UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

DATE: 10/3112013 

SUBJECT: 2,4-DB: Human Health Risk Assessment Seeping Document in Support of 

Registration Review 

PC Code: 030801 DP Barcode: D41 0363 

MRID No.: Not Applicable 

Petition No.: Not Applicable 

Registration No.: Not Applicable 

Regulatory Action: Registration Review 

Seeping 

Assessment Type: Single Chemical, 

Non-Aggregate 

Reregistration Case No.: Not Applicable 

TXR No.: Not Applicable CAS No.: 94-82-6 

FROM: 

THROUGH: 

Wade Britton, MPH, Risk Assesso nd ORE 

Linda Tayl?r, Ph.D.: Toxicologisf r JL t J/ 

Peter Savma, Chemist ~ 1\ { 

Risk Assessment Branch V /VII 

Health Effects Division (HED; 7~09P) 

Michael Metzger, Branch Chi*e ( /} / } (} ) . A _ Risk 

Health 

Assessment 

Effects Division 

Branch 

(7509 

VI VI 

) 

cz.{ M , -/ V ~ 

TO: Jill Bloom, Chemical Review Manager 

Risk Management and Implementation Branch II (RMIB II) 

Pesticide Re-Evaluation Division (PRD; 7508P) 

Attached is the Health Effects Division's (HED) human health risk assessment seeping 

document for 2,4-DB to support the registration review process. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 2 of 22

Executive Summary 

HED has considered the available risk assessments for 2,4-DB and its toxicity, exposure and 

usage databases, as well as the most current Agency science and assessment policies. HED 

performed this evaluation in order to determine the scope of work necessary to support the 

established tolerances and existing registrations during registration review. The primary source 

of information for this evaluation was the most recent 2,4-DB human health risk assessment 

(HHRA) 1 conducted to support the January 2005 USEPA registration eligibility decision (RED) 

for 2,4-DB2

. 

2,4-DB [4-(2,4-dichlorophenoxy) butyric acid] is a plant growth regulator and herbicide 

registered for use on alfalfa, clover, peanuts, soybean, peppermint, spearmint, and trefoil. It is 

manufactured as the acid and the dimethylamine salt; no other salt or ester products are currently 

registered. End-use products are formulated either as soluble, emulsifiable, or flowable 

concentrates. 2,4-DB can be applied either as a broadcast application early season or a directed 

spray late season by aerial or ground equipment. Maximum label application rates for food/feed 

crops range from 0.25 lb ae (acid equivalents)/A to 1.5 lb ae/A. There are no registered 

residential uses of 2,4-DB. 

Because of similarities in metabolism and degradation in animals, plants, and the environment, 

2,4-DB and 2,4-DB-DMAS [4-(2,4-dichlorophenoxy)butyric acid dimethylamine salt], the 

dimethylamine salt form of 2,4-DB, were considered equivalent by HED in the 2004 HHRA. 

For the purpose of this document, 2,4-DB and 2,4-DB-DMAS are also considered equivalent, 

unless otherwise noted. 

The toxicity database for 2,4-DB and 2,4-DB-DMAS is incomplete, but sufficient for 

characterization of potential developmental, reproductive, carcinogenetic, mutagenic, neurotoxic, 

and immunotoxic effects. HED recommends that an inhalation toxicity study (870.3465) be 

required for the registration review of 2,4-DB. Once these data have been received and 

reviewed, their impact on the endpoints, points of departure (PODs), and uncertainty 

factors/safety factors (UF/SFs) for the inhalation exposure risk assessment will be evaluated. 

Further, the need for a dermal risk assessment and the endpoint selection for the chronic dietary 

risk assessment may need to be reevaluated, based on updated policies and procedures.

The major target organs for 2,4-DB and 2,4-DB-DMAS are the kidney and liver. Developmental 

toxicity was not observed in the rabbit but was observed in the rat at the same dose level where 

maternal mortality was observed. Reproductive toxicity was not observed in the rat. 2,4-DB is 

classified as “not likely to be a human carcinogen.” 2,4-DB does not appear to be a frankly 

neurotoxic chemical, and there is no concern for immunotoxicity. 2,4-DB and 2,4-DB-DMAS 

are not acutely toxic by the oral, dermal, and inhalation routes of exposure based on lethality 

studies. Neither chemical is a skin irritant; 2,4-DB is a mild eye irritant, but 2,4-DB-DMAS is a 

severe eye irritant and both are negative for dermal sensitization. 

 

1

 2,4-DB and 2,4-DB-DMA Human Health Risk Assessment. K. Farwell. 6/20/2004. D290486. 

2 http://www.epa.gov/opp00001/reregistration/24d/
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 3 of 22

The 2,4-DB residue chemistry database is incomplete and considered inadequate to support the 

registration review process. In order to address residue chemistry data deficiencies for 2,4-DB, 

HED recommends that additional geographically representative field trial data be required for 

clover forage and hay. Acute and chronic dietary risk assessments3

 were last performed to 

support the 2005 RED. There are currently no acute or chronic dietary (food and water) risks of 

concern for 2,4-DB. During registration review, updated dietary exposure assessments may be 

conducted if there are any significant changes to the toxicological PODs or FQPA and other UFs, 

or if clover hay and forage geographic distribution residue data are submitted. In addition, any 

significant changes to the estimated drinking water concentrations (EDWCs) may also 

necessitate changes to the dietary assessment. 

While there are currently no registered residential uses of 2,4-DB, residential bystander 

exposures resulting from off-site transport (i.e., spray drift or volatilization) may occur as a result 

of occupational applications of 2,4-DB. The need for a bystander risk assessment for 2,4-DB 

will be considered during registration review. 

The occupational exposure database is adequate to support the registration review process for 

2,4-DB. The previous occupational exposure assessment4

 covered all exposure scenarios 

relevant to 2,4-DB and no occupational handler or post-application risks of concern were 

identified with use of appropriate personal protective equipment (PPE). An updated 

occupational handler assessment may be performed if warranted by significant changes to the 

current scenario-specific surrogate handler-exposure data, PODs, or UFs. 

Chemical Use Profile 

2,4-DB is a plant growth regulator and herbicide used for the control of broadleaf weeds. It is 

registered for use on alfalfa, clover, peanuts, soybean, peppermint, spearmint, and trefoil with 

permanent tolerances established under 40 CFR §180.331(a). There are no residential uses 

registered for 2,4-DB. End-use products are formulated either as soluble, emulsifiable, or 

flowable concentrates. 2,4-DB can be applied either as a broadcast application early season, or a 

directed spray late season aerially or by ground equipment. 

The nomenclature and physiochemical properties of 2,4-DB can be found in Attachment 1. 

Hazard Identification/Toxicology 

2,4-DB and 2,4-DB-DMAS are members of the dichlorophenoxy class of compounds that 

function by mimicking the action of auxins, plant growth hormones. 

The toxicity database for 2,4-DB and 2,4-DB-DMAS is incomplete, but sufficient for 

characterization of potential developmental, reproductive, carcinogenetic, mutagenic, neurotoxic, 

and immunotoxic effects. In rats, 2,4-DB is rapidly absorbed orally and eliminated mainly in the 

 

3

 2,4-DB Acute and Chronic Dietary Exposure Assessments for the Reregistration Eligibility Decision. F. Fort. 

