UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MEMORANDUM 

Date: September 15, 2015 

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

SUBJECT: Tribufos: Human Health Draft Risk Assessment for Registration Review. 

PC Code: 074801 

Decision No.: 40L~o t; 

Petition No.: NA 

DP Barcode: D357537 

Registration No.: NA 

Regulatory Action: Reg 

Review 

Risk Assessment Type: Single Chemical Aggregate 

TXRNo.: NA 

Case No.: 2145 

CAS No.: 78-48-8 

MRID No.: NA 40 CFR: §180.299 

FROM: Evisabel Craig, Ph.D., DABT, Toxicologist/Risk Assessor, RAB YI 

Bridgett Bobowiec, Biologist, ORE Asse~or, J>AB VJ ~,..-4 

Sheila Piper, Chemist, RAB Vl ~~pA--

Health Effects Division (7509P) 

THROUGH: Donna Davis, Acting Branch Chief C'\1/1(/ /' - /\ ,/ 

Risk Assessment Branch YI ~VCA-V ~ 

Health Effects Division (7509P) 

A~~ 

::stina 

William Irwin

Swartz

, RARC 

, RARC 

Reviewer 

evicwe~k, (,,(}~ d\-11--. ~ ~ "YL 

TO: Kelly Sherman, Branch Chief 

Marianne Mannix, Chemical Review Manager 

Risk Management and Implementation Branch Ill 

Pesticide Re-Evaluation Division (PRO) (7508P) 

Page I of8-I 
1.0 Executive Sum1nary ............................................................................................................. 4 

2.0 1-IED Recommendations ....................................................................................................... 7 

2.1 Data Deficiencies ............................................................................................................. 7 

2.2 Tolerance Considerations .................................................. .... ........... .................... ............ 7 

2.2. l Enforcement Analytical Method .................................................................................. 7 

2.2.2 International Harrnonjzation ..................................................... .... .......... .......... ............ 7 

2.2.3 RecomJTiended Tolerances ........ .................... ...................... .......................................... 7 

2.3 Label Recommendations .................................................................................................. 8 

2.3. l Recommendations from Residue Reviews ... ................................................................ 8 

3.0 Introduction .......................................................................................................................... 8 

3. 1 Chemical Identity ................................................................................................. ......... ... 8 

3.2 Physical/Chemical Characteristics ................................................................................... 9 

3.3 Pesticide Use Pattern ........................................................................................................ 9 

3.4 Anticipated Exposure Pathways ......... ..... .................................... ....... ............................ 12 

3.5 Consideration of Environmental Justice ........................................................................ 12 

4. 0 Hazard Characterization and Dose-Response Assessment ............ .................................... 12 

4.1 Toxicology Studies Available for Analysis ................................................................... 13 

4.2 Absorption, Distribution, Metabolism, & Elimination (ADME) ................................... 13 

4.2. l Dermal Absorption ......... ............................................................................................ 14 

4.3 Toxicological Effects ..................................................................................................... 14 

4.3.1 Toxicology Profile ........................................................................................................... 14 

4.3 .2 Critical Durations of Exposure ..................................................... ....... .......... ............. 15 

4.4 Literature Review on Neurodevelopment Effects .......................................................... 15 

4.5 Safety Factor for Infants and Children (FQPA SF) ....................................... ................. 20 

4.5.1 Completeness of the Toxicology Database ................................................................ 21 

4.5.2 Evidence of Neurotoxicity .......................................................................................... 2 1 

4.5.3 Evidence of Sensiti vity/Susceptibility in the Developing or Young Animal. ............ 21 

4.5.4 Residual Uncertainty in the Exposure Database ......................................................... 21 

4.6 Toxicity Endpoint and Point of Departure Selections ..... ............................................. .. 22 

4.6. I Dose-Response Assessment ....................................................................................... 22 

4.6.2 Recommendation for Combinjng Routes of Exposures for Risk Assessment ........... 24 

4.6.3 Cancer Classification and Risk Assessment Recommendation .................................. 25 

4.6.4 Summary of Points of Departure and Toxicity Endpoints Used in Hwnan Risk 

Assessment. .................. ........ .... .................... ............................................................................. 25 

4.7 Endocrine Disruption ..................................................................................................... 26 

5.0 Dietary Exposure and Risk Assessment ....................... ........................ ............................. 27 

5.1 Metabolite/Degradate Residue Profile ........................................................................... 27 

5.2 Food Residue Profile ............................................................. ...................................... ... 28 

5.3 Water Residue Profile .................................................................................................... 28 

5.4 Dietary Risk Assessment.. ................................................ ........ ........... ....... ................ .... 29 

5.4. 1 Description of Residue Data Used in Dietary Assessment.. ....................................... 29 

5.4.2 Percent Crop Treated Used in Dietary Assessment.. .................................................. 30 

5.4.3 Acute Dietary Risk Assessment .......................... ......... ........ .... .... ... ....... .......... ...... .... 30 

5.4.4 Steady State Dietary Risk Assessment ......................................................... ... ........... 31 

5.4.5 Cancer Dietary Risk Assessment.. .............................................................................. 33 

6.0 Residential (Non-Occupational) Exposure/Risk Characterization .................................... 33 

Page 2 of84 
Page 3 of 84

6.1 Non-Occupational Bystander Post-Application Inhalation Exposure and Risk Estimates

........................................................................................................33

6.2 Non-Occupational Spray Drift Exposure and Risk........................................................ 34

6.2.1 Combined Risk Estimates from Lawn Deposition Adjacent to Applications.................. 35

7.0 Aggregate Exposure/Risk Characterization....................................................................... 39

7.1 Acute Aggregate Risk .................................................................................................... 39

7.2 Steady State Aggregate Risk.......................................................................................... 39

7.3 Cancer Aggregate Risk................................................................................................... 39

8.0 Cumulative Exposure/Risk Characterization..................................................................... 39

9.0 Occupational Exposure/Risk Characterization .................................................................. 40

9.1 Steady-State Handler Risk ............................................................................................. 40

9.2 Steady State Post-Application Risk................................................................................ 45

9.2.1 Occupational Post-application Inhalation Exposure/Risk Estimates.......................... 45

9.2.2 Occupational Post-application Dermal Exposure/Risk Estimates.............................. 45

10.0 Review of the Incident Report........................................................................................... 49

11.0 References.......................................................................................................................... 49

Appendix 1. Summary of OPP’s ChE Policy & Use of BMD Modeling .................................... 52

Appendix 2 – Results for BMD/BMDL modeling for Tribufos................................................... 53

Appendix 3. Toxicology Profile and Executive Summaries........................................................ 55

A.3.1 Toxicology Data Requirements.................................................................................. 55

A.3.2 Toxicity Profiles......................................................................................................... 56

 A.3.2 Hazard Identification and Endpoint Selection.............................................61

Appendix 4. IRL Sheet.....................................................................................82

Appendix 5. Summary of Steady State Non Cancer Occupational Handler Dermal Risk 

Estimates for Tribufos.......................................................................................83
Page 4 of 84

1.0 Executive Summary

Tribufos [S,S,S-tributyl phosphorotrithioate] is an organophosphate (OP) herbicide used as a 

defoliant on cotton plants prior to harvest and as a bottom defoliant to reduce or prevent losses 

from boll rot organisms. There are no registered non-occupational (residential uses) for tribufos. 

Humans may be exposed to tribufos in food and drinking water, since tribufos may be applied 

directly to growing crops and application may result in tribufos reaching surface and ground 

water sources of drinking water. There are no registered uses for tribufos that will result in direct 

non-dietary (i.e., residential) exposures. However, there is the potential for indirect non-dietary 

exposure (incidental oral and dermal) to tribufos related to spray drift. In an occupational 

setting, applicators may be exposed while handling the pesticide prior to application, as well as 

during application. There is a potential for post-application exposure for workers re-entering 

treated fields. Tribufos is formulated as an emulsifiable concentrate (EC). The maximum 

application rates for the use on cotton are 1.875 pounds (lbs) active ingredient (ai) per acre (A) 

for the states of California (CA) and Arizona (AZ), and 1.125 lb ai/A for all other states. 

Application can be made by ground or aerial equipment. Cotton harvesting must be mechanical. 

Tribufos labels have a restricted entry interval (REI) of 7 days and require mixer/loaders to wear 

coveralls over long-sleeved shirt, long pants, chemical resistant gloves, chemical-resistant 

footwear plus socks, chemical-resistant apron, and protective eyewear.

Tribufos is a member of the OP class of pesticides. Like other OPs, the initiating event in the 

mammalian adverse outcome pathway (AOP)/ mode of action (MOA), for tribufos involves 

inhibition of the enzyme acetylcholinesterase (AChE) via phosphorylation of the serine residue 

at the active site of the enzyme. This inhibition leads to accumulation of acetylcholine and 

ultimately to neurotoxicity in the central and/or peripheral nervous system. For tribufos, AChE 

inhibition is the most sensitive endpoint in the toxicology database in multiple species, durations, 

lifestages, and routes. OPs also exhibit a phenomenon known as steady state AChE inhibition. 

After repeated dosing at the same dose level, the degree of inhibition comes into equilibrium 

with the production of new, uninhibited enzyme. The “steady state” endpoint selection for 

tribufos overlaps HED’s traditional short-term exposure duration endpoint selection as well as 

being appropriately protective for workers that are exposed over longer periods of time (i.e., 

intermediate-term exposures) as well as chronic exposure. Therefore, steady state exposure 

assessments were conducted instead of the traditional chronic, long-term, intermediate-term,

and/or short-term assessments.

The toxicology database for tribufos is complete for risk assessment. There are acceptable data 

available for toxicity endpoint selection from studies across multiple lifestages, durations, and 

routes for both red blood cell (RBC) and brain AChE inhibition. Clinical signs of neurotoxicity 

can be found throughout the database of toxicity studies at doses much higher than those causing 

inhibition of AChE. Quantitative susceptibility was observed in pups compared to adults in the 

acute comparative ChE (CCA) studies but no quantitative or qualitative susceptibility was 

observed in repeated-dosing studies. However, the Food Quality Protection Act (FQPA) Safety 

Factor (SF) has been retained for infants, children, youths, and women of childbearing age for all 

exposure scenarios due to uncertainty in the human dose-response relationship for 

neurodevelopmental effects (See Section 4.4). As a result, a total uncertainty factor of 1000X 
Page 5 of 84

was applied for all exposure scenarios, except dietary exposures for the adult population 

subgroup 50-99 years old where the FQPA SF does not apply (total uncertainty factor = 100X) 

and inhalation exposures where the interspecies uncertainty factor has been reduced to 3X (total 

uncertainty factor = 300X).

Tribufos is classified as “likely” at high doses but “unlikely” to be a human carcinogen at low 

doses since all tumor increases in mice occurred only at the highest dose tested (48 mg/kg/day in 

males and 63 mg/kg/day in females) and were accompanied by severe ChE inhibition and 

increased mortality. In addition, there was no concern for mutagenicity, and no structural 

analogs of concern were identified. Tribufos is categorized as having high acute toxicity via the 

oral and dermal routes (Category II) and is moderately toxic via the inhalation route (Category 

III). Tribufos induces mild eye and skin irritation (Category IV) and is not a dermal sensitizer.

The residue chemistry database for tribufos is complete. Acceptable data were submitted 

depicting residues in the target crop, cotton. The qualitative nature of the residues of tribufos in 

plants (cottonseed) and livestock has been adequately identified/characterized and is understood. 

The HED Metabolism Assessment Review Committee (MARC) reviewed the tribufos toxicology 

and metabolism data and recommended that the residue of concern for both tolerance expression 

and risk assessment for plants and livestock should consist of tribufos only. In addition, HED 

previously determined that tolerances for tribufos residues in eggs and poultry tissues were not 

required based on the results of the respective poultry metabolism and feeding studies (CFR 

§180.6(a)(3)). These conclusions have not changed for the purpose of registration review.

Additionally, based on the livestock feeding study and on an updated livestock diet, there are no 

reasonable expectations of finite residues of tribufos in/on any livestock commodity. Therefore, 

the established tolerances for residues of tribufos in/on milk, eggs, cattle, goat, horse, hog, 

poultry, and sheep commodities (40 CFR §180.272) should all be revoked.

Tribufos is moderately mobile and persistent in the soil suggesting the potential for runoff and/or 

leaching. Based on the environmental fate parameters, there is the potential for tribufos to reach 

drinking water resources. The Environmental Fate and Effects Division (EFED) performed 

surface water and groundwater modeling using the highest application rates. The highest 

estimated drinking water concentrations were from surface water (Texas cotton [TX]) and to 

further refine the dietary assessment, average rates were considered.

The acute aggregate (food and drinking water) dietary exposure estimates are below HED’s level 

of concern (i.e., <100% of the acute population adjusted dose (aPAD)) for the U.S. population 

and all population subgroups. Combined dietary exposure from cottonseed oil and drinking 

water concentrations based on maximum rates in TX at the 95th percentile of exposure is 15% of 

the aPAD for the U.S. population and 48% of the aPAD for all infants (<1 yr old), the most 

highly exposed population subgroup. Also, the combined dietary exposure from cottonseed oil 

and drinking water concentrations based on average rates in TX at the 95th percentile of exposure

is 7% of the aPAD for the U.S. population and 22% of the aPAD for all infants (<1 yr old), the 

most highly exposed population subgroup. The food only exposure contributes <1 for and

drinking water only contributes 48% of the aPAD for all infants. 
Page 6 of 84

For the steady state assessment, 2-day average food residues, representing average food 

exposure, and 21-day forward-rolling water averages were compared to the steady state (ss) point 

of departure. Combined dietary exposure from cottonseed oil and drinking water concentrations 

based on maximum rates in TX at the 95th percentile of exposure is 72% of the ssPAD for the 

U.S. population and 226% of the ssPAD for all infants (<1 yr old), the most highly exposed 

population subgroup. The combined dietary exposure from cottonseed oil and TX cotton 

average rate for drinking water at the 95th percentile of exposure is 34% of the ssPAD for the 

U.S. population and 105% of the ssPAD for all infants (<1 yr old), the most highly exposed 

population subgroup. The food only exposure contributes <2% of the ssPAD and drinking water 

only contributes 226% of the ssPAD for all infants. 

There are no registered residential uses; therefore, the acute and steady state aggregate 

assessments are the same as the acute and steady state dietary assessments discussed above. 

A spray drift assessment was completed for the currently registered occupational use of tribufos

on cotton, to determine the potential exposure and risk for those exposed via drift from both 

aerial and ground applications. The spray drift assessment was completed using both registered 

maximum application rates for cotton (1.875 lbs ai/acre and 1.125 lb ai/A) for aerial and 

groundboom application methods. The resulting risk estimates were still a concern at 300 feet 

from the field edge (MOE < 1,000) for adult dermal exposures as well as children (1<2 years of 

age) combined dermal and incidental oral exposures (MOE < 1,000). 

For occupational handler exposure, a total aggregate risk index (ARI) approach was used as a 

risk metric since the toxicological effects for dermal and inhalation exposure routes were similar 

but the levels of concern (LOCs) are different (LOC = 1000 for dermal and 300 for inhalation 

exposure). The occupational handler assessment identified risk estimates of concern (ARIs < 1) 

for all mixer/loader and applicator scenarios despite the engineering control and application rate 

mitigations made in the previous RED.

For the occupational post-application assessment, HED relied on chemical-specific data 

depicting the residues of tribufos available for transfer to workers re-entering treated areas or 

conducting specific post-application activities. HED has determined that there are steady-state

duration post-application dermal risk estimates of concern (i.e., MOEs < 1,000) on the day of 

treatment (DAT 0). Current tribufos labels require a Restricted Entry Interval (REI) of 7 days 

after application, however, activities such as module builder operator, rakers, and trampers result 

in risks of concern up to as many as 30 days following application. 

This risk assessment relies 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 Handler Exposure Database (PHED) 1.1; the Agricultural Handlers Exposure Task 

Force (AHETF) database; the Agricultural Reentry Task Force (ARTF) database; and other 

registrant-submitted studies (MRID 42701601 and MRID 42685901) submitted to the Agency by 

the registrant 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 
Page 7 of 84

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 website1. 

2.0 HED Recommendations

2.1 Data Deficiencies

There are no residue chemistry or toxicology data deficiencies. A biomonitoring study was 

requested in the 2000 IRED to confirm risk management decisions. In 2001, the request for the 

biomonitoring study was placed on hold, pending submission of the requested neurotoxicity 

studies. These neurotoxicity studies have since been submitted and reviewed, and found not to 

change the dermal toxicity endpoint. Accordingly, a biomonitoring study is no longer required.

2.2 Tolerance Considerations

2.2.1 Enforcement Analytical Method

Adequate methods are available for enforcement and data collection purposes for residues of 

tribufos in cottonseed commodities and milk. The Pesticide Analytical Method Volume II (PAM 

II) lists four gas-liquid chromatography (GLC)/flame ionization detector methods (Methods I, II, 

A and C). Method B is available for analysis of cottonseed commodities and milk, and uses an 

electron capture detector. Method II is used to determine tribufos residues in milk. The limit of 

quantitation (LOQ) is 0.05 ppm for cottonseed and 0.002 ppm for milk. There is no single 

residue method included in PAM Vol. II for tribufos residues in livestock commodities. 

2.2.2 International Harmonization

There are no CODEX, Canadian, or Mexican maximum residue levels (MRLs) established for 

tribufos. Therefore, international harmonization is not an issue.

2.2.3 Recommended Tolerances

Permanent tolerances are currently established under 40 CFR§180.272 for residues of tribuphos 

(i.e. tribufos) in/on cotton and livestock commodities. However, based on the recalculated diets 

for ruminants and swine, there is no reasonable expectation of finite residues of tribufos on any 

livestock commodity (Category 3 of 40 CFR §180.6(a)). The established tolerances for residues 

of tribufos in/on cattle, goat, horse, hog, and sheep commodities (40 CFR §180.272) should thus 

be revoked. HED recommends that the residue definition for the tribufos tolerance expression 

should read as following:

“Tolerances are established for the residues of the defoliant tribuphos, including its 

metabolites and degradates, in or on the commodities in the listed below. Compliance 

 

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

http://www.epa.gov/pesticides/science/post-app-exposure-data.html
Page 8 of 84

with the tolerance levels specified below is to be determined by measuring only tribuphos 

(S,S,S-tributyl phosphorotrithioate), in or on the commodity.”

Table 2.2.2. Tolerance Summary for Tribuphos, §180.272

 Commodity Proposed Tolerance 

(ppm)

HED-Recommended 

Tolerance (ppm)

Comments 

(correct commodity 

definition)

Cattle, fat 0.15 revoke

Cattle, meat 0.02 revoke

Cattle, meat byproducts 0.02 revoke

Cotton, gin byproducts 40.0 40.0

Cotton, undelinted seed 4.0 4.0

Goat, fat 0.15 revoke

Goat, meat 0.02 revoke

Goat, meat byproducts 0.02 revoke

Hog, fat 0.15 revoke

Hog, meat 0.02 revoke

Hog, meat byproducts 0.02 revoke

Horse, fat 0.15 revoke

Horse, meat 0.02 revoke

Horse, meat byproducts 0.02 revoke

Milk 0.01 revoke

Sheep, fat 0.15 revoke

Sheep, meat 0.02 revoke

Sheep, meat byproducts 0.02 revoke

2.3 Label Recommendations

2.3.1 Recommendations from Residue Reviews

No label changes are needed for the residue chemistry.

3.0 Introduction

Tribufos is an OP herbicide registered for use on cotton as a defoliant. It was first registered in 

the United States in 1961 to defoliate cotton in preparation for machine harvesting.

The technical registrant for tribufos is Bayer CropScience. The most recent human health risk 

assessment for tribufos was conducted in 2000 (D266540, R. Travaglini, 06/26/2000). 

3.1 Chemical Identity

Table 3.1. Tribufos Nomenclature

Chemical Structure
Page 9 of 84

Table 3.1. Tribufos Nomenclature

Empirical Formula C12H27OPS3

Common Name Tribuphos; Tribufos

IUPAC name S,S,S-tributyl phosphorotrithioate

CAS Name S,S,S-tributyl phosphorotrithioate

CAS Registry 

Number 78-48-8

Chemical Class Organophosphate Defoliant 

3.2 Physical/Chemical Characteristics

Tribufos is a colorless to yellow liquid with a mercaptan-like odor. It is practically insoluble in 

water, but is completely miscible in dichloromethane, n-hexane, 2-propanol, and toluene. 

Tribufos is relatively stable to heat and under acidic conditions, but slowly hydrolyzes under 

alkaline conditions. Tribufos’ moderately low vapor pressure (1.7 x 10-6torr) and Henry’s Law 

constant (3.1 x10-7atm-m3/mol) indicate that the compound is semi-volatile. The water 

solubility, soil adsorption, hydrolysis and aerobic soil metabolism data suggest that tribufos is 

not likely to be a ground water contaminant.

Table 3.2. Tribufos Physical and Chemical Properties

Parameter

Value

Molecular Weight 314.5 g/mole

Water solubility (20°C)

2.3mg/L

Solvents - Completely miscible with n-hexane, 

dichloromethane,

toluene, and 2-propanol

Vapor pressure (20°C) 1.7 x 10-6 Torr

Melting and Boiling Point < -250C; 150 °C at 0.3 mm Hg

Density 1.02 g/cm3 @ 20oC

Henry’s Law Constant (25°C) 3.1 x 10-7atm-m3/mol

Octanol/water partition coefficient (25°C) 3.31 x 105; log Kow = 5.7

3.3 Pesticide Use Pattern

Tribufos is registered for use as a cotton defoliant. Tribufos accelerates the defoliation process 

by stimulating the formation of the abscission layer where the stem joins the stalk, causing the 

leaves and stems to drop cleanly to allow mechanical harvesting of the crop without staining the 

lint. Leaves may begin to drop in green condition 3 days after application with satisfactory leaf 

drop in 5- 7 days under favorable conditions. Tribufos is formulated as a liquid in emulsifiable 

concentrate (70% active ingredient, ai), and as a liquid technical grade (97% ai). Products 
Page 10 of 84

contain 6 lbs of tribufos per United States (US) gallon of product. Tribufos containing products 

currently registered include; 1) Folex 6 EC Cotton (EPA Reg. No. 5481-504), 2) Def 6 

Emulsifiable Defoliant (5481-903 or EPA Reg. No. 264-730) [AMVAC Chemical Corporation], 

3) DFT 6 EC Cotton Defoliant (EPA Reg. No. 34704-867) [Loveland Products, Inc.], and 4) 

Tribufos 6 (EPA Reg. No. 85678-15) [Red Eagle International Corporation]. Tribufos is applied 

by ground and aerial equipment. Application by chemigation is prohibited. California and 

Arizona have a higher single maximum application rate of 1.875 lb ai/A than the rest of the U.S. 

states with a maximum application rate of 1.125 lb ai/A.

Labels indicate mixer/loaders must wear personal protective equipment (PPE) including 

coveralls over long-sleeved shirt, long pants, chemical resistant gloves, chemical-resistant 

footwear plus socks, chemical-resistant apron, and protective eyewear. One product, FOLEX 6 

EC (EPA reg # 5481-504) also has a respirator requirement. Mixers and loaders for aerial 

applications must use a closed mechanical transfer system that meets requirements listed in the 

worker protection standard (WPS) for agricultural pesticides [40 CFR 170.240 (d)(4)] for 

providing dermal protection. Aerial applications must be conducted in an enclosed cockpit. 

Cotton must be mechanically harvested. Hand harvesting is prohibited. Module building 

equipment does not appear to be prohibited on tribufos labels. See Table 3.3 for use profile.
Tribufos Human Health Risk Assessment DP No. D357537

Page 11 of 84

Table 3.3. Summary of Directions for Use of Tribufos.

Registrant 

Company

Product 

and EPA 

(Reg. No)

Percent 

active 

ingredient 

(ai)

Application. 

Timing, 

Type, and 

Equip.

Application. 

Rate (lb 

ai/A)

Max. No. 

Application. 

per Season

REI PPE Limits and Directions

Cotton

AMVAC

CHEMICAL 

CORPORAT

ION

FOLEX 6 

EC 

(5481-504)

70.5 % ai. 

Contain 6 

lbs/gallon

Ground and 

Aerial

California 

and Arizona 

1.875 lb 

ai/A 

Other U.S. 

states 1.125 

lb ai/A.

2

7 

days

Mixers/loaders for aerial application must use 

mechanical transfer system. Applicators and 

other handlers must wear coveralls over 

baseline dermal wear plus gloves, protective 

eyewear, chemical resistant headgear for

overhead exposure. Chemical-resistant apron 

when cleaning, mixing or loading. In addition 

a PF10 respirator is required for enclosed 

areas and PF5 for outdoor applications.

Can be used for bottom or complete 

defoliation. Cotton must be 

mechanically harvested. Hand 

harvesting prohibited. Aerial

application must be in enclosed 

cockpits. Do not apply through any 

type of irrigation. Do not graze 

treated fields. Diesel oil may be 

used instead of water in adverse 

conditions. 

DEF 6 EC 

Defoliant 

5481-9023

(264-730)

Mixers/loaders for aerial application must use 

mechanical transfer system. Mixer/loaders, 

applicators must wear coveralls over baseline 

dermal wear plus gloves, protective eyewear, 

chemical resistant headgear of overhead 

exposure and chemical-resistant apron when 

cleaning, mixing or loading

DEF Technical Defoliant (5481-9022) 97.9 % ai.; For use in the manufacture of cotton defoliants. 

LOVELAND 

PRODUCTS, 

INC

DFT 6 EC 

Cotton 

Defoliant 

(34704-

867) 70.5 % ai. Ground and 

Aerial

California 

and Arizona 

1.875 lb 

ai/A 

Other U.S. 

states 1.125 

lb ai/A.

2

7 

days

Mixers/loaders for aerial application must use 

mechanical transfer system. Mixer/loaders, 

applicators must wear coveralls over baseline 

dermal wear plus gloves, protective eyewear, 

chemical resistant headgear of overhead 

exposure and chemical-resistant apron when 

cleaning, mixing or loading.

Can be used for bottom or complete 

defoliation. Aerial application must 

be in enclosed cockpits. Cotton 

must be mechanically harvested. 

Hand harvesting prohibited. Do not 

apply through any type of irrigation. 

Do not graze treated fields. Diesel 

oil may be used instead of water in 

adverse conditions. 

REDEAGLE 

INTERNATI

ONAL LLC

Tribufos 6 

(85678-15)

Tribufos Technical (85678-10) 99.5 % ai. For use in the manufacture of cotton defoliants

AXION AG 

PRODUCTS, 

LLC 

AX 

Tribufos 6

(89167-43)

70.5 % ai. 

Contain 6 

lbs/gallon

Ground and 

Aerial

California 

and Arizona 

1.875 lb 

ai/A 

Other U.S. 

states 1.125 

lb ai/A.

2

7 

days

Applicators and other handlers must wear 

coveralls over baseline dermal wear plus 

gloves, protective eyewear, chemical resistant 

headgear of overhead exposure. Chemicalresistant apron when cleaning, mixing or 

loading.

Can be used for bottom or complete 

defoliation. Aerial application must 

be in enclosed cockpits. Cotton 

must be mechanically harvested. 

Hand harvesting prohibited. Do not 

apply through any type of irrigation. 

Do not graze treated fields. Diesel 

oil may be used instead of water in 

adverse conditions. 
Tribufos Human Health Risk Assessment DP No. D357537

Page 12 of 84

3.4 Anticipated Exposure Pathways

The Pesticide Re-Evaluation Division has requested a registration review assessment of human 

health risk to support the registered uses of tribufos. Humans may be exposed to tribufos in food 

and drinking water, since tribufos may be applied directly to growing crops and application may 

result in tribufos reaching surface and ground water sources of drinking water. There are no 

registered uses for tribufos that will result in direct non-dietary (i.e., residential) 

exposures. However, there is the potential for indirect non-dietary exposure (incidental oral and 

dermal) to tribufos related to spray drift. In an occupational setting, applicators may be exposed 

while handling the pesticide prior to application, as well as during application. There is a 

potential for post-application exposure for workers re-entering treated fields. 

