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Table of Contents

Executive Summary......................................................................................................................................4

1.0 Occupational and Residential Exposure/ Risk Assessment ............................................................11

1.1 Criteria for Conducting Exposure Assessments..........................................................................11

1.2 Toxicological Endpoints.............................................................................................................11

1.3 Incident Report............................................................................................................................16

1.4 Use Profile ..................................................................................................................................19

2.0 Occupational Exposures and Risks.................................................................................................23

2.1 Occupational Handler Exposures and Risks...............................................................................23

2.1.1 Assumptions for Handler Exposure Scenarios....................................................................25

2.1.2 Exposure Data for Handler Exposure Scenarios.................................................................28

2.1.3 Occupational Handler Exposure Scenarios.........................................................................35

2.1.4 Occupational Handler Exposure and Risk Calculations .....................................................36

2.1.5 Occupational Handler and Exposure Risk Summary..........................................................38

2.1.6 Summary of Occupational Handler Exposure Data Gaps...................................................38

2.1.7 Occupational Handler Characterization ..............................................................................40

2.2 Occupational Handler Comparison Using Biomonitoring Data .................................................42

2.2.1 Occupational Handler Biomonitoring Exposure and Risk Calculations.............................44

2.2.2 Occupational Handler Chemical Specific Data...................................................................45

2.2.2 Occupational Handler Comparison Using Biomonitoring Data Risk Summary.................51

2.3 Commercial Seed Treatment.......................................................................................................51

2.3.1 Assumptions for Commercial Seed Treatment Exposure Scenarios...................................52

2.3.2 Commercial Seed Treatment Exposure and Risk Calculations...........................................53

2.3.3 Commercial Seed Treatment and Exposure Risk Summary ...............................................54

2.3.4 Commercial Seed Treatment and Exposure Risk Characterization ....................................54

2.4 Occupational Post-application Exposures and Risks..................................................................54

2.4.1 Occupational Post-application Exposure Scenarios............................................................54

2.4.2 Data/Assumptions for Post-application Exposure Scenarios..............................................57

2.4.2.1 Post-application Exposure Study Summaries ..................................................................59

2.4.3 Use of Post-application DFR Data.....................................................................................67

2.4.4 Occupational Post-application Exposure and Risk Summary....................................................69

2.4.5 Summary of Exposure Data Gaps.......................................................................................76
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2.4.6 Occupational Post-application Characterization ......................................................................77

3.0 Residential Exposures and Risks ....................................................................................................78

3.1 Residential Handler Exposures and Risks ..................................................................................79

3.2 Residential Post-application Exposures and Risks .....................................................................79

3.2.1 Data and Assumptions for Residential Handler Exposure Scenarios .................................79

3.2.3 Residential Post-application Risk Summary.......................................................................90

3.2.4 Residential Post-application Exposure and Risk Characterization .....................................91

3.3 Residential Bystander Exposure and Risk ..................................................................................92

3.3.1 Residential Bystander Exposure and Risk ..........................................................................96

3.3.2 Residential Bystander Exposure and Risk Characterization...............................................97

3.4 Spray Drift ..................................................................................................................................98

4.0 Evaluation of Chlorpyrifos Oxon....................................................................................................99

4.1 Oxon Study Summaries ............................................................................................................100

4.2 Interpretation and Application of Oxon Studies .......................................................................106
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Executive Summary

Chlorpyrifos (0,0-diethyl-0-3,5,6-trichloro-2-pyridyl phosphorothioate) is a broad-spectrum, 

organophosphate insecticide that was first registered in 1965 to control foliage- and soil-borne 

insect pests on a variety of food and feed crops. Chlorpyrifos is formulated as granular, 

microencapsulated, soluble concentrate/liquids, water dispersible granular in water soluble 

packets (WSP) and wettable powder packaged in WSP, as well as impregnated paints, cattle ear 

tags and insect bait stations. A wide variety of application techniques have been identified that 

could potentially be used to apply chlorpyrifos for agricultural use, such as tractor-drawn 

equipment, open and closed mixing/loading, and hand held equipment. Applications of 

chlorpyrifos also include soil incorporated/directed uses, bark, root dip, foliar, seed treatments, 

aerial and ground-based fogger mosquitocide and golf course turf. 

The purpose of this memorandum is to update the most recent comprehensive occupational and 

residential exposure/risk assessment of chlorpyrifos conducted in June 2000 (T. Leighton, 

D63893), “Agricultural and Occupational Exposure Assessment and Recommendations for the 

Reregistration Eligibility Decision Document for Chlorpyrifos.” The update includes a 

deterministic assessment of human health exposure/risk to chlorpyrifos from agricultural 

handling of the pesticide and subsequent activity in treated areas, exposures in a residential 

environment after occupational application, as well as bystander exposure/risk from field 

volatilization based on available monitoring data. 

A residential risk assessment for registered homeowner uses was performed in June 2000 (D. 

Smegal, D266562), “Occupational/Residential Handler and Postapplication Residential/NonOccupational Risk Assessment for Chlorpyrifos.” The findings of this analysis resulted in 

voluntary cancellation of all chlorpyrifos homeowner uses, except ant and roach baits in childresistant packaging. A phase out was also developed for termite treatment products (post- and 

pre-construction). Even though homeowner uses have been cancelled, uses remain for aerial and 

ground-based fogger adult mosquitocide and for golf course turf applications which could result 

in exposures to the general population. An assessment of these uses for potential residential 

post-application exposure is included in this memorandum. 

Exposure Data

A number of studies were considered in the development of the chlorpyrifos risk assessment that 

includes scenario- and/or chemical-specific handler exposure data for occupational uses. The 

studies consist of biological monitoring and passive dosimetry exposure data, as well as 

dislodgeable foliar residue (DFR) data. Chemical-specific occupational handler studies used for 

risk quantitation and/or characterization include: exposure data for alfalfa, Christmas trees, corn, 

citrus, cotton, ornamentals, and wheat. In all other cases, occupational handler exposure was 

addressed using PHED (Pesticide Handlers Exposure Database), the Outdoor Residential 

Exposure Task Force (ORETF), and recently completed exposure scenario monographs as 

conducted and submitted by the Agricultural Handlers Exposure Task Force (AHETF). 

Occupational post-application exposure/risk was addressed through the use of 7 DFR studies. 

The DFR studies include: emulsifiable concentrate formulations on sugarbeets, pecans, citrus, 

sweet corn, cotton, and turf; wettable powder formulations on almonds, apples, pecans, 
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cauliflower, tomato and turf; granular formulations on sweet corn and turf; a total release 

aerosol formulation on ornamentals; and a microencapsulated formulation on ornamentals.

Review of Human Research

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 studies, which comprise AHETF, 

ORETF and PHED, as well as the majority of chemical-specific handler exposure data were

determined to require a review of their ethical conduct, have received that review, and have been 

determined to be ethical. A single handler exposure study, “Evaluation of Chlorpyrifos 

Exposure to Workers During Loading and Application of Lorsban® 15 % Granular Insecticide 

During Corn Planting (MRID - 44483501),” was determined to have been conducted in a manner 

which prohibits its use by the Agency. 

Hazard Characterization

The toxicity database is considered substantially adequate to support chlorpyrifos human health 

risk assessments. Chlorpyrifos, like other OPs, binds to and phosphorylates the enzyme, 

acetlycholinesterase (AChE), in both the central (brain) and peripheral nervous systems leading 

to accumulation of acetylcholine and, ultimately, to clinical signs of toxicity. In 2000, the 

Agency concluded for chlorpyrifos that inhibition of cholinesterase (ChE) was the most sensitive 

effect in all of the animal species evaluated (rats, mice, rabbits dogs) and in humans, regardless 

of exposure duration. The Agency is maintaining at this time, based on available data, that 

cholinesterase inhibition (ChEI) provides the most sensitive dose-response information for 

deriving points of departure for chlorpyrifos.

Acute toxicity categories for the technical grade are Toxicity Category II for oral, dermal, and 

inhalation. It is a Toxicity Category III for primary eye and dermal irritation. The endpoints 

used in this document to assess chlorpyrifos exposure/risk include short- and intermediate-term 

endpoints. Long-term (greater than 6 months) exposure durations are not anticipated based upon 

the use pattern of chlorpyrifos. 

A route specific short- and intermediate-term dermal NOAEL of 5 mg/kg/day from a 21-day 

dermal rat study (MRID 40972801) has been identified, and therefore, a dermal absorption 

adjustment is not necessary. The dermal LOAEL in this study of 10 mg/kg/day is based on 

plasma and red blood cell (RBC) cholinesterase inhibition (ChE) of 45 and 16 percent, 

respectively. Evaluation of biomonitoring data was conducted based on a comparison of the 

absorbed dose as measured by urinary excretion of 3,5,6-trichloro-2-pyridinol (TCP), to the oral 

equivalent NOAEL of 0.15 mg/kg/day (i.e., 5 mg/kg/day from 21-day dermal study x 0.03 

dermal absorption) because the majority of exposure is via the dermal route. The dermal 

absorption factor of 3% was estimated based on the ratio of the oral LOAEL of 0.3 mg/kg/day 

from the rat developmental neurotoxicity study (MRIDs 44556901, 44661001) to the dermal 

LOAEL of 10 mg/kg/day from the 21-day rat dermal study for plasma and red blood cell 

cholinesterase inhibition. This absorption factor is comparable to the dermal absorption 

(minimum 1-3%) estimated from human data in Nolan et al. (1982, MRID 00249203) by back-
Page 6 of 107

calculating chlorpyrifos exposure based on urinary levels of TCP. Most of the absorbed dose in 

the worker biomonitoring study is the result of dermal exposure.

Short- and intermediate-term inhalation risk assessments were based on two subchronic 

inhalation toxicity studies (MRID Nos.40013901, 40166501, 40908401) in the rat. Using the 

Agency‟s Reference concentration (RfC) methodology, a human equivalent concentration (HEC) 

was calculated and used to assess both occupational and residential exposure/risks. The shortand intermediate-term inhalation HEC, 0.0057 mg/m3

, calculated for residential exposures was 

converted to a NOAEL of 0.56 mg/kg/day to allow for comparison to estimated occupational 

inhalation doses. The HECs are based on no effects on plasma or RBC ChE inhibition identified 

from the two rat inhalation studies. For residential bystander exposure, the HEC for acute 

residential bystander exposure is 0.62 mg/m3and 0.0057 mg/m3 for short- and intermediate-term 

exposure. Because the 90-day study was conducted 5 days per week at 6 hours/day, the shortand intermediate-term residential HEC was adjusted to represent continual, 24 hour/7 day 

exposure. In contrast, the occupational inhalation exposure was only adjusted to account for an 8 

hour workday because worker exposure is expected to occur during the course of an average 

workweek (8 hours/day and 5 days/week). 

For short-term incidental oral exposure scenario, the results of the 11 day repeat phase of the 

new oral CCA study (MRID 48139301) indicated inhibition of RBC ChE in male PND11 rats as 

the most sensitive endpoint. A BMDL10 of 0.1 mg/kg/day was derived from a BMD analysis of 

the dose- response data. For intermediate-term incidental oral exposure scenarios a BMDL10 of 

0.03 mg/kg/day was selected from pregnant (GD6-20) rats exposed to chlorpyrifos in the 

developmental neurotoxicity study (MRID 44556901, Hoberman et al. 1998a,b) on the basis of 

inhibition of RBC ChE in pregnant dams. This point of departure (PoD) was supported by a 

WOE evaluation of other studies including an oral gavage study in pregnant (GD6-LD10) rats 

(MRID 44648101) and the new CCA study. 

A 1x FQPA safety factor (SF) is being proposed for this preliminary assessment for acute and 

chronic oral exposure along with all dermal durations and repeated inhalation exposures since 

the PoDs are selected from sensitive endpoints (RBC AChE inhibition) in sensitive 

lifestages/sexes. For acute inhalation exposure (PoD) based on acute inhalation ChE study using 

adult rats), a 10X FQPA database uncertainty factor (UF) is applied to account for LOAEL to 

NOAEL extrapolation (a NOAEL was not identified in the study). 

A total uncertainty factor of 100X was applied to endpoints selected for the short- and 

intermediate-term incidental oral and dermal exposures [10X for interspecies extrapolation, 10X 

for intraspecies variation and a 1X FQPA factor (residential)].

For acute inhalation exposures, a total uncertainty factor of 300X was applied [3X for 

interspecies extrapolation (reduced from 10X because RfC methodology used), 10X for 

intraspecies variation and a 10X database uncertainty factor for extrapolation from a LOAEL to a 

NOAEL, and a 1X FQPA].
Page 7 of 107

For short-term and intermediate- term inhalation exposures, a total uncertainty factor of 30X was 

applied [3X for interspecies extrapolation (reduced from 10X because RfC methodology used), 

10X for intraspecies variation and a 1X FQPA].

Because the same adverse effect (i.e., ChEI) was seen following dermal, incidental oral and 

inhalation exposures, MOEs estimated for these routes of exposure can be combined. However, 

because the LOCs for dermal/incidental oral and inhalation routes of exposure are not the same 

(an MOE of 100 defines dermal/incidental oral while inhalation is defined by an MOE of 

30/300) an aggregate risk index (ARI) was required to combine or aggregate estimated MOEs. 

EPA identifies as a level of concern ARIs that fail to reach or exceed the level of 1. Lower ARIs 

raise a risk of concern. 

Occupational Handler Exposure and Risk

Short- (1 to 30 days) and intermediate-term (1 to 6 months) exposure and risks were calculated 

for occupational handlers of chlorpyrifos for a variety of exposure scenarios at differing levels of 

personal protection. HED used 3 major unit exposure data sources, AHETF, ORETF and PHED, 

as well as 2 non-chemical specific handler exposure studies, to assess occupational handler 

exposure. 

Of the 305 exposure scenarios assessed, 134 were not of concern at some level of personal 

protection (i.e., an ARI > 1). Ninety-one (91) exposure scenarios were not of concern when 

engineering controls were considered. The remaining 80 scenarios resulted in risks of concern to 

HED (i.e., an ARI < 1) at all levels of personal protection and engineering controls considered.

Occupational Handler Comparison Using Biomonitoring Data

In an effort to characterize occupational handler risk estimates calculated using surrogate, 

passive dosimetry exposure data, HED has presented a comparative analysis of these data and 

biomonitoring data available for applicable exposure scenarios. 5 chemical specific handler 

exposure studies submitted in the past in support of chlorpyrifos re-registration were reviewed 

and considered for use. Ethical conduct was considered as a part of this review and all but one 

were determined to be ethical relevant to the standards of the time that the studies were 

conducted. Comparative risk estimates were calculated using absorbed doses measured from the 

4 studies determined to be acceptable for quantitative risk assessment purposes. The comparison 

of handler risk estimates was limited based on the level of clothing and personal protection worn 

by the participants when the biomonitoring studies were conducted. 

Of the 4 exposure scenarios compared, 3 (mixing/loading liquids for airblast application, airblast 

applications, and groundboom applications) result in biomonitoring estimates of greater risk 

potential than those estimated using surrogate data (i.e., the estimated MOEs are lower). The 

analysis of the exposure scenario, mixing/loading liquids for aerial application, results in reduced 

risk potential (a 3.8X reduction in MOE estimate). Because a number of issues were identified 

which limit the utility of the available biomonitoring data, HED has determined that these data 

are best suited for characterization of the estimates calculated for representative exposure 

scenarios using the surrogate data.
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Commercial Seed Treatment

Short- and intermediate-term exposure and risks were calculated for occupational handlers

performing seed treatment activities in commercial settings and for occupational secondary 

handlers from planting chlorpyrifos-treated seeds. No chemical-specific handler exposure data 

were submitted in support of this use pattern. 

The majority, 61 of 64, occupational handler seed treatment exposure scenarios assessed 

(combined dermal and inhalation) resulted in risk estimates which were not of concern to HED 

(i.e., an ARI > 1) at some level of personal protection. The remaining 3 exposure scenarios 

resulted in an ARI < 1 at all level of personal protection considered and, therefore, are of concern 

to HED. All seed planter (secondary handler) combined short- and intermediate-term dermal and 

inhalation exposure scenarios assessed resulted in an ARI > 1 at some level of personal 

protection and, therefore, are not of concern to HED.

Occupational Post-application Exposure and Risk

HED has assessed short- and intermediate term occupational post-application dermal exposure 

and risk for any crops where reentry into an area previously treated with chlorpyrifos is 

anticipated. The assessment was completed using 7 chemical-specific registrant submitted DFR 

studies. The studies, which encompass the use of five different formulations and twelve different 

crops, have been extrapolated to other groups based on the nature of the crop and application 

method and used to calculate risks for post-application workers 

The MOEs estimated for liquid spray and granular formulation reentry are not of concern (i.e., an 

MOE ≥ 100) in the range of 0 to 4 days for lower to medium exposure activities and 0 to 8 days 

for high exposure activities, with the greater majority falling between 0 to 4 days when all 

exposure activities are considered. HED also estimated the MOEs for reentry into 

microencapsulated and total release fogger formulation treated greenhouses. These estimates 

range from 0 to > 35 days after treatment (the completion of the monitoring period) for all 

exposure activities considered. 

A quantitative occupational post-application inhalation exposure assessment was not performed 

for chlorpyrifos given that an inhalation exposure assessment was performed for 

occupational/commercial handlers and handlers are expected to have greater exposures. 

Residential Handler Exposure and Risk

Based upon review of all chlorpyrifos registered uses, only the roach bait products can be applied 

by a homeowner in a residential setting; however, a quantitative exposure/risk assessment for

application of the roach bait products was not conducted because HED expects handler exposure 

to be negligible. With roach bait stations the active ingredient is contained within the bait 

station, therefore, limiting contact with the active ingredient in the product. 
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Residential Post-application Exposure and Risk

Chlorpyrifos can be used in areas frequented by the general population including ant mounds, 

golf courses, and as an aerial and ground-based (thermal aerosol and fog machine) ULV 

mosquitocide applied by a public agency made in the vicinity of residential areas. As a result, 

individuals could be exposed by entering these areas if they have been previously treated. Postapplication homeowner exposure from residential ant mound treatment (applied by professional 

only) was not quantitatively assessed because contact with the mound is not anticipated. 

Estimated short-term adult and child dermal and child incidental oral exposure to turf following 

aerial and ground mosquito treatments are not concern to HED (i.e., an MOE ≥ 100). Combined 

child exposure estimates (dermal and incidental oral) to turf following aerial mosquito treatment 

are of concern to HED; however, combined estimates following ground treatment are not of 

concern. Acute adult and child inhalation exposure following aerial mosquito treatment is not of 

concern (i.e., an MOE > 30), but is of concern following ground treatment. 

Adult dermal exposure from golfing is not concern to HED (i.e., an MOE > 100) using any TTR 

region-specific data for the emulsifiable concentrate formulation at the 1.0 lb ai/A, or 0.25 

application rates. 

Residential Bystander Exposure and Risk

HED has assessed residential bystander exposure to chlorpyrifos based on the available chemical 

specific ambient and application site air monitoring data. The chlorpyrifos bystander 

volatilization inhalation exposure assessment includes acute and short-/intermediate-term 

exposure scenarios. The acute scenario compares the maximum air concentration detected in the 

monitoring studies to the acute HEC. The short-/intermediate-term scenario compares the 

arithmetic mean chlorpyrifos air concentration detected in the monitoring studies to the shortand intermediate-term HEC. 