7/13/2004. D291221. 

4

 2,4-DB: Occupational and Residential Exposure and Risk Assessment for the Reregistration Eligibility Decision 

(RED) Document. T. Dole. 5/3/2004. D300629 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 4 of 22

urine. 2,4-DB-DMAS dissociates to 2,4-DB, after which they share the same metabolic 

pathway. 2,4-D and conjugated dichlorophenol are the major metabolites for both chemicals. 

Although data specific to 2,4-DB-DMAS are limited to a rat subchronic oral toxicity study, a rat 

developmental toxicity study, a subchronic dermal toxicity study in rabbits, and a mutagenicity 

battery, the toxicity and effect dose levels for both chemicals are generally similar. 

The major target organs for 2,4-DB and 2,4-DB-DMAS are the kidney and liver. Kidney 

toxicity is manifested as decreased kidney weights and increased incidence of non-neoplastic 

pathology (tubular degeneration/infarcts) in rats, and increased kidney weight in dogs. Liver 

toxicity is manifested as increased liver weights, increased liver enzymes, and hepatocellular 

hypertrophy in rats and dogs. The effects are consistent across time and species and do not 

appear to progress over time. Although absorption and distribution of chlorophenoxy herbicides 

and other organic acids generally are similar across all mammalian species evaluated, the 

decreased capacity of the dog to eliminate organic acids results in higher blood levels and a 

longer elimination half-life relative to those found in the rat and other mammalian species. 

Consequently, effects are seen at lower dose levels in the dog than in the rat. Because of this 

difference, the rat is a better predictor of the potential toxicity of 2,4-DB and 2,4-DB-DMAS in 

humans than the dog, and doses and endpoints from rat studies were used in all 2,4-DB risk 

assessments. 

Developmental toxicity was not observed in the rabbit but was observed in the rat at the same 

dose level where maternal mortality was observed. Reproductive toxicity was not observed in 

the rat. The 10X FQPA SF for the protection of infants and children has been reduced to 1X for 

the following reasons: (1) There is no increased qualitative or quantitative susceptibility in the 

rat or rabbit developmental toxicity studies; (2) the doses and endpoints selected for risk 

assessment are protective of the qualitative susceptibility observed in the rat reproduction study; 

(3) the NOAEL and LOAEL from the rat reproduction study are well characterized; and (4) the 

databases are considered to be complete for the purpose of risk assessment. 

2,4-DB is classified as “not likely to be a human carcinogen” not requiring a quantitative cancer 

risk assessment. This is based on the lack of evidence of carcinogenicity in mice and rats, and 

there is no concern for mutagenicity. 2,4-DB does not appear to be a frankly neurotoxic 

chemical, and there is no concern for immunotoxicity. 2,4-DB and 2,4-DB-DMAS are not 

acutely toxic by the oral, dermal, and inhalation routes of exposure based on lethality studies. 

Neither chemical is a skin irritant; 2,4-DB is a mild eye irritant, but 2,4-DB-DMAS is a severe 

eye irritant and both are negative for dermal sensitization. 

Acute and subchronic neurotoxicity studies in rats and an immunotoxicity study, which are 

required in accordance with new data requirements for conventional pesticide registration (40 

CFR Part 158), have not been submitted. However, based on a weight-of-evidence (WOE) 

approach, considering all the available 2,4-DB hazard and exposure information, the HED 

Hazard and Science Policy Council (HASPOC) concluded that the acute and subchronic 

neurotoxicity studies and the immunotoxicity study are not needed for 2,4-DB at this time5

. 

 

5

 2,4-DB/2,4-DB-DMA: Summary of Hazard and Science Policy Council (HASPOC) Meeting of July 25, 2013: 

Recommendations on the Requirement of a Subchronic Inhalation Study, an Immunotoxicity Study, and the Acute 

and Subchronic Neurotoxicity Studies for 2,4-DB/2,4-DB-DMA. U. Habiba. 8/13/2013. TXR No. 0056727. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 5 of 22

Based on current policies, HED recommends that a repeat dose inhalation toxicity study in rats is 

required due to the potential for repeated inhalation exposure to aerosolized formulations of 2,4-

DB and 2,4-DB-DMAS. The toxicity study should be conducted using aerosolized 2,4-DBDMAS, the more caustic of the two forms. The HASPOC determined that a 28-day inhalation 

toxicity study is needed to better estimate risk for occupational inhalation exposure. 

The acute, subchronic, and chronic toxicity profile tables for 2,4-DB and 2,4-DB-DMAS are 

included in Attachment 2. Toxicity endpoints, PODs, and UF/SFs were selected for oral, dermal, 

and inhalation exposure pathways for the Human Health Risk Assessment conducted in 2004 

(D290486), and these are shown in Attachment 3. 

Upon receipt and review of the outstanding toxicology data and consideration of all available 

data, the endpoints for risk assessment and all uncertainty factors will be re-evaluated during 

registration review to reflect updated data and uncertainty factor policies, and a new risk 

assessment done, if necessary. 

Conclusions: The toxicity data base for 2,4-DB and 2,4-DB-DMAS is considered incomplete, 

but is adequate for assessing potential carcinogenetic, mutagenic, developmental, neurotoxic, 

immunotoxic, and reproductive effects. An inhalation toxicity study (870.3465) is anticipated to 

be required for registration review. Once those data have been received and reviewed, their 

impact on the endpoints, PODs, and UF/SFs for the inhalation exposure risk assessment will 

need to be evaluated. During registration review, the need for a dermal risk assessment and the 

endpoint selection for the chronic dietary risk assessment may need to be reevaluated, based on 

updated policies and procedures.

Dietary Exposure 

Residue Chemistry

For 2,4-DB, the nature of the residue is adequately understood in both plants and livestock based 

on the available metabolism and residue studies. The HED Metabolism committee concluded6,7

that the residue of concern for tolerance expression and risk assessment is 2,4-DB per se, and that 

2,4-D need not be included. In maintaining consistency with 2,4-D, tolerances, limits established 

for residues in 2,4-DB should be expressed as parent, including both free and conjugated, 

determined as the acid. 

The following tolerances were reassessed and recommended for the 2005 2,4-DB RED: alfalfa, 

forage (0.70 ppm); alfalfa, hay (2.0 ppm); clover, forage and clover, hay (insufficient data 

available); peppermint, tops and spearmint, tops (0.20 ppm); soybean, seed (0.50 ppm), soybean, 

forage (0.70 ppm), soybean, hay (2.0 ppm); peanut (0.05 ppm); trefoil, forage (0.70 ppm) and 

trefoil, hay (2.0 ppm). The RED reaffirmed that the appropriate tolerance for 2,4-DB is 0.05 ppm 

(LOQ) in the meat byproducts of cattle, goats, hogs, horses, and sheep. The 2,4-DB ruminant and 

poultry feeding studies that were performed bore residue levels in livestock commodities below 

 

6

 2,4-DB: Metabolism Committee Question Re: Regulation of Metabolites in Plant and Animal Commodities. D. 

Miller. 11/14/1995. D220888. 