3.5 Consideration of Environmental Justice

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

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://epa.gov/compliance/ej/resources/policy/exec_order_12898.pdf). As a part of every 

pesticide risk assessment, OPP considers a large variety of consumer subgroups according to 

well-established procedures. In line with OPP policy, HED estimates risks to population 

subgroups from pesticide exposures that are based on patterns of that subgroup’s food and water 

consumption, and activities in and around the home that involve pesticide use in a residential 

setting. Extensive data on food consumption patterns are compiled by the USDA under the 

Continuing Survey of Food Intake by Individuals (CSFII) and are used in pesticide risk 

assessments for all registered food uses of a pesticide. These data are analyzed and categorized 

by subgroups based on age and ethnic group. Additionally, OPP is able to assess dietary 

exposure to smaller, specialized subgroups and exposure assessments are performed when 

conditions or circumstances warrant. Whenever appropriate, non-dietary exposures based on 

home use of pesticide products and associated risks for adult applicators and for toddlers, youths, 

and adults entering or playing on treated areas post-application are evaluated. Further 

considerations are currently in development as OPP has committed resources and expertise to the 

development of specialized software and models that consider exposure to bystanders and farm 

workers as well as lifestyle and traditional dietary patterns among specific subgroups.

4.0 Hazard Characterization and Dose-Response Assessment

Tribufos is a member of the organophosphate (OP) class of pesticides which functions by plant 

esterase inhibition causing elevated ethylene levels and leaf abscission. Like other OPs, the 

initiating event in the mammalian adverse outcome pathway (AOP)/mode of action (MOA), for 

tribufos involves inhibition of the enzyme cholinesterase (ChE) via phosphorylation of the serine 

residue at the active site of the enzyme. This inhibition leads to accumulation of acetylcholine 

and ultimately to neurotoxicity in the central and/or peripheral nervous system (see Figure 1). 

For tribufos, ChE inhibition is the most sensitive endpoint in the toxicology database in multiple 

species, durations, lifestages, and routes. ChE inhibition is the focus of this hazard 
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characterization; the availability of reliable ChE inhibition dose response data is one of the key 

determinants in evaluating this toxicology database. 

Figure 1. Adverse outcome pathway for OPs

4.1 Toxicology Studies Available for Analysis

The toxicology database for tribufos is complete for risk assessment. There are acceptable 

studies available for toxicity endpoint selection; they include: 

 chronic oral toxicity studies in rats and dogs 

 carcinogenicity studies in rats and mice 

 developmental studies in rats and rabbits 

 reproduction study in rats 

 acute and subchronic neurotoxicity studies in rats

 developmental neurotoxicity (DNT) study in rats 

 acute and repeated comparative cholinesterase (ChE) studies in juvenile and adult 

rats

 repeated, gestational ChE study in pregnant rats and fetuses

 delayed neurotoxicity study in hens

 subchronic dermal toxicity in rabbits 

 subchronic inhalation study in rats 

 immunotoxicity study in rats 

 gene mutation, chromosome aberrations and unscheduled DNA synthesis studies 

 metabolism studies in rats

4.2 Absorption, Distribution, Metabolism, & Elimination (ADME)

Some OPs require metabolic activation to the oxon metabolite. However, tribufos is not one of 

these OPs (see chemical structure in Table 3.1). Instead, tribufos as the parent active ingredient, 

is a ChE-inhibiting chemical. Generally, absorption and distribution are rapid with extensive 

metabolism and no accumulation in the tissues. 

In a rat metabolism study (MRID 42034501), [1- C

14] tribufos (97% ai) was administered to male 

and female Sprague-Dawley rats (5 animals/sex/dose) by gavage as a single dose at levels of 5 or 

100 mg/kg, or as a single dose at 5 mg/kg following a 14-day pretreatment with unlabeled 

tribufos at 5 mg/kg/day. Five additional rats/sex received a single 5 mg/kg intravenous dose.

Target 

Tissue Dose

Phosphorylation 

of the active site 

of ChE

Neurotoxicity Accumulation 

of acetylcholine
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Fifty-five to 80% of tribufos was absorbed into the bloodstream. The majority of the radioactive 

dose was excreted within 72 hours through urine (55-80%), followed by feces (15-42%), with 

<1% in the expired air. Females showed more radioactive compound in the urine than males for 

each dose regimen. There was no significant tissue residue. Only two excreted compounds 

could be identified, unmetabolized tribufos, the major compound found in the feces and butylgamma-glutamylcysteinylglycine disulfide, a minor metabolite, in the urine. The data indicate 

complete metabolic breakdown of absorbed tribufos with extensive breakdown of the S-butyl 

moieties. No evidence of sulfonation or glucuronidation was observed based on treatment with 

sulfatase or b-glucuronidase.

4.2.1 Dermal Absorption

No guideline dermal penetration study was available; however, the dermal absorption factor was 

not needed for tribufos because a 21-day dermal toxicity study, which showed RBC ChE inhibition 

in rabbits at 2 mg/kg/day, was available (MRID# 42007201). The similarity of the dermal and oral 

PODs following BMD modeling indicates that tribufos is highly absorbed via the dermal route. 

4.3 Toxicological Effects

4.3.1 Toxicology Profile

The toxicology database for tribufos is complete with respect to the regulatory requirements and 

provides evidence that like other organophosphates (OPs), tribufos inhibits cholinesterase in 

dogs, rabbits, rats, and mice. Furthermore, ChE inhibition is readily seen via all exposure routes 

and at all durations. The toxicology profile for tribufos has acceptable dose-response data across 

multiple lifestages, durations, and routes for both RBC and brain ChE inhibition. Many of these 

studies have been evaluated using benchmark dose (BMD) modeling techniques. The data

indicate that RBC ChE inhibition is generally more sensitive than brain ChE inhibition. Also, 

RBC data provided the best fit for BMD modeling. There is no evidence to suggest sex-specific 

sensitivity or that pregnant females are more sensitive than non-pregnant females. Juveniles 

appear to be more sensitive than adults following acute, but not repeated dosing. Also, fetuses do 

not appear to be more sensitive than pregnant females. Dermal and inhalation studies with a 

ChE component are available and allow route-specific evaluations. 

In the subchronic neurotoxicity study, brain and RBC ChE activity were decreased starting at the 

mid-dose (2.89/3.54 mg/kg/day [M/F]) but no cholinergic signs were observed up to the highest 

dose tested (36.8/42.6 mg/kg/day [M/F]). In the developmental neurotoxicity study, rat brain and 

RBC ChE activity were decreased starting at 3.4 mg/kg/day while cholinergic signs (tremors) 

were observed at the highest dose tested (16.4 mg/kg/day) in pregnant rats. Decreased body 

weights, clinical signs (delayed surface righting reflex, decreased motor activity, decreased 

startle response) as well as decreased plasma (but not RBC or brain) ChE were also observed in 

the offspring at 16.4 mg/kg/day. Tribufos is not a developmental or a reproductive toxicant. In 

the developmental neurotoxicity study, neurotoxic effects were observed in the offspring at the 

same dose as maternal effects. In chronic studies conducted with mice, rats and dogs, toxicity 

was manifested as inhibition of plasma, RBC and/or brain ChE activity and ocular damage (rats 

only). Tribufos is not immunotoxic. 
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The HED Cancer Peer Review Committee (CPRC) [now designated as Cancer Assessment 

Review Committee (CARC)] classified tribufos as “likely” at high doses but “unlikely” to be a 

human carcinogen at low doses. All tumor increases in mice occurred only at the highest dose 

tested (48 mg/kg/day in males and 63 mg/kg/day in females) and were accompanied by severe 

ChE inhibition and increased mortality in the later weeks of the study (TXR# 0012242, E. Rinde 

and R. Zendzian, 22-MAY-1997). No tumors were found in rats. The CPRC recommended a 

non-quantitative approach for the purpose of risk characterization utilizing the most sensitive 

toxic endpoint. In addition, there was no apparent concern for mutagenicity, and no structural 

analogs of concern were identified. 

With regard to acute toxicity, technical tribufos is placed in Toxicity Category II by the oral and 

dermal routes and Category III by the inhalation route. Dermal and eye irritation response is 

assigned to Toxicity Category IV. Tribufos is not a dermal sensitizer. In an acute neurotoxicity 

study, a single oral administration to rats resulted in signs of neurotoxicity (decreased motor 

activity) and inhibition of plasma and RBC ChE activity in both sexes at the same dose.

4.3.2 Critical Durations of Exposure

One of the key elements in risk assessment is the appropriate integration of temporality between 

the exposure and hazard assessments. One advantage of an AOP understanding is that human 

health risk assessments can be refined, focused on the most relevant durations of exposure. OPs 

exhibit a phenomenon known as steady state ChE inhibition. After repeated dosing at the same 

dose level, the degree of inhibition comes into equilibrium with the production of new, uninhibited 

enzyme. At this point, the amount of ChE inhibition at a given dose remains consistent across 

duration. In general, OPs reach steady state within 2-3 weeks and as such, this is considered a 

relevant duration of exposure.

4.4 Literature Review on Neurodevelopment Effects

For the OPs, historically the Agency has used inhibition of AChE as the POD for human health 

risk assessment; at present time, this policy continues. This science policy is based on decades 

of work which shows that AChE inhibition is the initial event in the pathway to acute cholinergic 

neurotoxicity. The use of AChE inhibition data for deriving PODs was supported by the FIFRA 

SAP (2008, 2012) for chlorpyrifos as the most robust source of dose-response data for 

extrapolating risk and is the source of data for PODs for tribufos. A detailed review of the 

epidemiological studies used in this review can be found either in the 2014 chlorpyrifos risk 

assessment (D424485, D. Drew et al, 12/29/2014) or in the 2015 literature review for other 

organophosphates (OPP/USEPA; D331251; 9/15/15). 

Newer lines of research on OPs in the areas of potential AOPs, in vivo animal studies, and 

notably epidemiological studies in mothers and children, have raised some uncertainty about the 

agency’s risk assessment approach with regard to the potential for neurodevelopmental effects in 

fetuses and children. Many of these studies have been the subject of review by the agency over 

the last several years as part of efforts to develop a risk assessment for chlorpyrifos (D424485, 

D. Drew et al, 12/29/2014). Initially, the agency focused on studies from three US cohorts: 1) 
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The Mothers and Newborn Study of North Manhattan and South Bronx performed by the 

Columbia Children’s Center for Environmental Health (CCCEH) at Columbia University; 2) the 

Mt. Sinai Inner-City Toxicants, Child Growth and Development Study or the “Mt. Sinai Child 

Growth and Development Study;” and 3) the Center for Health Assessment of Mothers and 

Children of Salinas Valley (CHAMACOS) conducted by researchers at University of California 

Berkeley. The agency has evaluated these studies and sought external peer review (FIFRA SAP 

reviews in 2008 and 2012; federal panel, 20132) and concludes they are of high quality. In the 3 

US cohort epidemiology studies, mother-infant pairs were recruited for the purpose of studying 

the potential health effects of environmental exposures during pregnancy on subsequent child 

development. Each of these cohorts evaluated the association between prenatal chlorpyrifos 

and/or OP exposure (with adverse neurodevelopmental outcomes in children through age 7 

years. For the 2014 chlorpyrifos revised human health risk assessment (D424485, D. Drew et al, 

12/29/ 2014), EPA included epidemiologic research results from these three US prospective 

birth cohort studies but primarily focused on the results of CCCEH since this cohort has 

published studies on the association between cord blood levels of chlorpyrifos and 

neurodevelopmental outcomes. The agency retained the FQPA 10X Safety Factor (SF) in the

2014 chlorpyrifos revised risk assessment, in large part, based on the findings of these studies.

In the 2015 updated literature review (OPP/USEPA; D331251; 9/15/15), the agency conducted a 

systematic review expanding the scope of the 2012/2014 review focused on US cohort studies 

with particular emphasis on chlorpyrifos. The expanded 2015 review includes consideration of 

the epidemiological data on any OP pesticide, studies designs besides prospective cohort studies, 

and non-U.S. based studies. The updated literature review identified eight studies which were 

relevant (Bouchard et al., 2010; Fortenberry et al., 2014; Fortenberry et al., 2014a; Furlong et al., 

2014; Guodong et al., 2012; Oulhote et al., 2013; Zhang et al., 2014; Shelton et al., 2014). 

These eight studies have been evaluated in context with studies from the 2012/2014 review 

(D424485, D. Drew et al, 12/29/2014). Only a brief summary is provided below.

The OP exposure being assessed in many of these studies used concentrations of urinary dialkyl 

phosphate metabolites (DAPs) as the urinary biomarker. Total DAPs is a non-specific measure 

of OP exposure and is the sum of six separate molecules - three dimethyl alkylphosphate 

(DMAP) molecules of DMP, DMTP, DMDTP, and three diethyl alkylphosphate (DEAP) 

molecules of DEP, DETP, and DEDTP. Each metabolite is a breakdown product from multiple 

OPs (Table 4.4.1; CDC, 2008)3. Specifically, DMP, DMTP, and DMDTP are associated with 

18, 13, and 5 OPs, whereas DEP, DETP, and DEDTP are associated with 10, 10, and 4 OPs, 

respectively. Thus, using urinary DAPs alone as an exposure measure, it is not possible to 

separate the exposure and associated effects for single, specific OPs. 

Table 4.4.1.CDC Table of organophosphate pesticides and their dialkyl phosphate metabolites (2008). 

Pesticide DMP DMTP DMDTP DEP DETP DEDTP

Azinphos methyl X X X

Chlorethoxyphos X X

Chlorpyrifos X X

 

2 http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2008-0850-0170

3 http://www.cdc.gov/nchs/data/nhanes/nhanes_03_04/l26opd_c_met_organophosphorus_pesticides.pdf
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Chlorpyrifos methyl X X

Coumaphos X X

Dichlorvos (DDVP) X

Diazinon X X

Dicrotophos X

Dimethoate X X X

Disulfoton X X X

Ethion X X X

Fenitrothion X X

Fenthion X X

Isazaphos-methyl X X

Malathion X X X

Methidathion X X X

Methyl parathion X X

Naled X

Oxydemeton-methyl X X

Parathion X X

Phorate X X X

Phosmet X X X

Pirimiphos-methyl X X

Sulfotepp X X

Temephos X X

Terbufos X X X

Tetrachlorviphos X

Trichlorfon X

DMP = dimethylphosphate; DEP = diethylphosphate; DMTP = dimethylthiophosphate; DMDTP = 

dimethyldithiophosphate; DETP = diethylthiophosphate; DEDTP = diethyldithiophosphate.

For studies which measured urinary 3,5,6-trichloro-2-pyridinol (TCPy) (e.g., Fortenberry et al, 

2014; Eskenazi et al, 2007; Whyatt et al, 2009), this metabolite can be derived from chlorpyrifos, 

chlorpyrifos-methyl, and the herbicide triclopyr. TCPy is also the primary environmental 

degradate of chlorpyrifos, chlorpyrifos-methyl, and triclopyr; thus exposure can be found 

directly on food treated with these pesticides. CCCEH studies have largely used chlorpyrifos 

measured in cord blood as the specific biomarker (e.g., Lovasi et al, 2010; Whyatt et al, 2004; 

Rauh et al, 2011). The CHARGE study (Shelton et al, 2015) did not measure biomarkers but 

instead used geospatial analysis to focus on the residential proximity to OP exposure using data 

from the California Department of Pesticide Regulation, with five OPs accounting for a total of 

73% of the pesticide applied near residential settings (chlorpyrifos, acephate, diazinon,

bensulide, and dimethoate). 

Similarly, DAPs can be found directly on food following OP applications (Zhang et al, 2008; 

Chen et al, 2012). Specifically, studies have shown that DAPs may form as environmental 

degradates abiotic hydrolysis, photolysis, and plant metabolism (Zhang et al, 2008; Chen et al, 

2012; Racke et al, 1994). Furthermore, since these DAPs are excreted more rapidly and 

extensively than the parent OPs (Zhang et al, 2008; Forsberg et al, 2008), direct exposure to 

DAPs may lead to an overestimate of OP exposure when using urinary DAPs as a biomarker of 
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OP exposure. The agency recognizes that this is a source of uncertainty when using DAPs for 

assessing OP exposure and will continue to monitor this issue in future assessments. 

With respect to neurological effects near birth, the CHAMACOS and Mt. Sinai cohorts measured 

neurological effects at birth (the Brazelton index), and observed a putative association with total 

DEAP, total DMAP, and total DAP exposure (Engel et al., 2007; Young et al., 2005). Similarly, 

a Chinese study (Zhang et al., 2014) reported statistically significant associations between for 

total DEAPs, total DMAPs, and total DAPs prenatal OP pesticide exposure and neonatal 

neurodevelopment assessed 3 days after birth. However, another cross-sectional Chinese study, 

Guodong et al (2012), observed no association with these exposures and a developmental 

quotient score for 23-25 month old children.

The 3 US cohorts (CCCEH, Mt. Sinai, CHAMACOS) each reported evidence of impaired mental 

and psychomotor development, albeit not consistent by age at time of testing (ranging from 6 

month to 36 months across the three cohorts). Attentional problems and ADHD were reported 

by three prospective cohorts [Rauh et al, 2006; Eskenazi et al., 2007; Marks et al, 2010; and 

Fortenberry et al (2014, 2014a)] investigators with additional support from a case control study, 

Bouchard et al. (2010). The exposure metric varied among these studies. Specifically, 

Fortenberry et al (2014) found suggestive evidence of an association with TCPy and ADHD in 

boys whereas statistically significant associations were observed by Rauh et al (2006) with 

chlorpyrifos exposure and ADHD. Eskenazi et al (2007) reported associated with total DMAPs 

and total DAPs and ADHD; Marks et al (2010) reported associations with total DEAP, DMAP, 

and total DAP exposure and ADHD. In a national cross-sectional study of Canadian children 

2007-2009 data for age 6-11 years (Oulhote et al., 2013), there were no overall statistically 

significant associations observed between child urinary DEAP, DMAP, or total DAP metabolite 

levels and parentally reported behavioral problems. In contrast, Bouchard et al. (2010), looking 

at U.S. children age 8-15 years in the 2000-2004 National Health and Nutrition Examination 

Survey (NHANES), observed a positive association between attention and behavior problems 

and total DAPs and DMAPs, but not DEAPs. As part of their analysis, Oulhote et al (2013) 

noted that their outcome assessment for behavioral problems may not have been as sensitive as 

Bouchard et al (2010), which may in part account for the difference in the observed results from 

these studies. 

In addition, the 3 US cohorts and the CHARGE study have reported suggestive or positive 

associations between OP exposure and autism spectrum disorders (Rauh et al., 2006; Shelton et 

al., 2014; Eskenazi et al, 2007; Furlong et al., 2014). Specifically, Furlong et al (2014) 

documented suggestive evidence of an association between total DEAP exposure and reciprocal 

social responsiveness among blacks and boys. Eskenazi et al (2007) reported a statistically 

significant association between pervasive developmental disorder (PDD) and total DAP 

exposure, whereas Eskenazi et al (2010) reported non-significant, but suggestive, increased odds 

of PDD of 2.0 (0.8 to 5.1; p=0.14). Rauh et al (2006) documented a significant association 

between PDD and specifically chlorpyrifos exposure. Both PDD and reciprocal social 

responsiveness are related to the autism spectrum disorder. Using a different exposure 

assessment method (geospatial analysis and residential proximity to total OP exposure), Shelton 

et al (2014) also showed statistically significant associations between total OP exposure and 

ASD. While these studies vary in the magnitude of the overall strength of association, they have 
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consistently observed a positive association between OP exposure and ASD. Finally, CCCEH, 

Mt. Sinai, CHAMACOS have reported an inverse relation between the respective prenatal 

measures of chlorpyrifos and intelligence measures at age 7 years (Rauh et al. ,2011; Engel et al., 

2011; Bouchard et al., 2011). 

Across the epidemiology database of studies, the maternal urine, cord blood, and other 

(meconium) measures provide evidence that exposure did occur to the fetus during gestation but 

the actual level of such exposure during the critical window(s) of susceptibility is not known. 

While significant uncertainties remain about the actual exposure levels experienced by mothers 

and infant participants in the children’s health cohorts, it is unlikely that these exposures resulted 

in AChE inhibition. As part of the CHAMACOS study, Eskenazi et al. (2004) measured AChE 

activity and showed that no differences in AChE activity were observed. The biomarker data 

(chlorpyrifos) from the Columbia University studies are supported by the agency’s dose 

reconstruction analysis using the PBPK-PD model. Following the recommendation of the 

FIFRA SAP (2012), the agency conducted a dose reconstruction analysis of residential uses 

available prior to 2000 for pregnant women and young children inside the home. The PBPK-PD 

model results indicate for the highest exposure considered (i.e., indoor broadcast use of a 1% 

chlorpyrifos formulation) <1% RBC AChE inhibition was produced in pregnant women. While 

uncertainty exists as to actual OP exposure at (unknown) critical windows of exposure, EPA 

believes it is unlikely individuals in the epidemiology studies experienced RBC AChE inhibition.

A review of the scientific literature on potential modes of action/adverse outcome pathways 

(MOA/AOP)4leading to effects on the developing brain was conducted for the 2012 FIFRA SAP 

meeting (USEPA, 2012) and updated for the December 2014 chlorpyrifos revised risk 

assessment (D424485, D. Drew et al, 12/29/2014). In short, multiple biologically plausible 

hypotheses and pathways are being pursued by researchers that include targets other than AChE 

inhibition, including cholinergic and non-cholinergic systems, signaling pathways, proteins, and 

others. However, no one pathway has sufficient data to be considered more credible than the 

others. The fact that there are, however, sparse AOP data to support the in vitro to in vivo

extrapolation, or the extrapolation from biological perturbation to adverse consequence 

significantly limits their quantitative use in risk assessment. The SAP concurred with the agency 

in 2008 and 2012 about the lack of definable key events in a MOA/AOP leading to 

developmental neurobehavioral effects. However, since the 2014 literature review, there are no 

substantive changes in the ability to define and quantitate steps in an MOA/AOP leading from 

exposure to effects on the developing brain. Published and submitted guideline DNT laboratory 

animal studies have been reviewed for OPs as part of the 2012/2014 review (D424485, D. Drew 

et al, 12/29/2014) and the updated 2015 review (OPP/USEPA; D331251; 9/15/15). 

Neurobehavioral alterations in laboratory animals were often reported, albeit at AChE inhibiting 

doses, but there was generally a lack of consistency in terms of pattern, timing, or dose-response 

for these effects, and a number of studies were of lower quality. However, this information does 

provide evidence of long-lasting neurodevelopmental disorders in rats and mice following 

gestational exposure.

 

4 Mode of action (MOA) and adverse outcome pathways (AOPs) describe a set of measureable key events that make 

up the biological processes leading to an adverse outcome and the causal linkages between such events. 
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At this time, a MOA(s)/AOP(s) has/have not been established for neurodevelopmental outcomes. 

This growing body of literature does demonstrate, however, that OPs are biologically active on a 

number of processes that affect the developing brain. Moreover, there is a large body of in vivo

laboratory studies which show long-term behavioral effects from early life exposure, albeit at 

doses which cause AChE inhibition. EPA considers the results of the toxicological studies 

relevant to the human population, as qualitatively supported by the results of epidemiology 

studies. The agency acknowledges the lack of established MOA/AOP pathway and uncertainties 

associated lack of ability to make strong causal linkages and unknown window(s) of 

susceptibility. These uncertainties do not undermine or reduce the confidence in the findings of 

the epidemiology studies. The epidemiology studies reviewed in the 2012/2014 and 2015 

literature reviews represent different investigators, locations, points in time, exposure assessment 

procedures, and outcome measurements. Despite all these differences in study design, with the 

exception of two negative studies in the 2015 literature review (Guodong et al, 2012; Oulhote et 

al, 2013), authors have identified associations with neurodevelopmental outcomes associated 

with OP exposure across four cohorts and twelve study citations. Specifically, there is evidence 

of delays in mental development in infants (24-36 months), attention problems and autism 

spectrum disorder in early childhood, and intelligence decrements in school age children who 

were exposed to OPs during gestation. Investigators reported strong measures of statistical 

association across several of these evaluations (odds ratios 2-4 fold increased in some instances), 

and observed evidence of exposures-response trends in some instances, e.g., intelligence 

measures.

As section 408(b)(2)(C) of the FFDCA instructs EPA, in making its “reasonable certainty of no 

harm” finding, that in “the case of threshold effects, an additional tenfold margin of safety for the 

pesticide chemical residue and other sources of exposure shall be applied for infants and children 

to take into account potential pre- and postnatal toxicity and completeness of data with respect to 

exposure and toxicity to infants and children.” Section 408 (b)(2)(C) further states that “the 

Administrator may use a different margin of safety for the pesticide chemical residue only if, on 

the basis of reliable data, such margin will be safe for infants and children.” Given the totality of 

the evidence, there is sufficient uncertainty in the human dose-response relationship for 

neurodevelopmental effects which prevents the agency from reducing or removing the statutory 

10X FQPA Safety Factor. For the tribufos DRA, a value of 10X has been applied. Similarly, a 

database uncertainty factor of 10X will be retained for occupational risk assessments. The 

agency will continue to evaluate the epidemiology studies and pursue approaches for quantitative 

or semi-quantitative comparisons between doses which elicit AChE inhibition and those which 

are associated with neurodevelopmental outcomes prior to a revised human health risk 

assessment. 

4.5 Safety Factor for Infants and Children (FQPA SF)

As noted above, the lack of an established MOA/AOP makes quantitative use of the 

epidemiology studies in risk assessment challenging, particularly with respect to determining 

dose-response, critical duration of exposure, and window(s) of susceptibility. However, 

exposure levels in the range measured in the epidemiology studies are likely low enough that 

they are unlikely to result in AChE inhibition. Epidemiology studies consistently identified 

associations with neurodevelopmental outcomes associated with OP exposure such as delays in 
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mental development in infants (24-36 months), attention problems and autism spectrum disorder 

in early childhood, and intelligence decrements in school age children. Therefore, there is a need 

to protect children from exposures that may cause these effects; this need prevents the agency 

from reducing or removing the statutory FQPA Safety Factor. Thus, the FQPA 10X Safety 

Factor will be retained for tribufos for the population subgroups that include infants, 

children, youths, and women of childbearing age for all exposure scenarios. 

4.5.1 Completeness of the Toxicology Database

The database of toxicology studies for tribufos is complete and adequate to assess potential risk to 

infants and children. It consists of developmental studies in rat and rabbit, a developmental 

neurotoxicity study, a reproductive toxicity study, dermal and inhalation subchronic studies, and 

oral comparative ChE studies with three components (acute, repeated, gestational). 

As discussed in Section 4.4, there is uncertainty in the human dose-response relationship for 

neurodevelopmental effects and this warrants retention of the FQPA Safety Factor for the 

population subgroups that include infants, children, youths, and women of childbearing age for 

all exposure scenarios.

4.5.2 Evidence of Neurotoxicity

Tribufos is an OP with a neurotoxic AOP; neurotoxicity is the most sensitive effect in all species, 

routes, and lifestages. Consequently, ChE inhibition, as an indicator of neurotoxicity, is being used 

to derive the PoDs. CNS neuropathy was observed in hens and retinal damage was observed in 

the chronic rat studies but these effects occurred at doses much higher than the PoDs.

4.5.3 Evidence of Sensitivity/Susceptibility in the Developing or Young Animal

A reproductive toxicity study and a developmental neurotoxicity (DNT) study in rats as well as 

rat and rabbit developmental studies are available and do not show any quantitative 

susceptibility. The CCA studies are available and provide ChE dose-response data from 

potentially susceptible lifestages (i.e., fetuses, pregnant dams, post-natal pups). Quantitative

susceptibility was observed in pups compared to adults in the acute comparative ChE (CCA) 

studies but no quantitative or qualitative susceptibility was observed in repeated-dosing studies. 