Of the 24 acute ambient air concentrations assessed, 4 result in a risk of concern to HED (i.e., an 

MOE < 300). No short-/intermediate-term ambient data assessed result in a risk of concern. Of 

the 5 acute application site air concentrations assessed, 3 resulted in a risk of concern. Of the 5 

short- and intermediate-term application site air concentrations assessed, 4 resulted in risks of 

concern (i.e., an MOE < 30).

Spray Drift 

Spray drift is a potential source of exposure to residents nearby to spraying operations. This is 

particularly the case with aerial application, but, to a lesser extent, could also be a potential 

source of exposure from the ground application method employed for chlorpyrifos. The Agency 

has been working with the Spray Drift Task Force, EPA Regional Offices and State Lead 

Agencies for pesticide regulation and other parties to develop the best spray drift management 

practices (see the Agency‟s Spray Drift website for more information at 

http://www.epa.gov/opp00001/factsheets/spraydrift.htm). On a chemical by chemical basis, the 

Agency evaluates the need for interim mitigation measures for aerial applications for placement 
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on labels/labeling. The Agency has completed its evaluation of the new database submitted by 

the Spray Drift Task Force, a membership of U.S. pesticide registrants, and is developing a 

policy on how to appropriately apply the data and the AgDRIFT computer model to its risk 

assessments for pesticides applied by air, orchard airblast and ground hydraulic methods. After 

the policy is in place, the Agency may seek further refinements in spray drift management 

practices to reduce off-target drift with specific products with significant risks associated with 

drift.

A quantitative residential post-application (acute) inhalation exposure (spray drift) assessment 

was conducted for ground and aerial ULV mosquitocide applied by a public agency made in the 

vicinity of residential areas. HED used AgDRIFT (V 2.01), which is the model that was 

developed as a result of the efforts of the Spray Drift Task Force (SDTF), to determine residue 

deposition and the airborne concentration of chlorpyrifos anticipated from aerial product 

application. Based upon this assessment, it was determined that adult and child acute inhalation 

exposures following aerial mosquito treatments are not of concern to HED (i.e., an MOE > 300). 

Inhalation exposure from ground based ULV treatment was assessed by assuming that the entire 

active ingredient applied to a 1 acre area is airborne and available to be inhaled by a child or 

adult. The assessment of adult and child acute inhalation from ground mosquito treatment 

resulted in a risk estimate of concern to HED.

Chlorpyrifos Oxon

HED has considered the exposure potential for occupational and residential exposure to 

chlorpyrifos oxon. Workers reentering an environment previously treated with chlorpyrifos 

(occupational post-application) and the general population residing near chlorpyrifos application 

sites (bystanders) could potentially be exposed to the oxon as chlorpyrifos is degraded in the 

environment. Dermal exposure to the oxon could occur through contact with chlorpyrifos 

treated surfaces and inhalation exposure through airborne oxon. However, the likelihood of 

exposure to the oxon is slight due to its rapid deactivation to TCP. In an effort to further explore 

the potential for oxon exposure, HED has researched and reviewed all available information 

sources. Based upon this review, HED is requiring additional studies to address uncertainties 

regarding the formation of chlorpyrifos oxon in the air post-application and its formation and 

decay in greenhouses. 

Summary of Data Gaps

HED has considered all data sources available for the assessment of occupational and residential 

exposure and risk. It has been determined that no new data are required are required for 

chlorpyrifos; however, data gaps were identified for the evaluation of chlorpyrifos oxon. As 

summarized in the previous paragraph, HED intends to require additional studies to address 

uncertainties regarding the formation of chlorpyrifos oxon in the air post-application and its 

formation and decay in greenhouses. Several data gaps were identified in the Registration 

Eligibility Decision (RED) for the occupational and residential assessment of chlorpyrifos 

(finalized 7/31/06; IRED issued 2/2002). All but one of these have been satisfied, confirmation 

of the area treated for sod farm chlorpyrifos treatment. 
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1.0 Occupational and Residential Exposure/ Risk Assessment

This document serves to update the June 2000 occupational and residential exposure and risk 

assessment for the insecticide chlorpyrifos. It presents the results of the Health Effects 

Division‟s (HED) review of the potential human health effects of occupational and residential 

exposure to the chemical.

1.1 Criteria for Conducting Exposure Assessments

An occupational and/ or residential exposure assessment is required for an active ingredient if (1) 

certain toxicological criteria are triggered and (2) there is a potential for exposure to handlers 

during use or to persons entering treated sites after application is complete. Chlorpyrifos meets 

both criteria. There is potential for exposure to handlers from occupational applications and 

from subsequent activity in treated agricultural and residential areas. Therefore, a risk 

assessment is required for occupational handlers, as well as for occupational and residential postapplication exposures that can occur as a result of chlorpyrifos use.

1.2 Toxicological Endpoints

Table 1.2 summarizes the chlorpyrifos toxicity endpoints and PoDs selected from a re-evaluation 

of the database (including data submitted since the 2002 IRED/2006 RED). Based on the results 

of benchmark dose (BMD) analyses and weight of the evidence (WOE) consideration of all 

quality and reliable data, the most sensitive compartment from the most sensitive sex in both 

juvenile and adult rats were identified and used for endpoint selection and PoD determination for 

the following exposure scenarios. The description of the primary toxicity studies used for 

selecting toxicity endpoints and points of departure for various exposure scenarios are presented 

in Appendix A of the Chlorpyrifos: Preliminary Human Health Risk Assessment for Registration 

Review (6/27/2011, D388070). 

Consistent with risk assessment on other OP and NMCs compounds, the Agency has used a 

benchmark response (BMR) level of 10% and has thus calculated BMD10s and BMDL10s. 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. Extensive analyses conducted as part of 

the OP cumulative risk assessment (USEPA 2002) have demonstrated that 10% is a level that 

can be reliably measured in the majority of rat toxicity studies, and is generally at or near the 

limit of sensitivity for discerning a statistically significant decrease in ChE activity across the 

brain compartment and is a response level close to the background brain ChE level. The Agency 

uses the BMDL, not the BMD, for use as the PoD since the BMDL accounts for variability of the 

data.

The Agency has not performed BMD analysis on studies evaluating the effect of chlorpyrifos on 

the developing brain as these do not provide dose response data amenable to BMD modeling 

analysis. Specifically, these studies, in general, may include only a single dose at a particular 

age, do not report graded responses (i.e., all or nothing effect), and/or show non-monotonic dose 

response curves (e.g., response goes up then down). For these studies, the Agency simply 

considered the doses used. 
Page 12 of 107

Toxicological endpoints were selected for the assessment of short- and intermediate -term 

exposures. Long-term exposure durations are not anticipated based upon the use pattern of 

chlorpyrifos. Risk assessments are required for occupational handlers and occupational and 

residential post-application exposures that can occur as a result of chlorpyrifos use.

Incidental Oral

For short-term incidental oral exposure scenario, the results of the 11 day repeat phase of the 

new oral CCA study (MRID 48139301) indicated inhibition of RBC ChE in male PND11 rats as 

the most sensitive endpoint. A BMDL10 of 0.1 mg/kg/day was derived from a BMD analysis of 

the dose- response data. For intermediate-term incidental oral exposure scenarios a BMDL10 of 

0.03 mg/kg/day was selected from pregnant (GD6-20) rats exposed to chlorpyrifos in the 

developmental neurotoxicity study (MRID 44556901, Hoberman et al. 1998a,b) on the basis of 

inhibition of RBC ChE in pregnant dams. This PoD was supported by a WOE evaluation of other 

studies including an oral gavage study in pregnant (GD6-LD10) rats (MRID 44648101) and the 

new CCA study. 

A total uncertainty factor of 100X is appropriate for incidental oral exposures [10X for 

interspecies extrapolation, 10X for intraspecies variation and a 1X FQPA factor].

Dermal

A short-/intermediate-term dermal PoD was selected from a 21-day dermal toxicity study (MRID 

40972801) in rats based on plasma and red blood cell ChE inhibition (NOAEL = 5 mg/kg/day). 

The use of the 21-day dermal toxicity study is appropriate for durations up to 6 months as it is 

expected that steady state ChE inhibition would have been reached by approximately 21 days of 

dermal exposure. The Agency has previously shown (USEPA, 2001; preliminary 

organophosphate cumulative risk assessment) that at or near 3-4 weeks of exposure the degree 

of inhibition following repeated dosing with OPs does not change with increasing duration but 

instead remains approximately the same. 

For comparison to biomonitoring data in the risk assessment, which evaluates total exposure 

from oral, dermal and inhalation routes (in terms of absorbed dose), the 21-day rat dermal study 

is used with an adjustment for 3% dermal absorption to convert the NOAEL of 5 mg/kg/day 

resulting from topically applied chlorpyrifos to an internal absorbed NOAEL = 0.15 mg/kg/day. 

The dermal absorption factor of 3% was estimated based on the ratio of the oral LOAEL of 0.3 

mg/kg/day from the rat developmental neurotoxicity study (MRIDs 44556901, 44661001) to the 

dermal LOAEL of 10 mg/kg/day from the 21-day rat dermal study (MRID 40972801) for plasma 

and red blood cell cholinesterase inhibition. This absorption factor is comparable to the dermal 

absorption (minimum 1-3%) estimated from human data in Nolan et al. (1982, MRID 00249203) 

by back-calculating chlorpyrifos exposure based on urinary levels of TCP. Most of the absorbed 

dose in the worker biomonitoring study is the result of dermal exposure.

A total uncertainty factor of 100X is appropriate for dermal exposures [10X for interspecies 

extrapolation, 10X for intraspecies variation and a 1X FQPA factor].
Page 13 of 107

Inhalation

An acute inhalation PoD was selected from a recently submitted special acute inhalation study 

(2010, MRID 48139303) based on lung and plasma ChE inhibition (LOAEL = 3.7 mg/m3; 

NOAEL not established). In this special acute inhalation study, adult female rats (Crl:CD(SD)) 

were exposed nose only to atmospheric concentrations of 0, 3.7, 12.9, 22.1 or 53.5 mg/m3for six 

hours and allowed an additional 72 hours to recover. Using the Agency‟s Reference 

concentration (RfC) methodology, a human equivalent concentration (HEC) was calculated and 

used to assess acute bystander exposure and risks. The HEC for acute bystander exposure is 0.62 

mg/m3.

Short- and intermediate-term inhalation risk assessments were based on two subchronic 

inhalation toxicity studies (MRID Nos.40013901, 40166501, 40908401) in the rat. Using the 

Agency‟s Reference concentration (RfC) methodology, a human equivalent concentration (HEC) 

was calculated and used to assess both occupational and residential exposure/risks. The shortand intermediate-term inhalation HEC, 0.0057 mg/m3

, calculated for residential exposures was 

converted to a NOAEL of 0.56 mg/kg/day to allow for comparison to estimated occupational 

inhalation doses. The HECs are based on no effects on plasma or RBC ChE inhibition identified 

from the two rat inhalation studies. For residential bystander exposure, the HEC for acute 

residential bystander exposure is 0.62 mg/m3and 0.0057 mg/m3 for short- and intermediate-term 

exposure. Because the 90-day study was conducted 5 days per week at 6 hours/day, the shortand intermediate-term residential HEC was adjusted to represent continual, 24 hour/7 day 

exposure. In contrast, the occupational inhalation exposure was only adjusted to account for an 8 

hour workday because worker exposure is expected to occur during the course of an average 

workweek (8 hours/day and 5 days/week). 

For acute inhalation exposures, a total uncertainty factor of 300X was applied [3X for 

interspecies extrapolation (reduced from 10X because RfC methodology used which takes into 

consideration the pharmacokinetic differences between animals and humans), 10X for 

intraspecies variation, and a 10X FQPA database uncertainty factor (for extrapolation from a 

LOAEL to a NOAEL). 

For short-term and intermediate- term inhalation exposures, a total uncertainty factor of 30X was 

applied [3X for interspecies extrapolation (reduced from 10X because RfC methodology used), 

10X for intraspecies variation and a 1X FQPA (because the inhalation NOAEL is considered 

protective of pregnant females based on effects seen in the DNT at 0.3 mg/kg/day)].

Determination of Acute and Chronic Dietary PoDs for Chlorpyrifos Oxon

There is some potential for direct exposure to the oxon metabolite of chlorpyrifos, particularly 

from drinking water. BMD modeling of available oxon data for acute and repeated dosing 

studies was conducted (Appendix E, Tables 7 and 8). The purpose of this analysis is to 

determine the toxicological PoDs for the oxon (Table 4.8.2) and to assess the relative potency of 

chlorpyrifos and its oxon metabolite.
Page 14 of 107

A BMDL10 of 0.05 mg/kg/day associated with RBC ChE inhibition in male rat pups exposed to 

chlorpyrifos oxon (acute dosing CCA study using oxon) was selected for the acute dietary PoD 

for the oxon. 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 0.0005 mg/kg/day. 

Based on the FQPA factor of 1, the acute population adjusted dose (aPAD) is 0.0005 mg/kg/day.

The chronic dietary PoD for chlorpyrifos oxon is selected from a BMDL10 of 0.011 mg/kg/day 

from an 11 day repeat dosing CCA study using oxon and is based on inhibition of RBC ChE in 

adult female rats. A comparison of the resulting BMD10s for juvenile and for adult rats indicates 

that juvenile rats are no more sensitive to the oxon than are adult rats. The BMDL10 for adult 

rats (0.011 mg/kg/day) was selected for the PoD because it was lower than that of the juvenile 

rats (0.025 mg/kg/day) and would be considered protective for juveniles. Uncertainty factors of 

10X to account for interspecies extrapolation and 10X for intraspecies variation is applied to the 

BMDL10 to obtain an aRfD of 0.00011 mg/kg/day. Based on the FQPA factor of 1, the chronic 

population adjusted dose (cPAD) is 0.000011 mg/kg/day.

While the Agency uses the BMDL, not the BMD, for use as the PoD since the BMDL accounts 

for variability of the data, the BMD10 provides a point of comparison across studies and the 

BMD10 provides the basis for determining the relative toxicity of the chlorpyrifos oxon 

compared to chlorpyrifos. A toxicity factor for the oxon was calculated by dividing the 

chlorpyrifos BMD10 for the endpoint associated with the most sensitive compartment from the 

most sensitive sex for the duration of interest by the corresponding BMD10 for the oxon. Acute 

(all populations) toxicity factors of 8.8 (males) and 11.9 (females) were calculated from BMD 

analysis of inhibition of male and female pup RBC ChE (acute phase of the CCA study). The 

chronic toxicity factor of 18.0 was derived from BMD analysis of inhibition of RBC ChE in 

adult female rats (adult male rats not examined) observed in the repeated phase of the CCA 

study. The toxicity factors may be used in aggregate assessments where exposures to 

chlorpyrifos and the oxon are to be combined. Adjusting for relative toxicity will allow 

comparison of the combined exposures to a single PoD (since PoDs are different for chlorpyrifos 

and oxon). 

Table 1.2 Summary of Toxicity Endpoints Selected for Chlorpyrifos 

Exposure 

Scenario

Dose

(mg/kg/day) Endpoint Study

Dermal

Short-

(1 – 30 days) and 

Intermediate-Term 

(1-6 months)

NOAEL = 5

Absorbed dermal

NOAEL = 0.15

(for assessment of 

biomonitoring data)

UFA = 10x

UFH = 10x

FQPA SF = 1x

(residential)

Plasma and RBC ChE 

inhibition

21-day dermal study 

(NOAEL) and 4 day 

probe study (LOAEL) in 

adult rats (MRID

40972801)
Page 15 of 107

Table 1.2 Summary of Toxicity Endpoints Selected for Chlorpyrifos 

Exposure 

Scenario

Dose

(mg/kg/day) Endpoint Study

Occupational and 

residential LOC is an 

MOE = 100

Acute Inhalation

HEC = 0.62 mg/m3 

(residential)

UFA = 3x

UFH = 10x

UFL= 10x

FQPA SF = 1x 

(residential)

OR

UFA = 3x

UFH = 10x

FQPA UFDB = 10x

(LOAEL to NOAEL 

and lifestage sensitivity) 

(residential)

Residential LOC for 

MOE=300

Lung ChE Inhibition

Special 6 hour acute 

inhalation study (MRID 

48139303)

Inhalation

Short- (1 – 30 

days) and

Intermediate-Term 

(1 – 6 months)

NOAEL (HEC) = 0.56 

mg/kg/day 

(occupational)

HEC = 0.0057 mg/m3

(residential)

UFA = 3x

UFH = 10x 

FQPA SF = 1x

(residential)

Occupational and 

Residential LOC is an 

MOE = 30

Lack of effects in 2 rat 

inhalation studies at the 

highest dose tested : 43% 

plasma and 41% RBC ChE 

inhibition following oral 

doses of 0.3 mg/kg/day in 

the DNT study mg/kg/day in 

the DNT study

Two 90-day inhalation 

studies and the rat DNT 

study (MRIDs 40908401; 

40013901/ 40166501)

Incidental Oral 

Short-Term 

(1-30 days)

BMDL10 = 0.1

UFA = 10x

UFH = 10x

FQPA SF = 1x

Residential LOC is an 

Inhibition of RBC ChE in 

PND 11 male rats

11 day repeat oral CCA 

study in the rat (MRID 

48139301)
Page 16 of 107

Table 1.2 Summary of Toxicity Endpoints Selected for Chlorpyrifos 

Exposure 

Scenario

Dose

(mg/kg/day) Endpoint Study

MOE=100

Incidental Oral 

Intermediate-Term 

(1 – 6 months)

BMDL10 = 0.03

UFA = 10x

UFH = 10x

FQPA SF = 1x

Residential LOC is an 

MOE= 100

Inhibition of RBC ChE in 

rat dams (GD 6 – 20).

Weight of evidence from 

studies including:

Developmental 

neurotoxicity study 

in pregnant (GD 6 -

20) rats (MRID 

44556901)

Gavage study in 

pregnant (GD 6 –

LD10) rats (MRID 

44648101)

NOAEL = no observed adverse effect level. LOAEL = lowest observed adverse effect level. UF = uncertainty 

factor. UFA = extrapolation from animal to human (interspecies). UFH = potential variation in sensitivity among 

members of the human population (intraspecies). UFL = use of a LOAEL to extrapolate a NOAEL. UFDB = to 

account for the absence of key data (i.e., lack of a critical study) or other residual uncertainties as evidenced by 

available data. FQPA SF = FQPA Safety Factor. MOE = margin of exposure. LOC = level of concern. 

1.3 Incident Report

The incident review was conducted by HED considering different types and sources of human 

observational information (medical data/information, human incident data and epidemiological 

information) in an effort to consider different lines of evidence that may inform the re-evaluation 

of chlorpyrifos in this phase of Registration Review. The symptoms reported in the medical 

human observational data and human incident data reviewed for this report are mutually 

supportive. The symptoms reported for both types of observational data include cardiovascular, 

respiratory, gastrointestinal, nervous system and ocular symptoms. The Agency‟s risk mitigation 

measures including canceling all homeowner use product registrations (except ant bait stations), 

and canceling uses in schools and parks where children may be exposed appears to have led to a 

decrease in the number of incidents reported in residential areas. The databases that were 

reviewed (OPP Incident Data System, National Pesticide Information Center, NIOSH 

Sentinel Event Notification System for Occupational Risks [SENSOR], and California Pesticide 

Illness Surveillance Program Incident Data [CA PISP]) showed the number of incidents 

associated with chlorpyrifos significantly declined post-2002 and this correlates well with the 

phase out/cancellation of the almost all chlorpyrifos residential products as of December 31, 

2001. In addition, the Agency's findings are also consistent with other incident cases 

investigations of American Association of Poison Control Centers (AAPCC) data which have 

reported a decrease in the number of chlorpyrifos incidents that is temporally associated with the 

phase out/cancellation of most residential chlorpyrifos products. For AAPCC data, the mean 

annual number of reports for chlorpyrifos in the pre- and post-announcement phases was 214 and 
Page 17 of 107

39.3, respectively (Stone et al. 2009). In IDS, there were 43 incidents reported in 2002 and only 

six reported in 2009.