7

 2,4-DB: Results of the 1/16/96 Meeting of the HED Metabolism Committee. D. Miller. 01/29/96. D221954. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 6 of 22

the level of quantitation (0.05 ppm for all tissues and 0.01 ppm for eggs and milk) at all the 

feeding rates tested8

. There is no reasonable expectation of the transfer of residues of 2,4-DB 

from feedstuffs to livestock meat, fat or milk; therefore, the current use of 2,4-DB with respect to 

these commodities should be classified as Category 3 under 40 CFR 180.6(a), and tolerances for 

residues of 2,4-DB in milk and in meat and fat of cattle, goats, hogs, horses, and sheep are not 

required. 

Adequate tolerance enforcement methods are currently available. 2,4-DB is completely 

recovered (>80%) by FDA MultiResidue Test Method 402 (PAM Vol I, updated 10/97). The 

Pesticide Analytical Manual (PAM) Vol. II, lists Method I, a GC method with microcoulometric 

detection, for the enforcement of tolerances for 2,4-DB residues; this method is the PAM Vol. I 

method for chlorophenoxy acid residues in food. 

The 2,4-DB residue chemistry database is incomplete. Residue chemistry deficiencies were 

identified in the 2004 HHRA including revised labels to prohibit the grazing and feeding of 

soybean forage and hay; the development of plant and livestock tolerance enforcement methods; 

additional clover forage and hay field trials; and the replenishment of reference standards. 

Since the time of the 2004 HHRA, the majority of the residue chemistry deficiencies have been 

addressed. Label changes required to prohibit the grazing of soybean forage or the feeding of 

soybean hay within 60 days of application and the replenishment of standards have been 

completed. Further, HED determined that the plant and livestock tolerance enforcement method 

deficiency is satisfied because 2,4-DB is completely recovered by FDA Multiresidue Test 

Method 402 (PAM Vol I) for enforcement, and PAM Vol II lists a GC method with 

microcoulometric detection for confirmation. Finally, reference materials have been replenished 

and are valid through 9/22/2015. 

Additional data are needed to support established tolerances for clover forage and hay. HED 

recommends these data be required because the geographic distribution of the trials represented 

in the previously submitted data is inadequate. OCSPP Guidelines call for trials from Region 1 

(1 trial), Region 2 (1), Region 4 (1), Region 5 (3), Region 6 (1), Region 7 (1), Region 8 (1), 

Region 9 (1), Region 10 (1), and Region 11 (1). The data which have been provided thus far 

only address two geographically similar regions. As a result, ten additional trials are 

recommended from Region 1 (1), Region 2 (1), Region 4 (1), Region 5 (3), Region 6 (1), Region 

7 (1), Region 8 (1), and Region 9 (1), at the maximum labeled rate of 1.3 lb ae/A with a PHI of 

60 days. 

Food Exposures

Current tolerance levels are established at 0.2 ppm based on negligible residues found in alfalfa, 

clover, mint, peanut, trefoil, soybean, and soybean hay. As explained above, no tolerances for 

livestock commodities or food/feed processed commodities have been established. 

 

8

 2,4-DB [4-(2,4-cholorophenoxy) butyric acid] and 2,4-DB dimethylamine salt (PC Codes030801, 030819): 

Revised Product Chemistry and Residue Chemistry Summary Documents for the Registration Eligibility Decision 

Document (RED). D. Drew. 7/19/2004. D291212. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 7 of 22

Acute and chronic dietary risk assessments were last performed to support the 2005 RED for 2,4-

DB. The assessments were conducted using the Dietary Exposure Evaluation Model (DEEMFCID) Version 1.30. This model used food consumption data for assessment from the 

USDA’s Continuing Surveys of Food Intakes by Individuals (CSFII) from 1994-1996 and 1998. 

An unrefined (tolerance level and 100% crop treated) acute and chronic dietary risk assessment 

was conducted for all supported 2,4-DB food uses. Acute dietary risk estimates were determined 

for females 13 to 49 years old since this is the only population subgroup for which an acute 

dietary endpoint was identified. For 2,4-DB there is no endpoint attributable to a single dose 

from oral toxicity studies for acute dietary risk assessment for the general population including 

infants and children. Therefore, only females 13-50 years of age were determined to have an 

endpoint for acute dietary risk assessment based upon the supporting rat developmental toxicity 

study that was performed. 

Risk estimates at the 95th percentile of exposure were <1% of the aPAD and, therefore, are not of 

concern. Chronic dietary risk estimates were also determined to be below the 100% threshold 

level of concern for the general U.S. population (<1% of the cPAD) and all its population 

subgroups. The most highly exposed population subgroup was found to be all infants <1 year 

old with an exposure of 2.2% of the cPAD. 

Drinking Water Exposures

Drinking water exposures can potentially occur through run-off resulting from the outdoor 

applications of 2,4-DB made for controlling broadleaf weeds. To assess this potential exposure 

for the 2005 RED, highly conservative estimated drinking water concentrations (EDWCs) were 

generated using PRZM/EXAMS modeling for surface water and SCI-GROW modeling for 

groundwater9. No degradation products of 2,4-DB were included in this assessment. 

For the 2005 RED, EDWCs were not directly incorporated into DEEM in the dietary assessment. 

At the time, HED was instead calculating estimated drinking water levels of comparison 

(DWLOCs) and comparing them to EDWCs to determine whether or not the risk estimates were 

of concern. The DWLOC is the highest concentration of 2,4-DB that could occur in drinking 

water resulting in risk estimates below the level of concern considering the exposure from food 

sources. The resulting surface and ground water acute EDWCs of 319 ppb and 0.51 ppb were 

found to be below the calculated acute DWLOC of 18,000 ppb for females 13-49 years old, and 

are therefore not of concern. Chronic EDWCs of 72 ppb and 0.51 ppb for surface and 

groundwater were likewise found to be below the chronic DWLOCs of 1,050 ppb for the general 

U.S. population and 290 ppb for all infants < 1 year old, and are therefore not of concern. 

A new dietary assessment may be necessary if new drinking water estimates are provided, or if 

changes occur to the risk assessment endpoints, points of departure, or uncertainty factors. 

Conclusions: The 2,4-DB residue chemistry database is incomplete and inadequate to support 

registration review due to the need for obtaining additional geographically representative field 

 

9

 2,4-DB: Drinking Water Assessment for the Health Effects Division (HED) Reregistration Eligibility Decision 

Document. A. Al-Mudallal, Chemist. 7/8/2003. D279637. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 8 of 22

trial data for clover forage and hay. During registration review, updated dietary exposure 

assessments may be conducted upon submission of the additional clover forage and hay data, 

and/or if there are any significant changes to the toxicological PODs or FQPA and other UFs. In 

addition, any significant changes to the EDWCs may also necessitate changes to the dietary 

assessment. 

Residential (Non-Occupational) Exposure 

There are no registered residential uses of 2,4-DB and, therefore, a quantitative exposure/risk 

assessment for direct residential applications is not necessary during registration review. 

Residential Bystander Exposures 

Residential exposures resulting from off-site transport (i.e., spray drift or volatilization) may 

occur as a result of agricultural applications of 2,4-DB. The Agency is in the process of 

evaluating these types of exposures and may, as appropriate, develop policies and procedures to 

identify the need for and, subsequently, the way to incorporate these post-application exposures 

into the Agency's risk assessments. The need for spray drift and volatilization risk assessment 

for 2,4-DB will be examined during Registration Review.

