for this approach is that for 

pollutants exhibiting threshold effects, the objective of the AWQC is to ensure that an 

individual’s total exposure from all sources does not exceed that threshold level. Exposures 

outside the RSC include, but are not limited to, exposure to a particular pollutant from ocean 

fish and shellfish consumption (which is not included in the FCR), non-fish food consumption 

(e.g., fruits, vegetables, grains, meats, poultry), dermal exposure, and respiratory exposure.

EPA derived an RSC for each chemical included in this 2015 update by using the Exposure 

Decision Tree approach described in the 2000 Methodology (USEPA 2000a). To use that

approach, EPA compiled information for each chemical on its uses, chemical and physical 

properties, occurrences in other potential sources (e.g., air, food), and releases to the 

environment, as well as regulatory restrictions on other sources that are specific to the 

chemical (e.g., air quality standards, food tolerance levels). The ATSDR “Toxicological Profiles”

(ATSDR 2015) were the primary sources for this information. EPA used the Hazardous 

Substance Data Bank (HSDB) (USDHHS 2015) from the National Library of Medicine’s Toxicology 

Data Network (TOXNET) as the primary source for chemicals without ATSDR Toxicological 

Profiles. Both sources are peer-reviewed compilations of chemical information.

EPA used additional references, including the following, to obtain specific types of information 

and to supplement the information from ATSDR and the HSDB: 

• EPA’s Six-Year Reviews (drinking water data) (USEPA 2009a; USEPA 2009b). 

• FDA Total Diet Study (USFDA 2015). 

• FDA Everything Added to Food in the United States (USFDA 2013). 

• EPA National Lake Fish Tissue Study (USEPA 2009c). 

• EPA Toxic Release Inventory (USEPA 2015g). 

• International Bottled Water Association Standards of Quality (IBWA 2012). 

12
2,4-D 94-75-7 

• NOAA Mussel Watch (NOAA 2014). 

• Additional sources as needed.

To determine the RSC to be used in the AWQC calculation, EPA then used the information 

compiled for each chemical to address the questions posed in the Exposure Decision Tree. 

Some of the important items evaluated in the Exposure Decision Tree follow:

• The adequacy of the data available for each relevant exposure source and pathway. 

• The availability of sufficient information to characterize the likelihood of exposure to 

relevant sources. 

• Whether there are significant known or potential uses/sources other than the source of 

concern (i.e., ambient water and fish/seafood from those waters). 

• Whether information on each source is available to make a characterization of 

exposure. 

In cases where there is a lack of environmental or exposure data, or both, the Exposure 

Decision Tree approach results in a recommended RSC of 20 percent. This 20 percent value for 

the RSC may be replaced where sufficient data are available to develop a scientifically 

defensible alternative value. When appropriate, if scientific data demonstrating that sources 

and routes of exposure other than water and fish from inland and nearshore waters are not 

anticipated for the pollutant in question, the RSC may be raised to 80 percent based on the 

available data (USEPA 2000a). 

6.2 Chemical-specific RSC

2,4-D is a herbicide used to control broad-leaved weeds in cereals, grain crops, road sides, and 

farm buildings (USDHHS 2012). 2,4-D is currently registered as a pesticide by EPA and is in the 

registration review process (USEPA 2015c). Human exposure to 2,4-D might occur through

inhalation and ingestion of food and water (USDHHS 2012). The primary exposure routes for 

the general public are through food residues and water ingestion (USDHHS 2012). 

2,4-D has been detected in food products. It was detected at levels above the detection limit in 

one of eight whole-wheat bread samples (USFDA 2005). The average level of 2,4-D in ready-toeat foods tested repeatedly between 1982 and 1991 was 0.006 μg/g (USDHHS 2012). In 2008, 

2,4-D was detected in potatoes (22 out of 744 samples), but not detected in grape juice, 

spinach, or strawberries (USDA 2009). In 2009 and 2013, 2,4-D was not detected in fruit or

vegetables (USDA 2011; USDA 2014). EPA has set a 40 CFR part 180 pesticide tolerance level for 

this chemical in food and feed commodities (USGPO 2015). Maximum residue limits for 2,4-D 

were found in the Global MRL Database for various commodities in the United States; however, 

no recent monitoring data on imported foods are available (Bryant Christie Inc. 2015). Thus, 

based on its prevalence in sampled foods, ingestion of food is a potentially significant source of 

exposure to 2,4-D. 