However, chlorpyrifos occupational incidents, reported in CA PISP and SENSOR databases, 

appear to be constant over time, despite risk mitigation implemented including reduced 

application rates and seasonal maximum limits, increased retreatment intervals, increased PPE 

and/or use of engineering controls were required as well as increased reentry intervals (REIs) for 

a number of crops. In NIOSH SENSOR application of chlorpyrifos appears to lead to the most 

incidents, being responsible for 46% reported to NIOSH SENSOR from 1998 to 2007 and in CA 

PISP drift of chlorpyrifos from adjacent fields appears to be the cause of the most incidents, 

being responsible for 56% of the cases reported to CA PISP from 1999 to 2008. 

For NIOSH SENSOR from 1998 to 2007, there are 635 cases reported for chlorpyrifos in the 

database. Of these cases, 348 were reviewed because, in these cases, chlorpyrifos was used as a 

single chemical only and had a certainty classification of definite, probable, or possible, as these 

cases provide more certain information about the potential effects of chlorpyrifos exposure. 

There was one death due to suicide; eight cases were classified as high severity, 60 cases were 

classified as moderate severity, and 279 cases were classified as low severity. Of the 348 cases 

chlorpyrifos-only cases, 18 cases involved children six years old or younger. These incidents 

were mostly due to accidental ingestions, misapplications around the home and drift from nearby 

properties. The health effects reported include: neurological, gastrointestinal, respiratory, ocular, 

dermal, cardiovascular, and genitourinary. 

Generally, chlorpyrifos incidents involved workers in agricultural or professional application 

occupations, homeowners and individuals at work but their job was not related to pesticide 

application, and to individuals exposed through drift. 

On average, less than seven incidents are reported per year for chlorpyrifos in the NIOSH 

SENSOR database. The number of incidents reported appears to remain steady from 1998 to 

2007. Agricultural workers reported incidents that occurred due to drift from adjacent fields;

post-application exposure, chlorpyrifos application, mixing/loading chlorpyrifos, repair and 

maintenance of equipment, chlorpyrifos transportation and disposal, and spills/splashes/leaks of 

chlorpyrifos onto workers (see Figure 4). Application of chlorpyrifos appears to lead to the most 

incidents, being responsible for 46% reported to NIOSH SENSOR from 1998 to 2007.
Page 18 of 107

Figure 1.3a: Occupational exposure scenarios reported to NIOSH Sensor for chlorpyrifos

One hundred and sixty four cases were identified that are attributable to chlorpyrifos-only 

exposures were reported to the CA PISP between 1999 and 2008. Of these cases, 87 were 

occupational incidents and 77 were non-occupational incidents. The health effects reported 

include: neurological, gastrointestinal, respiratory, ocular, dermal, cardiovascular.

On average less than nine agricultural incidents are reported per year for chlorpyrifos in the CA 

PISP database. The number of incidents reported appears to remain steady from 1999 to 2008. 

Agricultural workers reported incidents that occurred due chlorpyrifos drift from adjacent fields; 

post-application, spill/splash/leak, mixer/loader, and applicator exposure, misuse, equipment 

malfunction, and equipment cleaning (See Figure 6). Unlike in SENSOR, where application was 

the most likely activity involved in incidents, in PISP drift of chlorpyrifos appears to be the 

cause of the most incidents, being responsible for 56% of the cases reported to PISP from 1999 

to 2008.
Page 19 of 107

Figure 1.3b: Occupational exposure scenarios reported to CA PISP for chlorpyrifos

In summary, chlorpyrifos incidents reported have declined substantially among residential users 

from 2002 to May 27, 2010, but the occupational incidents reported remain the same. The 

incidents reported have been reviewed and, overall, the numbers and severity of incidents 

associated with chlorpyrifos use are not of significant potential public health concern, but we 

will continue to monitor these incidents and remain alert for any changes in trend or patterns.

1.4 Use Profile

Chlorpyrifos is an organophosphate insecticide currently registered for the control of various 

insects. Targeted pests include aphids, cockroaches, cutworms, fleas, grasshoppers, ticks, etc. 

Chlorpyrifos is manufactured as granular, microencapsulated, soluble concentrate/liquids, water 

dispersible granular in water soluble packets (WSP) and wettable powder packaged in WSP

formulations, as well as impregnated paints, cattle ear tags, insect bait stations and total release 

foggers. Registered use sites include the following uses: food crops, including fruit and nut 

trees, many types of fruits and vegetables, and grain crops; and non-food crops such as forage, 

golf course turf, industrial sites, greenhouse and nursery production, sod farms, and wood 

products. Public health uses include aerial and ground-based fogger treatments to control 

mosquitoes. There are a wide range of application rates. HED has conducted a review of all 

active product labels. Tables 1.4a and 1.4b below summarize all agricultural and nonagricultural use sites identified for chlorpyrifos under this review, respectively. Maximum 

application rate (lb ai/A) and application equipment for each site are also identified. Various 

chlorpyrifos product formulations are represented by the application rates and methods. 
Page 20 of 107

Table 1.4a Summary of Maximum Application Rates for Chlorpyrifos Agricultural Uses

Crop or Target

Maximum 

Application Rate

(lb ai/A)

Application Equipment

Alfalfa 1.0 Aerial, Chemigation, Groundboom, 

Tractor Drawn Spreader

Asparagus 1.0 Aerial, Groundboom

1.5 Tractor Drawn Spreader

Beets (Table and Sugar Grown for 

Seed)

1.9 Aerial

Brassica Vegetable (Bok Choy, 

Broccoli, Broccoli Raab, Brussel 

Sprout, Cabbage, Cauliflower, Chinese 

Broccoli, Collards, Kale, and 

Kohlrabi)

1.0 Aerial

2.3 Groundboom, Tractor Drawn Spreader

Carrot (Grown for Seed) 0.94 Aerial, Groundboom

Citrus Fruit 6.0 (AZ and CA), 

3.5 (States other 

than AZ and CA)

Aerial, Airblast, Groundboom

1.0 Tractor Drawn Spreader

4.0 Backpack Sprayer, Handgun, Low 

Pressure Handwand

Clover (Grown for Seed) 1.0 Aerial, Groundboom

Corn (Field, Grown for Seed and 

Sweet)

1.5 Aerial, Chemigation

3.0 Groundboom

1.3 Tractor Drawn Spreader

Cotton 1.0 Aerial, Chemigation, Groundboom

Cranberry 1.5 Aerial, Chemigation, Groundboom

Fig (CA only) 2.0 Groundboom

Grapes 2.0 Airblast

1.0 Backpack Sprayer, Low Pressure 

Handwand

Legume Vegetables (Succulent or 

Dried, Except for Soybeans)

1.0 Groundboom

Mint (Peppermint and Spearmint) 2.0 Chemigation, Groundboom

Onion (Dry Bulb) 1.0 Groundboom, Tractor Drawn Spreader, 

Handgun

Peanut 2.0 Aerial, Groundboom, Tractor Drawn 

Spreader

Peppers 1.0 Groundboom

Pineapple (Non-bearing) 1.9 Groundboom, Airblast

Radish 2.8 Aerial, Groundboom, Tractor Drawn 
Page 21 of 107

Table 1.4a Summary of Maximum Application Rates for Chlorpyrifos Agricultural Uses

Crop or Target

Maximum 

Application Rate

(lb ai/A)

Application Equipment

Speader, Handgun

Radish (Grown for Seed) 0.94 Aerial, Groundboom 

Rutabaga 2.3 Aerial, Groundboom, Tractor Drawn 

Spreader

Sorghum 1.0 Aerial, Chemigation, Groundboom

3.3 Tractor Drawn Spreader

Soybean 1.0 Aerial, Chemigation, Groundboom

Strawberry 1.0 Aerial

2.0 Groundboom

Sugarbeet 1.0 Aerial, Chemigation, Groundboom

2.0 Tractor Drawn Spreader

Sugarbeet (Grown for Seed) 1.9 Aerial, Chemigation, Groundboom

Sunflower 2.0 Aerial, Groundboom

Sweet Potato 2.0 Groundboom, Tractor Drawn Spreader

Tobacco 2.0 Groundboom, Tractor Drawn Spreader

Tree Nuts (Almonds, Filberts, Pecans, 

Walnuts)

4.0 Groundboom, Handgun

2.0 Aerial, Airblast

0.03 (lb ai/gallon) Low Pressure Handwand, Backpack 

Sprayer

Tree Fruit (Apples, Cherry, Nectarine, 

Peach, Pear, Plum, Prune, Sour 

Cherry)

2.0 Aerial, Airblast

0.03 (lb ai/gallon) Backpack Sprayer, Drench/Dip, 

Handgun, Low Pressure Handwand

Turnip 2.3 Groundboom, Tractor Drawn Spreader

Wheat 1.0 Aerial, Chemigation, Groundboom

2.0 Groundboom

Table 1.4b Summary of Maximum Application Rates for Chlorpyrifos Non-Agricultural 

Uses

Crop or Target

Maximum 

Application Rate

(lb ai/A)

Application Equipment

Ants (Fire Ant Mound, Carpenter) 0.080 (lb ai/gallon) Backpack Sprayer, Handgun, Low 

Pressure Handwand

Cattle Ear Tags 0.0033 (lb ai/ ear 

tag) 

Hand

Christmas Trees (Nurseries and 

Plantations, Stumps)

1.0 Airblast

0.94 Aerial (Helicopter)

0.03 (lb ai/gallon) Backpack Sprayer, Handgun, Low 

Pressure Handwand

Golf Course Turf 1.0 Belly Grinder, Low Pressure 

Handwand, Push Type Spreader, 
Page 22 of 107

Table 1.4b Summary of Maximum Application Rates for Chlorpyrifos Non-Agricultural 

Uses

Crop or Target

Maximum 

Application Rate

(lb ai/A)

Application Equipment

Tractor Drawn Spreader, Turfgun

Grass Seed (Perennial Crops) 1.0 Groundboom

Greenhouse and Nursery Production 

(Bedding Plants, Containerized 

Ornamentals, Cut Flowers, Flowering 

Hanging Baskets, Pine Seedling 

Transplant, Potted Flowers, 

Ornamentals, Trees and Shrubs)

4.0 Aerial, Groundboom

1.1 Belly Grinder, Push Type Spreader, 

Tractor Drawn Spreader

1.0 Airblast

0.16 (lb ai/gallon) Backpack Sprayer, Handgun, Low 

Pressure Handwand

0.02 (lb ai/gallon) Soil Drench

0.01 (lb ai/can) Total Release Fogger

Mosquitocide (Outdoor Residential, 

Recreational, or Other Non-Cropland 

Areas)

0.010 Wide Area Aerial and Ground

Non-crop Areas (Commercial 

Indoor/Outdoor Industrial Sites, 

Commercial Livestock Holding and 

Housing, Dumpsters/ Trash Areas, 

Food Processing Plants, Grown for 

Seed, Industrial Plant Site Perimeter 

Treatments, Manufacturing Sites, 

Power Utilities, Railroad Box Cars, 

Railroad Equipment, Road Medians, 

Ship Holds, Sod Farms, 

Telecommunications, Warehouse 

Sites)

1.0 Aerial, Belly Grinder, Groundboom, 

Push Type Spreader, Tractor Drawn 

Spreader

0.11 (lb ai/gallon) Backpack Sprayer

0.080 (lb ai/gallon) Handgun, Low Pressure Handwand, 

Paint Brush/Roller

0.044 (lb ai/1,000 sq 

ft)

Shaker Container

0.018 (lb ai/1,000 sq 

ft)

Open Pour Bag

Ornamentals (Cut Flowers, Industrial 

Buildings/Plant Sites Perimeter 

Treatments and Road Medians, 

Evergreens, Field Grown Nursery 

Stock, Flowers, Greenhouses, Nonbearing Fruit Trees Shrubs, Nurseries, 

Trees, Vines, Woody) 

6.0 Belly Grinder, Push Type Spreader, 

Tractor Drawn Spreader

4.0 Groundboom

2.0 Aerial, Airblast

0.16 (lb ai/gallon) Backpack Sprayer, Handgun, High 

Pressure Handwand, Low Pressure 

Handwand

0.020 (lb ai/gallon) Drench/Dip

Roach Control Bait Stations 0.00040 (lb 

ai/gallon)

Hand

Sewer Manhole Walls 0.080 (lb ai/gallon) Backpack Sprayer, Low Pressure 
Page 23 of 107

Table 1.4b Summary of Maximum Application Rates for Chlorpyrifos Non-Agricultural 

Uses

Crop or Target

Maximum 

Application Rate

(lb ai/A)

Application Equipment

Handwand, Paint Brush/Roller

Total Release Fogger (Greenhouse 

Bedding Plants, Cut Flowers, 

Flowering Hanging Baskets, Potted 

Flowers, Ornamentals)

0.010 (lb 

ai/container)

Hand

Trees (Cottonwood and Poplar Trees 

Grown for Pulp, Conifer, Deciduous, 

Grown in Nurseries and Greenhouses) 

2.0 Airblast, Handgun, Low Pressure 

Handwand, Backpack Sprayer

1.9 Aerial

Turfgrass (Sod or Seed) 4.0 Aerial, Groundboom

1.0 Tractor Drawn Spreader

Wood Products (Fence Posts, 

Industrial Sites, Landscape Timbers, 

Logs, Manufacturing, Pallets, 

Processed Wood, Right of Way, 

Railroad Ties, Utility Poles, Wooden 

Containers) 

6.0 Belly Grinder, Push Type Spreader

0.17 (lb ai/gallon) Low Pressure Handwand

0.080 (lb ai/gallon) Backpack Sprayer, Handgun, Paint 

Brush/Roller

2.0 Occupational Exposures and Risks

It has been determined there is a potential for exposure in occupational scenarios from handling 

chlorpyrifos products during the application process (i.e., mixer/loaders, applicators, flaggers and 

mixer/loader/applicators, commercial seed treatment) and from entering areas previously treated 

with chlorpyrifos (e.g., post-application worker exposure). As a result, risk assessments have 

been completed for the occupational handler and post-application exposure scenarios.

This section includes the occupational aspects of the risk assessment. Occupational handler 

exposures and risks are addressed in the following Sections: 2.1: Occupational Handler 

Exposures and Risks, 2.2: Occupational Handler Comparison Using Biomonitoring Data, and 

2.3: Commercial Seed Treatment. Occupational post-application exposures and risks are 

addressed in Section 2.4: Occupational Post-application Exposure and Risks. 

2.1 Occupational Handler Exposures and Risks 

HED uses the term “handlers” to describe those individuals who are involved in the pesticide 

application process. HED believes that there are distinct tasks related to applications and that 

exposures can vary depending on the specifics of each task. Job requirements (e.g., the amount 

of chemical to be used in an application), the kinds of equipment used, and the target being 

treated can cause exposure levels to differ in a manner specific to each application event.
Page 24 of 107

HED uses exposure scenarios to describe the various types of handler exposures that may occur 

for a specific active ingredient. The use of scenarios as a basis for exposure assessment is very 

common as described in the U.S. EPA Guidelines for Exposure Assessment (U.S. EPA; Federal 

Register Volume 57, Number 104; May 29, 1992). Information from the current labels, use and 

usage information, toxicology data, and exposure data were all key components in the 

development of the exposure scenarios. 

HED has developed a series of general descriptions for tasks that are associated with pesticide 

applications. Tasks associated with occupational pesticide handlers are categorized using one of 

the following terms:

Mixer/Loaders: these individuals perform tasks in preparation for an application. For 

example, prior to application, mixer/loaders would mix the chlorpyrifos and load it into 

the holding tank of the airplane or groundboom equipment.

Applicators: these individuals operate application equipment during the release of a 

pesticide product into the environment. Applications can be made using equipment such 

as airplanes or groundboom equipment. 

Mixer/Loader/Applicators and/or Loader/Applicators: these individuals are involved 

in the entire pesticide application process (i.e., they do all job functions related to a 

pesticide application event). These individuals would transfer chlorpyrifos into the 

application equipment and then also apply it.

Flaggers: these individuals guide aerial applicators during the release of a pesticide 

product onto an intended target.

HED risk assessors must understand how exposures to chlorpyrifos occur (i.e., frequency and 

duration) and how the patterns of these occurrences can cause the effects of the chemical to 

differ (referred to as dose response). Wherever possible, use and usage data determine the 

appropriateness of certain types of risk assessments. Other parameters are also defined from use 

and usage data such as application rates and application frequency. HED always completes noncancer risk assessments using maximum application rates for each scenario because what is 

possible under the label (the legal means of controlling pesticide use) must be evaluated in order 

to ensure there are no concerns for each specific use.

A chemical can produce different effects based on how long a person is exposed, how frequently 

exposures occur, and the level of exposure. It is likely that chlorpyrifos exposure can occur in a 

variety of patterns. The Agency believes that occupational chlorpyrifos exposure can occur over 

a single day or up to weeks at a time, even though each crop or application target is generally 

treated only a few times per season. Intermittent exposures over several weeks and months are 

also anticipated. Some applicators may apply chlorpyrifos over a period of several weeks 

because to cover large acreages, they may be custom or professional applicators that are 

completing a number or applications within a region, or they may be applying chlorpyrifos over 

a period of several days. HED classifies exposures up to 30 days as short-term, exposures 
Page 25 of 107

greater than 30 days up to 6 months as intermediate-term and exposures greater than 6 months as 

long-term. 

Under current policy, both short- and intermediate-term assessments are completed for 

occupational scenarios in essentially all cases, because these kinds of exposures are likely and 

acceptable use/usage data are not available to justify disregarding intermediate-term scenarios. 

This current policy differs from the policy in effect when earlier chlorpyrifos assessments were 

completed, in which HED assumed that agricultural handling of pesticides like chlorpyrifos 

involved only short-term exposure. For chlorpyrifos, HED has completed both short-term and 

intermediate-term assessments because of likely extended periods of exposure in segments of the 

user population and due to in the absence of crop- and use-specific information indicating 

otherwise. Long-term exposure (essentially every working day over a year) is not anticipated 

based upon the use profile of chlorpyrifos. Dow AgroSciences submitted a draft survey plan to 

the Agency following discussions with HED about the potential usefulness of additional 

information regarding the exposure durations for occupational handlers of chlorpyrifos. HED 

reviewed the draft survey plan in a 12/2/2009 memorandum (W. Britton, D368391) and was of 

the view that “the survey may help clarify the nature of the user population who may be exposed 

over an intermediate-term allowing for a more informed risk management decision; however, it 

is unlikely to alter the Agency‟s intention to assess intermediate-term risks in future occupational 

assessments for chlorpyrifos, consistent with current policy.” HED suggested revisions to 

improve the survey‟s potential utility for risk characterization purposes. The review was based 

upon input from the HED, the Biological Economic Analysis Division, and OPP‟s human 

subjects research ethics staff. HED has received no further correspondence regarding the draft 

survey plan since the time of the memorandum. 