As discussed in Section 4.4, there is uncertainty in the human dose-response relationship for 

neurodevelopmental effects and this warrants retention of the FQPA Safety Factor for the 

population subgroups that include infants, children, youths, and women of childbearing age for 

all exposure scenarios.

4.5.4 Residual Uncertainty in the Exposure Database

There are no residual uncertainties in the exposure database. Since the dietary and non-dietary 

exposure estimates were based on several conservative assumptions, HED does not believe the 

exposure assessment underestimates risk. These assumptions include tolerance-level residues 

and 100% crop treated estimates for all commodities. Furthermore, conservative, upper-bound 
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assumptions were used to determine exposure through drinking water and residential sources, 

such that these exposures have not been underestimated.

4.6 Toxicity Endpoint and Point of Departure Selections

4.6.1 Dose-Response Assessment

Although multiple datasets provided a poor fit for BMD modeling (see appendix 2), sufficient 

information was available for endpoint selection. Table 4.5.4.1 summarizes the tribufos toxicity 

endpoints and PoDs selected from an evaluation of the database. This endpoint selection was 

based on a weight of the evidence evaluation using the following considerations:

 Relative sensitivity of the brain and RBC compartments: For tribufos, across most 

studies, durations, lifestages, and routes, the RBC compartment tends to be more 

sensitive than the brain. For the OP cumulative risk assessment, OPP used brain data in 

PoD derivations as these data represented the target tissue and tended to be less variable 

than RBC data. In the case of tribufos; however, the brain data in general did not model 

well because often there was no dose-response. In cases where brain data did model, 

RBC data provided more protective endpoints. 

 Potentially susceptible populations (fetuses, juveniles, pregnancy): The available ChE 

data across multiple lifestages (adults, pregnant adults, fetuses, juveniles) show evidence 

of quantitative life stage sensitivity following acute exposure. This finding has driven the 

selection of the PND 11 pup RBC BMDLs to derive the PoDs for the acute dietary 

endpoint.

 Route of exposure: It is preferred to match, to the degree possible, the route of exposure 

in the toxicity study with that of the exposure scenario(s) of interest. In the case of 

tribufos, there are oral, dermal, and inhalation studies which contain adequate doseresponse ChE data.

 Duration of exposure: It is preferred to match, to the degree possible, the duration of 

toxicity study with that of the exposure duration of interest. In the case of tribufos, there 

are single day and steady-state oral studies as well as steady-state dermal and inhalation 

studies.

 Consistency across studies: In cases where multiple datasets are available for a single 

duration, it is important to evaluate the extent to which data are consistent (or not) across 

studies. Although there were limited data sets that modeled, the tribufos database has 

generally consistent data across studies which allows the PoDs to be derived from several 

critical studies. 

Descriptions of the primary toxicity studies used for selecting toxicity endpoints and PoDs for 

various exposure scenarios are presented in Appendix 3 of this document. Summary tables of 

BMD analyses can be found in Appendix 2 and the technical details of the analysis can be found 

in the BMD memo (R. Bever and E. Holman, D423990, 6/17/2015).
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Consistent with risk assessments for other ChE-inhibiting compounds, OPP has used a 

benchmark response (BMR) level of 10% and has thus calculated BMD10s and BMDL10s (See 

Appendix 1 for summary of OPP’s ChE policy). The BMD10 is the estimated dose where ChE is 

inhibited by 10% compared to background. The BMDL10 is the lower confidence bound on the 

BMD10 value. As a matter of science policy, the agency uses the BMDL, not the BMD, for 

selection of the PoDs (USEPA, 2012). All BMD/BMDL modeling for all individual datasets 

was completed using USEPA BMD Software, version 2.4; an exponential model was used to fit 

the data, with the assumption of constant variance across each dataset. BMD results from the OP 

Cumulative Risk Assessment (CRA 2002, 2006) were included in the endpoint selection weight 

of evidence evaluation.

Acute Dietary (all populations, including females 13-49) 

As shown in Table 4.5.4.1, results of the acute CCA study (MRID# 48707704) with tribufos 

provide the best dataset for this exposure scenario (RBC ChE data in the PND 11 male pups). 

The BMDL10 of 1.05 mg/kg/day associated with RBC ChE inhibition was selected as a suitable 

PoD for the acute dietary (all populations) exposure scenario. The corresponding BMD10 was 

1.41 mg/kg/day.

RBC cholinesterase inhibition data were selected for the PoD because it provided the best fit for 

BMD modeling and RBC cholinesterase appears to be more sensitive than the brain ChE. 

Findings from the PND 11 pups represent the highly exposed sub-populations (infants and young 

children) and are, thus, appropriate for the PoD derivation. 

An uncertainty factor of 100X (10X to account for interspecies extrapolation and 10X for 

intraspecies variation) is applied to the BMDL10 to obtain an aRfD of 0.01 mg/kg/day. Based on 

the FQPA safety factor of 10X due to uncertainty in the human dose-response relationship for 

neurodevelopmental effects (See Section 4.4), the acute population adjusted dose (aPAD) is 

0.001 mg/kg/day, except for the population subgroup of adults 50-99 where the FQPA SF does 

not apply (aPAD for this subgroup = 0.01 mg/kg/day).

Steady-State Dietary (all populations)

Table 4.3.2.1 also shows that the PoD for the steady-state dietary (all populations) scenario was 

derived from the subchronic oral neurotoxicity study (MRID 45369101), based on RBC ChE 

inhibition in adult female rats. The BMDL10 of 0.19 mg/kg/day associated with RBC ChE was 

selected as a suitable PoD for the steady-state dietary exposure assessment (all populations). No 

difference in sensitivity to RBC ChE inhibition was observed between pups and adult animals 

following repeated exposure, thus the selected endpoint is protective of all populations. The 

corresponding BMD10 was 0.27 mg/kg/day; this value represents the lowest BMD in the 

database. 

An uncertainty factor of 100X (10X to account for interspecies extrapolation and 10X for 

intraspecies variation) is applied to the BMDL10 to obtain an ssRfD of 0.002 mg/kg/day. Based 

on the FQPA safety factor of 10X due to uncertainty in the human dose-response relationship for 
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neurodevelopmental effects (See Section 4.4), the steady-state PAD (ssPAD) is 0.0002 

mg/kg/day, except for the population subgroup of adults 50-99 where the FQPA SF does not 

apply (aPAD for this subgroup = 0.01 mg/kg/day).

Incidental Oral, Steady State

For the purpose of assessing potential risk associated with incidental oral exposure from steady 

state durations resulting from spray drift, HED selected the subchronic oral neurotoxicity study 

(MRID# 45369101), based on RBC ChE inhibition in adult female rats. Quantitation of 

incidental oral risks was performed using the BMDL10 value of 0.19 mg/kg/day and the BMD10

of 0.27 mg/kg/day. Because no difference in sensitivity was observed between pups and adult 

rats in repeated-dosing studies (see table A.5), these values are considered protective of the subpopulations of interest (infants, toddlers, young children) for incidental oral exposure. An 

uncertainty factor of 1000X (10X to account for interspecies extrapolation, 10X for intraspecies 

variation, and a 10X FQPA SF) is applied for incidental oral exposure. 

Dermal, Steady-State 

A 21-day repeated dose dermal study in rabbits (MRID# 42007201) that measured parameters in 

the functional observational battery (FOB) as well as ChE inhibition was the only study available 

for the dermal risk assessment. Consequently, the PoD for the steady-state exposure scenario 

was selected from this study and was based on RBC ChE inhibition (BMDL10 = 0.16 mg/kg/day)

in adult female rats. An uncertainty factor of 1000X (10X to account for interspecies 

extrapolation, 10X for intraspecies variation, and a 10X FQPA SF for residential assessments or 

a database uncertainty factor in occupational assessments due to uncertainty in the human doseresponse relationship for neurodevelopmental effects (see Section 4.4)) is applied for dermal 

exposure. 

Inhalation, Single Day and Steady-State

The PoD for the inhalation steady-state exposure scenario was selected from the 13-week 

inhalation study in rats (MRID# 42399801) based on RBC ChE inhibition in adult male rats

(BMDL10 = 0.79 mg/m3/day; BMD10 = 1.00 mg/m3/day). Inhalation dosimetry was applied to 

the derived PoDs in accordance with EPA guidelines (EPA, 1994) to obtain a human equivalent 

concentration (HEC) and human equivalent dose (HED). HECs were generated from this study 

because exposure was via an aerosol, which resulted in a regional deposited dose ratio (RDDR) 

of 2.926. HECs were then converted to HEDs, which are 0.1 mg/kg/day for the BMDL10 and 

0.12 mg/kg/day for the BMD, respectively. For steady state inhalation exposures, a total 

uncertainty factor of 300X was applied [3X for interspecies extrapolation, 10X for intraspecies 

variation and a 10X FQPA SF for residential assessments or a database uncertainty factor in 

occupational assessments due to uncertainty in the human dose-response relationship for 

neurodevelopmental effects (see Section 4.4)].

4.6.2 Recommendation for Combining Routes of Exposures for Risk Assessment

PoDs for the oral, dermal, and inhalation routes are all derived from RBC ChE inhibition. 

Therefore, there is a common endpoint and exposures can be combined.
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4.6.3 Cancer Classification and Risk Assessment Recommendation

The HED Cancer Peer Review Committee (CPRC) [now designated as Cancer Assessment 

Review Committee (CARC)] classified tribufos as “likely” at high doses but “unlikely” to be a 

human carcinogen at low doses since all tumor increases in mice occurred only at the highest 

dose tested (48 mg/kg/day in males and 63 mg/kg/day in females) and were accompanied by 

severe ChE inhibition and increased mortality in the later weeks of the study (TXR# 0012242, E. 

Rinde and R. Zendzian, 22-MAY-1997). The CPRC concluded that a low-dose linear 

extrapolation risk assessment based on the tumors is not recommended because of the severe 

accompanying toxicity (ChE inhibition) typical of OPs, which occurred at all doses tested in 

mice. Therefore, a non-linear approach (i.e., RfD) will adequately account for all chronic 

toxicity, including carcinogenicity, that could result from exposure to tribufos.

4.6.4 Summary of Points of Departure and Toxicity Endpoints Used in Human Risk 

Assessment

Table 4.5.4.1 Summary of Toxicological Doses and Endpoints and Points of Departure 

for Tribufos for Use in Dietary, Non-Dietary and Occupational Human Health Risk 

Assessments

Exposure

Scenario

Point of 

Departure

(mg/kg/day)

Uncertainty/FQPA 

Safety Factorsa

RfD, PAD, Level of 

Concern for Risk 

Assessment

Study and 

Toxicological

Effects

Acute Dietary 

(all populations, 

including 

females 13-49)

BMDL10 = 1.05 

mg/kg/day

UFA = 10x

UFH = 10x

FQPA SF = 10x

Acute RfD = 0.01 

mg/kg/day

Acute PAD = 0.001 

mg/kg/day

Acute CCA Study 

(MRID 48707704) in

the rat – PND 11

BMD10 = 1.41 

mg/kg/day

Inhibition of RBC ChE 

in male rat pups.

Steady-State 

Dietary (all 

populations)

BMDL10 = 0.19 

mg/kg/day

UFA = 10x

UFH = 10x

FQPA SF = 10x

Steady State RfD = 

0.002 mg/kg/day

Steady State PAD = 

0.0002 mg/kg/day

Subchronic oral 

neurotoxicity study 

(MIRD 45369101)

BMD10 = 0.27 

mg/kg/day

Inhibition of RBC ChE 

in adult female rats

Incidental Oral

Steady State 

BMDL10 = 0.19 

mg/kg/day

UFA = 10x

UFH = 10x

FQPA SF = 10x

LOC for MOE = 1000 Subchronic oral 

neurotoxicity study 

(MRID 45369101)

BMD10 = 0.27 

mg/kg/day
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BMD = Benchmark dose. BMDL = Benchmark dose lower confidence limit. HED = Human equivalent dose. Point 

of Departure (PoD) = A data point or an estimated point that is derived from observed dose-response data and used 

to mark the beginning of extrapolation to determine risk associated with lower environmentally relevant human 

exposures. LOC = level of concern. MOE = margin of exposure. UF = uncertainty factor. UFA = extrapolation from 

animal to human (interspecies). UFH = potential variation in sensitivity among members of the human population 

(intraspecies). UFDB = database uncertainty factor. FQPA SF = FQPA Safety Factor. PAD = population adjusted 

dose.

a FQPA SF retained for infants, children, youths, and women of childbearing age for all exposure scenarios due to 

uncertainty in the human dose-response relationship for neurodevelopmental effects (see Section 4.4). This includes 

all exposure scenarios, except the dietary exposure scenarios for the population subgroup adults 50-99 for which the 

FQPA SF has been reduced to 1X (aPAD = 0.01 mg/kg/day; ssPAD = 0.002 mg/kg/day). In occupational dermal 

and inhalation assessments, a 10X database uncertainty factor is applied due to this uncertainty. 

4.7 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 tribufos, 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), tribufos is subject 

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

Inhibition of RBC ChE 

in adult female rats

Dermal

Steady State

BMDL10 = 0.16 

mg/kg/day

UFA = 10x

UFH = 10x

FQPA SF/UFDB = 

10x

LOC for MOE = 1000

21-day dermal toxicity 

study (MRID 

42007201) in rabbits

BMD10 = 0.22

mg/kg/day

Inhibition of RBC ChE 

in adult female rats.

Inhalation

Steady State

BMDL10 = 0.79 

mg/m3

(HED = 0.10 

mg/kg/day)

UFA = 3x

UFH = 10x 

FQPA SF/UFDB = 

10x

LOC for MOE = 300

13-Week inhalation 

toxicity study (MRID 

42399801)

BMD10 = 1.00 mg/m3

(HED = 0.12 

mg/kg/day)

Inhibition of RBC ChE 

in adult male rats.

Cancer (oral, 

dermal, 

inhalation)

Classification: "Likely at high doses but unlikely to be carcinogenic to humans at low doses”
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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 

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, 20135and includes some 

pesticides scheduled for Registration Review and chemicals found in water. Neither of these lists

should be construed as a list of known or likely endocrine disruptors. 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.6

5.0 Dietary Exposure and Risk Assessment 

5.1 Metabolite/Degradate Residue Profile

The qualitative nature of the residues of tribufos in plants (cottonseed) and livestock has been 

adequately identified/characterized and is understood. The HED Metabolism Assessment 

Review Committee (MARC) reviewed the tribufos toxicology and metabolism data and 

recommended that the residue of concern for both tolerance expression and risk assessment for 

plants and livestock should consist of tribufos only. In addition, HED previously determined that 

tolerances for tribufos residues in eggs and poultry tissues were not required based on the results 

of the respective poultry metabolism and feeding studies (CFR §180.6(a)(3)). These conclusions 

have not changed for the purpose of registration review.

Table 5.1. Summary of Metabolites and Degradates of Tribufos.

Matrix Residues included in Risk 

Assessment

Residues included in Tolerance 

Expression 

Plants Primary Crop

Rotational Tribufos Tribufos

Crop

 

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

chemicals.

6 http://www.epa.gov/endo/
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Table 5.1. Summary of Metabolites and Degradates of Tribufos.

Matrix Residues included in Risk 

Assessment

Residues included in Tolerance 

Expression 

Poultry1 Not Required Not Required

Ruminant1 Tribufos Tribufos

Drinking Water Tribufos Not Applicable

1 Tolerances are no longer needed for livestock commodities.

5.2 Food Residue Profile

Tribufos is only registered for use on cotton, and dietary exposure to tribufos can occur via 

residues present in cottonseed oil or as a result of transfer of residues from livestock feed items 

(cotton gin-byproducts, cottonseed hulls and cottonseed meal) to meat and milk. Residues are 

most likely to be surface residues from ground or foliar application on cotton and quantifiable 

residues in harvested crops. Environmental fate data suggest that residues are persistent in soil. 

Based on the animal metabolism data, residues of tribufos may end up in ruminant tissues and 

milk at low levels as a result of feeding livestock treated feed items. However, since ruminants

are fed only 5% of cottonseed and cotton gin byproducts, the expected secondary residues are

minimal. Additionally, based on the livestock feeding study and on an updated livestock diet, 

there are no reasonable expectations of finite residues of tribufos in/on any livestock commodity.

Therefore, the established tolerances for residues of tribufos in/on milk, eggs, cattle, goat, horse, 

hog, poultry, and sheep commodities (40 CFR §180.272) should all be revoked.

5.3 Water Residue Profile

The estimated drinking water concentrations (EDWCs) for tribufos residues in surface water 

were generated using the Surface Water Concentration Calculator (SWCC) computer model, 

while the Pesticide Root Zone Model for Groundwater (PRZM-GW) and Screening 

Concentration in Ground Water (SCI-GROW) models were used to generate groundwater 

EDWCs. The residue of concern (ROC) for tribufos is limited to the parent compound. Some 

OPs require metabolic activation to the oxon metabolite. However, tribufos is not one of these 

OPs and is already in the oxon form.

The labeled maximum annual and single application rate in the U.S. (except CA and AZ) is 1.125 

lbs a.i./A, allowing only one application per season. To further refine the drinking water assessment

in response to a request from the HED, EFED utilized average application rates rather than the 

maximum application rates that were used in the initial assessment. Additionally, a lower spray 

drift fraction (13.5%, decreased from 16%) was utilized, per current EFED guidance (USEPA, 

2013). Only the scenario (Texas cotton OP; aerial spray application) which resulted in the 

maximum surface water EDWC in the prior assessment was modeled for this refinement. In this 

refinement, as in the initial assessment, EFED assumed that only a single growing season for cotton 

may occur within a single year and a single application is made (See Table 5.3).
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For the acute and steady state assessments, the entire 30-year distribution of estimated daily 

concentrations was incorporated into the Pesticide Root Zone Model and Exposure Analysis 

Modeling System (PRZM/EXAMs) and used in the probabilistic analyses. For steady state the 

TX cotton maximum and average rate daily time series were recalculated using the 21-day 

forward rolling averages. In the 21-day rolling average distributions, the first data point is the 

average of days 1-21, the second data point is the average of days 2-22, the third data point is the 

average of days 3-23, etc. 

Table 5.3. EDWCs for the Maximum and Average Use of Tribufos on Cotton 

Drinking Water 

Source (Model 

Used)

Use (Rate 

Modeled)

Maximum Estimated Drinking Water 

Concentration (EDWC)

Acute (μg/L) Chronic (μg/L)

Cancer 

(μg/L)

Maximum EDWC for the Use of Tribufos on Cotton at Maximum Application Rate

Surface water

(SWCC; TX 

Cotton OP 

scenario)

Cotton

1.125 lbs a.i./A x 1 

application/yr; 

aerial

7.26 3.34 2.46

Groundwater 

(PRZM-GW)

Cotton

1.875 lbs a.i./A x 1 

application/yr

1.83E-09 1.8E-10  ̄

Groundwater 

(SCI-GROW)

Cotton

1.875 lbs a.i./A x 1 

application/yr

1.12E-02 1.12E-02  ̄

Surface Water EDWCs for the Aerial Use of Tribufos on Cotton at the Average 

Application Rate

Surface water

(SWCC; TX 

Cotton OP 

scenario)

Cotton

0.54 lbs a.i./A x 1 

application/yr; 

aerial

3.36 1.56 1.15

Surface water

(SWCC; CA 

Cotton w/irrig 

scenario)

Cotton

0.64 lbs a.i./A x 1 

application/yr; 

aerial

1.362 0.90 0.67

1While the higher application rate for CA and AZ was also modeled, the lower rate modeled using the TX 

scenario resulted in higher surface water EDWCs. 

2Modeled as a single application at the average rate, although the average number of applications for CA is 

reported as 1.2.

5.4 Dietary Risk Assessment

5.4.1 Description of Residue Data Used in Dietary Assessment

Acute and steady state dietary (food and drinking water) exposure and risk assessments were 

conducted using the Dietary Exposure Evaluation Model software with the Food Commodity 

Intake Database (DEEM-FCID) Version 3.16. This software uses 2003-2008 food consumption 

data from the USDA’s NHANES/WWEIA. The drinking water estimates provided by EFED are 

modeled estimates, and were generated using the most conservative model available for the 

existing uses. The labeled maximum annual and average application rates for cotton were 
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considered as a refinement to the initial assessment. The partially refined assessments assumed 

anticipated residues (average residue in cottonseed oil), an empirical processing factor for 

cottonseed oil, and 100% crop treated (PCT) for the acute assessment and steady state 

assessments. 

5.4.2 Percent Crop Treated Used in Dietary Assessment

Percent crop treated information were not used in these assessments.

5.4.3 Acute Dietary Risk Assessment

The acute aggregate (food and drinking water) dietary exposure estimates are below HED’s level 

of concern (i.e., <100% of the aPAD) for the U.S. population and all population subgroups. 

Combined dietary exposure from cottonseed oil and drinking water concentrations based on 

maximum rates in TX at the 95th percentile of exposure is 15% of the aPAD for the U.S. 

population and 48% of the aPAD for all infants (<1 yr old), the most highly exposed population 

subgroup. Also, the combined dietary exposure from cottonseed oil and drinking water 

concentrations based on “average” rates in TX at the 95th percentile of exposure is 7% of the 

aPAD for the U.S. population and 22% of the aPAD for all infants (<1 yr old), the most highly 

exposed population subgroup. The food only exposure contributes <1 for and drinking water 

only contributes 48% of the aPAD for all infants. The summary results for the general 

population and all population subgroups are shown below in Tables 5.4.3 and 5.4.4.

Table 5.4.3. Results of Acute Dietary Exposure Analysis for Tribufos Food only and Drinking 

Water only, and Food plus Drinking Water using Maximum Rate EDWCs.

Population 

Subgroup

aPAD

(mg/kg/day)1

Food only Drinking Water only 

(Maximum Rate)

Food plus Drinking 

Water (Maximum 

Rate)

95th Percentile 95th Percentile 95th Percentile

Exposure 

(mg/kg/day)

% 

aPAD

Exposure 

(mg/kg/day)

% 

aPAD

Exposure 

(mg/kg/day)

% 

aPAD

General U.S. 

Population

0.001

0.000002 <1 0.000153 15 0.000154 15

All Infants 

(<1 year old)

0.001

0.000001 <1 0.000476 48 0.000476 48

Children 1-2 

years old

0.001

0.000005 <1 0.000232 23 0.000234 23

Children 3-5 

years old

0.001

0.000005 <1 0.000189 19 0.000192 19

Children 6-

12 years old

0.001

0.000004 <1 0.000141 14 0.000143 14

Youth 13-19 

years old

0.001

0.000002 <1 0.000123 12 0.000124 12

Adults 20-49 

years old

0.001

0.000002 <1 0.000151 15 0.000151 2

Adults 50-99 

years old

0.01

0.000001 <1 0.000138 1 0.000139 14
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Females 13-

49 years old

0.001

0.000002 <1 0.000153 15 0.000154 15

1 Highest exposure at the 95th percentile is in bold.

2 mkd = mg/kg/day. aPAD = acute population-adjusted dose.

Table 5.4.4. Results of Acute Dietary Exposure Analysis for Tribufos Food only and Drinking 

Water only, and Food plus Drinking Water using Average Rate EDWCs.

Population 

Subgroup

aPAD

(mg/kg/day)1

Food only Drinking Water only 

(Average Rate)

Food plus Drinking 

Water (Average Rate)

95th Percentile 95th Percentile 95th Percentile

Exposure 

(mg/kg/day)

% 

aPAD

Exposure 

(mg/kg/day)

% 

aPAD

Exposure 

(mg/kg/day)

% 

aPAD

General U.S. 

Population

0.001

0.000002 <1 0.000071 7 0.000072 7

All Infants 

(<1 year old)

0.001

0.000001 <1 0.000222 22 0.000222 22

Children 1-2 

years old

0.001

0.000005 <1 0.000108 11 0.000111 11

Children 3-5 

years old

0.001

0.000005 <1 0.000088 9 0.000091 9

Children 6-

12 years old

0.001

0.000004 <1 0.000066 7 0.000068 7

Youth 13-19 

years old

0.001

0.000002 <1 0.000057 6 0.000058 6

Adults 20-49 

years old

0.001

0.000002 <1 0.000070 7 0.000071 7

Adults 50-99 

years old

0.01

0.000001 <1 0.000064 <1 0.000065 <1

Females 13-

49 years old

0.001

0.000002 <1 0.000071 7 0.000072 7

1 Highest exposure at the 95th percentile is in bold.

2 mkd = mg/kg/day. aPAD = acute population-adjusted dose.

5.4.4 Steady State Dietary Risk Assessment

The DEEM acute module was used to conduct the steady state assessment. 2-day average food 

residues, representing average food exposure, and 21-day forward-rolling water averages were 

compared to the steady state (ss) point of departure. The 2-day average dietary exposure 

assessment incorporated anticipated residues for cottonseed oil, empirical processing factors for 

cottonseed oil, and 100 percent crop treated (CT).

Combined dietary exposure from cottonseed oil and drinking water concentrations based on 

maximum rates in TX at the 95th percentile of exposure is 72% of the ssPAD for the U.S. 

population and 226% of the ssPAD for all infants (<1 yr old), the most highly exposed 

population subgroup. The combined dietary exposure from cottonseed oil and TX cotton 

“average” rate for drinking water at the 95th percentile of exposure is 34% of the ssPAD for the 
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U.S. population and 105% of the ssPAD for all infants (<1 yr old), the most highly exposed 

population subgroup. The food only exposure contributes <2% of the ssPAD and drinking water 

only contributes 226% of the ssPAD for all infants. The summary results for the general 

population and all population subgroups are shown below in Table 5.4.5 and 5.4.6.

 Table 5.4.5. Results of Steady-State Dietary Exposure Analysis for Terbufos Food only, Drinking 

Water only, and Food plus Drinking Water-using Maximum Rate EDWCs.

Population 

Subgroup

ssPAD

(mg/kg/day)1

Food only Drinking Water only 

(Maximum Rate)

Food plus Drinking 

Water (Maximum 

Rate)

95th Percentile 95th Percentile 95th Percentile

Exposure 

(mg/kg/day)

% 

ssPAD

Exposure 

(mg/kg/day)

% 

ssPAD

Exposure 

(mg/kg/day)

% 

ssPAD

General 

U.S. 

Population 0.0002 0.000002 <1 0.000143 72 0.000144 72

All Infants 

(<1 year 

old) 0.0002 0.000001 <1 0.000451 226 0.000452 226

Children 1-2 

years old 0.0002 0.000005 2 0.000216 108 0.000218 109

Children 3-5 

years old 0.0002 0.000005 2 0.000172 86 0.000175 87

Children 6-

12 years old 0.0002 0.000003 2 0.000129 64 0.000131 65

Youth 13-19 

years old 0.0002 0.000002 <1 0.000114 57 0.000115 57

Adults 20-

49 years old 0.0002 0.000002 <1 0.000141 70 0.000142 71

Adults 50-

99 years old 0.002 0.000001 <1 0.000132 7

0.000132 7

Females 13-

49 years old 0.0002 0.000002 <1 0.000144 72 0.000144 72

1 Highest exposure at the 95th percentile is in bold.

2 mkd = mg/kg/day. aPAD = acute population-adjusted dose.

 Table 5.4.6. Results of Steady-State Dietary Exposure Analysis for Terbufos Food only, Drinking 

Water only, and Food plus Drinking Water-using Average Rate EDWCs.

Population 

Subgroup

ssPAD

(mg/kg/day)1

Food only Drinking Water only 

(Average Rate)

Food plus Drinking 

Water (Average Rate)

95th Percentile 95th Percentile 95th Percentile

Exposure 

(mg/kg/day)

% 

ssPAD

Exposure 

(mg/kg/day)

% 

ssPAD

Exposure 

(mg/kg/day)

% 

ssPAD

General U.S. 