13
2,4-D 94-75-7 

2,4-D is regulated under the Safe Drinking Water Act, and EPA’s drinking water standard 

(maximum contaminant level) for 2,4-D is 70 μg/L (USEPA 2014d). Based on EPA’s Six-Year 

Reviews (USEPA 2009a; USEPA 2009b), from over 22,000 surface water sources of drinking 

water, 2,4-D was detected in 0.64 percent of samples. Of more than 101,000 groundwater 

sources of drinking water, 2,4-D was detected in 0.283 percent of samples (USEPA 2009a). 

2,4-D was detected in 49 of 50 samples in the limited sampling of drinking water and 1 of 11 in 

the sampling of groundwater in the Pesticide Data Program (USDA 2014). The Standard of 

Quality for bottled water is 70 μg/L (IBWA 2012). Therefore, based on its prevalence, ingestion 

of drinking water is a potentially a significant source of exposure to 2,4-D.

Volatilization of 2,4-D from moist soil surfaces is not expected to be an important fate process 

because anions do not readily volatilize (USDHHS 2012). Volatilization from water surfaces is 

also not expected to be an important fate process based upon this compound’s acid 

dissociation constant of 2.73, which indicates 2,4-D will exist almost entirely in the ionized form 

at pH values of 5–9 (USDHHS 2012). 2,4-D exists primarily in the particulate phase in the 

atmosphere and is susceptible to both wet and dry deposition. Its detection in urban areas is 

generally related to spray drift (USDHHS 2012). Recent data from EPA’s 2013 Toxic Release 

Inventory indicate that 1,265 pounds of 2,4-D were released to the air in 2013 (USEPA 2015g). 

This chemical is listed as a hazardous air pollutant (USEPA 2013b). Given the types of releases, 

air is a potentially significant source of exposure to 2,4-D. 

The log Kow for 2,4-D is 2.81 (USDHHS 2012). The national-level BAF estimate for 2,4-D is 

13 L/kg, which indicates 2,4-D has a low potential for bioaccumulation (USEPA 2011b). 2,4-D 

was not a target chemical in EPA’s National Lake Fish Tissue Study (USEPA 2009c), and it is not 

included in NOAA’s Mussel Watch Survey (NOAA 2014). Recent exposure information regarding 

concentrations of 2,4-D in fish and shellfish is lacking. Thus, based on its low potential for 

bioaccumulation, exposure to this chemical from ingestion of fish and shellfish is not 

considered likely.

In summary, based on the physical properties and available exposure information for 2,4-D, air, 

non-fish food, and drinking water are potentially significant sources. Following EPA’s Exposure 

Decision Tree in the 2000 Methodology (USEPA 2000a), significant potential sources other than 

fish and shellfish from inland and nearshore waters and water ingestion exist (Box 8A in the 

Decision Tree), but information is not available to quantitatively characterize exposure from 

those different sources (Box 8B in the Decision Tree). Therefore, EPA recommends an RSC of 

20 percent (0.20) for 2,4-D.

7 Criteria Derivation: Analysis

Table 1 summarizes the model inputs used to derive the 2015 updated human health AWQC 

that are protective of exposure to 2,4-D from consuming drinking water and eating fish and 

shellfish (organisms) from inland and nearshore waters. The criteria calculations are presented 

below. These updated criteria recommendations are based on the 2000 Methodology (USEPA 

2000a) and the updated exposure assumptions described above. (See section 4, Exposure 

14
2,4-D 94-75-7 

Factors; section 5, Hazard Identification and Dose Response; and section 6, Relative Source 

Contribution.)

Table 1. Summary of Input Parameters for 2015 Human Health AWQC for 2,4-D 

Input Parameter Value

RfD 0.21 mg/kg-d

CSF No data

RSC 0.20

BW 80.0 kg

DI 2.4 L/d

FCR 0.022 kg/d

BAF 13 L/kg

7.1 AWQC for Noncarcinogenic Toxicological Effects

For consumption of water and organisms:

AWQC (μg/L) = toxicity value (RfD [mg/kg-d] × RSC) × BW (kg) × 1,000 (μg/mg)

 DI (L/d) + (FCR (kg/d) × BAF (L/kg))

 = 0.21 mg/kg-d × 0.20 × 80.0 kg × 1,000 μg/mg

 2.4 L/d + (0.022 kg/d × 13 L/kg)

 = 1,251 μg/L

 = 1,300 μg/L (rounded)