Occupational handler exposure assessments are completed by HED using different levels of risk 

mitigation. Typically, HED uses a tiered approach. The lowest tier is designated as the baseline 

exposure scenario (i.e., long-sleeve shirt, long pants, shoes, socks, and no respirator). If risks are 

of concern at baseline attire, then increasing levels of personal protective equipment or PPE (e.g., 

gloves, double-layer body protection, and respirators) are evaluated. If risks remain a concern 

with maximum PPE, then engineering controls (e.g., enclosed cabs or cockpits, water-soluble 

packaging, and closed mixing/loading systems) are evaluated. This approach is used to ensure 

that the lowest level of risk mitigation that provides adequate protection is selected, since the 

addition of PPE and engineering controls involves an additional expense to the user and – in the 

case of PPE – also involves an additional burden to the user due to decreased comfort and 

dexterity and increased heat stress and respiratory stress.

2.1.1 Assumptions for Handler Exposure Scenarios

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. 

The assumptions and factors used in the risk calculations include:

Occupational handler exposure estimates were based on surrogate data from: (1) the 

Pesticide Handlers Exposure Database (PHED), (2) the Outdoor Residential Exposure 

Task Force (ORETF), and proprietary data. Data from the Agricultural Handlers
Page 26 of 107

Exposure Task Force (AHETF) were used; however, at the time of this assessment, only 

a few exposure scenarios had been reviewed and approved for use by HED. The AHETF

scenarios were recently posted and are publically available on the EPA website as of 

4/8/2011 (http://www.epa.gov/pesticides/science/handler-exposure-data.html). This new 

data were included in the presentation of the most current data used to assess exposure 

and risk for occupational pesticide handlers. 

HED assumes the maximum application rates allowed by labels in its risk assessments.

The adverse effects for the short- and intermediate-term dermal and inhalation endpoints 

are based on studies where the effects were observed in males and females, therefore the 

average body weight of an adult handler (i.e., 70 kg) is used to complete the risk 

assessment. 

Risk estimates for dermal and inhalation exposure were combined for chlorpyrifos

because common toxicity endpoints (i.e., plasma and RBC ChE inhibition) were used to 

calculate risks. 

The LOC for short- and intermediate-term occupational dermal exposure (defined as an 

MOE = 100) differs from the LOC for short- and intermediate-term occupational 

inhalation exposure (defined as an MOE = 30). An aggregate risk index (ARI) is 

required to combine or aggregate risk from differing routes of exposure when the LOCs 

are not the same. EPA identifies as a level of concern ARIs that fail to reach or exceed 

the level of 1. Lower ARIs raise a risk of concern. 

Short- and intermediate-term occupational risk assessments were completed based on the 

toxicity endpoints identified. The same toxicological endpoint was selected for shortand intermediate-term dermal chlorpyrifos exposures; therefore risk results for all dermal 

durations are identical. The same applies to all short- and intermediate-term inhalation 

exposure risk estimates. Long-term exposure durations are not anticipated based upon 

the use pattern of chlorpyrifos.

An absorption rate was not used for short- and intermediate-term dermal risk assessment

because the dermal toxicological endpoint selected for both durations is derived from a 

21-day dermal study. 

Generic protection factors (PFs) were used to calculate exposures when data were not 

available. For example, a filtering face piece dust mask is assumed to have a PF of 5 (i.e. 

reduces inhalation exposure by 80%). A half face elastomeric respirator with appropriate 

cartridges is assumed to have a PF of 10 (i.e. reduces inhalation exposure by 90%). 

The average occupational workday is assumed to be 8 hours. 

HED assumes that 1% of the maximum application rate of each cattle ear tag and roach 

bait station applied transfer to the handler. 
Page 27 of 107

The daily areas treated were defined for each handler scenario (in appropriate units) by 

determining the amount that can be reasonably treated in a single day (e.g., acres, square feet, or 

gallons per day). When possible, the assumptions for daily areas treated are taken from the 

Health Effects Division Science Advisory Committee on Exposure SOP #9: Standard Values for

Daily Acres Treated in Agriculture, which was completed on July 5, 2000. However, standard 

values are not available for numerous scenarios. Assumptions for these scenarios are based on 

HED estimates and could be further refined by input from affected sectors. The following daily 

volumes handled and acres, excerpted from the policy, to be treated in each occupational 

scenario include:

Aerial: 1,200 acres for large field crops, 350 acres for other field crops, and 7,500 acres 

for mosquito control adulticide applications;

Groundboom: 200 acres for large field crops (e.g., corn and wheat), 80 acres treated for 

other field crop groundboom applications, and 40 acres on golf course turf;

Airblast: 40 acres for treated agricultural applications;

Flaggers: 350 acres for all crops;

Ground Fogger (Truck Mounted): 3,000 acres for mosquito control;

High Pressure Handwand and Handgun: 1,000 gallons of spray solution prepared when 

mixing/loading liquids for application or making the application, or application to 5 

acres;

Backpack Sprayer or Low Pressure Handwand: 40 gallons when 

mixing/loading/applying liquids, or application to 5 acres;

A study was submitted by Dow AgroSciences, “Chlorpyrifos Peach Tree Root Dip 

Application and Worker Exposure Study (MRID 46365501) and reviewed by HED. This 

study, which was based upon the interview of 51 peach tree growers, was conducted to 

provide information for the assessment of worker exposure when peach tree seedlings are 

dipped prior to transplanting. Based on this review, HED has determined that root 

dipping is a very minor use pattern, with acreages for peach tree planting operations 

ranging from 1 to 150. The average hours spent by an individual during dipping 

application is estimated at 5.3 hours per occurrence. The majority of peach tree growers 

either did not dip their trees or only did so on a once in 5-8 year frequency. For the 

purpose of quantitative risk assessment HED will assume the maximum acreage 

identified per application, 150. Because the use pattern is minor and done so 

infrequently, duration of exposure is anticipated to be short-term only; 

Paint Roller/Brush: 110 gallons for sewer manhole wall covers (2, 55 gallon drums), 3 

gallons applied for all other uses;

Cattle Ear Tag: 100 ear tags applied (100 cattle, at 1 tag per head);
Page 28 of 107

Roach Bait Station: 100 stations applied;

Total Release Fogger: 4 cans applied;

Drench/Dip: 1,000 gallons for applications to soil in containers; 100 gallons for root dip 

applications (MRID 463655501);

Shaker Container and Open Pour Bag: 1,000 square feet applied by hand dispersal to 

treated area;

Push Type Spreader: 5 acres treated per application;

Fire Ant Mounds: 25 mounds;

Belly Grinder: 1 acre treated per application.

2.1.2 Exposure Data for Handler Exposure Scenarios

HED uses unit exposure to assess handler exposures to pesticides. Unit exposures are estimates 

of the amount of exposure to an active ingredient a handler receives while performing various 

handler tasks and are expressed in terms of micrograms or milligrams of active ingredient per 

pounds of active ingredient handled. HED has developed a series of unit exposures that are 

unique for each scenario typically considered in our assessments (i.e., there are different unit 

exposures for different types of application equipment, job functions, and levels of protection). 

The unit exposure concept has been established in the scientific literature and also through 

various exposure monitoring guidelines published by the U.S. EPA and international 

organizations such as Health Canada and OECD (Organization for Economic Cooperation and

Development). HED identified 3 major surrogate unit exposure data sources for assessment of 

occupational handler exposure/risk. These data sources include: 

PHED (Pesticide Handlers Exposure Database);

Outdoor Residential Exposure Task Force (ORETF); and 

Agricultural Handlers Exposure Task Force (AHETF). 

In addition to surrogate data, 2 non-chemical specific exposure studies and 5 chemical specific 

handler exposure (biomonitoring and passive dosimetry) studies that were submitted in support 

of chlorpyrifos re-registration and have been reviewed and considered for use by the HED. The 

non-chemical specific exposure studies (described in detail below) were used when either 

exposure data were lacking or it was deemed the best available data source for assessment of the 

exposure scenario. 

Based on HED‟s review of the 5 chemical specific studies, a number of issues were identified 

which limit the utility of the available data. These include: the sufficiency of sample size, PPE 

worn by study participants, sufficient risk mitigation is not provided (e.g., additional PPE or 

engineering controls), and the studies do not encompass all handler uses of chlorpyrifos. Due to 
Page 29 of 107

the limitations outlined, HED has determined that these data are most useful as a tool for 

comparison to those estimates generated for representative exposure scenarios using surrogate 

data. A full description of the issues, a summary of the studies and the results of the 

comparative analysis are presented in Section 2.2, Occupational Handler Comparison Using 

Biomonitoring Data. 

A detailed description of the data source selected for each exposure scenarios assessed is 

presented in Appendix A/Table 2 which accompanies this document. Unit exposure values as 

selected from these exposure data sources at each level of personal protection and engineering 

control are presented in Appendix A/Table 3. Below are brief descriptions of the data sources 

used. 

PHED Version 1.1 (August 1998): PHED was designed by a task force of representatives from 

the U.S. EPA, Health Canada, the California Department of Pesticide regulation, and member 

companies of the American Crop Protection Association. PHED is a software system consisting 

of two parts – a database of measured exposures for workers involved in the handling of 

pesticides under actual field conditions and a set of computer algorithms used to subset and 

statistically summarize the selected data. Currently, the database contains values for over 1,700 

monitored individuals (i.e., monitoring units).

Users select criteria to subset the PHED database to reflect the exposure scenario being 

evaluated. The subsetting algorithms in PHED are based on the central assumption that the 

magnitude of handler exposures to pesticides are primarily a function of activity (e.g., 

mixing/loading, applying), formulation type (e.g., liquids, granulars), application method (e.g., 

aerial, groundboom), and clothing scenarios (e.g., gloves, double layer clothing).

Once the data for a given exposure scenario have been selected, the data are normalized (i.e., 

divided by) by the amount of pesticide handled resulting in standard unit exposures (milligrams 

of exposure per pound of active ingredient handled). Following normalization, the data are 

statistically summarized. The distribution of exposure for each body part (e.g., chest, upper arm) 

is categorized as normal, lognormal, or “other” (i.e., neither normal nor lognormal). A central 

tendency value is then selected from the distribution of the exposure for each body part. These 

values are the arithmetic mean for normal distributions, the geometric mean for lognormal 

distributions, and the median for all “other” distributions. Once selected, the central tendency 

values for each body part are composited into a “best fit” exposure value representing the entire 

body. 

The unit exposures calculated by PHED generally range from the geometric mean to the median 

of the selected data set. To add consistency and quality control to the values produced from this 

system, the PHED Task Force has evaluated all data within the system and has developed a set of 

grading criteria to characterize the quality of the original study data. The assessment of data 

quality is based on the number of observations and the available quality control data. While data 

from PHED provide the best available information on handler exposures, it should be noted that 

some aspects of the included studies (e.g., duration, acres treated, pounds of active ingredient 

handled) may not accurately represent labeled uses in all cases. HED has developed a series of 

tables of standard unit exposure for many occupational scenarios that can be utilized to ensure 
Page 30 of 107

consistency in exposure assessments. Unit exposures are used which represent different levels of 

personal protection as described above. Protection factors were used to calculate unit exposures 

for varying levels of personal protection if data were not available.

ORETF Handler Studies (MRID 44972201): A report was submitted by the ORETF (Outdoor 

Residential Exposure Task Force) that presented data in which the application of various 

products used on turf by homeowners and lawncare operators (LCOs) was monitored. All of the 

data submitted in this report were completed in a series of studies. Unit exposures from the 

following ORETF studies were used in this assessment and are summarized in a 4/30/01 HED 

memo (G. Bangs, D261948). The following studies were used to assess chlorpyrifos 

occupational handler exposure:

ORETF Study OMA001 - monitoring of LCO exposure scenarios while using a granular 

spreader 

ORETF Study OMA002 - monitoring of LCO exposure scenarios using a low pressure, high 

volume turf handgun

Table 2.1.2a below presents the ORETF unit exposure values used in the assessment of 

chlorpyrifos occupational handler exposure. 

Table 2.1.2a: Unit Exposure Values Obtained from ORETF Studies (MRID 44972201)

Type

Units (mg exposure/lb ai handled)

Dermal

Single Layer, Inhalation

Gloves

Single Layer, 

Gloves

Double Layer, 

Gloves

LCO Push Granular 

Spreader (OMA001) 0.35 0.22 0.11 0.0071

LCO Turfgun (WP 

Formulation) (OMA002) No Data 0.65 0.36 0.0066

All unit exposure values are geometric means except inhalation value for granular spreader. Double layer 

value calculated using a 50% protection factor. Turf gun, no glove data were not back calculated using a 

90 percent protection factor as it is deemed unreliable. Inhalation unit exposure values represent no 

respiratory protection as respirators are not considered feasible in the outdoor residential settings and, 

therefore, were not measured. 

AHETF: AHETF is a consortium of 24 agricultural chemical companies (which includes the 

primary registrant of chlorpyrifos, Dow AgroSciences) that formed a joint data development task 

force to design and develop a database on exposure of agricultural workers during the mixing, 

loading and/or application of pesticides. The data generated covers several types of 

mixing/loading systems, types of application equipment and types of formulations. Currently, 

the Agency has reviewed three studies conducted and submitted by the AHETF. HED has 

confirmed that the studies meet the study design benchmarks outlined in the AHETF Governing 

Document (AHETF, 2007) and is considered to be the most reliable data for assessing exposure 

and risk for these exposure scenarios. Of the studies reviewed, two were applicable to and used

for the assessment of occupational dermal and inhalation chlorpyrifos exposure potential. 

Reviews of the studies are summarized in the documents, “Health Effects Division (HED) 

Review of Agricultural Handler Exposure Task Force (AHETF) Monograph: Open Pour Mixing 
Page 31 of 107

and Loading Liquid Formulations (D373605, 3/11/11)” and “Health Effects Division (HED) 

Review of Agricultural Handler Exposure Task Force (AHETF) Monograph: Open Cab Ground 

Boom Application of Liquid Sprays (D373605, 3/11/11).” The study, Open Pour Mixing and 

Loading Dry Flowable Formulations (MRID 47947801) has also been reviewed by HED, but no 

dry flowable formulation is registered for chlorpyrifos occupational use. The AHETF scenarios 

were posted and are publically available on the EPA website as of 4/8/2011 

(http://www.epa.gov/pesticides/science/handler-exposure-data.html).

Table 2.1.2b below presents the AHETF unit exposure values used in the assessment of 

chlorpyrifos occupational handler exposure. 

Table 2.1.2b: Unit Exposure (ug /lb ai handled): Open Pour Mixing and Loading Liquids

Exposure Scenario Dermal Inhalation

SL_G DBL_G NR PF5 PF10

Open Pour Mixing and Loading 

Liquids 37.6 29.1 0.219 0.044 0.022

Open Cab Groundbom 

Application of Liquid Sprays 16.1 12.6 0.34 0.068 0.034

Non-Chemical Specific Exposure Data:

MRID 44793301/Exhibit 2 (15G Study): Exposure of Farmworkers To Terbufos (CL 92100) 

While Loading COUNTER 15G Systemic Insecticide-Nematicide With A Lock-N-Load Closed 

Handling System And Applying COUNTER 15G To Corn At Planting Time; (3/26/99) 

Authored by Joseph Higham; Completed by ABC Laboratories of Columbia MO, Agrisearch of 

Frederick MD, and American Cyanamid of Princeton NJ.; Project ID #s include: Exhibit 2 of 

EPA MRID 44793301, Terbufos 99-02, EXA 99-004, EXA 99-006, and RES 99-003, Sponsored 

by American Cyanamid.

The field portion of this study was conducted at 15 sites in central Nebraska during the 1998 

growing season. All totaled, 30 individual farm workers were monitored in this study. At each 

site, two farm workers (one loaded and the other completed the application) were monitored 

during the application, with soil incorporation, of terbufos 15G using “Lock-N-Load” type 

packaging, typical corn planters, and closed cab tractors (open cabs were used for 2 replicates). 

All planters were either 6 or 8 row devices. Corn was treated at an average application rate of 

1.23 lb ai/acre (the label maximum is 1.3 lb ai/acre) at planting. The total amount of active 

ingredient loaded in this study ranged from 66 to 96 pounds while the total amount of active 

ingredient applied in this study ranged from 35.8 to 81.9 pounds. The exposure time periods for 

loaders ranged from 0.37 to 0.78 hours while the time periods for the applicators ranged from 3.8 

to 8.7 hours. Exposure monitoring for dermal and inhalation exposure was completed using 

whole-body dosimetry, aqueous handwashes, aqueous glove washes, facial/neck wipe, inhalation 

monitoring in the breathing zone using personal sampling pumps, and inhalation monitoring in 

the breathing zone using personal sampling pumps pulling air through a respirator cartridge 

attached to the sampling device. All data except for the glove washes and the respirator cartridge 

protected inhalation monitoring data were used in the risk assessment. These data were not used 

in the assessment because the monitoring techniques are not considered to be acceptable 

approaches by the Agency.
Page 32 of 107

The loaders in this study used the “Lock-N-Load” type packaging as described above. 

Additionally, they wore normal work clothing, nitrile gloves, and a chemical-resistant apron (i.e., 

treated as a double layer of protection by the Agency for the purposes of this assessment). The 

applicators in this study used closed cabs as described above. Additionally, they wore normal 

work clothing, nitrile gloves, coveralls, protective eyewear, and an air purifying respirator with 

and organic vapor removing cartridge. 

The analytical results of this study indicate that terbufos was present in the 15G formulation used 

to make all applications in the study (avg = 105 % of the nominal 15 w/w concentration) and that 

terbufos was stable over time in the solutions used to dose the field fortification samples (avg = 

99.6 % of nominal). The detection and quantification limits established in the study and verified 

by the Agency are as follows:

Exposure Media Detection Limit Quantification Limit

Air Tubes 4.0 (ng/sample) 20.0 (ng/sample)

Whole-Body Dosimeters 0.20 (μg/sample) 1.0 (μg/sample)

Handwashes 0.10 (μg/sample) 0.50 (μg/sample)

Face/neck wipes 0.10 (μg/sample) 0.50 (μg/sample)

Laboratory recovery data are generated to ensure the daily performance of an analytical method 

during the analysis of field samples. The laboratory recovery data for each media from this 

study are summarized in the table below:

Exposure Media Fortification Range

(μg/sample) N

Results 

[avg ± sd (%) & CV]

Air Tubes 0.020 – 2000 12 88 ± 16 CV = 18.2

Whole-Body 

Dosimeters

1.0 - 25.0 18 92 ± 18 CV = 19.6

Handwashes 0.5 - 2.5 17 88 ± 11 CV = 12.5

Face/neck wipes 0.5 - 5.0 10 94 ± 9.4 CV = 10.0

Field and storage stability recovery data are generated to ensure the stability of residues during 

field sampling and over storage until analysis. In this study, the field recovery samples also 

provided data for storage stability as the samples were generated, stored, and analyzed 

concurrently with the field samples. The field recovery data for each media from this study are 

summarized in the table below:

Exposure Media Fortification Range

(μg/sample) N

Results

avg ± sd (%) & CV

Air Tubes 0.05 - 1.00 18 62 ± 16 CV = 25.8

Whole-Body 

Dosimeters

5.0 - 10.0 18 86 ± 8.0 CV = 9.3

Handwashes 2.5 - 5.0 18 79 ± 8.4 CV = 10.6

Face/neck wipes 2.5 - 5.0 18 62 ± 5.5 CV = 8.9
Page 33 of 107

Unit exposures were calculated and verified by the Agency. These values were used for risk 

assessment purposes by the Agency. The values were calculated by correcting for the average 

field recovery values summarized above. The unit exposure values used by the Agency in the 

risk assessment were only based on the use of protective gloves (i.e., glove wash data were not 

used in the risk assessment) and the use of no respiratory protection (i.e., inhalation monitoring 

data generated by pulling air through a respirator cartridge was not used in the risk assessment). 