Conclusions: There are no registered residential uses of 2,4-DB and, therefore, a quantitative 

exposure/risk assessment for direct residential application is not necessary during registration 

review. The potential for residential exposures resulting from off-site transport (e.g., spray drift 

or volatilization) may need to be quantitatively evaluated for 2,4-DB during the registration 

review process. 

Aggregate Risk Assessment 

In accordance with the Food Quality Protection Act (FQPA), HED must consider and aggregate 

pesticide exposures and risks from three major sources: food, drinking water, and residential 

exposures. There are no registered residential uses for 2,4-DB; therefore, a quantitative 

aggregate assessment for direct residential applications is not necessary during registration 

review. The acute and chronic aggregate risk assessments for 2,4-DB included food and drinking 

water only. 

Acute 

The 2004 HHRA acute aggregate risk assessment addressed exposure to 2,4-DB residues in food 

and water using the DWLOC approach described above. The resulting surface and ground water 

acute EDWCs of 319 ppb and 0.51 ppb were found to be below the calculated acute DWLOC of 

18,000 ppb for females 13-49 years old, and are therefore not of concern. 

Chronic 

Chronic DWLOCs were calculated based on chronic dietary exposures to 2,4-DB in food and 

water. The most highly exposure population subgroup was all infants < 1 year old. Chronic 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 9 of 22

EDWCs of 72 ppb and 0.51 for surface and groundwater were likewise found to be below the 

chronic DWLOCs of 1,050 ppb for the general U.S. population and 290 ppb for all infants < 1 

year old, and are therefore not of concern. 

Conclusions: In 2004 there were no aggregate (food and water) risk estimates of concern for 

2,4-DB. During registration review, updated aggregate assessments may be conducted as 

required to include any significant change that might occur for toxicological PODs, FQPA and 

uncertainty factors, dietary exposure estimates, or EDWCs. Emerging policies related to 

pesticide drift and volatilization assessments may dictate further changes to the existing 

aggregate risk assessment. 

Occupational Exposure 

Occupational Handler Exposure 

The active ingredient 2,4-DB, including its dimethylamine salt, 2,4-DB DMAS, are registered 

for use on alfalfa, peanuts, mint, and soybeans with one or two applications allowed per growing 

season. 2,4-DB is formulated as soluble, emulsifiable, or flowable concentrates. 2,4-DB can be 

applied either as a broadcast application early season, or a directed spray late season aerially or 

by ground equipment resulting in the potential for occupational exposures. 

Occupational handler exposure scenarios include: mixing/loading all formulations; applying 

formulations by aerial or groundboom equipment; and flagging. Occupational handler exposures 

are expected to be short- and intermediate-term in duration. Due to the lack of dermal hazard for 

short-term exposures to 2,4-DB, only inhalation exposures were assessed for this exposure 

duration. Both dermal and inhalation exposures were assessed for intermediate-term risks and 

these exposures were combined because the endpoint is based on the same study. 

As previously assessed for the 2004 RED, short-term inhalation and intermediate-term 

(combined dermal and inhalation) occupational handler exposure/risks were not of concern to the 

Agency (i.e. an MOE ≥ 100) at some level of PPE. 

Recommended unit exposures for occupational handler assessments have generally been derived 

from the Pesticide Handlers Exposure Database (PHED). However, recently both the Outdoor 

Residential Exposure Task Force (ORETF) and the Agricultural Handler Exposure Task Force 

(AHETF) have produced pesticide handler exposure monitoring data that the Agency is using in 

place of PHED (http://www.epa.gov/pesticides/science/handler-exposure-data.html). As more 

reliable data become available, the Agency will continue to replace existing data. Reviewers will 

incorporate new exposure monitoring data, as necessary, during registration review. 

Conclusions: An updated occupational handler assessment may be required under registration 

review if warranted to reflect the most recent occupational handler surrogate exposure data10 and 

by any significant change in exposure assessment methodology, toxicological PODs, or UFs. 

 

10 Outlined in the “Occupational Pesticide Handler Unit Exposure Surrogate Reference Table” available at 

http://www.epa.gov/pesticides/science/handler-exposure-data.html
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 10 of 22

Further, the need for a dermal exposure/risk assessment may be reevaluated during registration 

review. 

Occupational Post-application Exposure 

Occupational post-application dermal exposures can occur from post-emergent, broadcast foliar 

uses of 2,4-DB. 2,4-DB is typically applied once or twice per season and, therefore, it is 

anticipated that exposures are primarily short-term in nature; however, because there is no hazard 

for short-term dermal exposures these were not assessed for the 2004 RED. Although 

considered unlikely, intermediate-term post-application dermal exposure/risks were assessed to 

address exposures over this time course. The intermediate-term dermal POD was used to 

conduct this assessment. Due to the conservative nature of the assessment of intermediate-term 

post-application exposures to 2,4-DB, average (typical) application rates were used. Chemicalspecific dislodgeable foliar residue (DFR) data were not available for 2,4-DB; default 

dislodgeable residue inputs were used for foliar post-application exposure scenarios. 

As assessed for the 2004 RED, intermediate-term post-application dermal exposures were not of 

concern on the day of application (Day 0). 

Recommended transfer coefficients for occupational post-application assessments are derived 

from Agricultural Reentry Task Force (ARTF) exposure monitoring studies 

(http://www.epa.gov/pesticides/science/post-app-exposure-data.html). These transfer 

coefficients were updated in 2012 based on a reanalysis and submission of new monitoring data. 

Reviewers will incorporate new exposure monitoring data, as necessary, during registration 

review. 

In accordance with the updated Part 158 data requirements (2007), one or more DFR studies are 

required when a pesticide has residential or occupational uses that could result in postapplication dermal exposure. As part of the recent revision to the Health Effects Division’s 2012 

Standard Operating Procedures for Residential Pesticide Exposure Assessment, HED analyzed a 

number of DFR studies and selected a new default value for the fraction of the application rate 

available to be dislodged after a foliar application (FAR). This default value is 25% (previously 

20%). Of the analyzed DFR studies, the maximum FAR value seen was 89% or 3.6 times higher 

than the default residue transfer value. Therefore, a calculated MOE of approximately 4 times 

higher than the LOC, i.e. an MOE > 400 if the LOC = 100, using the default dislodged residue 

value should provide an adequate margin of safety for any potentially higher residues seen in a 

chemical-specific DFR study (Guidance for Requiring/Waiving Turf Transferrable Residue 

(TTR) and Dislodgeable Foliar Residue (DFR) Studies. 6/7/2012, Exposure Science Advisory 

Council). In a screening level assessment, some of the estimated dermal post-application MOEs 

for 2,4-DB were less than 4 times the LOC (< 400). It should be noted, however, that the 

assessment of intermediate-term post-application exposures to 2,4-DB was highly conservative 

for the following reasons: intermediate-term post-application exposure is unlikely given that 

2,4-DB is applied only once or twice per season; and the use of an intermediate-term POD from 

a subchronic oral study to assess dermal exposures, especially given the lack of short-term 

dermal hazard. Due to the conservative nature of the post-application exposure assessment and 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 11 of 22

lack of risks of concern (i.e., MOEs are > 100) using the default DFR data, chemical specific 

DFR data are not being required at this time. 