For consumption of organisms only:

AWQC (μg/L) = toxicity value (RfD [mg/kg-d] × RSC) × BW (kg) × 1,000 (μg/mg)

 (FCR (kg/d) × BAF (L/kg))

 = 0.21 mg/kg-d × 0.20 × 80.0 kg × 1,000 μg/mg

 (0.022 kg/d × 13 L/kg)

 = 11,748 μg/L

 = 12,000 μg/L (rounded)

7.2 AWQC for Carcinogenic Toxicological Effects

EPA identified no CSF sources through the systematic search described above. (See section 5, 

Hazard Identification and Dose Response.) Therefore, EPA was unable to derive AWQC for 

carcinogenic toxicological effects.

15
2,4-D 94-75-7 

7.3 AWQC Summary

EPA derived the AWQC for 2,4-D using a noncarcinogenic toxicity endpoint. The updated human 

health AWQC for 2,4-D are 1,300 μg/L for consumption of water and organisms and 

12,000 μg/L for consumption of organisms only (Table 2). These updated criteria replace EPA’s 

previously published value (USEPA 1986c; USEPA 2002c). 

Table 2. Summary of EPA’s Previously Recommended (1986) and Updated (2015) Human Health 

AWQC for 2,4-D 

1986 Human Health AWQC 2015 Human Health AWQC

Water and Organism 100 μg/L 1,300 μg/L

Organism Only --- 12,000 μg/L

These AWQC are intended to be protective of the general adult population from 

noncarcinogenic effects due to chronic (up to a lifetime) exposure to 2,4-D from ingesting water 

and/or consuming fish and shellfish from inland and nearshore waters.

8 Criteria Characterization

The updated 2015 human health AWQC for 2,4-D take into account current data on health 

effects and exposure input parameters, consistent with the 2000 Methodology (USEPA 2000a). 

The following paragraphs describe the individual influence of each of the revised inputs and 

exposure assumptions on the overall change in value.

Body Weight

EPA’s updated AWQC assume a higher BW compared to the previously recommended 2002 

criteria, reflecting a recent rise in average adult BW among the U.S. population. The updated 

BW assumption of 80.0 kg, based on recent survey data from the 1999−2006 NHANES data, is 

10 kg greater than the previous assumption of 70 kg. Assuming all other input parameters 

remain constant, a higher average BW in the AWQC calculations (Eqs. 1 and 2 above) results in 

higher AWQC. That is, as BW increases, the level of a contaminant in water at or below which 

negative health effects are not anticipated from a lifetime of exposure also increases.

Drinking Water Intake

The updated DI assumption is 2.4 L/d, which is higher than the previously recommended rate of 

2 L/d. Assuming all other input parameters remain constant, a higher DI assumption in the 

AWQC calculations (Eqs. 1 and 2 above) results in lower AWQC. That is, as DI increases, and 

thus overall exposure increases, the level of a contaminant in water at or below which negative 

health effects are not anticipated from a lifetime of exposure decreases.

16
2,4-D 94-75-7

Fish Consumption Rate

The updated FCR for fish and shellfish from inland and nearshore waters is 22.0 g/d; the 

TL-specific FCRs are 7.6 g/d, 8.6 g/d, and 5.1 g/d for TLs 2, 3, and 4, respectively. The previously 

recommended FCR was 17.5 g/d. Assuming all other input parameters remain constant, a 

higher FCR assumption in the AWQC calculations (Eqs. 1 and 2 above) results in lower AWQC. 

That is, as fish consumption increases, and thus overall exposure increases, the level of a 

contaminant in water at or below which negative health effects are not anticipated from a 

lifetime of exposure decreases.

Bioaccumulation Factor

The national BAF used in the updated AWQC (Eqs. 1 and 2 above) is 13 L/kg wet-weight. This 

BAF was derived using EPA’s 2000 Methodology (USEPA 2000a) and its Technical Support 

Document, Volume 2 (USEPA 2003a). The previous criterion for 2,4-D was derived prior to the 

current methodology and did not take bioaccumulation and fish consumption into 

consideration in its development (USEPA 1976; USEPA 2002b).