The unit exposure values used in the assessment reflect only the use of closed cabs during 

application.

The results of this study indicate very low exposure levels as would be expected given the 

exposure scenarios that were monitored (i.e., lower exposure activities with high levels of 

personal protection). There was a high percentage of values that were less than either the limit of 

detection (LOD) or limit of quantification (LOQ). In such cases, the values used to calculate the 

unit exposure values were either 1⁄2 of the LOD or LOQ as appropriate. The nondetect values 

presented in the table below fit both categories. It should be noted, however, that a majority of 

the whole-body dosimeters contained detectable residues of terbufos. These study results are 

summarized in the following table:

Table 2.1.2c: Unit Exposure Values Obtained from Tractor Closed Loading and Application Study of 

Granular Formulation (MRID 44793301)

Job Function Data Type

Dermal Data Inhalation Data

Unit Exposure

(mg/lb ai)

Nondetects

(%)

Unit Exposure

(mg/lb ai)

Nondetects

(%)

Loaders1

Minimum 1.21x10-5

WBD: 6.7

HW: 93.3

FNW: 100

5.28 x10-7

Geo. Mean 5.20 x10 46.7 -57.84 x10-6

Maximum 3.61x10-46.50 x10-5

Applicators2

Minimum 7.32 x10-6

WBD: 33.3

HW: 93.3

FNW: 100

4.25 x10-7

Geo. Mean 3.51 x10 100 -52.43 x10-6

Maximum 1.65 x10-42.04 x10-5

1 Unit exposure data for loaders are based upon workers wearing double layer clothing, chemical resistant gloves 

and no respirator. 

2 Unit exposure data for applicator are based upon workers wearing double layer clothing, chemical resistant gloves, 

and PF10 respirator. 

WBD = whole-body dosimeter, HW = handwash, FNW: face and neck wipe

MRID 45250702: Worker Exposure Study during Application of Regent 20GR in Banana 

Plantation: 

A total of 18 mixer/loader/applicator events during granular backpack (i.e., a specialized device 

manufactured by Horstine Farmery) or spoon application of fipronil to bananas were monitored 

during applications on three different days in June, 1994 on the same banana plantation in 

Cameroon. [Note: Only the spoon application data included in this study are used in the carbaryl 
Page 34 of 107

risk assessment as backpack granular applications have been assessed using the data presented 

above.] The 18 replicates were distributed over the 3 sampling days as follows: 6 spoon/hand 

applications on day 1; 4 spoon/hand applications on day 2; and 8 backpack events on day 3. 

Weather was typical of the application season in that it was hot and humid. Monitoring was 

completed using whole body dosimeters, cotton gloves, cotton caps, and personal sampling 

pumps equipped with filters. Regent 20GR was supplied in 22 pound boxes which were loaded 

directly into the backpack devices or buckets for the spoon applicators. The application rate for 

fipronil used in this study is 7.5 grams of Regent 20GR (i.e., 0.15 grams ai/plant) which is 

equivalent to about 0.26 lb ai/acre (0.00033 lb ai/plant) at approximately 800 plants per acre. The 

numbers of acres treated ranged from approximately 0.75 to 1 acre. The pounds of active 

ingredient handled ranged from about a quarter to half a pound per replicate. Each applicator 

wore whole body dosimeters that also served as the normal work clothing. PVC gloves were also 

worn over cotton gloves which served as the dosimeters. A protection factor of 50 percent was 

used by the Agency to calculate exposure levels under a layer of normal work clothing. 

Dosimeter samples were segmented into arms, legs, and torso for analysis.

Analysis of fipronil residues was completed with gas chromatography and electron capture 

detection. The limits of quantification (LOQ) were 9.7 μg per sample for all media used. The 

limit of detection (LOD) varied for each media. The LOD for the cotton gloves was 0.5 μg per 

sample, 0.10 μg per sample for the air filters, and 2.0 to 4.0 μg per sample for the whole body 

dosimeters depending upon the sample analyzed. Field and laboratory recovery data were 

generated for all media. Field recovery data were generated in a manner that addressed field 

sampling, field storage, transport, laboratory storage, and analysis. However, the laboratory 

recovery data were indeterminate because the sample media could not be identified for each 

reported result. The overall recovery values do appear to be quantitative. Residues were 

corrected for the overall average field recovery for each residue/matrix combination. Generally, 

recovery was adequate for all media/residue combinations (i.e., all correction factors were 

greater than 85 percent). 

Unit exposure values were calculated using the data from the study and a commercial 

spreadsheet program. The exposures that were calculated were normalized by the amount of 

chemical used, the duration of the application interval, and by the body weight of the individual 

applicators (see table below). The values are based on a 50 percent clothing penetration factor 

and are separated for each equipment type monitored in this study. For each normalization factor, 

the arithmetic mean, geometric mean, and various percentiles were calculated. No analyses were 

completed with these data to ascertain the exact type of distribution. The Agency typically uses 

the best fit values from the Pesticide Handlers Exposure Database which are representations of 

the central tendency. Considering the standard practice, the Agency will use the geometric mean 

for risk assessment purposes. 

Table 2.1.2.d: Unit Exposures Obtained From Fipronil Spoon Application Study 

(MRID 45250702)

Type

Dermal

(mg ai/lb handled)

Inhalation

(μg ai/lb handled)

Geometric Mean 1.98 45.0
Page 35 of 107

2.1.3 Occupational Handler Exposure Scenarios

It has been determined that exposure to pesticide handlers is likely during the occupational use of 

chlopyrifos in a variety of occupational environments. The anticipated use patterns and current 

labeling indicate several occupational exposure scenarios based on the types of equipment and

techniques that can potentially be used for chlorpyrifos applications. The quantitative 

exposure/risk assessment developed for occupational handlers is based on the following 

scenarios:

Mixer/Loaders:

(1a) Liquids for Aerial/ Chemigation Applications

(1b) Liquids for Airblast Applications

(1c) Liquids for Groundboom Applications

(1d) Liquids for Wide Area Aerial Applications

(1e) Liquids for Wide Area Ground Applications

(2a) Wettable Powder (in WSP) for Aerial/ Chemigation Applications

(2b) Wettable Powder (in WSP) for Airblast Applications

(2c) Wettable Powder (in WSP) for Groundboom Applications

(3a) Granulars for Aerial Applications

(3b) Granulars for Tractor Drawn Spreader Applications

(3c) Granulars with SmartBox® for Aerial Applications

(3c) Granulars with SmartBox® for Tractor Drawn Spreader Applications

(4a) Water Dispersible Granulars (in WSP) for Aerial/ Chemigation Applications

(4b) Water Dispersible Granulars (in WSP) for Airblast Applications

(4c) Water Dispersible Granulars (in WSP) for Groundboom Applications

Applicators:

(5a) Aerial Liquid Applications

(5b) Wide Area Aerial Applications

(5c) Aerial Granular Applications

(6a) Airblast Applications

(6b) Wide Area Ground Applications

(7a) Groundboom Applications

(7b) Tractor-drawn Granular Applications

(7c) SmartBox® Applications

(8) Paint Brush/Roller Applications

(9) Cattle Ear Tag Applications

(10) Bait Station Applications

(11) Total Release Fogger

Mixer/Loader/Applicators:

(12a) Liquid: Low Pressure Handwand Sprayer

(12b) Liquid: Wettable Powder (in WSP) for Low Pressure Handwand Sprayer
Page 36 of 107

(12c) Liquid: Microencapsulated for Low Pressure Handwand Sprayer

(12d) Liquid: Backpack Sprayer

(12e) Liquid: Wettable Powder (in WSP) for Backpack Sprayer

(12f) Liquid: Microencapsulated for Backpack Sprayer

(12g) Liquid: High Pressure Handwand Sprayer

(12h) Liquid: Handgun Sprayer

(12i) Liquid: Wettable Powder (in WSP) for Handgun Sprayer

(12j) Liquid: Microencapsulated for Handgun Sprayer

(12k) Liquid: Drench/Dip

(12l) Liquid: Paint/Brush Roller

(13a) Granulars: Shaker Container

(13b) Granulars: Open Pour Bag

(13c) Granulars: Push Type Spreader

(13d) Granulars: Bucket and Spoon

(13e) Granulars: Belly Grinder

(14a) Water Dispersible Granular (in WSP): Low Pressure Handwand Sprayer

(14b) Water Dispersible Granular (in WSP): Backpack Sprayer

(14c) Water Dispersible Granular (in WSP): Handgun Sprayer

Flaggers:

(15a) Flagging for Aerial Spray Applications

(15b) Flagging for Aerial Granular Applications

2.1.4 Occupational Handler Exposure and Risk Calculations

Chlorpyrifos occupational handler exposure and risk were calculated using the Margin of 

Exposure (MOE) approach, which is the ratio of the body burden to the toxicological endpoint of 

concern. Estimates of body burden are calculated considering application parameters (i.e., rate 

and area treated daily) along with unit exposures selected for the exposure scenario. Exposures 

are normalized by body weight and adjusted for absorption factors as appropriated to calculate 

dose levels. 

Daily Exposure: Daily dermal or inhalation handler exposures are estimated for each applicable 

handler task with the application rate, the area treated in a day, and the applicable dermal or 

inhalation unit exposure using the following formula:

Daily Exposure (mg ai/day) = 

Unit Exposure (mg ai/lb ai) x Application Rate (lb ai/A) x Daily Acres Treated (A/day)

Where: 

Daily Exposure = Amount (mg or μg ai/day) deposited on the surface of the skin 

that is available for dermal absorption or amount inhaled that is 

available for inhalation absorption;
Page 37 of 107

Unit Exposure = Unit exposure value (mg or μg ai/lb ai) derived from exposure 

data;

Application Rate = Normalized application rate based on a logical unit treatment, 

such as acres, square feet, or gallons. Maximum values are 

generally used (lb ai/A, lb ai/sq ft, lb ai/gal) and

Daily Area Treated = Normalized application area based on a logical unit treatment 

such as acres (A/day), square feet (sq ft/day), OR gallons per 

day (gal/day).

Daily Dose: The daily dermal or inhalation dose is calculated by normalizing the daily exposure 

by body weight and adjusting, if necessary, with an appropriate dermal or inhalation absorption 

factor. For all short- and intermediate-term exposure scenarios for chlorpyrifos, an average adult 

body weight of 70 kilograms was used, since the toxicological endpoint of concern (i.e., plasma 

ChE and RBC AChE inhibition) is not sex-specific. An absorption rate was not used for shortand intermediate-term dermal risk assessment because the short- and intermediate-term 

toxicological endpoint is derived from a dermal study. Because the short- and intermediate-term 

inhalation endpoints are based on an inhalation study, no absorption factor is necessary. Daily 

dose was calculated using the following formula:

Average Daily Dose (mg ai/kg/day) = Daily Exposure (mg ai/day) x (Absorption Factor (%/100)

Body Weight (kg)

Where:

Average Daily Dose = Absorbed dose received from exposure to a pesticide in a given

scenario (mg pesticide active ingredient/kg body weight/day);

Daily Exposure = Amount (mg ai/day) deposited on the surface of the skin that is 

available for dermal absorption or amount inhaled that is 

available for inhalation absorption;

Absorption Factor = A measure of the amount of chemical that crosses a biological 

boundary such as the skin or lungs (% of the total available 

absorbed); and

Body Weight = Body weight determined to represent the population of interest in 

a risk assessment (kg).

Margins of Exposure: Dermal and inhalation risks for each applicable handler scenario are 

calculated using a Margin of Exposure (MOE) approach, which is a ratio of the daily dose to the 

toxicological endpoint of concern. All MOE values were calculated separately for dermal and 

inhalation exposure levels using the formula below:

MOE = NOAEL or LOAEL (mg ai/ (kg/day))

Average Daily Dose (mg ai/ (kg/day))

Where:

MOE = Margin of Exposure, value used by HED to represent risk or how 

close a chemical exposure is to being a concern (unitless);

ADD = Average Daily Dose or the absorbed dose received from 
Page 38 of 107

exposure to a pesticide in a given scenario (mg pesticide active 

ingredient/kg body weight/day); and

NOAEL or LOAEL = Dose level in a toxicity study, where no observed adverse effects 

(NOAEL) or where the lowest observed adverse effects 

(LOAEL) occurred in the study

Risk values are presented for each route of exposure (i.e., dermal or inhalation) in each scenario, 

because risk mitigation measures are specific to the route of exposure. In addition, dermal and 

inhalation risk estimates are combined when toxicity endpoints are the same. 

The LOC for short- and intermediate-term occupational dermal exposure (defined as an MOE = 

100) differs from the LOC for short- and intermediate-term occupation inhalation exposure 

(defined as an MOE = 30). An ARI is required to combine or aggregate risk from differing 

routes of exposure when the LOCs are not the same. EPA identifies as a level of concern ARIs 

that fail to reach or exceed the level of 1. Lower ARIs raise a risk of concern. The following 

formula was used to calculate an ARI: 

Aggregate Risk Index (ARI) = 1 / ((1/estimated MOE for dermal exposure/ LOC for 

 dermal exposure (MOE = 100)) + (1/estimated MOE inhalation

 exposure/ LOC for inhalation exposure (MOE = 30))

2.1.5 Occupational Handler and Exposure Risk Summary

Short- and intermediate-term occupational handler exposure/risk estimates are presented in 

Appendix A/Table 1 which accompanies this document. Risk estimates consider all levels of 

personal protective equipment and engineering controls using surrogate data from AHETF, 

ORETF and PHED. 

Of the 305 exposure scenarios assessed, 134 were not of concern at some level of personal 

protection (i.e., an ARI > 1). Ninety-one (91) exposure scenarios were not of concern when 

engineering controls were considered. The remaining 80 scenarios resulted in risks of concern to 

HED (i.e., an ARI < 1) at all levels of personal protection and engineering controls considered.

2.1.6 Summary of Occupational Handler Exposure Data Gaps

No new data gaps have been identified for the assessment of occupational handler exposure/risk 

in addition to those identified in the Registration Eligibility Decision (RED) for chlorpyrifos 

(finalized 7/31/06; IRED issued 2/2002). Based on a 90-day progress report submitted (August 

28, 2003) by the technical registrant, Dow AgroSciences, many of the data requirements are 

anticipated to be supported by the generation of data by the Agricultural Handlers Exposure Task 

Force (AHETF). These data requirements include: 

Exposure data for seed treatment uses;

Exposure data for mixing wettable powders for aerial/chemigation application;

Exposure data for loading and applying granulars for aerial application; 

Exposure data for groundboom application; and 
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Human flaggers for aerial application. 

AHETF data for groundboom application and for mixing/loading liquid formulations have been 

submitted and reviewed by HED and used to estimate exposure/risk for occupational handlers 

from the exposure scenario and, therefore, satisfy the data requirement. AHETF data for seed 

treatment and planting treated seed has been submitted and is currently under review by HED. 

The requirement for exposure data for mixing wettable powders for aerial/chemigation 

applications should be waived because the formulation is no longer supported by the registrant. 

An exposure study for loading granular formulations is being planned under AHETF.

The remaining data requirements were addressed as well in the Dow AgroSciences 90-day 

progress report (8/28/03). The following describes the data requirement, the update from the 

progress report and current status: 

Exposure data for dip applications (e.g., preplant peaches)

Data were submitted by Dow AgroSciences, (MRID 46365501) Robertson, A. (2004) 

Chlorpyrifos Peach Tree Root Dip Application & Worker Exposure Study. Project Number: 

ASR091604. Unpublished study prepared by Dow AgroSciences LLC. 8 p. HED has reviewed 

this usage data and determined that the data requirement has been satisfied. 

Risk Assessment data for treated wood in residential structures.

Data were submitted by Dow AgroSciences, (MRID 46207702) Selman, F. (2004) Potential 

Exposure and Risk to Inhabitants of Homes Where Door and Window Frames Are Constructed 

with Lumber that has Been Pressure Treated with Chlorpyrifos. Project Number: GH/C/5720. 

Unpublished study prepared by Dow AgroSciences, LLC. 13 p. HED has reviewed these data

and determined that the requirement has been satisfied. 

Use pattern information for hydraulic handheld spray applications (amounts handled per 

day, per season; types of sprayers used).

In the 90-day progress report, Dow AgroSciences provided that, “the USDA Forest Service will 

develop usage data to confirm the actual amount of chlorpyrifos used on a daily and seasonal 

basis. They will also provide additional information describing the types of sprayer used in order 

for EPA to refine risk assessments. The final report will be completed and submitted to the EPA 

by the Forest Service by November 28, 2003 and DAS will cite these data.” As of a February 

2011 OPPIN Database search, no data have been submitted by Dow AgroSciences from the 

USDA Forest Service report. Therefore, this data requirement is not currently satisfied.

Use pattern information for high pressure hand-wand spray applications (amounts 

handled per day, per season; types of sprayers used).

In the 90-day progress report, Dow AgroSciences described that it was “working with the 

American Nursery and Landscape Association to provide actual use information and data for this 

application. We will cite data from the Agricultural Chemical Use – 2000 Nursery and 
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Floriculture Summary published April 2002 by USDA. These data will indicate that chlorpyrifos 

is used intermittently in small blocks or areas as a rotational tool and actual use is typically less 

than 100 gallons per day.” As of a February 2011 OPPIN Database search, no data have been 

submitted by Dow AgroSciences from the USDA Nursery and Floriculture Summary report. 

Therefore, this data requirement has not been satisfied. 

 

Usage data to confirm the acres treated on sod farms for mole crickets. 

In the 90-day progress report, Dow AgroSciences described that “DAS is developing a survey 

for sod farm managers to determine actual use data for use of chlorpyrifos to control mole 

crickets. These data can then be used by EPA to refine risk estimates. The present EPA 

assumption of 80 acres treated per day is an overestimate for this particular use.” As of a

February 2011 OPPIN Database search, no data have been submitted by Dow AgroSciences in 

support of this data requirement. Therefore, this data requirement has not been satisfied.

Exposure data for backpack spray application.

No data have been submitted by Dow AgroSciences in support of this data requirement. 

However, HED has determined that the PHED data exposure data available and used to assess 

this exposure scenario are acceptable and no data will be required. 

2.1.7 Occupational Handler Characterization

The occupational handler exposure and risk assessment for chlorpyrifos is based upon an array of 

calculations completed for all identified exposure scenarios using the short- and intermediateterm endpoints. HED completes both short- and intermediate-term assessments for occupational 

scenarios in essentially all cases, because exposures of these durations are likely and acceptable 

use/usage data are not available to justify deleting intermediate-term scenarios. HED identified 

305 different exposure scenarios which are defined based on the equipment used to make 

applications or the type of formulation used. Within each of these categories, different 

application rates and acres treated values were considered to evaluate the broad range of 

applications that may occur with each kind of equipment (e.g., a groundboom may be used for 

turf or agriculture). Finally, it should be noted that each calculation was completed at different 

levels of personal protection to allow for a more informed risk management decision. Even 

given the scope of the calculations that have already been completed, it is possible that some 

uses of chlorpyrifos that have not been quantitatively addressed in this document either due to 

lack of exposure data or other information and because chlorpyrifos is such a widely used 

chemical. 