Restricted Entry Interval (REI)

2,4-DB is Acute Toxicity Category III or IV for acute dermal, inhalation, and oral toxicity, and 

primary skin irritation. 2-4-DB is Acute Toxicity Category IV for primary eye irritation. 

However, 2,4-DB-DMAS is Acute Toxicity Category I for eye irritation. Therefore, regardless 

of the lack of occupational post-application exposures/risks of concern on Day 0, a 48 hour REI 

is required under the Worker Protection Standard (WPS). 

Conclusions: The occupational exposure database is complete for 2,4-DB. There were no 

occupational handler or post-application exposure estimates of concern. An updated 

occupational handler or post-application exposure assessment may be required under registration 

review if warranted by any significant change in exposure assessment methodology, 

toxicological PODs, or UFs. The need for occupational dermal exposure/risk assessment may be 

reevaluated during registration review. 

Public Health and Pesticide Epidemiology Data 

The Agency has reviewed the HED has evaluated the available incident data11 and concludes that 

based on the low frequency and severity of incident cases reported for 2,4-DB, there does not 

appear to be a concern at this time that would warrant further investigation. The Agency will 

continue to monitor the incident information and if a concern is triggered, additional analysis 

will be included in the risk assessment. 

Tolerance Assessment and International Harmonization 

Permanent tolerances are established under 40 CFR §180.331(a) for the registered uses of 2,4-

DB on alfalfa, clover, peanuts, soybean, peppermint, spearmint, and trefoil. There are currently 

no Mexican, Canadian or Codex MRLs established for 2,4-DB on any crop or livestock 

commodities and, as a result, international harmonization of maximum residue limits is not 

currently required. 

Environmental Justice 

Potential areas of environmental justice concerns, to the extent possible, were considered in the 

most recent human health risk assessment, in accordance with U.S. Executive Order 12898, 

"Federal Actions to Address Environmental Justice in Minority Populations and Low-Income 

Populations," http://www.epa.gov/fedreg/eo/eo12898.htm. The Office of Pesticide Programs 

(OPP) typically considers the highest potential exposures from the legal use of a pesticide when 

conducting human health risk assessments, including, but not limited to, people who obtain 

drinking water from sources near agricultural areas, the variability of diets within the U.S. 

(including different ages, regions, and ethnicities), and people who may be exposed when 

harvesting crops. Should these highest exposures indicate potential risks of concern, OPP further 

 

11 2,4-DB: Tier I Review of Human Incidents: E. Evans and S. Recore. 9/4/2013. D414546 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 12 of 22

refines the risk assessments to ensure that the risk estimates are based on the best available 

information. 

Cumulative Risk Assessment 

The FQPA requires the Agency to consider the cumulative risks of chemicals sharing a common 

mechanism of toxicity. 2,4-DB is a member of the alkylphenoxy herbicide class of pesticides. 

This class also includes MCPA, 2,4-D and 2,4-DP. Although the various phenoxy herbicides 

have similar structural components, HED has not made a determination that they have a common 

mode of toxic action. Therefore, a cumulative risk assessment has not been performed for these 

compounds. 

Human Studies 

Past 2,4-DB risk assessments rely in part on data from studies in which adult human subjects 

were intentionally exposed to a pesticide or other chemical. These data, which include studies 

from Pesticide Handlers Exposure Database Version 1.1 (PHED 1.1); the Agricultural Handler 

Exposure Task Force (AHETF) database; the ARTF database; and the Outdoor Residential 

Exposure Task Force (ORETF) database, are (1) subject to ethics review pursuant to 40 CFR 26, 

(2) have received that review, and (3) are compliant with applicable ethics requirements. For 

certain studies, the ethics review may have included review by the Human Studies Review 

Board. Descriptions of data sources, as well as guidance on their use, can be found at the 

Agency website12. 

Endocrine Disruption 

As required by FIFRA and FFDCA, EPA reviews numerous studies to assess potential adverse 

outcomes from exposure to chemicals. Collectively, these studies include acute, subchronic and 

chronic toxicity, including assessments of carcinogenicity, neurotoxicity, developmental, 

reproductive, and general or systemic toxicity. These studies include endpoints which may be 

susceptible to endocrine influence, including effects on endocrine target organ histopathology, 

organ weights, estrus cyclicity, sexual maturation, fertility, pregnancy rates, reproductive loss, 

and sex ratios in offspring. For ecological hazard assessments, EPA evaluates acute tests and 

chronic studies that assess growth, developmental and reproductive effects in different 

taxonomic groups. As part of its most recent registration decision for 2,4-DB, EPA reviewed 

these data and selected the most sensitive endpoints for relevant risk assessment scenarios from 

the existing hazard database. However, as required by FFDCA section 408(p), 2,4-DB is subject 

to the endocrine screening part of the Endocrine Disruptor Screening Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including pesticide 

active and other ingredients) may have an effect in humans or wildlife similar to an effect 

produced by a “naturally occurring estrogen, or other such endocrine effects as the Administrator 

may designate.” The EDSP employs a two-tiered approach to making the statutorily required 

 

12 http://www.epa.gov/pesticides/science/handler-exposure-data.html and

http://www.epa.gov/opp00001/science/residential-exposure-sop.html
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 13 of 22

determinations. Tier 1 consists of a battery of 11 screening assays to identify the potential of a 

chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or T) hormonal 

systems. Chemicals that go through Tier 1 screening and are found to have the potential to 

interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP where EPA 

will determine which, if any, of the Tier 2 tests are necessary based on the available data. Tier 2 

testing is designed to identify any adverse endocrine-related effects caused by the substance, and 

establish a dose-response relationship between the dose and the E, A, or T effect. 

Under FFDCA section 408(p), the Agency must screen all pesticide chemicals. Between 

October 2009 and February 2010, EPA issued test orders/data call-ins for the first group of 67 

chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. A second list of 

chemicals identified for EDSP screening was published on June 14, 201313 and includes some 

pesticides scheduled for Registration Review and chemicals found in water. For further 

information on the status of the EDSP, the policies and procedures, the lists of chemicals, future 

lists, the test guidelines and the Tier 1 screening battery, please visit our website.14

In the interim, EPA is making no human health or environmental safety findings associated with 

the EDSP screening of 2,4-DB. Before completing this Registration Review, the Agency will 

make an EDSP FFDCA section 408(p) determination. 

Risk Assessment Updates and Data Deficiencies 

During registration review, updated exposure assessments may be conducted if there are any 

significant changes to 2,4-DB toxicological endpoints, PODs, or FQPA and other UFs, and/or 

standard operating procedures, exposure data or assumptions. Data required and submitted 

during registration review will be considered for use in new 2,4-DB human health risk 

assessments. Any significant changes to the EDWCs may necessitate changes to the dietary 

exposure assessment. Further, the need for a residential bystander risk assessment of exposure 

resulting from off-site transport (i.e., spray drift or volatilization) will be considered during 

registration review. 

The following data deficiencies were identified for 2,4-DB: 

Toxicology: 870.3465: 90-day Inhalation study15

Residue Chemistry: 860.1500: Ten additional field trials on clover forage and hay are required 

as recommended in the following Regions: 1 (Region 1), 1 (Region 2), 1 (Region 4), 3 (Region 

5), 1 (Region 6), 1 (Region 7), 1 (Region 8), and 1 (Region 9). 