As an additional line of evidence, EPA used model-estimated BAFs from the Estimation Program 

Interface (EPI) Suite (USEPA 2012) to support field-measured or predicted BAFs developed using 

the four methods described above. The BCFBAF program within EPI Suite estimates fish BAFs by 

using Kow and biotransformation data from a model designed by Arnot and Gobas (2003). The 

model includes mechanistic processes for bioaccumulation, such as chemical uptake from the 

water at the gill surface and from the diet, chemical elimination at the gill surface, fecal 

egestion, growth dilution, and metabolic biotransformation. Other processes included in the 

calculations are bioavailability in the water column (only the freely dissolved fraction can 

bioconcentrate) and absorption efficiencies at the gill and in the gastrointestinal tract. The 

model requires the Kow of the chemical and the normalized whole-body metabolic 

biotransformation rate constant as input parameters to predict BAF values. The EPI Suite model 

estimates are as follows:

TL2 = 39.49 L/kg

TL3 = 44.92 L/kg

TL4 = 68.75 L/kg

The utilization of a national BAF rather than a BCF better represents the amount of a 

contaminant accumulating in an organism because it accounts not only for the organism’s 

exposure to the pollutant in the water column, but also from the food chain and surrounding 

environment as well as biotransformation of the pollutant in the organism due to metabolic 

processes.

Reference Dose

EPA selected an RfD of 0.21 mg/kg-d for 2,4-D based on EPA OPP registration documents 

(USEPA 2013a; USEPA 2014c). This RfD replaces the previous value of 0.01 mg/kg-d (USEPA 

1986c; USEPA 2002c). EPA used the RfD of 0.21 mg/kg-d to derive AWQC for noncarcinogenic 

17
2,4-D 94-75-7

effects. Assuming all other input parameters remain constant, an increase in the RfD in the 

AWQC calculations (Eqs. 1 and 2) results in higher AWQC.

Cancer Slope Factor

EPA did not select a CSF for 2,4-D and therefore did not derive AWQC for carcinogenic effects.

EPA did not derive AWQC for carcinogenic effects of 2,4-D in its previous criteria update (USEPA 

1986c; USEPA 2002c). 

Relative Source Contribution

An RSC of 20 percent is included in the AWQC calculation. This is the same RSC used in the 

previous AWQC (USEPA 1986c; USEPA 2002c).

9 Chemical Name and Synonyms

• Chlorophenoxy Herbicide (2,4-D) (CAS Number 94-75-7)

• Acetic Acid, (2,4-Dichlorophenoxy)-

• Acide 2,4-Dichloro Phenoxyacetique

• Acido(2,4-Dicloro-Fenossi)-Acetico

• Agrotect

• Amidox

• Amoxone

• Aqua-Kleen

• Bh 2,4-D

• Brush-Rhap

• B-Selektonon

• Chloroxone

• Crop Rider

• Crotilin

• 2,4-D

• D 50

• Dacamine

• 2,4-D Acid

• Debroussaillant 600

• Decamine

• Ded-Weed Lv-69

• Desormone

• (2,4-Dichloor-Fenoxy)-Azijnzuur

• Dichlorophenoxyacetic Acid

• 2,4-Dichlorophenoxyacetic Acid

• Dichlorophenoxyacetic Acid, 2,4-

• 2,4-Dichlorophenoxyacetic Acid

• (2,4-Dichlor-Phenoxy)-Essigsaeure

18
2,4

-D 94-75-7

• Dicopur • Dicotox • Dinoxol • DMA-4 • Dormone • 2,4-Dwuchlorofenoksyoctowy Kwas • Emulsamine Bk • Emulsamine E-3 • ENT 8,538 • Envert 171 • Envert DT • Esteron • Esterone Four • Estone • Farmco • Fernesta • Fernimine • Fernoxone • Ferxone • Foredex 75 • Formula 40 • Hedonal • Herbidal • Ipaner • Krotiline • Lawn-Keep • Macrondray • Miracle • Monosan • Moxone • NA 2765 • Netagrone • Netagrone 600 • NSC 423 • Pennamine • Pennamine D • Phenox • Pielik • Planotox • Plantgard • RCRA Waste Number U240 • Rhodia

19
2,4-D 94-75-7

• Salvo

• Spritz-Hormin/2,4-D

• Spritz-Hormit/2,4-D

• Transamine

• Tributon

• Trinoxol

• U 46DP

• U-5043

• Vergemaster

• Vidon 638

• Visko-Rhap

• Weed-Ag-Bar

• Weedar-64

• Weedatul

• Weed-B-Gon

• Weedez Wonder Bar

• Weedone LV4

• Weed-Rhap

• Weed Tox

• Weedtrol

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