The data used in the chlorpyrifos occupational handler risk assessment represent the best data 

and approaches that are currently available. While some of the data which have been used may 

not be of optimal quality, they represent the best available data for the scenario in question. In 

many cases, the Pesticide Handlers Exposure Database (PHED) was used to develop the unit 

exposure values. PHED data quality varies widely from scenarios that meet guideline 

requirements for studies to others where a limited number of poor quality data points are 

available. The results for each scenario should be reviewed in the context of the quality of these 
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data. PHED unit exposure values represent a central tendency of the data (i.e., geometric mean, 

median or arithmetic mean depending upon the distribution of the data). As such, the values 

based on the recent studies also are measures of central tendency (e.g., the geometric means were 

selected from each study for assessment purposes in most cases). HED used recently developed 

data from AHETF to assess the exposure scenarios mixing/loading liquid formulations and 

application of liquid sprays via open cab groundboom. HED has reviewed the data for the two 

studies and has confirmed that it meets study design benchmarks outlined in the AHETF 

Governing Document (AHETF, 2007) and is considered the most reliable data for assessing 

exposure and risk for these exposure scenarios. The efforts undertaken by AHETF represent a 

well-designed, concerted process to collect reliable, internally-consistent, and current exposure 

data in a way that takes advantage of and incorporates a more robust statistical design, better 

analytical methods, and improved data handling techniques. For the purpose of the assessment 

of the two exposure scenarios, HED has used the arithmetic mean unit exposure for short- and 

intermediate-term exposure durations as recommended in the study reviews (D373605). 

Along with the unit exposure values used in the assessment, other inputs include application 

rates and daily acres treated values. The application rates selected for occupational handler risk 

assessment represent the maximum amounts that are allowed by the label for all uses. The 

application rates that were selected for use in the risk assessment were defined based on a review 

of all current labels. The other key input for completing handler risk assessments used for 

defining how much chemical can be used in a day is how much can be treated in a day which is 

generally expressed as the number of acres treated per day. The values used for this parameter 

represent the HED‟s most current thinking.

In addition to the key sources of information considered above, there are many underlying 

factors that may impact the overall results of a risk assessment. For example, the protection 

factors used for adding additional levels of dermal and respiratory protection may impact the 

overall risk picture. The factors used in this assessment by HED have been in use for many 

years. There are exposure monitoring issues which must be considered. For example, in many 

cases the data included in PHED are based on the use of cotton gloves for hand exposure 

monitoring which is thought by many to have the potential to overestimate exposure because 

they potentially retain residue more than a bare hand would over the course of a work day. Such 

intangible elements of the risk assessment reflect many of the hidden uncertainties associated 

with exposure data. 

In summary, HED believes that the risk values presented in this occupational assessment 

represent the highest quality results that could be produced given the exposure, use, and 

toxicology data that are available. Risk managers and other interested parties should consider 

the quality of individual inputs when interpreting the results and make decisions accordingly. It 

is difficult to ascertain at what point on a distribution the values which have been calculated fall 

because the distributional data for exposure, application rates, acres treated and many other 

parameters are unrefined. HED does believe, however, that the risks represent conservative 

estimates of exposure because maximum application rates are coupled with high acreage 

estimates to define risk estimates that likely fall in the upper percentiles of the actual exposure 

distributions. Additionally, risk estimates are thought to be conservative even when measures of 
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central tendency are combined because values that would be considered to be in the lower 

percentile aspect of any input parameter have not been used in the calculations. 

2.2 Occupational Handler Comparison Using Biomonitoring Data

Occupational handler exposure estimates as presented in Section 2.1 were estimated based on 

surrogate data from AHETF, ORETF and PHED, as well as 2 non-chemical specific studies. In 

addition to these data, 5 chemical specific handler exposure studies submitted in the past in 

support of chlorpyrifos re-registration were reviewed and considered for use by the HED. Risk 

estimates have been calculated using absorbed doses (mg/kg) measured from the biomonitoring 

studies determined to be acceptable for quantitative risk assessment purposes. In order to 

characterize occupational risk estimates calculated using surrogate, passive dosimetry exposure 

data, HED has presented a comparative analysis of these data and biomonitoring data available 

for applicable exposure scenarios. Based on HED policy and consistent with Section 2.1, a shortand intermediate-term assessment was completed for all occupational scenarios because 

extended periods of exposure are likely and acceptable use/usage data are not available to justify 

removing intermediate-term scenarios and the dermal toxicity point of departure is the same for 

both durations. Evaluation of biomonitoring data was conducted based on a comparison of the 

absorbed dose as measured by urinary excretion of 3,5,6-trichloro-2-pyridinol (TCP), to the oral 

equivalent NOAEL of 0.15 mg/kg/day (i.e., 5 mg/kg/day from 21-day dermal study x 0.03 

dermal absorption) because the majority of exposure is via the dermal route. 

Occupational handler exposure/risk was estimated consistent with the methods presented in 

Section 2.1.1 and the methods as described in Section 2.1.4 above and makes use of the exposure 

data as presented in the Section 2.2.1 below. Comparative estimates using chemical specific 

handler exposure studies are limited to the level of clothing and personal protection worn by the 

participants when the studies were conducted. Comparative short- and intermediate term 

occupational handler exposure/risk using biomonitoring and surrogate exposure data are

presented in Appendix B which accompanies this document.

All chemical specific exposure studies required review for ethical conduct, received that review 

and all but one were determined to be ethical relevant to the standards of the time that the studies 

were conducted. In addition, all studies were reviewed as to their relevance in the current market 

in regard to product formulation. Based on this analysis, one study measured the mixing/loading 

of a wettable powder formulation which is no longer supported by the product registrant. 

Despite this limitation, the application of the mixed wettable powder formulation was considered 

for use because the formulation was mixed into a liquid form and is, therefore an acceptable 

surrogate. HED considered use of the 4 remaining handler exposure studies and weighed the 

strengths and weaknesses of each. A number of issues limit the utility of the studies, including: 

the sufficiency of sample size, PPE worn by study participants, sufficient risk mitigation is not 

provided (e.g., additional PPE or engineering controls), and the studies do not encompass all 

handler uses of the chemical. 

The number of monitored workers across the 4 studies range from 15 to 1. HED has typically 

relied on the criteria set forth by the Pesticide Assessment Guideline which recommend 15 

exposure measurements as a minimum for each exposure scenario. Only 1 of the 4 studies meets 
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this criterion. For example, a mixer/loader/applicator greenhouse study (MRID 43027901)

attempted to monitor 6 different greenhouse handler scenarios and, as a result was only able to 

collect 5 measurements for 2 exposure scenarios, 3 for another, and only 1 for each of the 

remaining 3 scenarios. 

Workers participating in the 4 reviewed studies wore a wide range of clothing and personal 

protective equipment. All workers wore an inner dosimeter which included t-shirt and briefs 

used to measure the penetration of chlorpyrifos through the outer dosimeter. The outer 

dosimeter was typically represented either by coveralls, or long sleeved shirt and long pants. 

For example, in the mixing/loading for aerial applications study (MRID 44739302), participants 

wore chemical-resistant gloves, apron and knee high boots. In another study, 

mixing/loading/applying for ornamentals in greenhouses, the workers wore chemical-resistant 

gloves, socks, rubber boots, and protective eyewear. In this same study, participants who 

conducted overhead applications also wore neoprene rain paints and a rain jacket over the 

coveralls, a half face respirator equipped with two organic vapour cartridges and pre-filters, and 

a face shield. The study authors attempted to account for the additional PPE through means of 

assigned penetration factors. It is possible that the additional PPE, despite correction, affect 

passive and biomonitoring results by reducing worker exposure to chlorpyrifos that would have 

otherwise deposited on the inner or outer dosimeters in their absence and likewise, result in an 

assessment which potentially underestimates human health exposure/risk for the exposure 

scenario. This is of particular concern in the greenhouse study because the personal protective 

equipment worn by study participants making overhead applications exceeds current labelled 

requirements. 

Because the 4 available studies were conducted with study participants wearing a specific 

combination of clothing and PPE, the utility of the data are limited to assessment of occupational 

handler exposure/risk which is represented by that level of clothing or personal protection. For 

example, in the mixing/loading for airblast application study (MRID 43138102) workers wore 

inner dosimeters (t-shirts and briefs), short sleeved shirt, long pants and outer dosimeters 

(coveralls), chemical resistant gloves and half-face respirator. Therefore, data from the study 

would be limited to the assessment of occupational handlers mixing/loading liquids with double 

layer clothing, gloves and PF10 respirator. Furthermore, since the available exposure studies are 

specific to particular handler activities and formulations (e.g., mixing/loading for liquid 

formulations) the utility of the data are limited to these parameters. 

In lieu of the issues outlined, HED has determined that of the 4 available studies reviewed, 3 

should be considered for quantitative risk assessment purposes (MRIDs 42974501, 43138102, 

and MRID 44739302). The mixing/loading liquids for aerial applications study (MRID 

44739302) was used to present a comparative biomonitoring estimate for the mixing/loading 

liquids for aerial applications exposure scenario at double layer clothing, gloves and no respirator 

level of personal protection. The mixing/loading liquids for and airblast application study 

(MRID 43138102) was used to present a comparative estimate for the mixing/loading liquids for 

airblast application and airblast application exposure scenario at single layer clothing, gloves and 

PF10 respirator level of personal protection. Study MRID 42974501 measured exposure from 

the mixing/loading and application of a wettable powder formulation and mixing/loading of a 

liquid formulation for ground boom application. Because the wettable powder formulation is no 
Page 44 of 107

longer supported, only the subsequent liquid application by ground boom exposure data was used 

to present a comparative estimate at single layer, gloves and PF10 respirator level of personal 

protection. A summary of all studies considered is presented in Section 2.2.2: Occupational 

Handler Chemical Specific Data. 

2.2.1 Occupational Handler Biomonitoring Exposure and Risk Calculations

Chlorpyrifos occupational handler biomonitoring exposure and risk MOE estimates were 

calculated using measures of absorbed dose as calculated from the urinary metabolic measures 

from the 3 available studies. Pre- and post-exposure urine measurements were conducted in the 

studies and measured for 3,5,6-trichioropyridinol (TCP), the primary urinary metabolite of 

chlorpyrifos. Measures of TCP in all studies were corrected for background (pre-exposure) 

concentrations of TCP and the completeness of daily urinary collection. 

Although study participants were instructed to avoid exposure to chlorpyrifos prior to the studies, 

measureable quantities of TCP were commonly found in pre-study urine specimens. The preexposure urine levels were considered to represent background and the participants were 

assumed to have had some unknown steady state exposure to chlorpyrifos. The daily measured 

TCP levels were therefore, each corrected for the TCP residues detected in the sample from the 

measured interval prior to the exposure event. In addition, the completeness of urine collection 

was determined in all studies by analyzing urine samples for creatinine content. The creatinine 

concentration and urine volumes were used to determine total daily creatinine output. The 

results were compared to and corrected based on to an average daily creatinine excretion rate of 

1.8 g/day (95% range of 1.1 to 2.5 g/day) based on standard literature values (Documenta Geigy, 

1970). 

Total chlorpyrifos (μg) absorbed after exposure in the studies was calculated using two methods. 

In both methods, TCP was corrected for background chlorpyrifos exposure and incomplete urine 

collection. The method resulting in the highest measures of chlorpyrifos absorbed was used to 

represent total chlorpyrifos for each monitored worker. 

In the first method (urinary excretion factor method), the amount of chlorpyrifos absorbed is 

estimated by dividing the cumulative amount of TCP excreted in the urine by the urinary 

excretion factor, 0.370. This factor was based on the kinetics of TCP in human volunteers given 

single oral and dermal doses of chlorpyrifos. The urinary excretion factor corrects for the 

following: a) only 72% of an oral dose of chlorpyrifos was excreted in the urine as TCP b) the 

percentage of the urinary TCP expected to be eliminated during the time interval during which 

urine was collected, and c) the difference in molecular weights for TCP and chlorpyrifos (i.e., 

198 and 350.6). 

In the second method (kinetic model method), the amount of chlorpyrifos absorbed is estimated 

by fitting the amount of TCP excreted in the individual urine specimens to the one compartment 

pharmacokinetic model that described the time-course of TCP in urine of volunteers following 

application of chlorpyrifos to their forearm (Nolan et al., 1984). Based on Nolan et al. it was

assumed that 72% of the absorbed chlorpyrifos would be excreted in the urine as TCP. A 
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computer program, “SIMUSOLV*,” was used to perform optimization for the parameters of this 

model. 

2.2.2 Occupational Handler Chemical Specific Data

MRID - 42974501: Shurdut, B.A. et al. 1993. Lorsban® 4E and 50W insecticides: assessment 

of Chlorpyrifos exposures to applicators, mixer/loaders and reentry personnel during and 

following application to low crops.

LORSBAN® 4E was applied using boom type sprayers at 2 lb ai/acre to Christmas trees in 

Michigan and LORSBAN® 50W was applied to cauliflower and tomatoes at 1 lb ai/acre in 

Arizona and Florida, respectively. A total of 18 workers were monitored in the study (9 

mixer/loaders and 9 applicators). Passive dosimetry and biological monitoring (urine and blood) 

were conducted to determine potential inhalation and dermal exposure as well as total absorbed 

dose. Mixer/loaders and applicators wore, as a minimum, t-shirt, briefs, coveralls, socks and 

chemical resistant boots. In addition, the mixer/loaders wore a half face respirator, gloves and 

goggles. 

Passive dosimetry evaluated by measuring the deposition of chlorpyrifos on coveralls, and the tshirt and briefs underneath. To accomplish this, cotton coveralls were used as whole body 

dosimeters to measure the amount of chlorpyrifos deposited on the outer surface of the worker's 

clothing. In addition, cotton underwear (T-shirt and briefs) were used as dosimeters to measure 

chlorpyrifos that penetrated through the outer coverall. To assess exposure to the head, a cotton 

denim patch was attached to a baseball hat. Hand rinses were used to determine the deposition 

of chlorpyrifos to the surface of the hands.

Biological monitoring was used to evaluate the amount of chlorpyrifos absorbed. Urine 

specimens were collected at approximately 12-hour intervals over a six-day period and were 

analyzed for TCP. Blood plasma cholinesterase levels were also monitored in worker exposure 

volunteers by means of pre- (baseline) and post-exposure blood sampling. Blood plasma ChE 

measures resulted in an average increase of baseline to post-exposure of 1.6 % and ranged from a 

14% reduction to a 23% increase. 

Although the dermal exposure data collected for the mixer/loader/applicator portion of the 

chlorpyrifos groundboom treated cauliflower, tomatoes, and Christmas trees do not meet all of 

the criteria specified in Subdivision U (currently referred to as Series 875 Group A), the data are 

of sufficient quality to be used for quantitative risk assessment purposes. However, results from 

the wettable powder formulation have not been considered because it is no longer supported. 

The product is now formulated as a wettable powder in WSP. 

The monitoring data from the study are presented in Table 2.2.2a below for comparison of the 

passive dosimetry to the absorbed dose. 

Table 2.2.2a: Passive and Absorbed Doses (mg/lb ai): Mixing/Loading for and Applying 

Wettable Powder Formulation with Groundboom and Mixing/Loading Liquid Formulation 

(MRID 42974501)
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Table 2.2.2a: Passive and Absorbed Doses (mg/lb ai): Mixing/Loading for and Applying 

Wettable Powder Formulation with Groundboom and Mixing/Loading Liquid Formulation 

(MRID 42974501)

Mixer/Loader of the 50W Formulation (No Longer Supported)

Mean ± SD

Amount Handled (lb ai) 33 ± 14 (n = 6)

Biological Monitoring (mg/kg) 0.012 ± 0.0069 (n = 6)

Passive Dosimetry -- Total absorbed 

(3% dermal and 100% inhalation oral

equivalent) dose (mg/kg)

0.033 ± 0.024 (n = 6)

Mixer/Loader of the 4E Formulation

Mean ± SD

Amount Handled (lb ai) 107 ± 17 (n = 3)

Biological Monitoring (mg/kg) 0.0078 ± 0.010 (n = 3)

Passive Dosimetry -- Total absorbed 

(3% dermal and 100% inhalation oral 

equivalent) dose (mg/kg)

0.0096 ± 0.016 (n = 3)

Applicator - Groundboom Open Cab

Mean ± SD

Amount Handled (lb ai) 33 ± 14 (n = 6)

Biological Monitoring (mg/kg) 0.0023 ± 0.0014 (n = 6)

Passive Dosimetry -- Total absorbed 

(3% dermal and 100% inhalation oral 

equivalent) dose (mg/kg)

0.0023 ± 0.0012 (n = 6)

MRID - 43138102: Honeycutt, R.C. & Day, E.W. Jr. 1994. Evaluation of the Potential 

Exposure of Workers to Chlorpyrifos During Mixing and Loading, Spray Application, and 

Clean-up Procedures During the Treatment of Citrus Groves with Lorsban® 4E Insecticide.

Passive dosimetry (dermal and inhalation) and biological monitoring samples (blood and urine 

analysis) were collected for 15 open pour liquid mixer/loader monitoring units and 15 open cab 

airblast applicator monitoring units in Ventura, Kern and Tulare Counties California. The 

applications were made to citrus groves (i.e., lemons and oranges) using Lorsban® 4E at the 

maximum label rate of 6 lb ai/acre. Mixer/loader, applicator and clean up workers dressed in 

inner dosimeters (t-shirts and briefs), short sleeved shirt, long pants and outer dosimeters (handmade denim coveralls), chemical resistant gloves, boots, goggles and respirator. Workers were 

monitored for chlorpyrifos exposure during and after the Lorsban® 4E application. Also, face 

and neck exposure was monitored using head patch samples. Hand exposure was monitored 

using hand washing techniques. Respiratory exposure of chlorpyrifos to workers was monitored 

using personal air monitoring pumps. 
Page 47 of 107

Biological monitoring was used to evaluate the amount of chlorpyrifos absorbed. Urine 

specimens were collected at approximately 12-hour intervals over a four-day period. The 

samples were analyzed for TCP. Blood cholinesterase levels (plasma and RBC) were also 

monitored in worker exposure volunteers by means of pre- (baseline) and post-exposure blood 

sampling. Plasma and RBC ChE measures resulted in a reduction of baseline to post-exposure of 

0.98 and 4.2 %, respectively. Blood plasma ChE levels ranged from a 31% reduction to a 38 % 

increase. Blood RBC ChE levels ranged from a 31% reduction to a 40 % increase. 

The study meets the acceptability criteria outlined in Subdivision U of the Pesticide Assessment 

Guidelines. The quality control/quality assurance aspects of the passive dosimetry were 

adequate for the dermal whole-body dosimetry and inhalation. The quality control/quality 

assurance aspects of the biological monitoring were sufficient. The data from this study are of

sufficient scientific quality to be used in the assessment.

The monitoring data from the study are presented in Table 2.2.2b below for comparison of the 

passive dosimetry to the absorbed dose. 