 

13 See http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0477-0074 for the final second list of 

chemicals. 

14 http://www.epa.gov/endo/ 15 28-day inhalation study will be sufficient. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 14 of 22

References 

Table 1. Memoranda Relevant to Registration Review1.

Author Barcode Date Title

K. Farwell D290486 6/20/2004 2,4-DB and 2,4-DB-DMA Human Health Risk Assessment 

E. Evans and S. 

Recore D414546 9/4/2013 2,4-DB: Tier I Review of Human Incidents 

F. Fort D291221 7/13/2004 2,4-DB Acute and Chronic Dietary Exposure Assessments 

for the Reregistration Eligibility Decision 

T. Dole D300629 5/3/2004 

2,4-DB: Occupational and Residential Exposure and Risk 

Assessment for the Reregistration Eligibility Decision 

(RED) Document 

D. Drew D291212 7/19/2004 

2,4-DB [4-(2,4-cholorophenoxy) butyric acid] and 2,4-DB 

dimethylamine salt (PC Codes030801, 030819): Revised 

Product Chemistry and Residue Chemistry Summary 

Documents for the Registration Eligibility Decision 

Document (RED). 

U. Habiba TXR No. 

0056727 8/13/2013 

2,4-DB/2,4-DB-DMA: Summary of Hazard and Science 

Policy Council (HASPOC) Meeting of July 25, 2013: 

Recommendations on the Requirement of a Subchronic 

Inhalation Study, an Immunotoxicity Study, and the Acute 

and Subchronic Neurotoxicity Studies for 2,4-DB/2,4-DBDMA. 

A. Al-Mudallal D279637 7/8/2003 

2,4-DB: Drinking Water Assessment for the Health Effects 

Division (HED) Reregistration Eligibility Decision 

Document 

D. Miller D220888 11/14/1995 2,4-DB: Metabolism Committee Question Re: Regulation 

of Metabolites in Plant and Animal Commodities 

D. Miller D221954 1/29/1996 2,4-DB: Results of the 1/16/96 Meeting of the HED 

Metabolism Committee 

1. This table only includes references to final versions. Previous versions of these documents are available, but have been superseded. 

Attachments: 

Attachment 1: Chemical Identity and Physicochemical Properties 

Attachment 2: 2,4-DB and 2,4-DB-DMAS Acute, Subchronic, and Chronic Toxicity Profile 

Attachment 3: 2,4-DB Toxicological Doses and Endpoints 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 15 of 22

Attachment 1. Chemical Identity and Physicochemical Properties 

The chemical structures of 2,4-DB and 2,4-DB-DMAS, and physiochemical properties of 2,4-

DB are presented in Tables A.1 and A.2 below. 

Table A.1. Chemical Structure and Nomenclature of 2,4-DB and 2,4-DB-DMAS 

Compound Chemical Structure 

Common name 2,4-DB

Company code number None specified

IUPAC name 4-(2,4-dichlorophenoxy)butyric acid

Chemical abstract name 4-(2,4-dichlorophenoxy)butanoic acid

CAS registry number 94-82-6

Compound Chemical Structure 

Common name 2,4-DB-DMAS

Company code number None specified

IUPAC name 4-(2,4-dichlorophenoxy)butyric acid dimethylamine salt

Chemical abstract name 4-(2,4-dichlorophenoxy)butyric acid dimethylamine salt

CAS registry number 2758-42-1

Table A.2. Physiochemical Properties of 2,4-DB 

Parameter Value Reference 

Melting point/range 113.5-117.5 °C D198970, 9/30/94, F. Toghrol 

pH (20 °C) 4.31 (1% w:v) D198970, 9/30/94, F. Toghrol 

Density (22 °C) 1.461 D198970, 9/30/94, F. Toghrol 

Water solubility (25 °C) 46 ppm D161375, 7/9/91, F. Toghrol 

Solvent solubility (25 °C) 100 g/L in acetone 

50 g/L in ethanol D161375, 7/9/91, F. Toghrol 

Vapor pressure 7.1 x 10-7 mm Hg at 23.6 °C DEB No. 6823, 8/3/90, G. Makhijani 

Dissociation constant, pKa 3.0 x 10-5 DEB No. 6823, 8/3/90, G. Makhijani 

Octanol/water partition 

coefficient, Log(KOW) 2.62 DEB No. 6823, 8/3/90, G. Makhijani 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 16 of 22

Table A.2. Physiochemical Properties of 2,4-DB 

Parameter Value Reference 

UV/visible absorption spectrum Three distinct bands at 202, 229, 

and 284 nm; no appreciable 

differences in the extinction 

coefficients between acidic, 

neutral, and basic solutions. 

D286840, 1/9/03, S. Malak 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 17 of 22

Attachment 2. 2,4-DB and 2,4-DB-DMAS Acute, Subchronic, and Chronic Toxicity Profile 

Acute Toxicity - 2,4-DB Technical

Guideline

No. Study Type MRID(s) Results Toxicity Category

870.1100 Acute Oral 00128854 

0092159 

LD50 = 1935 mg/kg 

LD50 = 1715 mg/kg 

III 

870.1200 Acute Dermal 0128854 LD50 = > 2000 mg/kg III 

870.1300 Acute Inhalation 41774001 LC50 > 2.3 mg/L IV 

870.2400 Primary Eye Irritation 0128854 

00092160 

Eye irritation with complete 

clearing by day 7 

III 

870.2500 Primary Skin Irritation 0128854 No irritation IV 

870.2600 Dermal Sensitization 43593904 Not a dermal sensitizer — 

Acute Toxicity - 2,4-DB-DMAS (26%)

Guideline

No. Study Type MRID(s) Results Toxicity Category

870.1100 Acute Oral 41224401 LD50 = 3583 mg/kg III

870.1200 Acute Dermal (rabbit) 41224402 LD50 > 2000 mg/kg III

870.1300 Acute Inhalation 41370101 LC50 > 7.98 mg/L IV

870.2400 Primary Eye Irritation 41958001 Persistent corneal opacity, iritis, 

erythema

I 

870.2500 Primary Skin Irritation 250871 Irritation score = 0.99 IV

870.2600 Dermal Sensitization 43968911 Not a dermal sensitizer — 

Subchronic, Chronic, and Other Toxicity Profile – 2,4-DB

Guideline No./ 

Study Type

MRID No. (year)/ 

Classification /Doses Results

870.3100 

90-Day oral toxicity 

rats 

00104739 (1969) 

acceptable/guideline 

0, 316, 1000 or 3160 ppm 

(0, 15.8, 50 or 158 

mg/kg/day 

LOAEL =1000 ppm (50 mg/kg/day)., based on decreased 

body weight gain, increased relative liver and kidney weight, 

microscopic alterations in the liver, kidneys and stomach. 