Population

0.00001 0.000002 <1 0.000067 33 0.000068 34
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All Infants 

(<1 year old)

0.00001 0.000001 <1 0.000211 105 0.000211 105

Children 1-2 

years old 0.00001 0.000005 2 0.000101 50 0.000103 52

Children 3-5 

years old 0.00001 0.000005 2 0.000080 40 0.000083 41

Children 6-12 

years old 0.00001 0.000003 2 0.000060 30 0.000062 31

Youth 13-19 

years old 0.00001 0.000002 <1 0.000053 27 0.000054 27

Adults 20-49 

years old 0.00001 0.000002 <1 0.000066 33 0.000066 33

Adults 50-99 

years old 0.0001 0.000001 <1 0.000061 3

0.000062 3

Females 13-

49 years old 0.00001 0.000002 <1 0.000067 34 0.000068 34

1 Highest exposure at the 95th percentile is in bold.

2 mkd = mg/kg/day. aPAD = acute population-adjusted dose.

5.4.5 Cancer Dietary Risk Assessment

Tribufos is classified as “Likely at high doses, but unlikely to be carcinogenic to humans at low 

doses” (TXR# 0012242, E. Rinde and R. Zendzian, 22-MAY-1997). The steady state dietary 

exposure and risk estimates are protective of potential carcinogenicity.

6.0 Residential (Non-Occupational) Exposure/Risk Characterization

There are currently no registered residential uses for tribufos; therefore a residential handler and 

post-application exposure assessment was not performed. However, an assessment was 

conducted in consideration of potential bystander exposures to spray drift following tribufos 

applications to cotton. 

6.1 Non-Occupational Bystander Post-Application Inhalation Exposure and Risk 

Estimates

Volatilization of pesticides may be a source of post-application inhalation exposure to 

individuals nearby pesticide applications. The agency sought expert advice and input on issues 

related to volatilization of pesticides from its Federal Insecticide, Fungicide, and Rodenticide Act 

Scientific Advisory Panel (SAP) in December 2009, and received the SAP’s final report on 

March 2, 2010 (http://www.epa.gov/scipoly/SAP/meetings/2009/120109meeting.html). The 

agency has evaluated the SAP report and has developed a Volatilization Screening Tool and a 

subsequent Volatilization Screening Analysis

(http://www.regulations.gov/#!docketDetail;D=EPA-HQ-OPP-2014-0219). 

During Registration Review, the agency will utilize this analysis to determine if data (i.e., flux 

studies, route-specific inhalation toxicological studies) or further analysis is required for tribufos.
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6.2 Non-Occupational Spray Drift Exposures and Risk Estimates

Off-target movement of pesticides can occur via many types of pathways and it is governed by a 

variety of factors. Sprays that are released and do not deposit in the application area end up offtarget and can lead to exposures to those it may directly contact. They can also deposit on 

surfaces where contact with residues can eventually lead to indirect exposures (e.g., children 

playing on lawns where residues have deposited next to treated fields). The potential risk 

estimates from these residues can be calculated using drift modeling onto 50 feet wide lawns 

coupled with methods employed for residential risk assessments for turf products.

The approach to be used for quantitatively incorporating spray drift into risk assessment is based 

on a premise of compliant applications which, by definition, should not result in direct exposures 

to individuals because of existing label language and other regulatory requirements intended to 

prevent them.7 Direct exposures would include inhalation of the spray plume or being sprayed 

directly. Rather, the exposures addressed here are thought to occur indirectly through contact 

with impacted areas, such as residential lawns, when compliant applications are conducted. 

Given this premise, exposures for children (1 to 2 years old) and adults who have contact with 

turf where residues are assumed to have deposited via spray drift thus resulting in an indirect 

exposure are the focus of this analysis analogous to how exposures to turf products are 

considered in risk assessment. 

In order to evaluate the drift potential and associated risks, an approach based on drift modeling 

coupled with techniques used to evaluate residential uses of pesticides was utilized. Essentially, a 

residential turf assessment based on exposure to deposited residues has been completed to 

address drift from the agricultural applications of tribufos. In the spray drift scenario, the 

deposited residue value was determined based on the amount of spray drift that may occur at 

varying distances from the edge of the treated field using the AgDrift (v2.1.1) model and the 

Residential Exposure Assessment Standard Operating Procedures Addenda 1: Consideration of 

Spray Drift Policy. Once the deposited residue values were determined, the remainder of the 

spray drift assessment was based on the algorithms and input values specified in the recently 

revised (2012) Standard Operating Procedures For Residential Risk Assessment (SOPs). 

A screening approach was developed based on the use of the AgDrift model in situations where 

specific label guidance that defines application parameters is not available.8 AgDrift is 

appropriate for use only when applications are made by aircraft, airblast orchard sprayers, and 

ground boom sprayers. When AgDrift was developed, a series of screening values (i.e., the Tier 

1 option) were incorporated into the model and represent each equipment type and use under 

varied conditions. The screening options specifically recommended in this methodology were 

selected because they are plausible and represent a reasonable upper bound level of drift for 

common application methods in agriculture. These screening options are consistent with how 

spray drift is considered in a number of ecological risk assessments and in the process used to 

develop drinking water concentrations used for risk assessment. In all cases, each scenario is to 

be evaluated unless it is not plausible based on the anticipated use pattern (e.g., herbicides are 

 

7 This approach is consistent with the requirements of the EPA’s Worker Protection Standard.

8http://www.agdrift.com/ 
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not typically applied to tree canopies) or specific label prohibitions (e.g., aerial applications are 

not allowed). 

In many cases, risks are of concern when the screening level estimates for spray drift are used as 

the basis for the analysis. In order to account for this issue and to provide additional risk 

management options additional spray drift deposition fractions were also considered. These drift 

estimates represent plausible options for pesticide labels.

6.2.1 Combined Risk Estimates from Lawn Deposition Adjacent to Applications

The spray drift risk estimates are based on an estimated deposited residue concentration as a 

result of the screening level agricultural application scenarios. Tribufos is used on cotton and 

can be applied via ground boom and aerial equipment at a maximum application rate of 1.875 lbs 

ai/acre for the states CA and AZ and 1.125 lb ai/acre for other US states. The recommended drift 

scenario screening level options are listed below: 

 Groundboom applications are based on the AgDrift option for high boom height and 

using very fine to fine spray type using the 90th percentile results. 

 Aerial applications are based on the use of AgDrift Tier 1 aerial option for a fine to 

medium spray type and a series of other parameters which will be described in more 

detail below (e.g., wind vector assumed to be 10 mph in a downwind direction for entire 

application/drift event).9

In addition to the screening level spray drift scenarios described above, additional results are 

provided using viable drift reduction technologies (DRTs). In particular, different spray qualities 

have been considered as well as the impact of other application conditions (e.g., boom height, 

use of a helicopter instead of fixed wing aircraft, crop canopy conditions).

For adults, when an endpoint is not sex-specific (i.e., the endpoints are based on developmental 

or fetal effects) a body weight of 80 kg is typically used in risk assessment; however, in this case, 

a female-specific body weight of 69 kg was used. While the endpoint of concern, RBC AChE 

inhibition, is not sex-specific, the female-specific body weight was used to protect for pregnant 

women due to uncertainty in the human dose-response relationship for neurodevelopmental 

effects (See Section 4.4). 

Adult (dermal only) and children 1 to <2 years of age (combined dermal and incidental oral) risk 

estimates indicate that there are concerns for the indirect tribufos exposure on turf near aerial and 

ground applications to cotton. Dermal and incidental oral risk estimates for children 1 to <2 

years old were combined because the toxicity endpoint for each route of exposure is the 

inhibition of brain cholinesterase (ChE). The total applicable LOC is 1,000, so MOEs < 1,000 

represent risk estimates of concern. Adult dermal risk estimates from indirect exposure to 

tribufos related to spray drift result in buffers of over 300 feet for groundboom and aerial 

applications for the application rates of 1.875 lb ai/A and 1.125 lb ai/A. Children’s (1 to <2 

 

9 AgDrift allows for consideration of even finer spray patterns characterized as very fine to fine. However, this 

spray pattern was not selected as the common screening basis since it is used less commonly for most agriculture.
Tribufos Human Health Risk Assessment DP No. D357537

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years old) combined dermal and incidental oral risk estimates from indirect exposure to tribufos 

are of concern at 300 feet for groundboom and aerial applications based on the screening level 

scenarios for both application rates. Appropriate DRTs such as changing the spray type/nozzle 

configuration to coarser spray applications may result in less drift and reduced risk concerns (i.e., 

higher MOEs) from aerial applications. Similarly, using coarser sprays and lowering boom 

height for ground boom sprayers reduces risk concerns. Spray drift analysis results are presented 

in Tables 6.2.1.1 and 6.2.1.2.
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Tables: 6.2.1.1: Adult Dermal Spray Drift Analysis for Tribufos

Crop/Rate 

Group

Spray Type/ Nozzle 

Configuration

Application

Rate (lb 

ai/A)

Estimated or 

Adjusted 

TTR 

(ug/cm2)

At Edge 10 Feet 25 Feet 50 Feet 75 Feet 100 Feet 125 Feet 150 Feet 200 Feet 250 Feet 300 Feet

Dermal MOE Dermal 

MOE

Dermal 

MOE Dermal MOE Dermal MOE

Dermal 

MOE

Dermal 

MOE

Dermal 

MOE

Dermal 

MOE

Dermal

MOE

Dermal 

MOE

Aerial

Fine to Medium

1.875 0.2084063

1 1 1 2 2 3 3 4 5 6 7

Medium to Coarse 1 1 2 2 3 5 6 7 9 12 15

Coarse to Very Coarse 1 2 2 4 5 7 9 11 15 20 25

Very Fine to Fine 1 1 1 1 1 1 1 1 2 2 2

AT401, M, 10 mph, 37% SD 1 1 1 2 3 3 4 5 6 8 9

WASP, M, 10 mph, 37% SD 1 1 2 2 3 4 5 6 8 9 11

AT401, C, 10 mph, 25% SD 1 1 2 3 4 5 7 8 12 15 18

WASP, C, 10 mph, 25% SD 1 2 2 4 5 7 9 11 15 20 22

AT401, VC, 10 mph, 20% SD 1 2 3 5 6 9 11 14 20 25 33

WASP, VC, 10 mph, 20% SD 1 2 3 5 8 10 14 16 25 28 33

Groundboom

High Boom Very fine to Fine 1 2 4 6 8 10 12 14 18 25 28

Low Boom Very fine to Fine 2 6 10 15 20 25 28 33 39 49 65

High Boom Fine to 

Medium/Coarse 4 10 15 22 28 33 39 39 49 65 65

Low Boom Fine to 

Medium/Coarse 6 16 25 33 39 49 65 65 98 98 98

Aerial

Fine to Medium

1.125 0.1250438

1 2 2 3 3 4 5 6 8 10 12

Medium to Coarse 2 2 3 4 6 7 9 11 16 20 25

Coarse to Very Coarse 2 3 4 6 9 12 15 18 25 33 41

Very Fine to Fine 1 1 1 1 1 2 2 2 3 3 3

AT401, M, 10 mph, 37% SD 1 2 2 3 4 5 7 8 10 13 16

WASP, M, 10 mph, 37% SD 2 2 3 4 5 7 8 10 13 16 18

AT401, C, 10 mph, 25% SD 2 2 3 5 7 9 11 14 19 25 30

WASP, C, 10 mph, 25% SD 2 3 4 6 9 12 14 18 25 33 36

AT401, VC, 10 mph, 20% SD 2 3 5 7 11 14 18 23 33 41 54

WASP, VC, 10 mph, 20% SD 2 4 6 9 13 17 23 27 41 47 54

Groundboom

High Boom Very fine to Fine 2 4 6 9 13 16 19 23 30 41 47

Low Boom Very fine to Fine 4 10 16 25 33 41 47 54 65 82 110

High Boom Fine to 

Medium/Coarse 7 17 25 36 47 54 65 65 82 110 110

Low Boom Fine to 

Medium/Coarse 10 27 41 54 65 82 110 110 160 160 160

Bolded number are risk estimates of concern (MOE <1,000)
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Tables: 6.2.1.2: Tribufos Combined Incidental Oral and Dermal Spray Drift Analysis for Children (Ages 1<2 Years Old) 

Crop/Rate 

Group

Spray Type/ Nozzle 

Configuration

Application 

Rate (lb 

ai/A)

Estimated 

or Adjusted 

TTRt

(ug/cm2)

At Edge 10 Feet 25 Feet 50 Feet 75 Feet 100 Feet 125 Feet 150 Feet 200 Feet 250 Feet 300 Feet

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Combined 

MOE

Aerial

Fine to Medium

1.875 0.208406

0 1 1 1 1 2 2 2 3 3 4

Medium to Coarse 1 1 1 1 2 3 3 4 5 7 9

Coarse to Very 

Coarse 1 1 1 2 3 4 5 6 9 11 14

Very Fine to Fine 0 0 0 0 1 1 1 1 1 1 1

AT401, M, 10 mph, 

37% SD 0 1 1 1 1 2 2 3 4 4 5

WASP, M, 10 mph, 

37% SD 1 1 1 1 2 2 3 3 4 5 6

AT401, C, 10 mph, 

25% SD 1 1 1 2 2 3 4 5 7 9 10

WASP, C, 10 mph, 

25% SD 1 1 1 2 3 4 5 6 9 11 13

AT401, VC, 10 mph, 

20% SD 1 1 2 3 4 5 6 8 11 14 19

WASP, VC, 10 mph, 

20% SD 1 1 2 3 5 6 8 9 14 16 19

Groundboom

High Boom Very 

fine to Fine 1 1 2 3 5 6 7 8 10 14 16

Low Boom Very fine 

to Fine 1 4 6 9 11 14 16 19 23 28 38

High Boom Fine to 

Medium/Coarse 2 6 9 13 16 19 23 23 28 38 38

Low Boom Fine to 

Medium/Coarse 3 9 14 19 23 28 38 38 56 56 56

Aerial

Fine to Medium

1.125 0.125043

1 1 1 2 2 3 3 4 5 6 7

Medium to Coarse 1 1 2 2 3 4 5 7 9 12 14

Coarse to Very 

Coarse 1 2 2 4 5 7 9 10 14 19 24

Very Fine to Fine 1 1 1 1 1 1 1 1 2 2 2

AT401, M, 10 mph, 

37% SD 1 1 1 2 2 3 4 5 6 7 9

WASP, M, 10 mph, 

37% SD 1 1 2 2 3 4 5 6 7 9 10

AT401, C, 10 mph, 

25% SD 1 1 2 3 4 5 7 8 11 14 17

WASP, C, 10 mph, 

25% SD 1 2 2 4 5 7 8 10 14 19 21

AT401, VC, 10 mph, 

20% SD 1 2 3 4 6 8 10 13 19 24 31

WASP, VC, 10 mph, 

20% SD 1 2 3 5 8 10 13 16 24 27 31

Groundboom

High Boom Very 

fine to Fine 1 2 3 5 8 9 11 13 17 24 27

Low Boom Very fine 

to Fine 2 6 9 14 19 24 27 31 38 47 63

High Boom Fine to 

Medium/Coarse 4 10 14 21 27 31 38 38 47 63 63

Low Boom Fine to 

Medium/Coarse 6 16 24 31 38 47 63 63 94 94 94

Bolded number are risk estimates of concern. (MOE <1,000)
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7.0 Aggregate Exposure/Risk Characterization

In accordance with the FQPA, when there are potential residential exposures to a pesticide, 

aggregate risk assessment must consider exposures from three major routes: oral, dermal, and 

inhalation. There are three sources for these types of exposures: food, drinking water, and 

residential uses. In an aggregate assessment, exposures from relevant sources are added together 

and compared to quantitative estimates of hazard (e.g., a NOAEL or PAD), or the risks 

themselves can be aggregated. When aggregating exposures and risks from various sources, 

HED considers both the route and duration of exposure. The registered tribufos uses are not 

anticipated to result in residential exposure. 

7.1 Acute Aggregate Risk

The acute aggregate risk assessment is equivalent to the acute dietary risk assessment discussed 

in Section 5.4.3 above. 

7.2 Steady State Aggregate Risk

The steady state aggregate risk assessment is equivalent to the steady state dietary risk 

assessment discussed in Section 5.4.4 above. 

7.3 Cancer Aggregate Risk

Tribufos is classified as “likely” at high doses but “unlikely” to be a human carcinogen at low 

doses. The Agency has determined that the quantification of risk using a non-linear approach

(i.e., RfD) will adequately account for all chronic toxicity, including carcinogenicity, that could 

result from exposure to tribufos.

8.0 Cumulative Exposure/Risk Characterization

OPs, like tribufos, share the ability to inhibit AChE through phosphorylation of the serine residue 

on the enzyme leading to accumulation of acetylcholine and ultimately cholinergic neurotoxicity. 

This shared MOA/AOP is the basis for the OP common mechanism grouping per OPP’s 

Guidance For Identifying Pesticide Chemicals and Other Substances that have a Common 

Mechanism of Toxicity (USEPA, 1999). The 2002 and 2006 CRAs used brain AChE inhibition 

in female rats as the source of dose response data for the relative potency factors and PODs for 

each OP, including tribufos. Prior to the completion of Registration Review, OPP will update 

the OP CRA on AChE inhibition to incorporate new toxicity and exposure information available 

since 2006. 

As described in Section 4.4, OPP has retained the FQPA Safety Factor for OPs, including 

tribufos, due to uncertainties associated with neurodevelopmental effects in children and 

exposure to OPs. There is a lack of an established MOA/AOP for the neurodevelopment 

outcomes which precludes the agency from formally establishing a common mechanism group 

per the Guidance For Identifying Pesticide Chemicals and Other Substances that have a 

Common Mechanism of Toxicity (USEPA, 1999) based on that outcome. Moreover, the lack of a 
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recognized MOA/AOP and other uncertainties with exposure assessment in the epidemiology 

studies prevent the agency from establishing a causal relationship between OP exposure and 

neurodevelopmental outcomes. The agency will continue to evaluate the epidemiology studies 

associated with neurodevelopmental outcomes and OP exposure prior to the release of the 

revised PRA. During this period, the agency will determine whether or not it is appropriate to 

apply the draft guidance document entitled, Pesticide Cumulative Risk Assessment: Framework 

for Screening Analysis for the neurodevelopment outcomes. 

9.0 Occupational Exposure/Risk Characterization

9.1 Steady-State Handler Risk

HED uses the term handlers to describe those individuals who are involved in the pesticide 

application process. HED believes that there are distinct job functions or tasks related to 

applications and exposures can vary depending on the specifics of each task. Job requirements 

(amount of chemical used in each application), the kinds of equipment used, the target being 

treated, and the level of protection used by a handler can cause exposure levels to differ in a 

manner specific to each application event. 

Based on the anticipated use patterns and current labeling, types of equipment and techniques 

that can potentially be used, occupational handler exposure is expected from the registered uses 

of tribufos. The quantitative exposure/risk assessment developed for occupational handlers is 

based on the following scenarios: 

 Mixer/Loader for aerial and groundboom equipment;

 Applicators for aerial and groundboom equipment; and

 Flagger for aerial spray applications.

The above exposure scenarios best represent the registered use pattern (i.e., cotton defoliant 

application) for tribufos. Chemigation applications are prohibited on the product labels. 

Occupational Handler Exposure Data and Assumptions

A series of assumptions and exposure factors served as the basis for completing the occupational 

handler risk assessments. Each assumption and factor is detailed below on an individual basis.

Application Rate: Product labels indicate that tribufos can be used at a maximum single 

application rate of 1.875 lbs ai/A for the states CA and AZ and 1.125 lb ai/A for other states. 

The occupational handler assessment was completed assuming both single maximum application 

rates.

Unit Exposures: It is the policy of HED to use the best available data to assess handler exposure. 

Sources of generic handler data, used as surrogate data in the absence of chemical-specific data, 

include PHED 1.1, the AHETF database, or other registrant-submitted occupational exposure 

studies. Some of these data are proprietary (e.g., AHETF data), and subject to the data 

protection provisions of FIFRA. The standard values recommended for use in predicting handler 
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exposure that are used in this assessment, known as “unit exposures”, are outlined in the 

“Occupational Pesticide Handler Unit Exposure Surrogate Reference Table10”, which, along with 

additional information on HED policy on use of surrogate data, including descriptions of the 

various sources, can be found at the Agency website11. 

A chemical-specific handler exposure study (MRID 42685901) was reviewed and used in the 

2000 risk assessment. Data from the study were combined as per HED’s policy with other 

surrogate data from the Pesticide Handlers Exposure Database (PHED) to increase the sample 

size and numbers of studies (T. Leighton, D252221, January 26, 1999). The study was designed 

to determine the dermal and inhalation exposures to the workers. The study was conducted in 

California and Mississippi with application rates ranging from 1.127 lb ai/A to 1.875 lb ai/A. 

The worker exposures in this study, and subsequent MOEs, were determined from dosimetry 

data. The exposure assessment component of the study on which OPP is relying does not

measure or identify a toxic effect; therefore, review of the exposure assessment research by the 

Human Studies Review Board (HSRB) is not required (M. Lydon, Ethics Review of Exposure 

Research from 1993 Study of Worker Exposure to Tribufos during Aerial and Ground 

Application of DEF 6 to Cotton, MRID 42685901, 07/02/2015). 

Area Treated or Amount Handled: Based on HED EXPOSAC Policy 9.1, the amount treated or 

handled per day was assumed to be: 

 200 acres for mixing/loading ground boom applications and applying via ground 

equipment on cotton;

 1,200 acres for mixing/loading for aerial applications and applying via aircraft on cotton; 

and

 350 acres for flagging for aerial application on cotton crops.

 

Body Weight: For adults, when an endpoint is not sex-specific (i.e., the endpoints are based on 

developmental or fetal effects) a body weight of 80 kg is typically used in risk assessment; 

however, in this case, a female-specific body weight of 69 kg was used. While the endpoint of 

concern, RBC AChE inhibition, is not sex-specific, the female-specific body weight was used to 

protect for pregnant women due to uncertainty in the human dose-response relationship for 

neurodevelopmental effects (See Section 4.4). 

Since the steady-state dermal point of departure PoD was based on a route-specific toxicity 

study, no dermal absorption factor was necessary to estimate exposure. A route-specific 

inhalation study is available and was used in the assessment.

Exposure Duration: HED typically classifies exposures from 1 to 30 days as short-term and 

exposures 30 days to six months as intermediate-term. Exposure duration is determined by many 

things, including the exposed population, the use site, the pest pressure triggering the use of the 

pesticide, and the cultural practices surrounding that use site. For the organophosphate 

pesticides, the match between the toxicological endpoint selection and the exposure duration is 

different than HED’s approach for conventional chemicals. The endpoint selection for 

organophosphate pesticides involves a “single day” and “steady state” approach based on an 

 

10 Available: http://www.epa.gov/opp00001/science/handler-exposure-table.pdf

11 Available: http://www.epa.gov/pesticides/science/handler-exposure-data.html
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individual chemical’s cholinesterase inhibition. The steady state approach is appropriate for 

tribufos given the toxicological and exposure profile. The “steady state” endpoint selection for 

tribufos overlaps HED’s traditional short-term exposure duration endpoint selection and is 

considered protective for occupational handlers that apply commercially over longer periods of 

time (i.e., intermediate-term exposures).

Existing Mitigation/Personal Protective Equipment: Estimates of dermal and inhalation 

exposure were calculated based on the mitigation of agricultural uses from the July 2006 tribufos

RED. Current tribufos labels require closed systems for mixers/loaders for aerial applications

and all applicators and handlers must wear coveralls over baseline dermal wear in addition to 

gloves, protective eyewear, chemical resistant headgear of overhead exposure and chemicalresistant apron when cleaning, mixing or loading. 

Combining Exposures/Risk Estimates:

A total aggregate risk index (ARI) was used since the LOC values for dermal exposure (1,000) 

and inhalation exposure (300) are different. The target ARI is 1; therefore, ARIs of less than 1 

are of concern. The aggregate risk index (ARI) was calculated as follows.

Aggregate Risk Index (ARI) = 1÷ [(Dermal LOC ÷ Dermal MOE) + (Inhalation LOC ÷ Inhalation MOE)]

Summary of Occupational Handler Non-Cancer Exposure and Risk Estimates

HED has no data to assess exposure to pilots using open cockpits. The only data available is for 

exposure to pilots in enclosed cockpits. Therefore, risks to pilots are assessed assuming the use 

of engineering controls (enclosed cockpits) and baseline attire (long-sleeve shirt, long pants, 

shoes, and socks); pilots are not required to wear protective gloves. 

HED does not have unit exposure data for assessing dermal and inhalation exposure for tribufos 

flaggers with engineering controls. HED has presented flagger risk estimates at double layer 

baseline PPE (coveralls over long-sleeve shirt, long pants, shoes, socks, gloves) and PF10 

respirator.

The Agency matches quantitative occupational exposure assessment with appropriate 

characterization of exposure potential. While HED presents quantitative risk estimates for 

human flaggers where appropriate, agricultural aviation has changed dramatically over the past 

two decades. According the 2012 National Agricultural Aviation Association (NAAA) survey of 

their membership, the use of GPS for swath guidance in agricultural aviation has grown steadily 

from the mid 1990’s. Over the same time period, the use of human flaggers for aerial pesticide 

applications has decreased steadily from ~15% in the late 1990’s to only 1% in the most recent 

(2012) NAAA survey. The Agency will continue to monitor all available information sources to 

best assess and characterize the exposure potential for human flaggers in agricultural aerial 

applications.

In the Appendix 5, dermal and inhalation risk estimates are presented individually with varying 

PPE levels ranging from baseline (i.e., no gloves and no respirator) through PF10 respirator and 

engineering controls. Dermal MOEs appear to be the driver and range from 0.17 to 3.9 with 
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labeled PPE 0.57 to 9.6 with ECs. Inhalation MOEs ranged from 54 to 1,000 with labeled PPE 

(See Appendix Table A.5.1 and Table A.5.2 highlighted MOEs). 

All of the occupational handler exposure scenarios resulted in combined dermal and inhalation 

risk estimates of concern (i.e., ARIs are < 1) at the label-defined PPE for occupational handlers, 

and all additional levels of PPE and ECs assessed. Occupational handler risk estimates are 

presented in the following table 9.1.1. 
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Table 9.1.1. Steady-State Occupational Exposure and Risk Estimates for Tribufos Total ARI 

Exposure Scenario Application 

Rate1

Amount 

Handled / 

Area Treated2

ARI3

Worker 

Activity

Formula

tion

Applic

ation 

Equip

ment

Value Units Value Units SL/No G 

+ No-R

SL/G + 

No-R

DL/G + 

No-R

SL/No 

G + 

PF5 R

SL/G 

+ PF5 

R

DL/G + 

PF5 R

SL/No 

G + 

PF10 R

SL/G 

+ PF10 

R

DL/G 

+ PF10 

R

EC

M/L L/SC/EC

Aerial

1.875

lb ai/A

1200

A

0.00002 0.00013 0.00017 0.00002 0.00013 0.00017 0.00002 0.00013 0.00017 0.00057

1.125 0.00004 0.00022 0.00028 0.00004 0.00022 0.00028 0.00004 0.00022 0.00028 0.00095

Ground

boom

1.875

200

0.00013 0.00078 0.00100 0.00013 0.00078 0.00100 0.00013 0.00078 0.00100 0.00340

1.125 0.00022 0.00130 0.00170 0.00022 0.00130 0.00170 0.00022 0.00130 0.00170 0.00570

Applicator

Spray

Aerial 1.875

lb ai/A

1200

A

No Data No Data No Data No 

Data

No 

Data

No 

Data

No 

Data

No 

Data

No 

Data 0.00240

Aerial 1.125 1200 No Data No Data No Data No 

Data

No 

Data

No 

Data

No 

Data

No 

Data

No 

Data 0.00390

Ground

boom 1.875 200 0.00037 0.00180 0.00230 0.00037 0.00180 0.00230 0.00037 0.00180 0.00230 0.00580

Ground

boom 1.125 200 0.00062 0.00300 0.00380 0.00062 0.00300 0.00390 0.00062 0.00300 0.00390 0.00960

Flagger Aerial

1.875

lb ai/A 350 A

0.00150 0.00140 0.00160 0.00150 0.00140 0.00160 0.00150 0.00140 0.00160 No Data

1.125 0.00250 0.00230 0.00260 0.00250 0.00230 0.00260 0.00250 0.00230 0.00260 No Data

1. Based on registered label (e.g. Reg. No. 5481-504, 5481-903 or EPA Reg. No. 264-730, 34704-867, and 85678-15).

2. Exposure Science Advisory Council Policy #9.1

3. Total Aggregate Risk Index (ARI) = 1÷ [(Dermal LOC ÷ Dermal MOE) + (Inhalation LOC ÷ Inhalation MOE). Bolded numbers are risk estimates of concern (ARI<1). Highlighted risk 

estimates are label defined PPE.