Table 2.2.2b: Passive and Absorbed Doses (mg/lb ai): Open Mixing/Loading and Applying 

Liquid Formulation for Airblast Application (MRID 43138102)

Mixer/Loader - Open Mixing/Loading

Mean ± SD

Amount Handled (lb ai) 78 ± 22 (n = 15)

Biological Monitoring (mg/kg) 0.010 ± 0.021 (n = 15)

Passive Dosimetry -- Total absorbed (3% dermal 

and 90% PF for the inhalation for the respirator) 

dose (mg/kg)

0.0062 ± 0.0062 (n = 15)

Applicator - Open Cab

Mean ± SD

Amount Handled (lb ai) 78 ± 22 (n = 15)

Biological Monitoring (mg/kg) 0.013 ± 0.024 (n = 15)

Passive Dosimetry -- Total absorbed (3% dermal 

and 90% PF for the inhalation for the respirator) 

dose (mg/kg)

0.0059 ± 0.0060 (n = 15)

MRID - 44483501: R. F. Bischoff 1998. Evaluation of Chlorpyrifos Exposure to Workers 

During Loading and Application of Lorsban® 15 % Granular Insecticide During Corn Planting.

Passive dosimetry (dermal and inhalation) and biological (urine) monitoring samples were 

collected for 16 combined monitoring units of loading and applying Lorsban® 15G during corn 

planting. The test subjects loaded the granular product in row planters (8 to 12 row planters) and 

accompanied the tractor driver (i.e., farmer) in the enclosed cab. 

HED is unable to make use of this study as reliance on this study in actions under FIFRA or 

FFDCA is prohibited per Kelly Sherman‟s August 26, 2010 memorandum, “Ethics Review of 

Worker Exposure Study.”
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MRID 44739302: Knuteson et. al. 1999. Evaluation of Potential Exposure to Workers Mixing 

and Loading Lorsban®-4E Insecticide Products for Aerial Application.

Exposures were estimated based on both passive dosimetry measurements and biomonitoring of 

urinary TCP. This study characterizes exposures to 14 workers during the mixing and loading of 

Lorsban®-4E or Lorsban® 4E-SG, a 45 percent emulsifiable concentration insecticide for aerial 

application to cotton, alfalfa and wheat. Each worker mixed and loaded enough product to cover 

a 500 acre per day target rate (170 to 250 lb ai and 42.5 to 62.5 gallons product for wheat and 

500 lbs ai and 125 gallons of product for cotton and alfalfa). Lorsban® was applied at the 

maximum label registered application rates of 0.5 lb active ingredient (ai) per acre for wheat, and 

1.0 lb ai per acre for cotton and alfalfa. The study examined exposures to a total of 15 workers, 

five for wheat in Dalhart, Texas, five for cotton in Gila Bend, Arizona and five for alfalfa in Gila 

Bend, Arizona. The mixing/loading exposure period ranged from 40 to 131 minutes, with an 

average of 89 minutes. The workers wore cotton overalls, a cotton t-shirt, briefs, and socks, 

chemical resistant gloves, chemical resistant apron and knee-high chemical resistant boots, 

goggles and a hat during the mixing/ loading operation.

A summary of the study results are provided below. The majority of the exposure data meet the 

criteria specified in Series 875 Group A. Only minor issues were identified. The study 

evaluated 15 workers, however one of the workers dropped out of the study the day after 

exposure and another performed and activity outside of mixing/loading and, therefore these were 

not included in the passive dosimetry or biomonitoring results. The monitoring data from the 

study are presented in Table 2.2.2c below for comparison of the passive dosimetry to the 

absorbed dose. 

Table 2.2.2c: Passive and Absorbed Doses (mg/lb ai): Mixing/Loading and 

Applying Liquid Formulation for Aerial Application (MRID 44739302)

Mean ± SD

Amount Handled (lb ai) 398 ± 136 (n = 13)

Biological Monitoring (mg/kg) 0.0015 ± 0.0014 (n = 13)

Passive Dosimetry -- Total absorbed (3% 

dermal and 100% inhalation oral 

equivalent) dose (mg/kg)

0.00097 ± 0.00052 (n = 13)

. 

MRID 43027901: Contardi et. al. 1993. Evaluation of Chlorpyrifos Exposures During 

Mixing/Loading and Application of EMPIRE 20 Insecticide to Ornamental Plants in Commercial 

Greenhouses

This study was designed to examine chlorpyrifos dermal and inhalation exposure to 16 handlers 

applying EMPIRE 20 (a flowable microencapsulated formulation of chlorpyrifos) to ornamentals 

in a greenhouse. The greenhouse was located in the Columbus, Ohio area and the study was 

conducted on five different days. Each monitoring unit was responsible for performing all tasks 

associated with typical application of the test product, including mixing, loading, application, 
Page 49 of 107

and clean-up (in two instances, however, clean-up activities were not performed). Exposure was 

estimated based on both passive dosimetry measurements and biomonitoring of urinary TCP. 

Ten of the workers applied EMPIRE 20 to plants on the floor or on benches and six applied 

EMPIRE 20 to hanging overhead plants. Mixing/loading activities involved the handler pouring 

the test product (packaged in 16 oz plastic bottles) directly into the mix tank. The application 

activities varied, but generally consisted of the applicator walking down each row while spraying 

the plants on one side only, usually holding the spray nozzle in one hand and the hose in the 

other. Application equipment used by the handlers was not clearly specified in the study report. 

Using best professional judgement in interpretation of application equipment, there were 10 

handgun spray equipment handlers, 1 electrostatic sprayer handler, 3 handlers operating 

backpack/low pressure handwand sprayers and 2 handlers making applications using a fogger 

sprayer. Following the application, each handler (except workers 7 and 8) performed clean-up 

activities which involved flushing the tank mix and hose/nozzle with water and rolling up the 

hose. 

Each handler wore a clean pair of briefs and T-shirt, long-sleeve cotton coveralls, chemicalresistant nitrile gloves, socks, rubber boots, and protective eyewear. The overhead applicators 

also wore neoprene rain paints and a rain jacket over the coveralls, a 3M half face respirator 

equipped with two organic vapour cartridges and pre-filters, and a face shield. One floor/bench 

worker (worker 4) also wore neoprene pants and all floor/bench workers wore a baseball cap. 

While this study meets many of the criteria specified in Series 875 Group A, there are several 

issues which limit the utility of the study for quantitative risk assessment purposes. These issues 

include the following:

Limited Sample - The principal concern is that the study has been determined to contain 

an insufficient number of workers. Applications were conducted with 4 different types of 

sprayers to three different plant orientations (bench, floor, overhead). Among the 16 

workers observed there were:

o 10 handgun spray equipment handlers (5 bench/floor, 5 overhead)

o 1 electrostatic sprayer handler (1 overhead)

o 3 handlers operating backpack/low pressure handwand sprayers (3 bench/floor)

o 2 handlers making applications using a fogger sprayer (1 bench/floor and 1 

overhead application). 

Clothing Penetration Factors - Overhead applicators wore neoprene rain pants and rain 

jackets over long-sleeve cotton coveralls, briefs, t-shirt, gloves socks, rubber boots and a 

half face respirator. While clothing penetration factors were calculated to determine 

passive dosimetry despite the addition of coveralls, neoprene pants and jackets, 

penetration factors calculated for 2 overhead applicators are questionable because the 

inner dosimeter t-shirt/brief and outer dosimeter torso samples were both less than the 

limit of detection. 

Inhalation PPE - The overhead handlers OV respirators that are presumed to provide a 90% 
Page 50 of 107

protection factor via inhalation. Therefore, the inhalation component of exposure would not have 

been captured by the biomonitoring of those handlers. 

Inner Dosimeters – For 3 bench/floor workers, the inner dosimeter t-shirt/brief samples 

were less than the detection limit. These workers were used to calculate average 

penetration factors for each handler type, which were then used to calculate dermal 

exposure. 

Inhalation Measures - The inhalation measures reported for single overhead electrostatic 

sprayer workers the loading sample was reported as higher than the total in the study 

report, therefore invalidating results for worker. 

The monitoring data from the study are presented in Table 2.2.2d below for comparison of the 

passive dosimetry to the absorbed dose. 

Table 2.2.2d: Passive and Absorbed Doses (mg/lb ai): Mixing/Loading and 

Applying Liquid Formulation for Greenhouse Application (MRID 43027901)

Bench/Floor – Handgun

Mean ± SD

Amount Handled (lb ai) 0.49 ± 0.23 (n = 5)

Biological Monitoring (mg/kg) 0.0025 ± 0.000059 (n = 5)

Passive Dosimetry -- Total absorbed (3% 

dermal and 100% inhalation oral equivalent) 

dose (mg/kg)

0.00070 ± 0.00036 (n = 5)

Bench/Floor – Backpack/Low Pressure Handwand

Mean ± SD

Amount Handled (lb ai) 0.11 ± 0.04 (n = 3)

Biological Monitoring (mg/kg) 0.011 ± 0.00010 (n = 3)

Passive Dosimetry -- Total absorbed (3% 

dermal and 100% inhalation oral equivalent) 

dose (mg/kg)

0.00014 ± 0.000086 (n = 3)

Bench/Floor - Fogger

Mean ± SD

Amount Handled (lb ai) 13 (n = 1)

Biological Monitoring (mg/kg) 0.25 (n = 1)

Passive Dosimetry -- Total absorbed (3% 

dermal and 100% inhalation oral equivalent) 

dose (mg/kg)

0.30 (n = 1)
Page 51 of 107

Table 2.2.2d: Passive and Absorbed Doses (mg/lb ai): Mixing/Loading and 

Applying Liquid Formulation for Greenhouse Application (MRID 43027901)

Overhead - Handgun

Mean ± SD

Amount Handled (lb ai) 0.66 ± 0.29 (n = 5)

Biological Monitoring (mg/kg) 0.0019 ± 0.000064 (n = 5)

Passive Dosimetry -- Total absorbed (3% 

dermal and 100% inhalation oral equivalent) 

dose (mg/kg)

0.0089 ± 0.0092 (n = 5)

2.2.2 Occupational Handler Comparison Using Biomonitoring Data Risk 

Summary

Appendix B, which accompanies this document, presents the comparative short- and 

intermediate-term occupational handler exposure/risk estimates. Risk estimates compare the 

level of personal protection measured in biomonitoring exposure studies and the corresponding 

level estimated using surrogate exposure data. Evaluation of biomonitoring data was conducted 

based on a comparison of the absorbed dose as measured by urinary excretion of 3,5,6-trichloro2-pyridinol (TCP), to the oral equivalent NOAEL of 0.15 mg/kg/day (i.e., 5 mg/kg/day from 21-

day dermal study x 0.03 dermal absorption) because the majority of exposure is via the dermal 

route. 

Of the 4 exposure scenarios compared, 3 (mixing/loading liquids for airblast application, airblast 

applications, and groundboom applications) result in biomonitoring estimates of greater risk 

potential than those estimated using surrogate data (i.e., the estimated MOEs are lower). The 

analysis of the exposure scenario, mixing/loading liquids for aerial application, results in reduced 

risk potential (a 3.8X reduction in MOE estimate). Because a number of issues were identified 

which limit the utility of the available biomonitoring data, HED has determined that these data 

are best suited for characterization of the estimates calculated for representative exposure 

scenarios using the surrogate data.

2.3 Commercial Seed Treatment

Based upon the registered seed treatment uses of chlorpyrifos, occupational handler exposure is 

expected for individuals involved in commercial seed treatment. Post-application exposure to 

agricultural workers is not expected. 

Occupational handlers may experience short- and intermediate-term exposure to chlorpyrifos 

while performing seed treatment activities in commercial settings. In addition, occupational 

secondary handlers may experience short- and intermediate-term exposure while planting 

chlorpyrifos-treated seeds. No chemical-specific handler exposure data were submitted in 

support of this use pattern. In order to assess commercial seed treatment and seed planting 

activities, unit exposure data were taken from HED ExpoSAC Policy 14: SOPs for Seed 

Treatment. The amount of active ingredient handled depends on the application rate (lb ai/lb 
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seed) and the pounds of seed treated in a day (or the pounds of seed that can be planted in a day), 

all of which vary depending upon the seed type. Values for the amount of seed treated and 

planted per day were obtained from HED ExpoSAC Policy 15. Short- and intermediate-term 

commercial seed treatment is presented in Appendix C/Table 1 which accompanies this 

document. Short- and intermediate-term secondary handler exposure is presented in Appendix 

C/Table 2.

Potential occupational exposure scenarios from the use of chlorpyrifos as a commercial seed 

treatment include:

Mixing, loading, applying liquid formulations

Bagging treated seed

Sewing bags

Multiple activities

Typically, for large-scale commercial seed treatments, workers perform only those specific 

individual tasks listed above; however, it is assumed that workers also may perform multiple 

activities throughout the day. As a result a “multiple activities” scenario (i.e. where one worker 

performs all seed treatment tasks such as loading/applying, sewing, bagging, cleaning, 

calibration, repair, forklift driver, etc.) was addressed.

Planting Treated Seed

Potential occupational exposure scenarios from the use of chlorpyrifos as a commercial seed 

treatment include: planting treated seed (secondary handler).

Planting treated seed consists of the farmer purchasing bags of treated seed, placing the seed in 

the hopper and applying seed to fields (considered a secondary handler exposure scenario).

2.3.1 Assumptions for Commercial Seed Treatment Exposure Scenarios

A series of assumptions and exposure factors served as the basis for completing the occupational 

risk assessment for commercial seed treatment exposure scenarios. The assumptions and factors 

used in the risk calculations include:

Chemical-specific data for assessing exposure during commercial seed treatment and 

seed planting were not submitted to the Agency in support of this Section 3 application. 

Unit exposures are from ExpoSAC Policy 14: SOPs for Seed Treatment (May 1, 2003). 

Unit exposure values from ExpoSAC Policy 14 are based on data for open 

mixing/loading/application systems. 

HED assumes the maximum application rates allowed by labels in its risk assessments.

The amount of seed treated per day at a commercial facility (lb seed/day):

Beans (Field, Green, Kidney, Lima, Navy, Snap, String, Wax), Corn (Sweet) –
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194,000

Cotton (Delinted) – 160,000

Triticale – 71,800

Cucumber, Pumpkin – 5,500

Amount of seed treated or planted per day (secondary handler) as based on ExpoSAC 

Policy No. 15:

Beans (Lima, Snap), Triticale – 100

Beans (Navy) – 45

Cotton – 18

Corn (Field, Sweet) – 15

Cucumber – 5

Pumpkin - 4

The adverse effects for the short- and intermediate-term dermal and inhalation endpoints 

are based on studies where the effects were observed in males and females, therefore the 

average body weight of an adult handler (i.e., 70 kg) is used to complete the risk 

assessment. 

A short- and intermediate-term occupational risk assessment was completed based on the 

toxicity endpoints identified. The same toxicological endpoint was selected for shortand intermediate-term dermal chlorpyrifos exposures; therefore risk results for all dermal 

durations are identical. The same applies to all short- and intermediate-term inhalation 

exposure risk estimates. Long-term exposure durations are not anticipated based upon 

the use pattern of chlorpyrifos.

An absorption rate was not used for short- and intermediate-term dermal risk assessment 

because the short and intermediate-term toxicological endpoint is derived from a dermal 

study. Because the short- and intermediate-term inhalation endpoints are based on an 

inhalation study, no absorption factor is necessary. 

The LOC for occupational dermal exposure (defined as an MOE = 100) differs from the 

LOC for occupation inhalation exposure (defined as an MOE = 30). An aggregate risk 

index (ARI) is required to combine or aggregate risk from differing routes of exposure 

when the LOCs are not the same. An ARI ≥ 1 is not of concern to HED. 

2.3.2 Commercial Seed Treatment Exposure and Risk Calculations

Commercial seed treatment exposure and risk was calculated using the formulas and MOE 

approach described in Section 2.1.4: Occupational Handler Exposure and Risk Calculations. It 

should be noted that for commercial seed treatment, the application rate is presented in units, lbs 

ai/lb seed and daily amount handled is presented in units, lbs seed/day. All other inputs remain 

as is described in the previous Section. 
Page 54 of 107

2.3.3 Commercial Seed Treatment and Exposure Risk Summary

The majority, 61 of 64, occupational handler seed treatment exposure scenarios assessed 

(combined dermal and inhalation) resulted in risk estimates which were not of concern to HED 

(i.e., an ARI > 1) at some level of personal protection. The remaining 3 exposure scenarios 

resulted in an ARI < 1 at all level of personal protection considered and, therefore, are of concern 

to HED. All seed planter (secondary handler) combined short- and intermediate-term dermal and 

inhalation exposure scenarios assessed resulted in an ARI > 1 at some level of personal 

protection and, therefore, are not of concern to HED.

Commercial seed handler combined short- and intermediate-term dermal and inhalation exposure 

and risk for each exposure scenario is presented in Appendix C/Table 1 which accompanies this 

document. Secondary handler combined short- and intermediate-term dermal and inhalation 

exposure and risk is presented in Appendix C/Table 2 which accompanies this document. 

2.3.4 Commercial Seed Treatment and Exposure Risk Characterization

HED is currently in the process of revising ExpoSAC Policy No. 15: Amount of Seed Treated or 

Planted per Day (March 2, 2004) in efforts to update the daily amount of seed treated in 

commercial seed treatment facilities. With the increasing demand for hybridized treated seeds, 

there have been modifications not only in the seed treatment equipment, but in the market shares 

of new and currently registered pesticide products. HED understands that due to the high cost of 

producing hybrid seeds, direct seeding is done in a precise manner which requires lower volumes 

of seed per acre. However, this does not provide a direct correlation to the amount of seed which 

is treated in the commercial seed treatment facilities. Over the past several years, HED has 

received submissions from Industry for the use of several seed treatment pesticides which 

provide a range of daily amounts of vegetable seeds treated (1,000 to 5,500 lbs). The amount 

chosen for cucumber and pumpkin seeds (5,500 lbs) is assumed to be conservative.

2.4 Occupational Post-application Exposures and Risks

HED uses the term post-application to describe exposures to individuals that occur as a result of 

being in an environment that has been previously treated with a pesticide (also referred to as 

reentry exposure). HED believes that there are distinct job functions or tasks related to the kinds 

of activities that occur in previously treated areas. Job requirements (e.g., the kinds of jobs to 

cultivate a crop), such as the nature of the crop or target that was treated, and how the chemical 

residues degrade in the environment can cause exposure levels to differ over time. Each factor 

has been considered in this assessment.

2.4.1 Occupational Post-application Exposure Scenarios

Chlorpyrifos uses are varied as it can be used on agricultural crops and in a variety of other 

outdoor occupational settings, as well as indoor greenhouses. As a result, a wide array of 

individuals can potentially be exposed by working in areas that have been previously treated. 

HED is concerned about the kinds of exposures one could receive in the workplace. 
Page 55 of 107

When assessing post-application exposures to agricultural crops, HED uses a concept known as 

the transfer coefficient (TC) to numerically represent the post-application exposures an 

individual would receive (generally presented as cm2/hour). The TC concept has been 

established in the scientific literature and through various exposure monitoring guidelines 

published by the U.S. EPA and international organizations. The relationship or correlation 

between field residues and exposure (the statistical and mathematical underpinning of transfer 

coefficients) was then investigated in the 1980s (USEPA, 1984; USEPA 1998, Popendorf et al, 

1980; Popendorf et al, 1982; Zweig et al 1984; Zweig et al 1985). Conceptually, a transfer 

coefficient may be thought of as a „contact factor‟ determining a worker‟s exposure depending 

on how long they work, what activity they are doing, and how much residue is available for 

contact and transfer. The establishment of TCs also forms the basis of the work of the 

Agricultural Reentry Task Force. Mathematically, a transfer coefficient is expressed as follows:

TC for a given crop / activity 

sq. cm

hour

Dermal Exposure 

m g

day

Time spent doing activity 

hours

day

* Dislodgeable Foliar Residue 

m g

sq. cm

The transfer coefficient is essentially a measure of the contact with a treated surface one would 

have while doing a task or activity. These values are defined by calculating the ratio of an 

exposure for a given task or activity to the amount of pesticide on leaves, or other surfaces, that 

can rub off on the skin resulting in an exposure. For post-application exposures, the amounts of 

pesticide that can rub off on the skin are measured using techniques that specifically determine 

the amount of residues on treated leaves or other surfaces (referred to as transferable residues), 

rather than the total residues contained both on the surface and absorbed into treated leaves

(Iwata, 1977). HED has developed a series of standard TCs that are unique for a variety of job 

tasks or activities that are used in lieu of chemical- and scenario-specific data.