NOAEL = 316 ppm (15.8 mg/kg/day) 

870.3150 

90-Day oral toxicity 

dogs 

00092165 (1969) 

acceptable/guideline 

capsules at 8, 25 or 80 

mg/kg/day; group of 4 

dogs/sex was also treated 

with capsules at 2.5 

mg/kg/day for three 

LOAEL = 25 mg/kg/day, based on death (sacrifice in 

extremis), clinical signs of toxicity, decreased body weight, 

decreased hematology parameters, altered clinical chemistry 

parameters, increased relative weight of liver, kidney and 

spleen, and gross and microscopic necropsy evidence of 

generalized toxicity. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 18 of 22

Subchronic, Chronic, and Other Toxicity Profile – 2,4-DB

Guideline No./ 

Study Type

MRID No. (year)/ 

Classification /Doses Results

months. NOAEL = 8 mg/kg/day 

870.3200 

21-day dermal - 

rabbit 

41551301 (1989) 

acceptable/guideline 

0, 500, 1000, 2000 

mg/kg/day 

The LOAELs for systemic toxicity and dermal irritation 

were not established. The NOAELs for systemic toxicity and 

dermal irritation were 2000 mg/kg/day (HDT). 

870.3700a 

Prenatal 

developmental in 

rats 

MRID 41382701 (1990) 

acceptable/guideline 

0, 31.25, 62.5 or 125 

mg/kg/day 

Maternal LOAEL = 62.5 mg/kg /day, based on decreased 

body weight (days 6-16) and body weight gain (days 0-20). 

One maternal death was attributed to treatment at 125 

mg/kg/day. 

Maternal NOAEL = 31.25 mg/kg/day. 

Developmental LOAEL = 125 mg/kg/day, based on 

decreased fetal body weight, skeletal malformations/ 

variations, microphthalmia, retroesophageal aortic arches. 

Developmental NOAEL = 62.5 mg/kg/day. 

870.3700b 

Prenatal 

developmental in 

rabbits 

MRID 41529902 (1990) 

acceptable/guideline 

 0, 15, 30 or 60 

mg/kg/day 

Maternal LOAEL = 60 mg/kg/day, based on mortality, 

clinical signs of toxicity (loss of coordination, reduced 

activity, ataxia, prostration, and/or yellow discharge from 

eyes) and decreased body weight gains. 

Maternal NOAEL = 30 mg/kg/day. 

Developmental LOAEL was not determined. 

Developmental NOAEL = 60 mg/kg/day (HDT). 

870.3800 

Reproduction and 

fertility effects 

MRID 40257503 (1986) 

acceptable/guideline 

0,60, 300 or 1500 ppm 

(Males: 0, 5, 25 or 112 

mg/kg/day and Females: 

0, 6, 30 and 133 

mg/kg/day) 

Parental LOAEL = 1500 ppm (112 mg/kg/day in males and 

133 mg/kg/day in females), based on increased water 

consumption, decreased food consumption and body weight, 

increased food conversion ratio, organ weight changes and 

macroscopic renal findings. 

Parental NOAEL = 300 ppm ( 25 mg/kg/day in males and 30 

mg/kg/day in females). 

Offspring LOAEL = 1500 ppm (112 mg/kg/day in males and 

133 mg/kg/day in females), based on increased mortality, 

decreased mean litter weight and increased incidence of 

necropsy findings (small thymus, distended bladder with 

bloody/dark urine and ocular opacity). 

Offspring NOAEL = 300 ppm (25 mg/kg/day in males and 

30 mg/kg/day in females). 

No effect upon reproductive parameters. 

870.4100b 

Chronic toxicity in 

dogs 

MRID 42006301 (1990) 

acceptable/guideline 

0, 75, 225 or 450 

(reduced from 675 ppm 

after week 6) (0, 2.39, 

6.07 and 12.94 mg/kg/day 

for males; 2.15, 7.06 and 

14.16 mg/kg/day for 

females) 

LOAEL = 75 ppm (2.39 and 2.15 mg/kg/day in males and 

females, respectively) based on clinical pathology changes 

(increased BUN, ALT and AST), decreased liver and kidney 

weights (females) and microscopic changes in the kidney. 

NOAEL has not been established. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 19 of 22

Subchronic, Chronic, and Other Toxicity Profile – 2,4-DB

Guideline No./ 

Study Type

MRID No. (year)/ 

Classification /Doses Results

870.4200 

Carcinogenicity in 

mice 

40257502 (1987) 

acceptable/guideline 

0, 25, 250 or 750 ppm 

(equivalent to 0, 3.75, 

37.5 and 112.5 mg/kg/day 

based on 7 ppm in the 

diet equals 1 mg/kg/day) 

LOAEL = 750 ppm (112.5 mg/kg/day), based on increased 

mortality in males and increased absolute and relative organ 

weights (liver and kidney in both sexes, heart in males), 

gross pathological changes in liver and kidney (males) and 

increased amyloidosis and hepatocellular enlargement 

(males). 

NOAEL = 250 ppm (37.5 mg/kg/day) 

870.4300 

Chronic Toxicity/ 

Carcinogenicity in 

rats 

MRID 40257501(1987) 

acceptable/guideline 

0, 60, 600 or 1800 ppm 

(equivalent to 0, 3, 30 or 

90 mg/kg/day based on 1 

ppm equals 0.05 mg/kg) 

LOAEL in males = 1800 ppm (90 mg/kg/day) based on 

decreased body weight, body weight gain and food 

consumption, clinical pathology changes and necropsy 

alterations (decreased kidney weights, increased incidence 

of kidney foci and infarcts and liver mononuclear infiltration 

and spongy degeneration). 

LOAEL in females = 600 ppm (30 mg/kg/day), based on 

decreased body weight gain and food consumption. 

NOAEL in males = 600 ppm (30 mg/kg/day) 

NOAEL in females = 60 ppm (3 mg/kg/day). 

870.5265 

Gene Mutation - 

Ames assay 

MRID 40257504 (1987) 

Acceptable/guideline 

0, 1, 10, 100, 500, 1000, 

2500, 5000, or 10000 

μg/plate 

TA1535, TA1537, 

TA1538, TA98, TA100 

Negative for mutagenic activity with and without S9 

870.5300 

Forward mutation 

assay 

(CHO/HGPRT) 

MRID 40257505 (1987) 

Unacceptable/nonguideline 

Negative without S9; weakly or presumptively positive with 

S9 

870.5375 

In vitro

chromosomal 

aberration 

MRID 40257506 (1987) 

Acceptable/guideline 

250, 500, 750, 1000 

μg/mL 

Without S9 positive; with S9, negative. 

Significant increase in chromosomal aberrations in CHO 

cells at 750 and 1000 μg/mL without S9 (17.25 hr 

exposure); increases at 250 and 500 μg/mL not statistically 

significant; no increase at up to 1000 μg/mL with S9 with 2 

hr exposure; ppt at 1890 μg/mL 

870.5550 

Unscheduled DNA 

synthesis 

MRID 40257507 (1987) 

Acceptable/guideline 

0.997-99.7 μg/mL 

No evidence of UDS induction at doses at which there was 

>50% cell viability; lethal at >99,7 μg/mL; some evidence 

of cytotoxicity at 24.9-99.7 μg/mL 

870.7485 

Metabolism and 

pharmacokinetics in 

rats 

41981601 (1991) 

acceptable/guideline 

Single i.v. and oral doses 

and multiple oral dosing. 

Radioactivity recovery in urine was 71.5-97.6% of dose and 

3.5-13.8% in feces. Major metabolites were 2,4-D and 

conjugated 2,4-DCP. 

870.7485 

Metabolism and 

pharmacokinetics in 

rats 

MRID 44774101(1999) 

acceptable/guideline 

Single oral dose and 2 

bile-duct cannulated 

males. 