SL = single layer, DL = double layer, G = gloves, EC = engineering controls, R = respirator
Tribufos Human Health Risk Assessment DP No. D357537

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9.2 Steady State Post-Application Risk

HED uses the term post-application to describe exposures that occur when individuals are 

present in an environment that has been previously treated with a pesticide (also referred to as reentry exposure). Such exposures may occur when workers enter previously treated areas to 

perform job functions, including activities related to crop production, such as scouting for pests 

or harvesting. Post-application exposure levels vary over time and depend on such things as the 

type of activity, the nature of the crop or target that was treated, the type of pesticide application, 

and the chemical’s degradation properties. In addition, the timing of pesticide applications, 

relative to harvest activities, can greatly reduce the potential for post-application exposure.

9.2.1 Occupational Post-application Inhalation Exposure/Risk Estimates

There are multiple potential sources of post-application inhalation exposure to individuals 

performing post-application activities in previously treated fields. These potential sources 

include volatilization of pesticides and resuspension of dusts and/or particulates that contain 

pesticides. The Agency sought expert advice and input on issues related to volatilization of 

pesticides from its Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel 

(SAP) in December 2009, and received the SAP’s final report on March 2, 201012. The Agency 

has evaluated the SAP report and has developed a Volatilization Screening Tool and a 

subsequent Volatilization Screening Analysis13. During Registration Review, the agency will 

utilize this analysis to determine if data (i.e., flux studies, route-specific inhalation toxicological 

studies) or further analysis is required for tribufos.

In addition, the Agency is continuing to evaluate the available post-application inhalation 

exposure data generated by the Agricultural Reentry Task Force. Given these two efforts, the 

Agency will continue to identify the need for and, subsequently, the way to incorporate 

occupational post-application inhalation exposure into the agency's risk assessments.

9.2.2 Occupational Post-application Dermal Exposure/Risk Estimates

Occupational Post-application Dermal Exposure Data and Assumptions

A series of assumptions and exposure factors served as the basis for completing the occupational 

post-application risk assessments. Each assumption and factor is detailed below on an individual 

basis.

Exposure Duration: The endpoint selection for organophosphate pesticides involves a “single 

day” and “steady state” approach based on an individual chemical’s cholinesterase inhibition. 

The steady state approach is appropriate for tribufos given the toxicological and exposure profile. 

The “steady state” endpoint selection for tribufos overlaps HED’s traditional short-term exposure 

 

12 http://www.epa.gov/scipoly/SAP/meetings/2009/120109meeting.html

13 http://www.regulations.gov/#!docketDetail;D=EPA-HQ-OPP-2014-0219. 
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duration endpoint selection as well as being appropriately health protective for workers that are 

exposed over longer periods of time (i.e., intermediate-term exposures).

Transfer Coefficients: It is the policy of the HED to use the best available data to assess postapplication exposure. Sources of generic post-application data, used as surrogate data in the 

absence of chemical-specific data, are derived from ARTF exposure monitoring studies, and, as 

proprietary data, are subject to the data protection provisions of FIFRA. HED standard values 

for “transfer coefficients”, are presented in the EXPOSAC Policy 314” which, along with 

additional information about the ARTF data, can be found at the Agency website15. Policy 3 

includes the standard transfer coefficient (TC) values for mechanically harvesting cotton based 

on a tribufos chemical specific study (MRID No. 42701601). The study was reviewed by EPA 

(B. Tarplee, D227007, 02/12/1997) and then re-reviewed in 1999 (T. Leighton, D252221 

01/26/1999) and was a two part worker monitoring study that determined dermal and inhalation 

exposures of workers conducting mechanical harvesting activities for cotton, and, in addition, 

collected cotton boll samples for residues. The transfer coefficient values are as follows; picker 

operator 2,400 g/hr, module builder operator 900 g/hr, rakers 2,400g/hr, and trampers 5,050 g/hr. 

The exposure assessment component of the study on which OPP is relying does not measure or 

identify a toxic effect; therefore, review of the exposure assessment research by the Human 

Studies Review Board (HSRB) is not required. If OPP needs to rely on this study’s other type of 

research, namely its research on cholinesterase levels, then HSRB review of the cholinesterase 

research would be required (M. Lydon, Ethics Review of Exposure Research from 1993 Study of 

Worker Exposure to Tribufos during Harvesting of Cotton Treated with DEF 6, MRID 

42701601, 07/2/2015). Because tribufos is a defoliant, other post-application activities such as 

weeding and scouting were not considered in this assessment.

Table 9.2.2.1: Anticipated Post-Application Activities and Dermal Transfer Coefficients.

Crops Policy Crop 

Group Category

Transfer 

Coefficients Activities

g/hr

Cotton

Field/ Row 

Crop, 

Low/Medium

2,400 Harvesting, Mechanical, Picker Operator Harvesting, 

900 Mechanical, Module Builder Operator

2,400 Harvesting, Mechanical, Raker

5,050 Harvesting, Mechanical, Tramper

Application Rate: The occupational post-application assessment was conducted at the maximum 

single application rates of 1.125 lb ai/A and 1.875 lb ai/A.

Body Weight: For adults, when an endpoint is not sex-specific (i.e., the endpoints are based on 

developmental or fetal effects) a body weight of 80 kg is typically used in risk assessment; 

however, in this case, a female-specific body weight of 69 kg was used. While the endpoint of 

concern, RBC AChE inhibition, is not sex-specific, the female-specific body weight was used to 

 

14 Available: http://www.epa.gov/pesticides/science/ExpoSAC_policy3.pdf

15 Available: http://www.epa.gov/pesticides/science/post-app-exposure-data.html
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protect for pregnant women due to uncertainty in the human dose-response relationship for 

neurodevelopmental effects (See Section 4.4). 

Absorption Factor: Since the steady-state dermal PoD was based on a route-specific toxicity 

study, no absorption factor was necessary to estimate exposure.

Exposure Time: The average occupational workday is assumed to be 8 hours. 

Dislodgeable Cotton Boll Residues: As stated above in the transfer coefficient section, a 

chemical-specific study was submitted (MRID 42701601, Eberhart, D.; Ellisor, G. (1993) 

Evaluation of Worker Exposure to Tribufos During Harvesting of Cotton Treated with DEF 6: 

Lab Project Number: DE202302: 105103. Unpublished study prepared by Miles Inc. 307 p.). 

Triplicate cotton samples (50 grams each) from four fields at two different geographical areas 

(California and Mississippi) were collected at various days after treatment (DAT) of DEF 6 at 

the maximum application rate of 1.875 lb ai/acre. Cotton boll samples were collected prior to 

treatment DAT 0, and up to DAT 20. This assessment uses the ground application to the 

California site for DFR analysis because it was the most complete data set from the four sites. 

This analysis generated an initial concentration value of 1.803 μg/g bolls on day one after 

treatment. Table 9.2.2.2 below provides a summary of the data for use in the post-application 

exposure calculations. 

Table 9.2.2.2. DFR Values and Regression Analysis Results for Cotton Treated with DEF 6 (EPA REG # 

3125-282) Tribufos (MRID# 42701601).

Cotton

Input/Parameter Site: California Ground Application

Application Rate (lb ai/A)a 2.5 pints/acre or 1.875 lb ai/acre

Spray Volume (GPA) 20 GPA

Measured Average Day 0 Residue (μg/cm2) 1.803 μg/g bolls

a. The applications were conducted using ground equipment.

Occupational dermal post-application risk assessments were conducted at application rates of 

1.875 lb ai/A for the states CA and AZ and for the lower rate of 1.125 lb ai/A using the 

standardized dermal transfer coefficients for mechanically harvesting cotton and the dislodgeable 

foliar residue values collected from the ground CA application. Using this information there 

were risks estimates of concern (MOE < 1,000) for the mechanical module picker, mechanical 

picker, raker and tramper from DAT 0 through DAT 30 for the higher application rate of 1.875 

lb ai/A and for the lower application rate. See Table 9.2.2.3 and 9.2.2.4 for post application risk 

estimates.
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Table 9.2.2.3: Summary of Occupational/Commercial Mechanically Harvesting Post-application Risk Estimates for Tribufos on Cotton 1.125 lb ai/A with Steady State PoD

Activity

Transfer 

Coefficient 

(cm2/hr)1

MOE2

DAT (Day After Treatment)

0 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Module 

Builder 

Operator

900 1.4 5.5 6.7 8.1 9.8 12 15 18 21 26 32 38 46 56 68 83 100 120 150 180 22

0

270 320 390 480

Picker 

Operator 2400 0.53 2.1 2.5 3 3.7 4.5 5.4 6.6 8 9.7 12 14 17 21 26 31 38 46 56 68 82 100 120 150 180

Raker 2400 0.53 2.1 2.5 3 3.7 4.5 5.4 6.6 8 9.7 12 14 17 21 26 31 38 46 56 68 82 100 120 150 180

Tramper 5050 0.25 0.98 1.2 1.4 1.8 2.1 2.6 3.1 3.8 4.6 5.6 6.8 8.3 10 12 15 18 22 26 32 39 47 58 70 85

1. DFR and TC values for each activity taken from taken from chemical-specific study data (MRID 42685901 and MRID 42701601).

2. MOE = POD (mg/kg/day) / Daily Dermal Dose. Daily Dermal Dose = [DFR/TTR (μg/g boll\) x TC x 0.001 mg/μg x 8 hrs. /day]  69 (kg). 

Table 9.2.2.4: Summary of Occupational/Commercial Mechanically Harvesting Post-application Risk Estimates for Tribufos on Cotton 1.875 lb ai/A with Steady State PoD

Activity

Transfer 

Coefficient 

(cm2/hr)1

MOE2

DAT (Day After Treatment)

0 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Module 

Builder 

Operator

900 0.85 3.3 4 4.9 5.9 7.2 8.7 11 13 16 19 23 28 34 41 50 61 73 89 110 130 160 190 240 290

Picker 

Operator 2400 0.32 1.2 1.5 1.8 2.2 2.7 3.3 4 4.8 5.8 7.1 8.6 10 13 15 19 23 28 33 41 49 60 73 88 110

Raker 2400 0.32 1.2 1.5 1.8 2.2 2.7 3.3 4 4.8 5.8 7.1 8.6 10 13 15 19 23 28 33 41 49 60 73 88 110

Tramper 5050 0.15 0.59 0.71 0.87 1.1 1.3 1.6 1.9 2.3 2.8 3.4 4.1 5 6 7.3 8.9 11 13 16 19 23 28 35 42 51

1. DFR and TC values for each activity taken from taken from chemical-specific study data (MRID 42685901 and MRID 42701601).

2. MOE = POD (mg/kg/day) / Daily Dermal Dose. Daily Dermal Dose = [DFR/TTR (μg/g boll\) x TC x 0.001 mg/μg x 8 hrs. /day]  69 (kg). 
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Restricted Entry Interval

Tribufos is classified as Toxicity Category II via the dermal route and for eye irritation and 

Toxicity Category IV for skin irritation potential. It is not a skin sensitizer. Under 40 CFR 

156.208 (c) (2), if the active ingredient is an organophosphorus ester that inhibits cholinesterase 

and that may be applied outdoors in an area where the average rainfall is less than 25 inches per 

year, the restricted interval should be listed as 72 hours. The current tribufos restricted entry 

interval (REI) was changed from 24 hours to 7 days per the 2000 IRED mitigation, however,

there remains risk estimates of concern for all mechanical post-application activities using the 

updated PoDs and SOPs.

10.0 Review of the Incident Report

Tribufos was previously reviewed in 2008 in the Updated Review of Tribuphos Incident Reports

(M. Hawkins and J. Cordova, 10/30/2008). In this earlier review, Main Incident Data System 

(IDS) was queried from 2002 to October 2008. Two incidents were reported, one was classified 

as major and one was classified as moderate. This current Tier I update looks at IDS incident 

reports from 2007 through 2014 and found two additional single active ingredient (ai) cases 

which were both minor in severity (incident report reference). SENSOR-Pesticides identified 19 

single ai cases from 1998 to 2010. Based on the low frequency and relatively low severity of 

incident cases reported for Tribufos in both IDS and National Institute of Occupational Safety 

and Health Sentinel Event Notification System for Occupational Risk (NIOSH SENSOR) 

Pesticides in the current analysis, there does not appear to be a concern at this time with respect 

to incident cases 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.

11.0 References

Bouchard MF, Chevrier J, Harley KG, Kogut K, Vedar M, Calderon N, et al. Prenatal Exposure 

to Organophosphate Pesticides and IQ in 7-Year-Old Children. Environ Health Perspectives. 

2011;119:1189-1195.

Engel SM, Berkowitz GS, Barr DB, Teitelbaum SL, Siskind J, Meisel SJ, Wetmur JG, Wolff 

MS. Prenatal Organophosphate Metabolite and Organochlorine Levels and Performance on the 

Brazelton Neonatal Behavioral Assessment Scale in a Multiethnic Pregnancy Cohort. American 

Journal of Epidemiology. 2007;165:1397-1404.

Engel SM, Wetmur J, Chen J, Zhu C, Barr DB, Canfield RL, Wolff MS. Prenatal Exposure to 

Organophosphates, Paraoxonase 1, and Cognitive Development in Childhood. Environmental 

Health Perspectives. 2011;119:1182-1188. 

Eskenazi B, Harley K, Bradman A, Weltzien E, Jewell NP, Barr DB, Furlong CE, Holland NT. 

Association of in Utero Organophosphate Pesticide Exposure and Fetal Growth and Length of 

Gestation in an Agricultural Population. Environmental Health Perspectives. 2004;115:792–798.
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Eskenazi B, Marks AR, Bradman A, Harley K, Barr DB, Johnson C, Morga N, Jewell NP. 

Organophosphate Pesticide Exposure and Neurodevelopment in Young Mexican-American 

Children. Environmental Health Perspectives. 2007;115:792–798.

FIFRA Scientific Advisory Panel. (2012). Transmittal of the Meeting Minutes from the FIFRA 

Scientific Advisory Panel Meeting of April 10-12, 2012. FIFRA Scientific Advisory Panel, 

Office of Science Coordination and Policy, Office of Prevention, Pesticides and Toxic 

Substances, U.S. Environmental Protection Agency. Washington, DC. Available: 

http://www.epa.gov/scipoly/sap/meetings/2012/041012meeting.htmll

Fortenberry, GZ, Meeker, JD, Sánchez, BN, Barr, DB, Panuwet, P, Bellinger, D, Schnaas, L, 

Solano-González, M, Ettingera, AS, Hernandez-Avila, M, Hu, H, Tellez-Rojoj, MM. Urinary 

3,5,6-trichloro-2-pyridinol (TCPY) in pregnant women from Mexico City: Distribution, temporal 

variability, and relationship with child attention and hyperactivity. International Journal of 

Hygiene and Environmental Health 217 (2014): 405–412.

Fortenberry, GZ, Meeker, JD, Sánchez, BN, Barr, DB, Peterson, K, Bellinger, D, Schnaas, L, 

Solano-González, M, Ettinger, AS, Hernandez-Avila, M, Hu, H, Tellez-Rojoj, MM. Paraoxonase 

I polymorphisms and attention/hyperactivity in school-age children from Mexico City, Mexico. 

(2014a.) Environmental Research 132:342–349.

Furlong, Melissa A., Engel, Stephanie M., Boyd Barr, Dana, Wolff, Mary S. Prenatal exposure to 

organophosphate pesticides and reciprocal social behavior in childhood. 2014. Environment 

International 70:125-131.

Guodong, Ding, Pei, Wang, Ying, Tian, Jun, Zhang, Yu, Gao, Xiaojin, Wang, Rong, Shi, 

Guoquan, Wang, and Xiaoming, Shen. Organophosphate Pesticide Exposure and 

Neurodevelopment in Young Shanghai Children. Environmental Science and Technology 

46:2911-2917, 2012.

Oulhote, Youssef and Bouchard, Maryse F. Urinary Metabolites of Organophosphate and 

Pyrethroid Pesticides and Behavioral Problems in Canadian Children. Environmental Health 

Perspectives. 121:1378-1384 (2013) 

Rauh VA, Garfinkel R, Perera FP, Andrews HF, Hoepner L, Barr DB, Whitehead R, Tang D, 

Whyatt RW. Impact of Prenatal Chlorpyrifos Exposure on Neurodevelopment in the First 3 

Years of Life Among Inner-City Children. Pediatrics. 2006;18(6): 1835-1859.

Rauh V, Arunajadai S, Horton M, Perera F, Hoepner L, Barr DB, et al. 2011. Seven-Year 

Neurodevelopmental Scores and Prenatal Exposure to Chlorpyrifos, a Common Agricultural 

Pesticide. Environmental Health Perspectives. 2011;119:1196-1201.

Shelton, Janie F., Geraghty, Estella M., Tancredi, Daniel J., Delwiche, Lora D., Schmidt, 

Rebecca J., Ritz, Beate, Hansen, Robin L., and Hertz-Picciotto, Irva. Neurodevelopmental 

disorders and prenatal residential proximity to agricultural pesticides: The CHARGE Study. 

Environmental Health Perspectives 122:1103–1109, 2014.

U.S. Environmental Protection Agency. (2008). Science Issue Paper: Chlorpyrifos Hazard and 

Dose Response Characterization. 

http://www.epa.gov/scipoly/sap/meetings/2008/091608_mtg.htm
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U.S. Environmental Protection Agency. (2012). Draft Issue Paper: Scientific Issues Concerning 

Health Effects of Chlorpyrifos. http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP2012-0040-0002

Young JG, Eskenazi B, Gladstone EA, Bradman A, Pedersen L, Johnson C, Barr DB, Furlong 

CE, Holland NT. Association Between In Utero Organophosphate Pesticide Exposure and 

Abnormal Reflexes in Neonates. NeuroToxicology. 2005;26:199–209.

Zhang, Ying, Han, Song, Liang, Duohong, Shi, Xinzhu, Wang, Fengzhi, Wei Liu, Zhang, Li, 

Chen, Lixin, Gu, Yingzi, Tian, Ying. Prenatal exposure to organophosphate pesticides and 

neurobehavioral development of neonates: A birth cohort study in Shenyang, China. PlosOne 

47:80–85, 2014.

Tribufos: Human Health Risk Assessment. D266540, June 26, 2000, R. Travaglini.

Tribufos: Occupational and Residential Exposure Assessment for the Registration Review Risk 

Assessment. D424281, 9/15/2015, B. Bobowiec.

Tribufos: Updated Dietary Burden for Livestock Commodities, D427650, June 4, 2015, S. Piper.

Tribufos: Acute and Steady State Aggregate Dietary (Food and Drinking Water) Exposure and 

Risk Assessments to Support Registration Review. D42796, September 8, 2015, S. Piper.

Tribufos: Tier 1 Update Review of Human Incidents for Preliminary Risk Assessment. 

D428263, November 25, 2014, S. Recore.

Tribufos: Drinking Water Exposure Assessment in Support of the Registration Review for the 

Organophosphate Tribufos. D424579, January 20, 2015, C. Sutton.

Tribufos: Amendment to Refine the Drinking Water Exposure Assessment in Support of the 

Registration Review for the Organophosphate Tribufos. July 14, 2015, D424903, C, Sutton.

Tribufos: Benchmark Dose Analysis of Acute, Subchronic, and Chronic Studies to Support 

Derivation of Points of Departure. D423990, 06/17/15. R. Bever & E. Holman.

Ethics Review of Exposure Research from 1993 Study of Worker Exposure to Tribufos during 

Aerial and Ground Application of DEF 6 to Cotton. 07/01/2015. M. Lydon.

Ethics Review of Exposure Research from 1993 Study of Worker Exposure to Tribufos during 

Harvesting of Cotton Treated with DEF 6. 07/02/2015. M. Lydon.
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Appendix 1. Summary of OPP’s ChE Policy & Use of BMD Modeling

OPP’s cholinesterase (ChE) policy (USEPA, 200016) describes the manner in which ChE data are 

used in human health risk assessment. The following text provides a brief summary of that 

document to provide context to points of departure selected. 

Acetylcholinesterase (AChE) can be inhibited in the central or peripheral nervous tissue. 

Measurements of AChE or ChE inhibition in peripheral tissues (e.g., liver, diaphragm, heart, 

lung etc) are rare. As such, experimental laboratory studies generally measure brain (central) 

and blood (plasma and red blood cell, RBC) ChE. Blood measures do not represent the target 

tissue, per se, but are instead used as surrogate measures for peripheral toxicity in studies with 

laboratory animals or for peripheral and/or central toxicity in humans. In addition, RBC 

measures represent AChE, whereas plasma measures are predominately butyryl-ChE (BuChE). 

Thus, RBC AChE data may provide a better representation of the inhibition in target tissues. As 

part of the dose response assessment, evaluations of neurobehavior and clinical signs are 

performed to consider the dose response linkage between AChE inhibition and apical outcomes.

Refinements to OPP’s use of ChE data have come in the implementation of BMD approaches in 

dose response assessment. Beginning with the OP CRA, OPP has increased its use of BMD 

modeling to derive PoDs for AChE inhibiting compounds. Most often the decreasing 

exponential empirical model has been used. 

OPP does have not a defined benchmark response (BMR) for OPs. However, the 10% level has 

been used in the majority of dose response analyses conducted to date. This 10% level 

represents a 10% reduction in AChE activity (i.e., inhibition) compared to background (i.e., 

controls). Specifically, the BMD10 is the estimated dose where ChE is inhibited by 10% 

compared to background. The BMDL10 is the lower confidence bound on the BMD10. 

The use of the 10% BMR is derived from a combination of statistical and biological 

considerations. A power analysis was conducted by the Office of Research and Development 

(ORD) on over 100 brain AChE datasets across more than 25 OPs as part of the OP CRA 

(USEPA, 2002). This analysis demonstrated that 10% is a level that can be reliably measured in 

the majority of rat toxicity studies. In addition, the 10% level is generally at or near the limit of 

sensitivity for discerning a statistically significant decrease in ChE activity in the brain 

compartment and is a response level close to the background brain ChE level. With respect to 

biological considerations, a change in 10% brain AChE inhibition is protective for downstream 

clinical signs and apical neurotoxic outcomes. With respect to RBC AChE inhibition, these data 

tend to be more variable than brain AChE data. OPP begins its BMD analyses using the 10% 

BMR for RBC AChE inhibition but BMRs up to 20% could be considered on a case by case 

basis as long as such PoDs are protective for brain AChE inhibition, potential peripheral 

inhibition, and clinical signs of neurotoxicity.

 

16 USEPA (2000) Office of Pesticide Programs, US Environmental Protection Agency, Washington DC 20460. 

August 18, 2000 Office of Pesticide Programs Science Policy of The Use of Data on Cholinesterase Inhibition

for Risk Assessments of Organophosphorous and Carbamate Pesticides.
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Appendix 2 – Results for BMD/BMDL modeling for Tribufos

Please note that confidence in the accuracy of reported BMD10 and BMDL10 values were not the same for all submitted tests or 

specific data sets within each test. The most reliable values are shown in bold. Less trusted values are indicated in gray. 

For all the tables: NDR = No Dose Response; --- = a trustworthy value could not be calculated, usually due to too much variance; 

CCA = Comparative Cholinesterase Assay; M# = Model number chosen; C = constant variance model; NC = modeled without 

constant variance; FT# = Failed statistical test number x; RD = Repeated Dose (study); RF = Range Finder

Appendix Table 2.1. BMD results of single dose studies in rats.

Test Sex & age Compartment

BMD Results

BMD10 (mg/kg) BMDL10 (mg/kg) Comments

MRID 48709901

Acute CCA RF, 24h

Male PND11 Brain --- --- Too much variance

Female PND11 Brain 5.56 4.31 M2, C, Too much variance

MRID 48709901

Acute CCA RF, 24h

Male PND11 RBC --- --- Too much variance

Female PND11 RBC 1.22 0.98 M4 a, NC

MRID 48709901

Acute CCA RF, 48h

Male PND11 Brain 6.47 5.51 M2, C, Too much variance

Female PND11 Brain 6.90 6.02 M2, NC

MRID 48709901

Acute CCA RF, 48h

Male PND11 RBC 2.95 1.70 M4, NC

Female PND11 RBC 3.25 2.43 FT3, M2, NC, Too much variance

MRID 48709904

Acute CCAa, 24h

Female Brain 77.97 56.03 M2, C, Not Accurate

Female RBC 5.98 4.68 FT3, M2, NC

MRID 48709904

Acute CCA RF, 6h

Male PND11 Brain 9.21 4.74 FT3, M5 a, NC

Female PND11 Brain 8.62 5.68 FT3, M5, NC, Accuracy?

MRID 48709904

Acute CCA RF, 6h

Male PND11 RBC 1.41 1.05 M4, NC

Female PND11 RBC --- --- NF

a The Chi-Square test for fit is not valid, because degrees of freedom ≤0.
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Appendix Table 2.2. BMD results of repeated dose oral studies in rats.

Test Sex & age Compartment

BMD Results

BMD10 (mg/kg) BMDL10 (mg/kg) Comments

MRID 48709901

RD CCA RF, 6h

Male PND11 Brain 2.40 2.19 M2, C, Too much variance

Female PND11 Brain 2.59 2.07 M4, C

MRID 48709901

RD CCA RF, 6h

Male PND11 RBC --- --- Too much variance

Female PND11 RBC --- --- Too much variance

MRID 48709905

RD CCA a, 24h

Female Brain --- --- NDR

Female RBC 0.54 0.31 M4, C, Not accurate

MRID 48709905

RD CCA, 6h

Male PND11 Brain 3.49 2.69 M3, C

Female PND11 Brain 2.20 1.79 M4, C

MRID 48709905

RD CCA, 6h

Male PND11 RBC 4.08 0.66 M3, NC, Too much variance

Female PND11 RBC 0.47 0.29 M4, NC, Not accurate

MRID 48709906

Gestational CCA, 24h

Dam Brain 3.70 3.05 M3, C

Dam RBC 5.32 1.06 M5 c, NC

MRID 48709906

Gestational CCAb, 24h

Fetus Brain --- --- FT1

Fetus RBC 22.30 13.92 FT3, M2, NC, Too much variance

MRID 45369101 

Subchr. NT 4W

Male RBC 0.28 0.21 FT3, M4, NC

Female RBC 0.27 0.19 M4, NC

MRID 42007201 21D 

Dermal Tox

Male Brain 6.67 4.97 M4

Female Brain 7.73 6.50 FT3, M2, NC

MRID 42007201 21D 

Dermal Tox

Male RBC 0.87 0.48 M4, C, Too much variance

Female RBC 0.22 0.16 M4, NC

MRID 42399801 

Subchr. Inhal. 13W

Male RBC 1.00 mg/m30.79 mg/m3 FT3, M4, NC

Female RBC 1.01 mg/m30.85 mg/m3 FT3, M4, NC

a Adult male not tested

b Fetus not more sensitive than dam

c The Chi-Square test for fit is not valid, because degrees of freedom ≤0
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Appendix 3. Toxicology Profile and Executive Summaries

A.3.1 Toxicology Data Requirements

The requirements (40 CFR 158.500) for the non-food use for Tribufos are in Table A.1. Use of the new guideline 

numbers does not imply that the new (1998) guideline protocols were used.

Table A.3.1 Toxicology Data Requirements for Tribufos

Study

Technical

Required Satisfied

870.1100 Acute Oral Toxicity.......................................................

870.1200 Acute Dermal Toxicity..................................................

870.1300 Acute Inhalation Toxicity..............................................

870.2400 Primary Eye Irritation....................................................

870.2500 Primary Dermal Irritation..............................................

870.2600 Dermal Sensitization .....................................................

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

870.3100 Oral Subchronic (rodent)...............................................

870.3150 Oral Subchronic (nonrodent).........................................

870.3200 21/28-Day Dermal.........................................................

870.3250 90-Day Dermal..............................................................

870.3465 90-Day Inhalation..........................................................

yes

yes

yes

no

CR

yes

yes

yes

yes

yes

870.3700a Developmental Toxicity (rodent) ..................................

870.3700b Developmental Toxicity (nonrodent) ............................

870.3800 Reproduction .................................................................

yes

yes

yes

yes

yes

yes

870.4100a Chronic Toxicity (rodent)..............................................

870.4100b Chronic Toxicity (nonrodent)........................................

870.4200a Oncogenicity (rat)..........................................................

870.4200b Oncogenicity (mouse) ...................................................

870.4300 Chronic/Oncogenicity ...................................................

yes

no

yes

yes

yes

yes

yes

yes

yes

yes

870.5100 Mutagenicity—Gene Mutation - bacterial.....................

870.5300 Mutagenicity—Gene Mutation - mammalian................

870.5375 Mutagenicity—Structural Chromosomal Aberrations...

870.5395 Mutagenicity—Mammalian erythrocyte micronucleus.

870.5550 Mutagenicity—Unscheduled DNA synthesis............

yes

yes

yes

yes

no

yes

no

yes

no

yes

870.6100a Acute Delayed Neurotoxicity (hen)...............................

870.6100b 90-Day Neurotoxicity (hen) ..........................................

870.6200a Acute Neurotoxicity Screening Battery (rat).................

870.6200b 90-Day Neurotoxicity Screening Battery (rat) ..............

870.6300 Developmental Neurotoxicity .......................................

CR

no

yes

yes

CR

yes

yes

yes

yes

yes

870.7485 General Metabolism ......................................................

870.7600 Dermal Penetration........................................................

870.7800 Immunotoxicity .............................................................

yes

CR

yes

yes

yes

yes

Special Studies 

Comparative Cholinesterase (rat) ...............................

yes yes

 CR = conditionally required
Tribufos Human Health Risk Assessment DP No. D357537

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A.3.2 Toxicity Profiles

Table A.3.2.1 Acute Toxicity of Tribufos

Guideline

Number Study Type MRID Results

Toxicity 

Category

870.1100 Acute Oral - Rat 41954903 LD50 =192-235 mg/kg II

870.1200 Acute Dermal - Rabbit 41954902

LD50 =>1000 mg/kg (m)

<2000 mg/kg (f) II

870.1300 Acute Inhalation - Rat 41782301

LC50 =4650 mg/m3(m)

2460 mg/m3(f) III

870.2400 Primary Eye Irritation -Rabbit 42441601

Signs of irritation present 1 hour 

after exposure but resolved by 

day 7 IV

870.2500 Primary Skin Irritation - Rat 41896203

Mild to moderate erythema, dry 

cracked skin, edema IV

870.2600 Dermal Sensitization-Guinea Pig 41618812 negative NA

The toxicology data for repeated-dose toxicity studies with tribufos are listed in Table A. 2.2. 

Table A.3.2.2 Subchronic, Chronic and Other Toxicity Profile - Tribufos

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

870.3100 90-Day oral toxicity 

rodents

See Subchronic 

Neurotoxicity study 

870.6200b

870.3200 21-Day dermal 

toxicity 

(New Zealand 

White) rabbits

42007201 (1991)

Acceptable/guideline

0, 2, 11, or 29 mg/kg/day

Dermal and systemic NOAEL: 2 mg/kg/day

Dermal and systemic LOARL: 11 mg/kg/day based 

on dermal irritation, muscle fasciculation and anal 

staining in males and females.

NOAEL = Not established

LOAEL = 2 mg/kg/day based on decreased plasma 

ChE in male rats and decreased RBC ChE in female 

rats.

BMD10 = 0.22 mg/kg/day

BMDL10 = 0.16 mg/kg/day based on RBC ChE 

inhibition in adult female rats.

870.3465 90-Day inhalation 

toxicity

(Wistar) rats

42399801 (1992)

Acceptable/guideline

0, 0.93, 2.43, 12.2, or 59.5 

mg/m3

(0, 0.3, 0.9, 4.5 or 22 

mg/kg/day)

NOAEL = 2.43 mg/m3(0.9 mg/kg/day)

LOAEL = 12.2 mg/m3(4.5 mg/kg/day) based on 

decreased RBC ChE in male and female rats.

BMD10 = 1.00 mg/m3(HED = 0.12 mg/kg/day)
Tribufos Human Health Risk Assessment DP No. D357537

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Table A.3.2.2 Subchronic, Chronic and Other Toxicity Profile - Tribufos

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

BMDL10 = 0.79 mg/m3(HED = 0.10 mg/kg/day) 

based on RBC ChE inhibition in adult male rats

870.3700a Developmental 

toxicity

(SD) rats

40190601 (1986)

Acceptable/guideline

0, 1, 7, 28 mg/kg/day

Maternal toxicity NOAEL = 1 mg/kg/day.

Maternal toxicity LOAEL = 7 mg/kg/day based on 

decreased plasma and RBC ChE.

Developmental toxicity NOAEL = 28 mg/kg/day

Developmental toxicity LOAEL = Not established

870.3700b Developmental 

toxicity in 

(American Dutch) 

rabbits

40190602 (1987) 

Acceptable/Guideline

0, 1, 3, or 9 mg/kg/day

Maternal toxicity NOAEL = Not established

Maternal toxicity LOAEL = 1 mg/kg/day based on 

decreased plasma and RBC ChE.

Developmental toxicity NOAEL = 9 mg/kg/day

Developmental toxicity LOAEL = Not established

870.3800 Reproductive 

toxicity

(SD) rats

42040101 (1991)

Acceptable/Guideline

0, 4, 32 or 250 ppm (0, 0.2, 

1.7, or 15 mg/kg/day) for two 

generations

Parental/Systemic NOAEL = Not established

Parental/Systemic LOAEL = 4ppm (0.2 mg/kg/day)

based on decreased plasma (F0 females) and RBC 

ChE (F0 females, adult F1 males/females).

Reproductive NOAEL = 32 ppm (1.7 mg/kg/day)

Reproductive LOAEL = 250 ppm (15 mg/kg/day) 

based on increased number of litters with still born 

pups and pup death (F1 and F2 generation); decrease 

in F1 and F2 pup body weight; and increased F1

gestation period.

870.4100b Chronic toxicity 

dogs

42007203 (1991) 

Acceptable/Guideline

0, 4, 16, or 64 ppm (0, 0.1, 

0.4, 1.7 mg/kg/day in males 

and 0, 0.1, 0.4, or 2.0 

mg/kg/day in females) for 52 

weeks

Systemic NOAEL = 64 ppm (1.7 mg/kg/day in 

males and 2.0 mg/kg/day in females)

Systemic LOAEL = Not established

Cholinesterase Inhibition NOAEL = 16 ppm (0.4 

mg/kg/day)

Cholinesterase Inhibition LOAEL = 64 ppm (1.7 

mg/kg/day) based on statistically significantly 

decreased RBC ChE in males and females.

870.4200a Chronic toxicity/

Carcinogenicity

(Fisher) rat

42335101 (1992)

Acceptable/Guideline

0, 4, 40, or 320 ppm (0, 0.2, 

1.8, or 16.8 mg/kg/day for 

male rats and 0, 0.2, 2.3, or 

21.1 mg/kg/day for female 

rats) for 24 months. 

Systemic NOAEL = 40 ppm (1.8 mg/kg/day for 

males and 2.3 mg/kg/day for females).

Systemic LOAEL = 320 ppm (16.8 mg/kg/day for 

males and 21.1 mg/kg/day for females) based on 

ocular damage.

Cholinesterase Inhibition NOAEL = 4 ppm (0.2 

mg/kg/day for males and females)

Cholinesterase Inhibition LOAEL = 40 ppm (1.8 

mg/kg/day for males and 2.3 mg/kg/day for females) 

based on statistically significantly decreased RBC 

ChE.
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Table A.3.2.2 Subchronic, Chronic and Other Toxicity Profile - Tribufos

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

No evidence of carcinogenicity was observed in

male or female rats. 

870.4200b Carcinogenicity

mice

41171001 (1989)

Acceptable/Guideline

0, 10, 50, or 250 ppm (0,

1.64, 8.28, or 48.02 

mg/kg/day for males and 0, 

2.08, 11.14, or 63.4

mg/kg/day for females) for 

90 weeks.

Systemic NOAEL = Not established

Systemic LOAEL = 10 ppm (1.64 mg/kg/day in 

males and 2.08 mg/kg/day in females) based on 

decreased MCV/MCH in males and decreased 

hematocrit in females 

Cholinesterase Inhibition NOAEL = Not established

Cholinesterase Inhibition LOAEL = 10 ppm (1.64 

mg/kg/day in males and 2.08 mg/kg/day in females) 

based on statistically significantly decreased plasma, 

RBC and brain ChE.

Evidence of carcinogenicity was observed at the 

highest dose only (250 ppm = 48.02 mg/kg/day in 

males and 63.4 mg/kg/day in females), with 

increased hemangiosarcomas and adenocarcinomas 

of the small intestine in males and increased 

alveolar/bronchiolar adenomas in females.

870.6100a Acute delayed 

neurotoxicity-hen

Not available. Literature references and an acceptable subchronic study in the hen 

show that tribufos produces organophosphate-induced delayed neuropathy. 

Therefore, an acute study in the hen is not required.

870.6100b Delayed Subchronic 

Neurotoxicity Study 

Hen

42007202 (1991)

Acceptable/Guideline

0, 2.6, 11, or 42 mg/kg/day 

for 90 days

Neuropathy NOAEL = 11 mg/kg/day

Neuropathy LOAEL = 42 mg/kg/day based on 

delayed neuropathy observed in the brain and spinal 

cord of hens.

Cholinesterase Inhibition NOAEL = Not established

Cholinesterase Inhibition LOAEL = 2.6 mg/kg/day 

based on whole blood ChE inhibition

870.6200a Acute Neurotoxicity 

(Wistar) Rat 

(Oral gavage)

45194401 (2000)

Acceptable/Guideline

0, 2, 20, or 100 mg/kg (single 

dose).

Systemic NOAEL = 2 mg/kg

Systemic LOAEL = 20 mg/kg based on decreased 

motor activity.

Cholinesterase Inhibition NOAEL = 2 mg/kg

Cholinesterase Inhibition LOAEL = 20 mg/kg based 

on plasma and RBC ChE inhibition.

870.6200b Subchronic 

Neurotoxicity

(Wistar) Rat 

(Oral dietary)

45369101 (2001)

Acceptable/Guideline

0, 2, 40 or 500 ppm (00.14, 

2.89, or 36.8 mg/kg/day in 

males and 0, 0.17, 3.54. or 

42.6 mg/kg/day in females) 

for 13 weeks.

Systemic NOAEL = 2 ppm (0.17 mg/kg/day in 

females)

Systemic LOAEL = 40 ppm (3.54 mg/kg/day in 

females), based on decreased body weight and food 

consumption in females.

Cholinesterase Inhibition NOAEL = 2 ppm (0.14 

mg/kg/day in males and 0.17 mg/kg/day in females)

Cholinesterase Inhibition LOAEL = 40 ppm (2.89 in 

males and 3.54 mg/kg/day in females) based on ChE 
Tribufos Human Health Risk Assessment DP No. D357537

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Table A.3.2.2 Subchronic, Chronic and Other Toxicity Profile - Tribufos

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

inhibition in plasma, and RBCs (both sexes) and 

brain (in females only).

BMD10 = 0.27 mg/kg/day

BMDL10 = 0.19 mg/kg/day based on RBC ChE 

inhibition in adult female rats

870.6300 Developmental 

Neurotoxicity 

(Wistar) Rat 

(Oral dietary)

45499501 (2001)

Acceptable/Non-guideline

0, 4, 40 or 200 ppm (0, 0.4, 

3.4, or 16.4 mg/kg/day) from 

gestation day 0 through 

postnatal day 21.

Maternal systemic NOAEL = 40 ppm (3.4 

mg/kg/day).

Maternal systemic LOAEL = 200 ppm (16.4 

mg/kg/day) based on clinical signs (tremor) and 

decreased body weight.

Maternal cholinesterase Inhibition NOAEL = 4 ppm 

(0.4 mg/kg/day)

Maternal cholinesterase Inhibition LOAEL = 40 ppm 

(3.4 mg/kg/day) based on decreases in plasma, RBC

and brain ChE activity.

Offspring systemic NOAEL = 40 ppm (3.4 

mg/kg/day)

Offspring systemic LOAEL = 200 ppm (16.4 

mg/kg/day), based on decreased body weight, body 

weight gain, delay in preputial separation, delayed 

surface righting reflex, decreased motor activity, 

decreased startle response amplitude, decreased 

absolute brain weights, and changes in the brain 

morphometrics (cerebrum and cerebellum).

Offspring cholinesterase Inhibition NOAEL = 200 

ppm (16.4 mg/kg/day)

Offspring cholinesterase Inhibition LOAEL = Not 

established

870.7485 Metabolism and 

pharmacokinetics

(SD) rat

42034501 (1991)

Acceptable/Guideline

Rats received single oral 

dose, 5mg/kg or 100 mg/kg 

or 5 mg/kg/day X 14 days 

cold tribufos followed by 5 

mg/kg [1-C14] tribufos.

Fifty-five to 80% was absorbed of which 90+% was 

excreted in 72 hours. There was no significant tissue 

residue. Absorbed material was extensively and 

completely metabolized.

Tribufos is already in the oxon form

870.7800 Immunotoxicity

Female (Wistar) rat

(oral dietary)

48709904 (2011)

Acceptable/Non-guideline

0, 5, 50, or 500 ppm (0, 0.43, 

4.32, or 44.62 mg/kg/day) for 

4 weeks

Immunotoxicity NOAEL = 500 ppm (equivalent to 

44.62 mg/kg/day)

Immunotoxicity LOAEL = Not established

 Cholinesterase Inhibition NOAEL= 5 ppm (0.43 

mg/kg/day)

Cholinesterase Inhibition LOAEL = 50 ppm (4.32 

mg/kg/day) based on significant decreased in RBC

ChE activity.

Special Studies- Comparative Cholinesterase Studies
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Table A.3.2.2 Subchronic, Chronic and Other Toxicity Profile - Tribufos

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

Comparative 

Cholinesterase 

Studies (PND 11 

rats single & 

repeated dosing--

dose-range finding)

(oral gavage)

48709901 (2011)

Acceptable/Non-guideline

Single-dosing study: 0, 20, 

40 or 50 mg/kg. 

Cholinesterase assessments 

made at 24 and 48 hours 

post-dosing.

Repeated-dosing study:

0, 5, 10, 15, or 20 mg/kg/day 

for 11 consecutive doses. 

Cholinesterase assessments 

made 6 hours after the last 

dose.

Single-dosing study:

Inhibition of erythrocyte ChE in males and females 

at 20 mg/kg after 24 hours.

Repeated-dosing study:

Inhibition of erythrocyte and brain ChE at 

5/mg/kg/day in males and females.

Comparative 

Cholinesterase 

Studies (PND 11 

rats single dosing 

study-time to peak 

effect)

(oral gavage)

48709902 (2011)

Acceptable/Non-guideline

Single oral dose of 50 mg/kg

Time to peak effect = 6-8 hours based on erythrocyte 

and brain ChE inhibition; brain ChE inhibition 

decreased by 48 hours.

Comparative 

Cholinesterase 

Studies (adult and 

pup rats single 

dosing study)

(oral gavage)

48709904 (2011)

Acceptable/Non-guideline

Pups: 0, 2, 10 or 50 mg/kg

Adults (females only): 0, 2, 

10, or 80 mg/kg

Cholinesterase assessments 

made at 24 hours post-dosing 

in adults and 6 hours postdosing in pups

Pups:

NOAEL: Not established

LOAEL: 2 mg/kg based on RBC ChE inhibition

Adults:

NOAEL: 10 mg/kg

LOAEL: 80 mg/kg based on brain and RBC ChE 

inhibition

BMD10 = 1.41 mg/kg/day

BMDL10 = 1.05 mg/kg/day based on RBC ChE 

inhibition in male pups

Comparative 

Cholinesterase 

Studies (adult and 

pup rats- repeated 

dosing study)

(oral gavage)

48709905 (2011)

Acceptable/Non-guideline

0, 0.1, 1.0, or 5.0 mg/kg/day 

for 11 days

(females only for adult 

group)

Cholinesterase assessments 

made at 24 hours post-dosing 

in adults and 6 hours postdosing in pups

Pups:

NOAEL: 1.0 mg/kg/day

LOAEL: 5.0 mg/kg/day based on RBC and brain 

ChE inhibition

Adults:

NOAEL: 0.1 mg/kg/day

LOAEL: 1.0 mg/kg/day based on RBC ChE 

inhibition

Gestational 

Cholinesterase 

inhibition

48709906 (2011)

Acceptable/Non-guideline

Maternal systemic NOAEL = 7 mg/kg/day

Maternal systemic LOAEL = 28 mg/kg/day based on

decreased food consumption.
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Table A.3.2.2 Subchronic, Chronic and Other Toxicity Profile - Tribufos

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

rats

(oral gavage)

0, 1 (adjusted 0.3-0.8 

mg/kg/day), 7, or 28 

mg/kg/day from GD 6-19

Cholinesterase assessments 

made at 24 hours postdosing.

Maternal cholinesterase Inhibition NOAEL = 0.3-0.8 

mg/kg/day

Maternal cholinesterase Inhibition LOAEL = 7 

mg/kg/day based on decreases in brain ChE activity.

Offspring systemic NOAEL = 7 mg/kg/day

Offspring systemic LOAEL = 28 mg/kg/day based 

on decreased body weight in males

Offspring cholinesterase Inhibition NOAEL 28

mg/kg/day

Offspring cholinesterase Inhibition LOAEL = Not 

established

ChE = cholinesterase; HED = human equivalent dose; RBC = red blood cell.

PoD Conversion Calculations

As summarized in Section 4.5.4, inhalation PoDs were derived using the following conversion 

calculations:

Inhalation BMDL10

The BMDL10 of 0.79 mg/m3(male RBC ChE result from BMD modeling summarized in 

appendix 1) is first duration-adjusted as follows:

PoDadj = (BMDL10 in mg/L) (daily duration adjustment) (weekly duration adjustment)

PoDadj = (0.00079 mg/L) (6h rat exposure/8h expected human) (5 day rat/5 day expected human)

PoDadj = 0.00059 mg/L

The PoDadj is converted to an HEC as follows:

HEC = (regional deposited dose ratio or RDDR) (PoDadj)

HEC = (2.926) (0.00059 mg/L) = 0.0017 mg/L

The HEC is then converted to an HED using the following formula:

HED = HEC (mg/L) x CF(L/hr/kg bw) x D (hr) x AF X A

AF is the ratio of the exposure specific scenario ventilation rate (8.3 L/min) and the default value 

of 13.8 L/min

CF is the default ventilation rate of 13.8 L/min converted to hours and normalized to a body 

weight of 70 kg
Tribufos Human Health Risk Assessment DP No. D357537

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D is the human daily duration (8 hrs)

A is the absorption ratio as compared to the oral route (assumed to be 1)

Therefore,

HED = (0.0017) (11.8) (8) (0.6) (1)

HED = 0.10 mg/kg/day

Inhalation BMD

Similarly, the BMD of 1.00 mg/m3(male RBC ChE result from BMD modeling summarized in 

appendix 1) is converted to an HED as follows:

PoDadj = (0.001 mg/L) (6h rat exposure/8h expected human) (5 day rat/5 day expected human)

PoDadj = 0.00075 mg/L

HEC = (2.926) (0.00075 mg/L) = 0.0022 mg/L

HED = (0.0022) (11.8) (8) (0.6) (1) = 0.12 mg/kg/day

A.3.3 Hazard Identification and Endpoint Selection

A.3.3.1 Acute Dietary Reference Dose (aRfD) – All Populations

Study Selected: Acute Comparative Cholinesterase (CCA) Study in Rats- Single Dose

MRID No.: 48707704

Dose and Endpoint for Risk Assessment: A BMDL10 of 1.05 mg/kg/day associated with RBC 

ChE inhibition in PND 11 rat male pups was selected as a suitable PoD for the acute dietary (all 

populations, including females 13-49) exposure scenario. The corresponding BMD10 was 1.41

mg/kg/day. 

Comments about Study/Endpoint/Uncertainty Factors: RBC ChE inhibition was selected 

for the PoD since the RBCs were more sensitive than the brain, and the RBC ChE data were 

more robust than brain ChE data. Data from the PND 11 pups represent the highly exposed subpopulations (infants and young children) and thus are appropriate for PoD derivation. 

Additionally, RBC ChE data were the most sensitive endpoint and indicated a quantitative 

susceptibility of the pups compared to adult rats in an acute exposure scenario. An uncertainty 

factor of 100X (10X to account for interspecies extrapolation and 10X for intraspecies variation) 

is applied to the BMDL10 to obtain an aRfD of 0.01 mg/kg/day. Based on the FQPA safety 

factor of 10X, the acute population adjusted dose (aPAD) is 0.001 mg/kg/day.

Acute RfD (All Populations) = (BMDL10) 1.05 mg/kg/day = 0.01 mg/kg/day

 (UF) 100

 aPAD = aRfD = 0.01 mg/kg/day = 0.001 mg/kg/day

 FQPASF 10

A.3.3.2 Steady State Dietary Reference Dose (ssRfD) –All Populations 
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Study Selected: Subchronic Oral Neurotoxicity Study in Rats

MRID No.: 45369101

Dose and Endpoint for Risk Assessment: A BMDL10 of 0.19 mg/kg/day associated with RBC 

ChE inhibition in adult female rats was selected since the RBCs were more sensitive than the 

brain, and the RBC ChE data were more robust than brain ChE data. It is, therefore, considered 

as a suitable PoD for the steady-state dietary (all populations) exposure scenario. The 

corresponding BMD10 was 0.27 mg/kg/day. 

Comments about Study/Endpoint/Uncertainty Factors: Although pups are more sensitive 

than adult rats to ChE inhibition in acute studies, in repeated-dosing studies ChE is similarly 

inhibited in pup and adult rats (see table A.5). This is likely due to maturation of enzymes in the 

pups over the study duration (PND 11-PND21). Therefore, the available ChE data at durations 

of 4 weeks in adult female rats provide an endpoint that is protective of highly exposed subpopulations (infants and young children) and, thus, are appropriate for PoD derivation. An 

uncertainty factor of 100X (10X to account for interspecies extrapolation and 10X for 

intraspecies variation) is applied to the BMDL10 to obtain an ssRfD17 of 0.0019 mg/kg/day. 

Based on the FQPA safety factor of 10X, the steady-state population adjusted dose (ssPAD) is 

0.0002 mg/kg/day.

ssRfD (All Populations)= ssPAD = (BMDL10) 0.19 mg/kg/day= 0.002 mg/kg/day

 (UF) 100

 ssPAD = ssRfD = 0.002 mg/kg/day = 0.0002 mg/kg/day

 FQPASF 10

A.3.3.3 Incidental Oral Exposure (Steady State)

Study Selected: Subchronic Oral Neurotoxicity Study in Rats

MRID No.: 45369101

Dose and Endpoint for Risk Assessment: A BMDL10 of 0.19 mg/kg/day associated with RBC 

ChE inhibition in adult female rats was selected since the RBCs were more sensitive than the 

brain, and the RBC ChE data were more robust than brain ChE data. It is, therefore, considered 

as a suitable PoD for the steady-state dietary (all populations) exposure scenario. The 

corresponding BMD10 was 0.27 mg/kg/day. 

Comments about Study/Endpoint/Uncertainty Factors: Although pups are more sensitive 

than adult rats to ChE inhibition in acute studies, in repeated-dosing studies ChE is similarly 

inhibited in pup and adult rats (see table A.5). This is likely due to maturation of enzymes in the 

pups over the study duration (PND 11-PND21). Therefore, the available ChE data at durations 

of 4 weeks in adult female rats provide an endpoint that is protective of highly exposed subpopulations (infants and young children) and, thus, are appropriate for PoD derivation. An 

uncertainty factor of 1000X (10X to account for interspecies extrapolation and 10X for intraspecies variation and 10X FQPASF) is applied.

 

17 ssRfD: Reference dose derived from steady state duration
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A.3.3.4 Dermal Exposure (Steady State) 

Study Selected: 21-Day Dermal Study in Rabbits

MRID No.: 42007201

Dose and Endpoint for Risk Assessment: A BMDL10 of 0.16 mg/kg/day associated with RBC 

ChE inhibition in adult female rats, following dermal exposure, was selected as a suitable PoD 

for assessing the potential risk associated with dermal exposure (short-term intermediate). The 

corresponding BMD10 was 0.22 mg/kg/day. 

Comments about Study/Endpoint/Uncertainty Factors: This is the only dermal study 

available for tribufos and was an acceptable/guideline study. RBC ChE inhibition was selected 

for the PoD since the RBCs were more sensitive than the brain through the dermal route of 

exposure. An uncertainty factor of 100X (10X to account for interspecies extrapolation and 10X 

for intraspecies variation) is applied to the BMDL10.

A.3.3.5 Inhalation Exposure (Steady State) 

Study Selected: 13-Week Inhalation Toxicity Study in Rats

MRID No.: 42399801

Dose and Endpoint for Risk Assessment: A BMDL10 of 0.79 mg/m3(HED = 0.10 mg/kg/day) 

associated with RBC ChE inhibition in adult male rats following inhalation exposure was 

selected as a suitable PoD for assessing the potential risk associated with inhalation exposure 

(short-term intermediate). The corresponding BMD10 was 1.00 mg/m3(HED = 0.12 mg/kg/day). 

Comments about Study/Endpoint/Uncertainty Factors: The particle size distribution from 

this study suggests that most particles are within the respirable range for an aerosol (1-3 μm), 

thus the BMD and BMD10 were converted to human equivalent concentrations (HECs) followed 

by conversion to human equivalent doses (HEDs). An uncertainty factor of 30X (3X to account 

for interspecies extrapolation and 10X for intraspecies variation) is applied to the BMDL10. 

A.3.4 Executive Summaries

A.3.4.1 Subchronic Toxicity

870.3100 90-Day Oral Toxicity – Rat 

See Subchronic Neurotoxicity study 870.6200b [MRID 45369101]

870.3150 90-Day Oral Toxicity – Non-Rodent 

Subchronic oral toxicity studies in non-rodent species are not available. However, these studies 

are not required because acceptable chronic studies are available in the dog.

870.3200 21/28-Day Dermal Toxicity – Rabbit

In a 21-day dermal toxicity study (MRID 42007201), groups of New Zealand White rabbits 
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(10/sex/dose) received repeated dermal applications of tribufos at doses of 0, 2, 10 or 25 

mg/kg/day nominal (0, 2, 11 or 29 mg/kg/day actual), six hours per day, five days per week over 

a period of 21 days. Polyethylene glycol 400 was used as the solvent and applied neat to the 

control animals. The application site in the clipped back of the rabbits approximated 240 cm2. 

The day after the last treatment, 5 animals/sex/group were sacrificed, the remaining animals were 

sacrificed 14 days after the last treatment. Animals were observed daily for clinical signs and the

application site graded for signs of irritation. Body weight and food consumption were 

determined weekly. Ophthalmic examination was performed on both eyes of the survivors on 

week 3 of treatment. Blood samples were collected 24 hours after the last dose and at sacrifice. 