Biliary excretion and enterohepatic recirculation occurred. 

Metabolite G was identified 4-(2,4-dichlorophenoxy)-3-

hydroxybutyric acid. The distribution of metabolites in the 

present study was comparable to the distribution observed in 

the previous rat metabolism study. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 20 of 22

Subchronic, Chronic, and Other Toxicity Profile – 2,4-DB-DMAS

Guideline No./ 

Study Type

MRID No. (year)/ 

Classification /Doses Results

870.3200 

21-day dermal - 

rabbit 

41529901 (1990) 

acceptable/guideline 

0, 250, 750 or 1500 

mg/kg/day 

The systemic LOAEL was not established. The systemic 

NOAEL was 1500 mg/kg/day. The dermal irritation LOAEL 

was 750 mg/kg/day based on erythema, desquamation and 

fissuring which were more severe than control lesions. The 

dermal NOAEL was 250 mg/kg/day. 

870.3100 

90-Day oral toxicity 

rodents 

41775401 (1990) 

acceptable/guideline 

 0, 60, 600 or 1800 ppm 

(0, 4.16, 41.86 and 

130.37 mg/kg/day for 

males and 0, 4.96, 51.36 

and 153.64 mg/kg/day for 

females) 

LOAEL = 600 ppm (41.86 mg/kg/day in males and 51.36 

mg/kg/day in females), based on decreased body weight and 

body weight gain (females), increased kidney weights and 

increased incidence of tubular degeneration (females). 

NOAEL = 60 ppm (4.16 mg/kg/day in males and 4.96 

mg/kg/day in females). 

870.3700a 

Prenatal 

developmental in 

rats 

42536101, 42595201 

(1991) 

acceptable/guideline 

0, 31.25, 62.5 or 125 

mg/kg/day 

Maternal LOAEL = 62.5 mg/kg/day, based on emaciation 

and decreased body weight gain and food consumption. 

There were 3 maternal deaths attributed to treatment at 125 

mg/kg/day. 

Maternal NOAEL = 31.25 mg/kg/day. 

Developmental LOAEL = 125 mg/kg/day, based on an 

increase in early resorptions per dam, skeletal 

malformations, microphthalmia, and a decrease in fetal 

weight. 

Developmental NOAEL = 62.5 mg/kg/day. 

870.5300 

In vitro Forward 

mutation assay 

(CHO/HGPRT) 

MRID 41810701 (1990) 

Acceptable/guideline Did not induce forward mutation at the HGPRT locus in 

CHO cells with or without metabolic activation 

870.7600 

Dermal penetration 

rat 

44729501 (1998) 

Dermal absorption factor = 23%. This value is from a 120 

ug/cm2

 exposure determined at 96 hours after a wash at 10 

hours. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 21 of 22

Attachment 3. 2,4-DB Toxicological Doses and Endpoints 

Summary of Toxicological Doses and Endpoints - 2,4-DB 

Exposure

Scenario

Dose for Risk 

Assessment and 

Uncertainty 

Factor

Special FQPA Safety 

Factor and Level of 

Concern

Study and Toxicological Effects

Acute Dietary 

(Females 13-50 

years of age) 

NOAEL = 62.5 

mg/kg/day 

UF = 100 

Acute RfD = 0.6 

mg/kg/day 

FQPA SF = 1X 

aPAD = acute RfD

 FQPA SF 

= 0.6 mg/kg/day 

Rat developmental toxicity. 

LOAEL = 125 mg/kg/day based on 

skeletal variations/ malformations, 

microphthalmia, post-implantation loss 

Acute Dietary 

(General 

population 

including infants 

and children) 

None N/A No appropriate endpoint attributable to a 

single dose from oral toxicity studies. 

Chronic Dietary 

(All populations) 

NOAEL= 3 

mg/kg/day 

UF = 100 

Chronic RfD = 0.03 

mg/kg/day 

FQPA SF = 1X 

cPAD = chronic RfD

 FQPA SF 

= 0.03 mg/kg/day 

Chronic/carcinogenicity study in rats. 

LOAEL = 30 mg/kg/day based on 

decreased body weight gain and food 

consumption in females1

Short-Term 

Dermal 

(1 to 30 days) 

None N/A 

Quantitation not required. No systemic 

toxicity via the dermal route and there are 

no developmental concerns. 

2

IntermediateTerm 

Dermal 

(1 to 6 months) 

Oral NOAEL = 15.8 

mg/kg/day 

Residential MOE = 100 

Occupational 

MOE = 100 

Subchronic rat toxicity (2,4-DB study) 

LOAEL = 50 mg/kg/day based on 

decreased body weight gain, increased 

relative liver/kidney weight, and 

microscopic changes 

2

Long-Term 

Dermal (> 6 

months) 

Oral NOAEL = 3 

mg/kg/day 

 (dermal absorption = 

23%) 

Residential MOE = 100 

Occupational 

MOE = 100 

Chronic/carcinogenicity study in rats. 

LOAEL = 30 mg/kg/day based on 

decreased body weight gain and food 

consumption in females. 

Short-Term 

Inhalation 

(1 to 30 days) 

Oral NOAEL = 31 

mg/kg/day 

(inhalation 

absorption = 100%) 

Residential MOE = 100 

Occupational 

 MOE = 100 

Rat developmental toxicity. 

LOAEL = 62.5 mg/kg/day based on 

decreased maternal body weight, body 

weight gain, and food consumption, and 

clinical signs (emaciation, few feces) 

Intermediate-Term 

Inhalation 

(1 to 6 months) 

Oral NOAEL = 15.8 

mg/kg/day 

(inhalation 

absorption = 100%) 

Residential MOE = 100 

Occupational 

MOE = 100 

Subchronic rat toxicity (2,4-DB study). 

LOAEL = 50 mg/kg/day based on 

decreased body weight gain, increased 

relative liver/kidney weight, and 

microscopic changes 

Long-Term 

Inhalation 

(>6 months) 

Oral NOAEL = 3 

mg/kg/day 

(inhalation 

Residential MOE = 100 

Occupational 

MOE = 100 

Chronic/carcinogenicity study in rats. 

LOAEL = 30 mg/kg/day based on 

decreased body weight gain and food 

consumption in females. 
2,4-DB: Human Health Risk Assessment Scoping Document in Support of Registration Review 

Page 22 of 22

Summary of Toxicological Doses and Endpoints - 2,4-DB 

Exposure

Scenario

Dose for Risk 

Assessment and 

Uncertainty 

Factor

Special FQPA Safety 

Factor and Level of 

Concern

Study and Toxicological Effects

absorption = 100%) 

Cancer None N/A Classified "not likely to be a human 

carcinogen". 1

Endpoint should be reevaluated, based on the minimal decrease in BW. 2Dermal assessment should be reevaluated based on 

current policy (no systemic toxicity at limit dose; no developmental concerns; no progression of kidney/liver toxicity with 

duration of exposure). 

UF = uncertainty factor, FQPA SF = FQPA safety factor, NOAEL = no observed adverse effect level, LOAEL = lowest observed 

adverse effect level, PAD = population adjusted dose, (a = acute, c = chronic), RfD = reference dose, MOE = margin of exposure,

LOC = level of concern, NA = Not Applicable