No mortality occurred at 2 or 11 mg/kg/day whereas one male and four females died or were 

sacrificed in extremis at 29 mg/kg/day. Mild to moderate dermal irritation was observed at 11 

and 29 mg/kg/day in both sexes. Signs of dose-related toxicity (muscle fasciculations, anal stain)

were observed in both sexes at 11 and 29 mg/kg/day, with a greater effect at the higher dose. At 

termination, dose-related inhibition of plasma, erythrocyte (RBC) and brain cholinesterase (ChE)

activity was observed in both sexes at all dose levels. Statistically-significant (p<0.05) inhibition

of ChE was observed in plasma (males) and RBC (females) at 2 mg/kg/day and in all 

compartments (plasma, RBC and brain) at 11 and 29 mg/kg/day in both sexes. No recovery was 

observed in RBC and brain ChE activity at 33 to 34 days (14 days post-dose). 

The dermal and systemic NOAEL for tribufos in rabbits is 2 mg/kg/day and the LOAEL is 

11 mg/kg/day based on mild to moderate dermal irritation, fasciculations and anal stain in 

both males and females. The NOAEL for cholinesterase inhibition is not identified and the 

LOAEL for cholinesterase inhibition is 2 mg/kg/day based on decreased plasma (males) 

and RBC (females) cholinesterase activity. 

This study is classified as Acceptable/Guideline and does satisfy the guideline requirements for 

a repeated-dose dermal study [OPPTS 870.3200 (§82-2)] in rabbits.

870.3465 Subchronic Inhalation – Rat

In a subchronic toxicity study (MRID 42399801), groups of Wistar rats (10/sex/concentration) 

were exposed via inhalation to an aerosol of tribufos at concentrations of 0, 0.93, 2.43, 12.2 or 

59.5 mg/m3actual (0, 0.3, 0.9, 4.5, 22 mg/kg/day), six hours per day, five days per week for 90 

days. Rats were weighed prior to the first exposure and weekly during the study. Appearance and 

behavior were evaluated daily. Blood samples were collected monthly and at necropsy for 

hematological and clinical chemistry tests. Electroretinographic (ERG) tests were performed 

during week 10 and prior to necropsy on 5 males and 5 females each from the control and high 

dose group.

No ChE inhibition was observed in either sex at 0.93 or 2.43 mg/m3. Plasma ChE inhibition was 

observed in males at 12.2 and 59.5 mg/m3and in females at 59.5 mg/m3. RBC ChE inhibition 

was observed at 12.2 and 59.5 mg/m3in both sexes. Brain ChE inhibition was seen at 59.5

mg/m3both sexes. The adrenals showed cortical fat deposition at 59.5 mg/m3in both sexes. 

Electroretinogram (ERG) was depressed (a- and b- waves) at 59.5 mg/m3in both sexes indicative 

of a toxic effect on the rods and cones of the retina.
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The NOAEL is 2.43 mg/m3(0.9 mg/kg/day) and the LOAEL is 12.2 mg/m3(4.5 mg/kg/day) 

based on RBC cholinesterase inhibition. 

This subchronic inhalation toxicity study in the rat is Acceptable/Guideline and does satisfy the 

guideline requirements for a subchronic inhalation study OPPTS 870.3465; OECD 413 in the rat.

A.3.4.2 Prenatal Developmental Toxicity

870.3700a Prenatal Developmental Toxicity Study – Rat

In a developmental toxicity study (MRID 40190601), pregnant Crl:COBS-CD (SD) rats received 

oral doses of tribufos at 0, 1, 7 or 28 mg/kg/day during gestation days (GD) 6 through 16. 

Animals were observed daily and weighed on GD 0, 6, 8, 10, 12, 15, and 20. Food consumption 

recorded on GD 1, 6, 8, 12, 15, and 20. Dams were sacrificed on GD16 (5 dams/dose) or GD 20 

(10 dams/dose), and blood and brain were collected for determination of cholinesterase activity. 

Dams were subjected to gross necropsy, and all fetuses examined externally. Approximately 

one-half of each litter was processed for visceral examination and the remaining one-half was 

processed for skeletal examination. Fetal brain cholinesterase was measured in 20 fetuses from 

each dose group.

All animals survived to terminal sacrifice. No clinical signs of toxicity were observed, except 

for salivation in two high dose animals on GD 9 and 12. Maternal body weights and food 

consumption were similar between the treated and control groups throughout the study. On 

GD16, plasma and RBC ChE were inhibited at 7 and 28 mg/kg/day, while brain ChE was 

inhibited at 28 mg/kg/day. On GD 20, RBC ChE was inhibited at 7 and 28 mg/kg/day, while 

brain ChE was inhibited at 28 mg/kg/day. Maternal necropsy was unremarkable. Therefore, the 

maternal toxicity NOAEL is 1 mg/kg/day and the maternal toxicity LOAEL is 7 mg/kg/day, 

based on inhibition of plasma and RBC cholinesterase activity.

No differences were observed between the treated and control groups for number of corpora 

lutea, number of implantation sites, live fetuses/dam, pre- and post-implantation losses, fetal 

body weights, or fetal sex ratios. No treatment-related external, visceral, or skeletal 

malformations/variations were observed in any fetus from any group. Therefore, the 

developmental toxicity NOAEL is 28 mg/kg/day and the developmental toxicity LOAEL 

was not identified.

This study is classified as Acceptable/Guideline and satisfies the requirements for a 

developmental toxicity study [OECD 870.3700 (§83-3a)] in rats.

870.3700b Prenatal Developmental Toxicity Study – Rabbit

In a developmental toxicity study (MIRD 40190602), pregnant American Dutch rabbits

(17/dose) were administered tribufos via oral gavage at 1, 3, or 9 mg/kg/day during gestation 

days (GD) seven through 19. Does were observed daily for morbidity and mortality. Body 
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weights were obtained on GDs 0, 7, 10, 14, 19, 21 and 28. Food consumption was measured on 

GDs 1, 6, 8, 12, 15, 20, 23, and 28. Blood was obtained for RBC and plasma ChE determinations 

on GD 20 (24 hours after administering the last dose) and on GD 28 (prior to sacrifice). Half the 

brain of each doe was collected at termination for brain ChE measurements. On GD 28, all does 

were sacrificed and the uteri were weighed and examined. All fetuses were dissected, sexed and 

eviscerated. All fetuses were examined for visceral and bone anomalies.

No treatment-related clinical signs were observed in does at any dose. Two low dose does and 

one high dose doe died of respiratory disease. One low dose doe and one high dose doe aborted 

during the study. No treatment-related effects were observed on food consumption. Statistically 

significant decreases in body weight gain were observed in does from the high-dose group. 

Plasma and RBC ChE activities were decreased in does from all dose groups at GD 20. RBC 

ChE activity remained decreased at GD 28. No treatment-related changes in brain ChE were 

reported. Therefore, the maternal toxicity LOAEL is 1 mg/kg/day based on inhibition of 

plasma and RBC ChE activity; a NOAEL is not established. 

No developmental toxicity was observed. Therefore, the developmental toxicity NOAEL for 

tribufos in rabbits is 9 mg/kg/day. The developmental toxicity LOAEL is not established.

This study is classified as Acceptable/Guideline and does satisfy the guideline requirements for 

a developmental toxicity study [OECD 870.3700 (§83-3b)] in rabbits.

A.3.4.3 Reproductive Toxicity

870.3800 Reproduction and Fertility Effects – Rat

In a two-generation reproduction study (MRID 42040101), Sprague-Dawley rats were fed diets 

containing tribufos at 0, 4, 32 or 260 ppm (0, 0.2, 1.7, or 15 mg/kg/day) for two successive 

generations. Thirty animals/sex/dose (F0) were fed tribufos in the diet for 10 weeks and then 

bred within their test groups. Thirty males and thirty females were selected from the weanlings 

of each test group to parent the second generation (F1). After 10 weeks on the test diet, F1 

animals were bred within their test group. Animals were observed daily for morbidity, mortality 

and clinical signs. Body weight and food consumption were measured weekly for each male and 

female. During gestation, female body weight was determined on days 0, 6, 13, and 20 of 

gestation and days 1, 4, 7, 14, and 21 of lactation. Food consumption in females was measured 

weekly during gestation, twice during week one of lactation and once a week during weeks 2 and 

3 of lactation. F0 and F1 males were sacrificed after the last F1 and F2 litters, respectively, were 

delivered or the dams had gone past day 24 of gestation. F0 and F1 adult females were sacrificed 

after each dam’s pups were weaned/died or when day 24 of gestation was reached. All adult 

animals were subjected to a complete gross necropsy.

Live and still born pups and sex distribution were determined for each litter. Littler counts were 

performed daily and pup weights obtained within 24 hours of birth and on days 4, 7, 14, and 21. 

Each litter was culled on day 4, with 8 pups (4 males, 4 females) selected for sacrifice and 

necropsy. Plasma and RBC ChE were determined on F0 and F1 adults during week 8 and just 
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prior to sacrifice while brain ChE was determined at termination. Plasma, RBC, and brain ChE 

were determined for one male and one female pup each from 10 litters at culling (day 4) and at 

weaning (day 21 post-partum).

No treatment-related effect on mortality was observed in adult rats. In F0 dams in the high-dose 

group, body weight was decreased on day 20 of gestation and throughout the lactation period 

while food consumption was decreased during lactation. F1 dams in the high-dose group 

exhibited decreased body weight throughout the lactation period and decreased food 

consumption on days 14 and 21 of lactation. Birth index, live birth index, viability index and 

litter size were significantly reduced in F0 dams in the high-dose group. Birth index, live birth 

index, fertility index, and viability index were significantly decreased in F1 dams in the highdose group. Plasma, RBC and brain ChE activity was significantly decreased in F0 and F1 adults 

in the mid- and high-dose groups. Plasma ChE activity was also significantly decreased in F0 

females in the low-dose group while RBC ChE activity was decreased in F0 females as well as 

F1 adults in the low-dose group. Therefore, the parental systemic toxicity LOAEL is 4 ppm 

(0.2 mg/kg/day), the lowest dose tested, based on inhibition of plasma (F0 females) and 

RBC (F0 females, adult F1 males/females) cholinesterase activity. A parental/systemic 

toxicity NOAEL is not established. 

The number of still births in F1 and F2 litters as well as the gestation length for F2 litters was 

significantly increased in the high-dose group. Mean body weights for F1 and F2 pups in the 

high-dose group were significantly reduced. Although brain and RBC ChE activity were 

significantly inhibited in F1 female pups at day 4 and F2 pups at day 21, no clear dose-response 

was observed. Plasma ChE was significantly inhibited on day 21 in F1 females in the mid- and 

high-dose groups. However, in accordance with current policy inhibition of plasma 

cholinesterase activity alone in the absence of clinical signs is not considered appropriate for use 

as the basis for a LOAEL. There was no evidence of increased susceptibility of pups over the 

adults. For reproductive toxicity, the NOAEL was 32 ppm (1.7 mg/kg/day) and the LOAEL 

was 260 ppm (15 mg/kg/day) based on a significant increase in the number of litters with 

still born pups and pup death (including cannibalism) through lactation; decrease in F1 

and F2 pup body weights; and significant increase in the gestation period for F2 litters.

This study is classified as Acceptable/Guideline and satisfies the guideline requirement for a 

reproduction study (OPPTS 870.3800 §83-4) in rats.

A.3.4.4 Chronic Toxicity

870.4100a (870.4300) Chronic Toxicity – Rat

See Carcinogenicity section below.

870.4100b Chronic Toxicity - Dog

In a chronic toxicity study (MRID 42007203), groups of four male and four female Beagle dogs 
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were fed diets containing tribufos at doses of 0, 4, 16 or 64 ppm (equivalent to 0, 0.1, 0.4, or 1.7 

mg/kg/day in males and 0, 0.1, 0.4, or 2.0 mg/kg/day in females, respectively) for 52 weeks. 

Individual body weights and feed consumption were recorded weekly and daily, respectively. 

Animals were observed twice daily for morbidity and mortality. Detailed physical examinations 

and body temperatures were taken weekly. Ophthalmic examinations were performed before 

dosing and prior to sacrifice. Clinical neurological examinations, clinical chemistry, hematology 

and urinalysis were performed on all dogs before dosing and at 3, 6, 9, and 12 months. Gross 

pathological examinations were performed on all animals at termination.

No abnormalities were observed during clinical, neurological or ophthalmological examinations. 

Therefore, the systemic NOAEL is 64 ppm (1.7 mg/kg/day in males and 2.0 mg/kg/day). A 

LOAEL is not established.

Inhibition of plasma ChE activity was observed in both sexes at 16 ppm. However, in accordance 

with current policy inhibition of plasma cholinesterase activity alone in the absence of clinical 

signs is not considered appropriate for use as the basis for a LOAEL. Inhibition of erythrocyte 

ChE activity was observed in both sexes at 64 ppm. A possible decrease in the number of 

erythrocytes at 64 ppm was observed in both sexes. No other toxic effects were observed. 

Therefore, the ChE inhibition NOAEL is 16 ppm (0.4 mg/kg/day) and the LOAEL is 64 

ppm (1.7 mg/kg/day in males; 2.0 mg/kg in females) based on decreased erythrocyte ChE 

activity.

This chronic oral toxicity study in dogs is classified as Acceptable/Guideline and satisfies the 

guideline requirement for a chronic oral toxicity study [OPPTS: 870.4100b (§83-1b)] in dogs.

A.3.4.5 Carcinogenicity

870.4200a Carcinogenicity Study - Rat

In a combined chronic toxicity/carcinogenicity study (MRID 4255360), Fischer rats 

(50/sex/dose) received diets containing tribufos at doses of 0, 4, 40 or 320 ppm (equivalent to 

0.0, 0.2, 1.8 and 16.8 mg/kg/day in males and 0.0, 0.2, 2.3 and 21.1 mg/kg/day in females, 

respectively) for 24 months. Body weights and food consumption and body temperature were 

determined weekly. Clinical observations were recorded daily and a detailed physical 

examination performed weekly. Prior to dosing and just prior to sacrifice, ophthalmic exams 

were conducted on all animals in the study. Blood was drawn for clinical pathology 

determinations at 3, 6, 12, 18 and 24 months of the study.

Complete bilateral retinal atrophy (obliteration) was observed at 12 months at the high dose (320 

ppm). At 24 months statistically-significant ocular damage at the high dose included cataract, 

lens opacity, corneal opacity, corneal neovascularization and bilateral retinal atrophy 

(obliteration). At doses of 4 and 40 ppm, terminal retinal atrophy was generally unilateral and 

histopathologically different from that seen at the high dose. Other treatment-related effects 

observed included: at 4 ppm decreased plasma ChE was observed in both sexes; at 40 ppm

decreased weight gain, cholesterol and calcium were observed in males; and decreased RBC 
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ChE, RBC count, hemoglobin, and hematocrit were observed in both sexes. At 320 ppm,

decreased weight gain was observed in the females. In addition, both sexes of rats at 320 ppm

exhibited the following effects: increased food consumption, cataract, lens and corneal opacity, 

corneal neovascularization, iritis/uveitis; decreased total protein, globulin, cholesterol, calcium; 

increased blood urea nitrogen (BUN); decreased brain ChE adrenals; vacuolar degeneration (12 

month); retinal atrophy (12 month); autolysis, vacuolar degeneration in the small intestines (12 

and 24 months); retinal atrophy, uveitis, cataract, neovascularization (24 month); atrophy of the 

optic nerve (24 month); vacuolar degeneration, hyperplasia of the small intestines (24 months). 

There was no evidence of carcinogenicity in rats. 

For chronic toxicity, the LOAEL is 320 ppm (16.8 mg/kg/day for males and 21.1 mg/kg/day 

for females), based on ocular damage in both sexes. The NOAEL is 40 ppm (1.8 mg/kg/day 

for males and 2.3 mg/kg/day for females). For ChE inhibition, the NOAEL is 4 ppm (0.2 

mg/kg/day for males and females), and the LOAEL is 40 ppm (1.8 mg/kg/day for males and 

2.3 mg/kg/day for females) based on decreased RBC ChE activity.

The chronic toxicity/carcinogenicity study in the rat is Acceptable/Guideline and satisfies the 

guideline requirements for a chronic toxicity/carcinogenicity oral study (OPPTS 870.4200a; §

83-1) in rats. 

870.4200b Carcinogenicity Study – Mouse

In a carcinogenicity study (MRID 41171001), CD-1 mice (50/sex/dose) were fed diets 

containing tribufos at doses of 0, 10, 50 or 250 ppm for 90 weeks. These doses were equivalent 

to 0, 1.64, 8.28 or 48.02 mg/kg/day in males and 0, 2.08, 11.14 or 63.4 mg/kg/day in females. 

Complete blood counts were performed in 10 mice/sex/dose at 12 months, 18 months and at 

termination or when animals were sacrificed in extremis. Plasma and RBC ChE activity was 

determined in 10 mice/sex/dose at 12 months, 18 month and at termination. Brain ChE activity 

was measured in the same mice at termination of the study. Gross pathology was performed on 

all mice found dead, sacrificed in extremis and at termination.

At 78 weeks, males in the 10 ppm group showed decreased mean corpuscular volume (MCV) 

and mean corpuscular hemoglobin (MCH) and, at week 90 females in the 10 ppm group showed 

decreased hematocrit. An increased number of males in the 50 ppm group showed paleness and 

hunched backs was well as decreased MCV and MCH at week 78 and decreased MCH at week 

90. At week 90 females in the 50 ppm group showed decreased RBC count, hemoglobin and 

hematocrit.

Histopathology of males in the 50 ppm group showed: adrenals amyloid, epididymis 

hyperspermatogenensis, small intestine amyloid and vacuolar degeneration epithelium, and 

spleen hematopoiesis. At 250 ppm loose stools were observed in females, enlarged abdomen in 

both sexes, increased mortality/decreased life span in both sexes, and increased food 

consumption and body weight in both sexes. Decreased RBC count, hemoglobin, hematocrit, 

MCV and MCH was observed in males and decreased RBC count, hemoglobin and hematocrit in 

females. There was evidence of carcinogenicity in mice only at the highest dose tested (250 ppm 

or 48.02 mg/kg/day in males and 63.4 mg/kg/day in females). Histopathology of males in the 250 
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ppm group showed: adrenals degeneration, liver hemangiosarcoma, rectum acute inflammation, 

necrosis aid ulcer, small intestine adenocarcinoma, dilated/distended and mucosal hyperplasia. 

In females in the 250 ppm group, histopathology showed: adrenals calcification and 

degeneration/pigmentation, caecum edema, liver hypertrophy, lung alveolar/bronchiolar 

adenoma, mesenteric lymph node congestion, rectum acute inflammation, necrosis and ulcer, and 

small intestine adenocarcinoma dilated/distended, mucosal hyperplasia). Statistically-significant 

decreases in plasma, RBC and brain ChE activity were observed in both sexes at all dose levels. 

The LOAEL is 10 ppm (1.64 mg/kg/day in males and 2.08 mg/kg/day in females) based on 

decreased MCV/MCH in males, decreased hematocrit in females as well as decreased 

plasma, erythrocyte and brain cholinesterase in both sexes. A NOAEL is not established.

The study is classified as Acceptable/Guideline and satisfies the guideline requirements for a 

carcinogenicity toxicity study in mice (OPPTS 870.4200; §83-5).

A.3.4.6 Mutagenicity

Mutagenicity Studies with Tribufos

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

870.5100 Gene Mutation -

Salmonella

41459101 (1990)

Acceptable/Guideline

TA98, TA100, TA1537 and 

TA1538 strains

667, 1000, 3333, 6667, and 

10000 g/plate

Not mutagenic with or without microsomal 

activation at concentrations up to 10,000 g/plate.

870.5300 Gene MutationMammalian cellsThis study is not available but the HED Carcinogenicity Peer Review Committee 

evaluated tribufos carcinogenicity data (TRX#0012242) and concluded that there is 

no apparent concern for mutagenicity. Therefore, this study is not needed.

870.5375 Chromosome 

Aberrations

Chinese Hamster 

Ovary cells

41459103 (1990)

Acceptable/Guideline

0.0005-50 l/mL

Negative for chromosomal aberrations with or 

without microsomal activation. Doses tested without 

activation showed toxicity at 0.025 l/mL and 0.05 

g/mL. Doses tested with activation showed toxicity 

at 0.05 l/mL and 0.1 l/mL.

870.5397 Mammalian 

Erythrocyte 

Micronucleus

This study is not available but the HED Carcinogenicity Peer Review Committee 

evaluated tribufos carcinogenicity data (TRX#0012242) and concluded that there is 

no apparent concern for mutagenicity. Therefore, this study is not needed.

870.5500 Unscheduled DNA 

Synthesis

Primary rat 

hepatocytes

41459102 (1990)

Acceptable/Guideline

0.0003-10 l/mL

Negative in primary rat hepatocytes at 

concentrations of 0.0001-0.006 l/mL. Higher 

concentrations were toxic.

A.3.4.7 Neurotoxicity

870.6100b Delayed Neurotoxicity Study – Hen
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In a delayed neurotoxicity study (MRID 42007202), tribufos was applied to the comb of 12 hens 

at doses of 0, 2.6, 11, or 42 mg/kg/day for 13 weeks (a total of 65 applications). Triorthocresolphosphate (TOCP) was utilized as a positive control at 18 mg/kg/day. Hens were observed 

daily during treatment for clinical signs and weighed on day 0 and weekly thereafter. Whole 

blood cholinesterase activity was determined prior to initiation of the study, at one week and at 

one, two and three months of exposure. All surviving hens were sacrificed 10-13 days after the 

last treatment to accommodate observation of possible recovery.

Body weight was significantly decreased in the 42 mg/kg/day dose group and in the TOCP 

positive control group. Histopathology indicative of organophosphate induced neuropathy 

(OPIDN) was observed primarily in the brain and spinal cord of hens at the highest dose tested 

(42 mg/kg/day). Whole blood cholinesterase inhibition was observed at the lowest dose tested 

(2.6 mg/kg/day). Therefore, the OPIDN NOAEL is 11 mg/kg/day and the LOAEL is 42 

mg/kg/day based on delayed neuropathy observed in the brain and spinal cord of hens. 

For Cholinesterase inhibition, the LOAEL is 2.6 mg/kg/day based on whole blood 

cholinesterase inhibition; a NOAEL is not established.

870.6200a Acute Neurotoxicity Screening Battery – Rat

In an acute neurotoxicity study (MRID No.: 45194401), four groups of 18/sex Wistar strain rats 

were dosed with 0, 2, 20 or 100 mg/kg of tribufos (98% purity in corn oil at 5 mL/kg). Twelve 

rats/sex per group were assessed for clinical signs, body weight, FOB and motor activity at 

pretest, estimated time for peak effects (six hours post dosing based on preliminary studies), and 

on days 7 and 14 postdosing. At termination the 6 rats/sex/group were perfused and assessed 

histologically for neuropathological effects. The remaining 6 rats/sex/group were assessed for 

cholinesterase at pretest and the time of peak effect. 

No lesions in the nervous system were attributed to tribufos treatment. At 20 mg/kg statistically 

significant decreased motor and locomotor activity was evident in males. In females statistical 

significance was not attained but decreases in motor activity and in locomotor activity were 

apparent. At 100 mg/kg the decreases in motor and locomotor activity were larger to indicate a 

dose response and the locomotor activity decrease in females was statistically significant. There 

was also decreased rearing and arousal. Body temperature was decreased 1.9 C and 2.8 C in 

males and females, respectively. Two deaths among the females on days 2 and 4 were attributed 

to tribufos. The LOAEL is 20 mg/kg based on decreased motor activity. The NOAEL is 2 

mg/kg. 

Plasma ChE was statistically significantly inhibited at 20 mg/kg and at 100 mg/kg when assessed 

6 hours after dosing. RBC ChE was also statistically significantly inhibited at 20 mg/kg and 100 

mg/kg. Brain ChE was statistically significantly inhibited in the 100 mg/kg female dose group

only. The LOAEL for cholinesterase inhibition is 20 mg/kg based on inhibition of both

plasma and RBC ChE. The NOAEL is 2 mg/kg. 

This acute neurotoxicity study is classified Acceptable/Guideline and satisfies the guideline 

requirement for an acute neurotoxicity study [OPPTS 870.6200a (§81-8)] in rats. 
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870.6200b Subchronic Neurotoxicity Screening Battery – Rat

In a subchronic neurotoxicity study (MRID 45369101), Tribufos (98.0-98.1% purity) was 

administered to 18 young adult Wistar rats/sex at dietary dose levels of 0, 2, 40, or 500 ppm 

(equivalent to mean doses of 0, 0.14, 2.89, or 36.8 mg/kg/day for males and 0, 0.17, 3.54, or 42.6 

mg/kg/day for females, respectively) for 13 weeks. Neurobehavioral assessment (functional 

observational battery (FOB) and motor activity testing) was performed in 12 animals/sex/group 

at pretest and at weeks 4, 8 and 13. Cholinesterase was assessed in 6 rats/sex/dose for plasma 

and erythrocytes at pre-treatment and at weeks 4 and 13, and in whole brain at week 13. At 

study termination, 6 control and high dose animals/sex/group were perfused in situ for 

neuropathological examination of brain and peripheral nervous system tissues. 

Mean body weights and food consumption were decreased for females in the 40 ppm group and 

for males and females in the 500 ppm group. Treatment-related clinical signs were limited to red 

nasal staining in one high-dose male on day 4, perianal staining in one high-dose male on day 11,

and urine staining in two high-dose females from days 53-88. FOB effects included 

sluggish/minimal movement and decreased approach response in the open field in high-dose 

males and a decreased number of rearing counts in the open field was noted in high-dose 

females, both at week 4 only. Slight yellow urine staining was noted for 1/12 high-dose females 

at week 8 and for 2/12 high-dose females at week 13. Although statistical significance was not 

attained, these effects are considered treatment-related. Motor activity was decreased in highdose males and females during weeks 4, 8, and 13. Locomotor activity was significantly

decreased in high-dose males and females at weeks 4, 8, and 13, with decreases averaging 32-

55% for males and 48-53% for females. Neuropathology indicated retinal degeneration (mostly 

bilateral) was noted in 5/6 high-dose males and 6/6 high-dose females but was not seen in the 

controls or the mid dose. The LOAEL in females is 40 ppm (3.54 mg/kg/day) based on 

decreased body weight and decreased food consumption. The NOAEL is 2 ppm (0.17 

mg/kg/day). The LOAEL for males is 500 ppm (36.8 mg/kg/day) based on decreased body 

weight, decreased food consumption, and motor activity decrease and retinal degeneration. 

The NOAEL is 40 ppm (2.89 mg/kg bw/day). 

RBC and plasma ChE were significantly inhibited at 40 ppm and 500 ppm in both males and 

females. Brain cholinesterase activity was decreased in females at 40 ppm and at 500 ppm in 

males and females. The NOAEL for ChE inhibition is 2 ppm (0.14 mg/kg/day in males and 

0.17 mg/kg/day in females) and the LOAEL is 40 ppm (2.89 ppm in males and 3.54 

mg/kg/day in females) based on ChE inhibition in plasma and RBC (both sexes) and brain 

(females only).

The study is classified as Acceptable/Guideline and satisfies the guideline requirement for a 

subchronic neurotoxicity study in rats (870.6200b). 

870.6300 Developmental Neurotoxicity – Rat
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In a developmental neurotoxicity study (MRID 45499501), Tribufos (98.0-98.1% a.i.) was 

administered to parent female Wistar rats in the diet at concentrations of 0, 4, 40 or 200 ppm 

from gestation day 0 through postnatal day 21. The average daily intake of Tribufos was 0, 0.4, 

3.4-3.5, and 16.4-18.2 mg/kg bw/day during gestation and 0, 0.6-1.0, 6.1-9.9, and 33.5-55.4 

mg/kg/day during lactation, for the 0, 4, 40, and 200 ppm groups, respectively. Two studies, 

with a two-week difference in start date, were conducted under similar conditions. The main 

study (00-D72-AG) included 20 parent females/dose level, and the satellite study (00-D2-AS) 

included 10 parent females/dose level and was utilized to provide samples to assess 

cholinesterase activity. A Functional Operational Battery (FOB) was performed on 10 

dams/dose from the main study on gestation days 6, 13, and 20 and on and lactation days 4, 11, 

and 21. On postnatal day 4, litters were culled to yield four males and four females (as closely as 

possible). Offspring from the main study, representing at least 20 litters/dose, were allocated for 

detailed clinical observations (abbreviated FOB), assessment of motor activity, assessment of 

auditory startle response habituation, assessment of learning and memory, electroretinography, 

and neuropathology at study termination (day 75 of age). Animals from the satellite study were 

used to provide blood and brain samples for the offspring (postnatal days 11 and 21) and dams 

(lactation day 21) for measurement of cholinesterase (ChE) activity.

No treatment-related effects were seen on survival, food consumption or reproductive 

parameters. Dams exhibited clinical signs of toxicity (slight tremors) at parturition which are 

likely associated with the observed ChE inhibition. Maternal body weight during lactation was 

decreased at each time point recorded for the high-dose group, with average differences of 8-

12% compared to the control. 

The maternal systemic LOAEL is 200 ppm (16.4 mg/kg/day) based on clinical signs 

(tremor) and decreased body weight during lactation. The maternal systemic NOAEL is 40 

ppm (3.4 mg/kg/day).

Plasma, brain and RBC ChE activity was decreased in maternal animals at 40 and 200 ppm. 

Therefore, the maternal ChE LOAEL is 40 ppm (3.4 mg/kg/day) based on decreases in 

plasma, RBC and brain ChE activity. The maternal ChE NOAEL is 4 ppm (0.4 mg/kg 

bw/day).

Treatment had no adverse effects on offspring survival. No treatment-related effects on body 

weight or body weight gains were seen at 4 or 40 ppm pups. At the high dose (200 ppm), pup 

body weights were significantly decreased on PNDs 4, 11 and 21. Preputial separation for highdose males was delayed relative to controls. Additionally, the development of a surface-righting 

reflex in high dose animals was significantly delayed. Motor activity was decreased in males 

and females at the high dose on PND 13. Conversely, motor activity was increased in males and 

females at the high dose on PND 17. There were no apparent effects on motor activity in males 

or females on PND 21 and PND 60. Startle response amplitude for high-dose males and females 

was decreased compared to controls on day 21, but not on subsequent test days. No treatmentrelated effects were seen on learning and memory. Absolute brain weights of high-dose males 

and females were decreased compared to controls on days 11 and 21. The anterior to posterior 

lengths of the cerebrum were marginally decreased for high-dose males and females compared to 

controls on PND 11. The anterior to posterior length for the cerebellum was also marginally 
Tribufos Human Health Risk Assessment DP No. D357537

Page 75 of 84

decreased in high-dose males and females, although the effect did not reach statistical 

significance

The offspring systemic LOAEL is 200 ppm (16.4 mg/kg/day), based on decreased body 

weight, body weight gain, delay in preputial separation, delayed surface righting reflex, 

decreased motor activity, decreased startle response amplitude, decreased absolute brain 

weights, and changes in the brain morphometrics (cerebrum and cerebellum). The 

offspring systemic NOAEL is 40 ppm (3.4 mg/kg/day).

For the day 21 offspring, the only effect was a 21-22% decrease in plasma cholinesterase in 

high-dose males and female. RBC and brain cholinesterase activity was not adversely affected 

in the offspring. In accordance with the current policy inhibition of plasma cholinesterase 

activity alone in the absence of clinical signs is not considered to be appropriate for use as the 

basis for a LOAEL. Therefore, the offspring cholinesterase NOAEL is 200 ppm (16.4 

mg/kg/day), the highest dose tested. An offspring cholinesterase NOAEL was not 

established.

This study is classified Acceptable/Non Guideline and may be used for regulatory purposes, 

however it does not satisfy the guideline requirement for a developmental neurotoxicity study in 

rats (OPPTS 870.6300, §83-6).

A.3.4.8 Metabolism

870.7485 Metabolism – Rat

In a rat metabolism study (MRID 42034501), [1- C

14] tribufos (97% ai) was administered to male 

and female Sprague-Dawley rats (5 animals/sex/dose) by gavage as a single dose at levels of 5 or 

100 mg/kg, or as a single dose at 5 mg/kg following a 14-day pretreatment with unlabeled 

tribufos at 5 mg/kg/day. Five additional rats/sex received a single 5 mg/kg intravenous dose.

Fifty-five to 80% of tribufos was absorbed, with >90% excreted in 72 hours. The majority of the 

radioactive dose was excreted in urine (55-80%), followed by feces (15-42%), with <1% in the 

expired air. Females showed more radioactive compound in the urine than males for each dose 

regimen. There was no significant tissue residue. Only two excreted compounds could be 

identified, unmetabolized tribufos in the feces and butyl-gamma-glutamylcysteinylglycine 

disulfide in the urine. The data indicate complete metabolic breakdown of absorbed tribufos with 

extensive breakdown of the S-butyl moieties, probably to two carbon fractions. No evidence of 

sulfonation or glucuronidation was observed following treatment with sulfatase or bglucuronidase.

This metabolism study in the rat is classified Acceptable/Guideline and satisfies the guideline 

requirement for a metabolism study [OPPTS 870.7485 (§85-1)] in the rat. 

A.3.4.9 Immunotoxicity
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870.7800 Immunotoxicity

In an immunotoxicity study (MRID #48709907), Tribufos was administered to female Han 

Wistar rats (10/dose) in the diet at dose levels of 0, 5, 50, or 500 ppm (equivalent to 0, 0.43, 4.32, 

or 44.62 mg/kg/day, respectively) for 4 weeks. The positive control group (8 females) was

administered 50 mg/kg bw/day of cyclophosphamide (CP) by intraperitoneal injection on study 

Day 27. During the study, clinical condition, bodyweight, food and water consumption, organ 

weight, brain cholinesterase, and macroscopic pathology were evaluated. Four days before 

sacrifice, all animals in all groups received a single intravenous dose of sheep red blood cells 1.0 

mL/animal (2 x 108 SRBC/mL) in 0.9% saline. At sacrifice (Day29), selected organs were 

removed and weighed (spleen, brain and thymus). Brain and blood cholinesterase levels were 

determined. The anti-SRBC IgM concentration was measured with Plaque Forming Cell (PFC) 

Assay.

There were no premature deaths and no treatment-related clinical signs. At 500 ppm there was 

significant reduction in weight gain during the first 11 days of treatment and at week 4. Food 

and water consumption were reduced only in week one. Erythrocyte cholinesterase (ChE) 

activity was significantly reduced at 50 ppm and 500 ppm (66% and 91%, respectively) in week 

4. At 500 ppm, brain ChE activity was reduced 78%. There was increased spleen weight at 500 

ppm. Gross pathology showed splenomegaly only in one animal at 500 ppm; there were presence 

of pale contents in jejunum in the majority of animals at 500 ppm. The systemic LOAEL is 50 

ppm (equivalent to 4.32 mg/kg/day) based on significant decreased erythrocyte ChE

activity. The systemic NOAEL is 5 ppm (equivalent to 0.43 mg/kg/day).

There were no statistically significant decreases in the numbers of PFC/10 6 viable cells or 

PFC/spleen in treated groups when compared to the vehicle control group. High dose group 

receiving 500 ppm had significantly increased cells/spleen. High inter-individual variability was 

noted in all the treatment groups as well as in the control group. Evaluation of the individual 

animal data of this study did not show any trend or distribution that would demonstrate 

significant suppression of anti-SRBC antibody response. The positive control group had a 

statistically significant decrease in the PFC response. This confirmed the ability of the test 

system to detect immuno-suppressive effects and confirmed the validity of the study design.

The Natural Killer (NK) cells activity was not evaluated in this study. The toxicology database 

for Tribufos technical does not reveal any evidence of immunotoxicity. The overall weight of 

evidence suggests that the chemical does not directly target the immune system. Under HED 

guidance a NK cell activity assay is not required at this time. The NOAEL for immunotoxicity 

is 500 ppm (equivalent to 44.62 mg/kg/day), the highest dose tested. The immunotoxicity 

LOAEL is not established.

This immunotoxicity study is classified acceptable/guideline and satisfies the guideline

requirement for an immunotoxicity study (OPPTS 870.7800) in rats.

A. 3.4.10 Special Studies
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Comparative Cholinesterase Studies

This series of non-guideline cholinesterase (ChE) inhibition studies (MRIDs 48709901-

48709906) was undertaken to evaluate differences between PND-11 through PND-21 pups of 

both sexes and young adult female rats, and between dams and their fetuses, with regard to 

erythrocyte (RBC) or brain ChE following administration of tribufos. All studies used Crl: CD 

Sprague Dawley rats and the same batch of tribufos (Batch No. NX81AX0015; 98.2% a.i.) 

delivered in corn oil. All doses were administered by oral gavage, and the actual doses were 

quantified by gas chromatography using electron capture detection. In addition to assessing ChE 

inhibition, most of the studies also included determinations of clinical signs, mortality, body 

weight, food consumption, and necropsy with macroscopic examinations. 

Acute and Repeat Dose – Range Finding Studies in PND 11 Rat Pups

In pilot dose range-finding studies (MRID 48709901) neonate 11 day old rat pups were 

administered acute (0, 20, 40 or 50 mg/kg/day) or repeated (11 consecutive doses of 0, 5, 10, 

15, or 20 mg/kg/day) gavage doses of Tribufos. The small number of animals per dose precludes 

making firm conclusions. However some reported effects included decreased body weights and 

weight gains, appearance of being underactive, cold to touch, increased body tone, unsteady gait, 

body tremor and irregular breathing or vocalization. The signs were reported in both acute and 

repeat dosing with only flat posture and splayed hindlimbs in the repeat dosing phase. 

RBC and brain (but not plasma) ChE assessments were made at 24 and 48 hours following a 

single dose of Tribufos. RBC ChE was inhibited in males and females at 20 and 50 mg/kg/day 

after 24 and 48 hours. Brain ChE was inhibited at 20 mg/kg/day in females but not in males after 

24 hours. Brain ChE was inhibited in both males and females at 40 and 50 mg/kg/day after 48 

hours. In the repeated dose study, assessments for RBC and brain ChE were made about 6 hours 

after the last dose. RBC and brain ChE were significantly inhibited in both males and females 

starting at 5 mg/kg/day.

 

Overall conclusion: This study supports a maximum tolerated dose of 50 mg/kg tribufos for a 

single oral dose and 15 mg/kg/day for repeat dosing in an 11 day multiple dosing study for 11 

day old Sprague-Dawley rat pups. 

This study is classified as acceptable/non-guideline and provides information for doses for 

definitive acute and repeat dosing comparative cholinesterase studies. It is noted that 

assessment for plasma ChE inhibition was not included. 

Acute Oral Dosing in Young Rats to Determine Time to Peak Effect

In an acute study (MRID 48709902) to determine the time to peak effect on clinical signs and 

cholinesterase (ChE) inhibition, groups of 15/sex neonate rats (Sprague-Dawley, aged 11 days), 

were given a single oral dose of 50 mg/kg bw Tribufos. Three pups/sex were scheduled for 

observation and sacrificed at 2, 4, 6, 8, 24 and 48 hours post dosing. ChE was determined in red 

blood cells (RBC) and brains.
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Two pups appearing in poor condition were sacrificed. Clinical signs reported (i.e underactivity, 

reduced body temperature) do not provide data that can delineate the time to peak of effect. 

Body weights trended to be reduced at 24 and 48 hours in both sexes. Brain weight and gross 

pathology were not affected. 

RBC ChE was inhibited in both sexes at all times. Brain ChE was inhibited in both sexes after 4 

hours and maximum inhibition was noted in males at 6 hours and in females at both 6 and 8 

hours postdosing. Brain ChE inhibition declined to about 50% by 48 hours but RBC ChE 

remained at about 88%. 

Conclusion: Based on observed RBC and brain ChE inhibition, 6-8 hours is a reasonable 

selection for the time to peak effect for the definitive study. 

This study is classified as acceptable/non-guideline and provides information on the time 

course for ChE inhibition. 

Acute Oral Dosing to Young Adult Rats to Determine Time to Peak Effect

In an acute oral toxicity study (MRID 48709903), groups of 24 young adult female (aged 48-52 

days) SD rats were given a single oral dose of Tribufos at doses of 0 or 80 mg/kg bw to 

determine the time to a peak effect for systemic toxicity and cholinesterase inhibition. The study 

was conducted in 2 phases (Phase I, 24 hrs. used 15 animals/group) (Phase II, 48 hrs. used 9 

animals/group). Clinical observations were made prior to sacrifice and at 2, 4, 6, 8, and 24 hours 

(3 per time) post dosing in Phase I, and at 24, 36, and 48 hours post dosing in Phase II. ChE was 

determined in red blood cells (RBCs) and brains. 

There were no outstanding clinical signs observed, or effects on mortality, body weight and gain, 

brain weight, or gross pathology. RBCs ChE was significantly reduced by 90% in Phase I at 24 

hours and in Phase II by 83% and 89% between 24 and 36 hours respectively. Brain ChE

inhibition in Phase I was 15% at 24 hours, while in Phase II inhibition was 15% at 24 hours and 

20% at 36 hours. 

Conclusion: The brain and RBC inhibition time to peak effect appears to be between 24 and 36 

hours. 

This study is classified as acceptable/non-guideline as a pilot study. The study is considered to 

have provided information on time frames in which to search for the values of the time to peak

effect for RBC and brain ChE inhibition. 

Acute Oral Dosing Adult and Pup Comparative ChE 

In a comparative cholinesterase inhibition study (MRID 48709904), groups of 8 young adult SD 

rats(female only, aged 48-52 days) and pups (11 days old male and female) were administered 

Tribufos at doses of 0, 2, 10 or 50 (pups only) or 80 (adults only) mg/kg bw. Clinical 

observations were made prior to dosing and at 3 and 6 hours and 24 (adults only). RBC and 
Tribufos Human Health Risk Assessment DP No. D357537

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brain cholinesterase (ChE), brain weights and gross pathology were determined at times of 

sacrifice or at 6 hours for the pups and 24 hours for the adults. 

Clinical signs were not noted in adult females. The 10 mg/kg neonates exhibited under activity 

at 6 hours post dosing and at 50 mg/kg under activity and uncoordinated gait was noted as early 

as 3 hours and signs of being unsteady at 6 hours. There were no effects on mortality, brain 

weight, or gross pathology. Body weights of male and female pups were slightly lower (males 

6.7 to 11% and females 7.3 to 8.9%) for all doses without a dose response and were not 

definitely related to treatment and will not be considered in the LOAEL statement. 

The LOAEL for systemic effects in pups is 10 mg/kg based on under activity. The NOAEL 

for pups is 2 mg/kg. The LOAEL for systemic effects in adults is not established since no 

effects were noted. The NOAEL is > 80 mg/kg. 

RBC ChE was inhibited for the 2, 10 and 50 mg/kg female pup dose groups and for the 10 and 

50 mg/kg male pup dose groups. In female adults there was inhibition at 24 hours in the 80 

mg/kg dose group only. Brain ChE was inhibited in male and female pups at 10 and 50 mg/kg. 

Adult female brain ChE was inhibited only at 80 mg/kg. The LOAEL for ChE inhibition in 

pups is < 2 mg/kg based on RBC inhibition in females. The NOAEL is not established. The 

LOAEL for adults for both RBC and brain ChE inhibition is 80 mg/kg. The NOAEL is 10 

mg/kg. 

Conclusion: Pups are more sensitive to the inhibitory effects of tribufos than adults. 

This comparative cholinesterase study is classified as acceptable/non-guideline and established 

that pups are more sensitive to both systemic and ChE inhibitory effects of tribufos. 

Repeat Dose Comparative Sensitivity Study in Young Adult Female and 11 Day Old 

Neonatal CD Rats by Oral Gavage

In a repeat dose comparative sensitivity study (MRID 48709905) in SD rats, tribufos was 

administered via gavage at dose levels of 0, 0.1, 1.0 or 5.0 mg/kg bw/day to 8 young adult 

females/group and 8 male and 8 female neonatal pups (11 days old)/group for 11 consecutive 

days. Clinical signs of toxicity, mortality, body weights, cholinesterase (ChE) measurements 

[erythrocyte (RBC) and brain determinations], brain weights and macroscopic pathology data 

were obtained.

In adults, salivation was noted in 1 female in the 1.0 and one in the 5.0 mg/kg/day 1-2 h after 

dosing. However, salivation was not reported as an adverse effect in the subchronic neurotoxicity 

study (MRID No.: 45369101) at dietary doses up to 4.6 mg/kg/day. No other effects were noted 

in the adult females. In pups, reactions were noted at 5 mg/kg/day 1-2 hours postdosing 

including four males and 5 females with unsteady muscle reaction and one male and five females 

with prostrate posture as well as under activity. At terminal sacrifice, pup male body weight was 

statistically significantly decreased in the 5.0 mg/kg/day group. Female pup body weight at 

termination was statistically significantly (p<0.05) decreased in the 0.1, 1.0 and 5.0 mg/kg/day 

group. There was, however, no dose response to allow for a firm conclusion that body weight 
Tribufos Human Health Risk Assessment DP No. D357537

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was affected in female pups. The LOAEL for systemic toxicity in young adult females was 

not established. The NOAEL is > 5 mg/kg/day. The LOAEL for systemic toxicity in pups 

is 5 mg/kg/day based on unsteady muscle reaction, prostrate appearance and underactivity. 

The NOAEL is 1 mg/kg/day. 

RBC ChE was statistically significantly inhibited in adults in the 1.0 and 5.0 mg/kg/day group. 

RBC ChE was statistically significantly inhibited only in the 5.0 mg/kg/day group in pup males 

and females. Thus, the adults are more sensitive than the pups to inhibition of RBC ChE by 

tribufos. Brain ChE was not inhibited in adults. Brain ChE was statistically significantly 

inhibited in the 5.0 mg/kg/day group in pup males and females. Thus, the pups are more 

sensitive than the adults to the inhibition of brain ChE by tribufos. The ChE LOAEL is 1.0 

mg/kg/day based on RBC ChE inhibition in young adult females. The NOAEL is 0.1 

mg/kg/day. The ChE LOAEL for male and female pups is 5.0 mg/kg/day based on 

inhibition of RBC and brain ChE. The NOAEL is 1.0 mg/kg/day.

 

This repeat dose comparative sensitivity study in young adult female and 11 day old neonatal 

rats is acceptable/non-guideline. 

Table A.5. Comparison of Tribufos RBC ChE inhibition in rat female adults vs female 

pups following repeated-dosinga

Dose (mg/kg/day) % ChE inhibition 

Female adults

% ChE inhibition

Female Pups (PND11)

0 - -

0.1 10 12

1.0 18 18

5.0 64 69

aMIRD 48709905

Gestational ChE Inhibition- Rat

In a gestational cholinesterase (ChE) inhibition study (MRID 48709906), tribufos was 

administered to pregnant SD rats (10/dose) by gavage at targeted doses of 0, 1 (nominal), 7, or 

28 mg/kg/day on gestation days (GD) 6-19. Because of low test substance concentrations 

measured in the 1 mg/kg/day formulations, the actual time-weighted dose was between 0.3 and 

0.8 mg/kg/day. On GD 20, blood samples were collected from the dams and from the umbilical 

cord of all litters for the analysis of erythrocyte ChE activity. At necropsy on GD 20, each dam’s 

uterus was removed via cesarean section and its contents examined. Brains were removed from 

the dams and fetuses for the determination of brain ChE activity. Fetuses were examined for 

external malformations. 

A statistically significant decrease in mean body weight gain (not adjusted for gravid uterine 

weight) was observed in the 28 mg/kg/day group over the treatment period. Mean food 

consumption in the 28 mg/kg/day group was significantly lower during GD 6-9 and GD 18-

19.There were no treatment-related effects on maternal mortality or clinical signs, or on 

reproductive parameters, including mean number of corpora lutea, implantations, resorptions, 

pre- and post-implantation losses, number of live young, and sex ratio. The LOAEL for 
Tribufos Human Health Risk Assessment DP No. D357537

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maternal toxicity in rats is 28 mg/kg/day based on decreased body weight gain and 

decreased food consumption. The NOAEL is 7 mg/kg/day. 

There were no observed fetal external abnormalities. Mean male, female, and overall fetal 

weights, and mean litter weights in the 28 mg/kg/day group were slightly decreased (approx. 6% 

each), reaching statistical significance for male fetal weight only. The LOAEL for offspring

toxicity in rats is 28 mg/kg/day based on decreased body weight in males. The NOAEL is 7 

mg/kg/day.

Maternal erythrocyte and brain ChE were significantly decreased at 7 and 28 mg/kg/day. At 28 

mg/kg/day, fetal erythrocyte (from the umbilical cord) and brain ChE activities were lower than 

the control but did not reach statistical significance. The LOAEL for maternal brain and 

erythrocyte ChE inhibition is 7 mg/kg/day. The NOAEL is between 0.3 and 0.8 mg/kg/day. 

The LOAEL for offspring erythrocyte and brain ChE inhibition is not determined. The 

NOAEL is 28 mg/kg/day, the highest dose tested. 

This gestational exposure cholinesterase inhibition study is classified acceptable/non-guideline.
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Appendix 3. IRL Sheet

International Residue Limits 

Summary of US and International Tolerances and Maximum Residue Limits 

US Canada Mexico1 Codex

Residue Definition:

40CFR 180.272

Tribuphos (S,S,Stributylphosphorotrithioate) 

None None

Commodity Tolerance (ppm) /Maximum Residue Limit (mg/kg)

Commodity US Canada Mexico1 Codex

Cattle, fat 0.15 None None None

Cattle, meat 0.02

Cattle, meat byproducts 0.02

Cotton, gin byproducts 40

Cotton, undelinted seed 4.0

Goat, fat 0.15

Goat, meat 0.02

Goat, meat byproducts 0.02

Hog, fat 0.15

Hog, meat 0.02

Hog, meat byproducts 0.02

Horse, fat 0.15

Horse, meat 0.02

Horse, meat byproducts 0.02

Milk 0.01

Sheep, fat 0.15

Sheep, meat 0.02

Sheep, meat byproducts 0.02

Completed by: M. Negussie; 06/19/15

1 Mexico adopts US tolerances and/or Codex MRLs for its export purposes.
Tribufos Human Health Risk Assessment DP No. D357537

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Appendix 5. Summary of Steady State Non Cancer Occupational Handler Dermal Risk Estimates for Tribufos

Appendix 5 Table A.5.1.: Tribufos Steady State Non Cancer Occupational Handler Dermal Risk Estimates on Cotton.

Exposure Scenario Application 

Rate2

Area 

Treated3

Dermal Unit Exposures (ug/lb 

ai)1 Dermal Dose (mg/kg/day)4 Dermal MOE5

Worker 

Activity Formulation Application 

Equipment Value Units Value Unit SL/No G

SL/G DL/G EC SL/No 

G

SL/G DL/G EC SL/No 

G

SL/G DL/G EC

M/L L/SC/EC

Aerial

1.875

lb 

ai/A

1200

A

220 37.6 29.1 8.6 7.170 1.230 0.949 0.281 0.022 0.13 0.17 0.57

1.125 220 37.6 29.1 8.6 4.300 0.736 0.570 0.168 0.037 0.22 0.28 0.95

Groundboom

1.875

200

220 37.6 29.1 8.6 1.200 0.204 0.158 0.047 0.13 0.78 1 3.4

1.125 220 37.6 29.1 8.6 0.717 0.123 0.095 0.028 0.22 1.3 1.7 5.7

Applicator Spray

Aerial

1.875

lb 

ai/A

1200

A

No 

Data

No 

Data

No 

Data 2.08 No Data

No 

Data

No 

Data 0.068 No Data

No 

Data

No 

Data 2.4

1.125 No 

Data

No 

Data

No 

Data 2.08 NoData

No 

Data

No 

Data 0.041 No Data

No 

Data

No 

Data 3.9

Groundboom

1.875

200

78.6 16.1 12.6 5.1 0.428 0.088 0.069 0.028 0.37 1.8 2.3 5.8

1.125 78.6 16.1 12.6 5.1 0.257 0.053 0.041 0.017 0.62 3 3.9 9.6

Flagger Spray Aerial

1.875

lb 

ai/A 350 A

11 12 10.6 No 

Data 0.105 0.114 0.101 No Data 1.5 1.4 1.6 No Data

1.125 11 12 10.6 No 

Data 0.063 0.069 0.060 No Data 2.5 2.3 2.6 No Data

1 PHED/ORETF (Eng. Controls [EC]).

2

. Based on registered label (e.g. Reg. No. 5481-504, 5481-903 or EPA Reg. No. 264-730, 34704-867, and 85678-15).

3. Exposure Science Advisory Council Policy #9.1.

4. Dermal Dose = Dermal Unit Exposure (ug/lb ai) x Conversion Factor (0.001 mg/ug) x Application Rate (lb ai/acre or gal) x Area Treated or Amount Handled (A) / BW (69 kg).

5. Dermal MOE = Dermal POD (0.16 mg/kg/day)/Dermal Dose (mg/kg/day). Bolded numbers are risk estimates of concern. (MOE<1,000). Highlighted risk estimates are current label 

PPE mitigation.
Tribufos Human Health Risk Assessment DP No. D357537

Page 84 of 84

Appendix 5: Table A.5.2: Tribufos Steady State Non Cancer Occupational Handler Inhalation Risk Estimates on Cotton.

Exposure Scenario Application 

Rate2

Amount 

Handled / 

Area 

Treated3

Inhalation Unit Exposures 

(ug/lb ai)1

Inhalation Dose (mg/kg-day)4Inhalation MOE5

Worker 

Activity Formulation Application Equipment Value Units Value Unit No-R

PF5 

R

PF10 

R

EC No-R

PF5 

R

PF10 

R

EC NoR

PF5 

R

PF10 

R

EC

M/L L/SC/EC

Aerial

1.875

lb ai/A

1200

A

0.219 0.044 0.022 0.083 0.007 0.001 0.001 0.003 28 140 280 130

1.125 0.219 0.044 0.022 0.083 0.004 0.001 0.000 0.002 47 230 470 210

Groundboom

1.875

200

0.219 0.044 0.022 0.083 0.001 0.000 0.000 0.000 170 840 1700 750

1.125 0.219 0.044 0.022 0.083 0.001 0.000 0.000 0.000 280 1400 2800 1300

Applicator Spray

Aerial

1.875

lb ai/A

1200

A

No 

Data

No 

Data

No 

Data 0.005 No Data

No 

Data

No 

Data 0.000 No Data

No 

Data

No 

Data 630

1.125 No 

Data

No 

Data

No 

Data 0.005 No Data

No 

Data

No 

Data 0.000 No Data

No 

Data

No 

Data 1000

Groundboom

1.875

200

0.340 0.068 0.034 0.043 0.002 0.000 0.000 0.000 54 270 540 1500

1.125 0.340 0.068 0.034 0.043 0.001 0.000 0.000 0.000 90 450 900 2400

Flagger Spray Aerial

1.875

lb ai/A 350 A

0.350 0.070 0.035 No 

Data 0.003 0.001 0.000 No Data 100 510 1000 No Data

1.125 0.350 0.070 0.035 No 

Data 0.002 0.000 0.000 No Data 170 850 1700 No Data

1. PHED/ORETF (Eng. Controls [EC]).

2. Based on registered label (e.g. Reg. No. 5481-504, 5481-903 or EPA Reg. No. 264-730, 34704-867, and 85678-15).

3. Exposure Science Advisory Council Policy #9.1.

4. Inhalation Dose = Inhalation Unit Exposure (ug/lb ai) x Conversion Factor (0.001 mg/ug) x Application Rate (lb ai/acre or gal) x Area Treated or Amount Handled (A) / BW (69 kg).

5. Inhalation MOE = Inhalation HED (0.10 mg/kg/day)/ Inhalation Dose (mg/kg/day). Bolded numbers are risk estimates of concern (MOE<300). ). Highlighted risk estimates are current label 

PPE mitigation.