As is done for occupational handler risk assessment, the first step in the post-application risk 

assessment process is to identify the kinds of individuals that are likely to be exposed to 

chlorpyrifos after application. Because of U.S. agricultural diversity and in order to do this in a 

consistent manner, the Agency has adopted a method of clustering crops, crop growth stages, and 

post-application activities into groups that are expected to result in comparable exposure. The 

Agency also considers whether individuals are exposed to pesticides as part of their employment 

(referred to as occupational risk assessments). Common examples include: agricultural 

harvesting and scouting activities, crop maintenance tasks such as irrigating and weeding, and 

turf maintenances such as mowing. The next step in the risk assessment process is to define how 

and when chemicals are applied in order to determine the level of transferable residues to which 

individuals could be exposed over time. Wherever available, use and usage data are included in 

this process to define values such as application rates and application frequency. HED always 

completes risk assessments using maximum application rates for each scenario because what is 

possible under the label (the legal means of controlling pesticide use) must be evaluated, for 

complete stewardship, in order to ensure HED has no concern for the specific use. Additionally, 

whenever HED has additional information, such as typical or average application rates or 

frequency data, it uses the information to further evaluate the overall risks associated with the 

use of the chemical. In order to define the amount of transferable residues to which individuals 
Page 56 of 107

can be exposed, HED relies on chemical- and crop-specific studies as described in HED 

guidelines for exposure data collection (Series 875, Occupational and Residential Exposure Test 

Guidelines: Group B - Post-application Exposure Monitoring Test Guidelines). HED has also 

developed a standard modeling approach that can also be used to predict transferable residues 

over time in lieu of chemical- and scenario-specific data (best described in HED‟s SOPs for

Residential Exposure Assessment). All occupational post-application exposure scenarios were 

evaluated using chlorpyrifos-specific DFR dissipation data.

Next, assessors must understand how exposures to chlorpyrifos occur (i.e., frequency and 

duration) and how the patterns of these occurrences can alter the effects of the chemical in the 

population after being exposed (referred to as dose response). This is supported by the fact that 

several areas within a work environment may be treated at different times. For example, parts of 

agricultural fields might be treated over several weeks, because the projected harvest date is 

different for the different fields. Therefore, individuals working in those fields or exposed to 

might be exposed from contact with treated areas over an extended period of time that could be 

categorized as an intermediate-term exposure. In these situations, typically both short-term 

intermediate-term dermal exposures and risks are considered. Long-term post-application 

exposure durations to chlorpyrifos are not anticipated.

Based on the Agency's current practices, a quantitative post-application inhalation exposure 

assessment is not typically performed for a chemical when it is characterized by low acute 

inhalation toxicity (Toxicity Category III and IV) and low vapor pressure. Chlorpyrifos does not 

fit into these categories as it is classified as Toxicity Category II for inhalation toxicity and has a 

moderate vapor pressure of 1.9 x 10-5 mm Hg at 25o C. The inhalation exposure potential from 

occupational/commercial post-application activities may be elevated based upon these criteria. 

A quantitative occupational post-application inhalation exposure assessment was not performed 

for chlorpyrifos; however, an inhalation exposure assessment was performed for 

occupational/commercial handlers. It is expected that while many of these handler inhalation 

exposure estimates are of concern to HED, exposure and risk from occupational post-application 

inhalation would be of no greater concern than occupational handler inhalation estimates. 

The use of personal protective equipment or other types of equipment to reduce exposures for 

post-application workers is not considered a viable exposure/risk mitigation strategy for the 

regulatory process. This is described in some detail in EPA‟s Worker Protection Standard 

(40CFR170). As such, an administrative approach is used by HED to reduce the risks and is 

referred to as the Restricted Entry Interval or REI. The REI is the time period follow a pesticide 

application during which entry into the treated area is restricted. REIs from post-application risk 

levels are generally calculated in the risk assessment process on a chemical-, crop-, and activityspecific basis. To establish REIs, post-application risks are considered on varying days after an 

application. REIs for chlorpyrifos uses are currently based upon the most recent assessment of 

post-application exposure conducted in June 2000 (T. Leighton, D63893), “Agricultural and 

Occupational Exposure Assessment and Recommendations for the Reregistration Eligibility 

Decision Document for Chlorpyrifos.” HED has used the basic approach described above since 

the mid 1980s for calculating post-application risks to pesticides. From that time to the present, 

several revisions and modifications were made to HED policies as data which warranted such 

changes became available. In 1995, HED issued a Data Call-In for post-application agricultural 
Page 57 of 107

data that prompted the formation of the Agricultural Reentry Task Force (ARTF). In 2001 the

ARTF submitted final reports on 27 worker reentry exposure studies, numerous studies in 

support of the surrogate compounds used in the reentry studies (method validation, field stability, 

storage stability, etc.) and a report on its comprehensive survey of growers

(www.exposuretf.com). As a result of the need for the revision and using the latest data, HED 

developed a revised policy entitled the Science Advisory Council for Exposure (ExpoSAC) 

Policy Number 3.2: Agricultural Transfer Coefficients (5/5/11). The revision to this policy 

entailed linking worker activities to more specific crop/agronomic groupings and making better 

use of the available occupational post-application exposure data. HED used the updated policy 

to develop scenarios for occupational post-application exposures. Chlorpyrifos uses were 

identified in the following crop groupings from the policy:

Low Berry (e.g., strawberry, cranberry);

 Field/row crops, low/medium (e.g., alfalfa, clover, soybean);

Field/row crops, tall (e.g., corn, sorghum, sunflowers);

Trees/fruit, deciduous (e.g., apples, pears);

Trees/fruit, evergreen (e.g., Christmas trees, citrus); 

Trees/nut (e.g., almonds, filberts, pecans, walnuts);

Turf/sod (e.g., golf courses, sod farms);

 Ornamentals/nurseries (e.g., conifers, non-bearing trees, ornamentals);

Vegetable/root (e.g., carrot and radish (grown for seed));

Vegetable/fruiting (e.g., pepper);

Vegetable/head and stem brassica (e.g., broccoli, cabbage, cauliflower);

Vegetables/leafy (e.g., bok choy, collard greens, kale, kohlrabi);

 Vegetables/stem and stalk (e.g., asparagus); 

 Vine/trellis (e.g., grapes); and

Nursery crops (e.g., bedding plants, woody ornamentals, potted flowers.)

Within each agronomic group, a variety of cultural practices are required to maintain the 

included crops. These practices are varied and typically involve light to heavy contact with 

immature plants as well as with more mature plants. HED selected transfer coefficient values in 

its revision of Policy 3.2 to represent this range of exposures within each agronomic group. 

2.4.2 Data/Assumptions for Post-application Exposure Scenarios

A series of assumptions and exposure factors served as the basis for completing the occupational 

post-application worker risk assessments for agricultural scenarios. Each assumption and factor 

is detailed below on an individual basis. In addition to these values, transfer coefficient values 

were used to calculate risk estimates. Several chemical-specific residue dissipation studies were 

were used in the development of the risk estimates. The transfer coefficients were taken from 

HED‟s revised policy entitled the Science Advisory Council for Exposure (ExpoSAC) Policy 

Number 3.2: Agricultural Transfer Coefficients. The assumptions and factors used in the risk 

calculations are presented below:

There are many factors that are common to handler and post-application risk assessments 
Page 58 of 107

such as body weights, duration, and ranges of application rates. Please refer to the 

assumptions and factors in Section 2.1.1: Assumptions for Handler Exposure Scenarios 

for further information concerning these values which are common to both handler and 

post-application risk assessments. In the post-application risk assessment, generally only 

maximum application rates were considered because of the complexity of the 

calculations.

A total uncertainty factor of 100X was applied to the endpoint selected for short- and 

intermediate-term dermal exposures [10X for interspecies extrapolation, 10X for 

intraspecies variation and a 1X FQPA factor (residential)].

The available dislodgeable foliar residue (DFR) and turf transferable residue (TTR) data 

for chlorpyrifos were used to complete all post-application risk assessments involving 

agricultural crops and turf grass. The chemical-specific residue data are described in

detail below in Section 2.4.2.1. 

Exposure calculations reflect chemical-specific residue dissipation rates over time 

coupled with surrogate transfer coefficients as outlined in HED‟s revised policy. 

Chlorpyrifos is used on numerous agriculture crops but only 7 residue studies were 

submitted that meet current HED guidelines for sampling techniques and data quality. 

A pseudo-first order kinetics analysis was used to analyze chlorpyrifos residue 

dissipation over time as outlined in EPA‟s draft Series 875 Post-application Exposure 

Monitoring Guidelines. A more sophisticated curve-fitting approach is not warranted 

because any possible refinements associated with such an approach would be 

inappropriate to apply broadly to all crops and associated activities. 

When extrapolating the available DFR data to other crops, HED adjusted (normalized) 

the data for differences in application rate using a simple proportional approach. This 

approach seems to be the most appropriate given the data which are available and is 

commonly practiced in HED post-application risk assessments.

Occupational post-application exposure were calculated assuming a central tendency 

workday duration estimate of 8 hours per day as reviewed by the FIFRA SAP in 2008.

Commercial Seed Treatment

The potential for post-application exposures following the planting of chlorpyrifos-treated seeds 

is unlikely because sustained levels of contact with treated seed after it has been placed in the 

soil or other planting media would not be expected because no routine cultural practice required 

for the production of agricultural commodities involves such an activity as defined in the 

“no/low contact” criteria in the Worker Protection Standard (WPS). Therefore, no quantitative 

post-application assessment is required for exposure to treated seeds that have already been 

planted.
Page 59 of 107

Occupational Post-application Inhalation 

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, 2010 

(http://www.epa.gov/scipoly/SAP/meetings/2009/120109meeting.html). The Agency is in the 

process of evaluating the SAP report as well as available post-application inhalation exposure 

data generated by the Agricultural Reentry Task Force and may, as appropriate, develop policies 

and procedures, to identify the need for and, subsequently, the way to incorporate occupational 

post-application inhalation exposure into the Agency's risk assessments. 

However, based on the Agency's current practices, a quantitative post-application inhalation 

exposure assessment is not typically performed for a chemical when it is characterized by low 

acute inhalation toxicity (Toxicity Category III and IV) and low vapor pressure. Chlorpyrifos 

does not fit into these categories as it is classified as Toxicity Category II for inhalation toxicity 

and has a moderate vapor pressure of 1.9 x 10-5 mm Hg at 25o C. The inhalation exposure 

potential from occupational/commercial post-application activities may be elevated based upon 

these criteria. A quantitative occupational post-application inhalation exposure assessment was 

not performed for chlorpyrifos; however, an inhalation exposure assessment was performed for 

occupational/commercial handlers. It is expected that while many of these handler inhalation 

exposure estimates are of concern to HED, exposure and risk from occupational post-application 

inhalation would be of no greater concern than occupational handler inhalation estimates. 

Chlorpyrifos can be used in indoor facilities as well as agricultural/commercial outdoor uses. 

Indoor use sites for chlorpyrifos include greenhouse use, indoor commercial uses (e.g., 

warehouses, indoor industrial sites) and commercial seed treatment facilities. HED has not 

assessed post-application inhalation exposure for greenhouses due to requirements for high air 

exchange rates and ventilation regulations. The Worker Protection Standard for Agricultural 

Pesticides contains requirements for protecting workers from inhalation exposures during and 

after greenhouse applications through the use of ventilation requirements [40 CFR 170.110, (3) 

(Restrictions associated with pesticide applications)]. Furthermore, HED assumes that 

commercial applicators do not typically return to the treated areas after an indoor use site 

pesticide application and thus an occupational post-application inhalation exposure assessment 

was not performed for commercial applicators. Seed treatment assessments provide quantitative 

inhalation exposure assessments for seed treaters and secondary handlers (i.e. planters). It is 

expected that these exposure estimates would be protective of most post-application inhalation 

exposure scenarios

2.4.2.1 Post-application Exposure Study Summaries

The registrant submitted 7 chemical-specific dislodgeable foliar residue (DFR) studies in support 

of the re-registration of chlorpyrifos. The studies encompass the use of 5 different formulations 

and 12 different crops. Specifically, the DFR studies examine the use of 1) emulsifiable 
Page 60 of 107

concentrate formulations on sugarbeets, pecans, citrus, sweet corn, cotton, and turf; 2) wettable 

powder formulations on almonds, apples, pecans, cauliflower, tomato and turf; 3) granular 

formulations on sweet corn and turf; 4) a total release aerosol formulation on ornamentals; and 5) 

a microencapsulated formulation on ornamentals. The submitted studies were reviewed by HED. 

Despite limitations (see summary of all DFR and TTR studies in Appendix D which 

accompanies this document), HED recommended the use of all or some of the data in the studies 

to assess post-application risks to chlorpyrifos except for the tomato DFR data. More 

information on the available DFR studies is summarized below. 

For each DFR data set, the raw data are corrected for field and laboratory fortification recoveries 

as appropriate. The data are then log-transformed. A semi-log regression analysis is performed 

on the log-transformed DRF data (i.e., loge DFR vs time) using individual residue data (unless 

only averages are available) collected from the first interval after the last application through 

either the last day of sampling or out to the first sampling interval where all or the majority of the 

residues are below the level of quantification (LOQ). Residues which are <LOQ are assigned a 

value of 1⁄2 LOQ for calculation purposes. From the regression analysis, a dissipation rate (slope) 

and predicted dislodgeable foliar residue data for each site and crop is determined. For most 

trials, the actual DFR values were significantly higher than the predicted DFR values in the first 

sampling intervals after the application. In these cases using the predicted values in the risk 

assessment would underestimate exposure. Therefore, in the risk assessment, the predicted DFR 

value was replaced with the actual DFR value in the risk calculations.

MRID 42974501 (Cauliflower and Tomato)

This DFR study (MRID 42974501), performed in conjunction with a passive dosimetry and 

biomonitoring study, examined the residues of chlorpyrifos on cauliflower and tomato. Three 

test sites were established in Arizona for the cauliflower trials and three tests sites were 

established in Florida for the tomato trials. Each test plot received a single application of 

Lorsban® 50W (a wettable powder formulation) using tractor-mounted spray equipment at a 

nominal rate of 1 lb ai/A. Samples were taken from the control plot and treated plots using a 

Birkenstrand leaf punch (producing leaf discs approximately 2.5 cm2in area) at the following 

nominal sampling intervals: the day prior to application, following the application, and at day 1, 

2, 3, 5, 7, 14, 21, and 30 after the application. A total of 50 leaf discs were collected for each 

sample, totalling 250 cm

2

(front and back of 2.5 cm2singe puch x 50 leaf punches). The residues 

found within a given plot at a given time interval was the average of 3 samples collected within 

that plot. Samples were analyzed for residues of chlorpyrifos. A validated LOQ or LOD was not 

provided. Concurrent and method validation samples were analyzed. The average recoveries 

were 110±15.4% and 104±10.3%, respectively. The study corrected the field study samples for 

method validation recoveries, only if the recoveries were <100%. Field fortification data for the 

tomato sites were extremely variable, with a range of 12.7% to 990%. The study report suggests 

that either unknown interferences or poor techniques were responsible for the variability. For 

cauliflower, the overall average recovery was 99.2%. The field study samples were not 

corrected for field fortification recovery. 

The DFR values for cauliflower and tomatoes were reported as an average of three samples. 

Raw data was not provided. The DFR data for tomatoes were not used in the assessment due to 

uncertainty in the results based on the extremely variable field fortification recoveries and also 
Page 61 of 107

because data from two of the sites were erratic. The three cauliflower sites were located in 

Arizona and have similar dissipation rates, therefore average daily DFR values were combined 

for the three sites and the regression was performed on the combined DFR values. A summary 

of the actual and predicted DFR values can be found in Appendix D/Tables 1 and 2 (which 

accompany this document), respectively. The result of statistical analysis for the individual and 

combined cauliflower DFR data from the 3 Arizona sites after groundboom application is 

presented in Table 2.4.2.2a below.

Table 2.4.2.2a. Cauliflower DFR Dissipation Data After Groundboom Application (MRID

42974501)

Lorsban®50W (Wettable Powder)

AZ - Site ABC AZ - Site DEF AZ - Site GHI AZ – All Sites

r

2

0.99 0.93 0.93 0.93

Slope (Ln DFR vs. t) -0.19 -0.198 -0.23 -0.21

T1/2 (days) 3.59 3.49 2.98 3.33

Notes: The nominal application rate was 1 lb ai/A. 

MRID 43062701 (Citrus)

This DFR study (MRID 43062701), performed in conjunction with a worker exposure and 

biomonitoring study, examined the residues of chlorpyrifos on citrus. Three test sites were 

established in California (Site No. 2 and 5 in Tulare County and Site No. 6 in Kern County). 

Lorsban® 4E Insecticide (an emulsifiable concentrate formulation), was applied using an airblast 

spray application to citrus trees. One application was made at each test site at a nominal rate of 6 

lb ai/A. At each sampling interval, duplicate samples were taken from the control plots and 

duplicate samples from three subplots were taken from the treated plots using a Birkenstrand leaf 

punch. Leaf samples were collected prior to and following the application and at 1, 4, 5, 7, 14, 

21, 35, and 43 days after the application for Site No. 2, prior to and following the application and 

at 1, 2, 7, 14, and 40 days after the application for Site No. 5 and prior to and following the 

application and at 1, 2, 7, 14, 21, and 35 days after the application for Site No. 6. Samples were 

analyzed for residues of chlorpyrifos. The overall average concurrent recovery for Site No. 2 was 

101% ± 17.2% with recoveries ranging from 67.8% to 126% (n=26). The overall average 

concurrent recovery for Site No. 5 and No. 6 was 91.8% ± 10.1% with recoveries ranging from 

76.6% to 112% (n=29). A validated LOQ was not provided. The limit of detection (LOD) was 

set at approximately three (3) times the signal noise ratio for an analytical run. The LOD ranged 

from 0.0032 to 0.016 μg/cm2. The field study samples did not require correction for field 

fortification recovery. Average field fortification recoveries were greater than 100% for Site No. 

5 and No. 6. Field fortification data from Site No. 2 indicated that the spiking solutions had 

degraded causing erratic data for field fortification recoveries, therefore, these recoveries were 

not used to correct the raw residue data and Site No. 5 and Site No. 6 field fortification 

recoveries were applied to Site No. 2 DFRs. 

Actual average DFR values are provided in Appendix D/Table 3 which accompanies this 

document. The chlorpyrifos residues on leaves demonstrated a biphasic pattern of dissipation. 

Residue levels dropped dramatically in the first two days, and then dissipation of chlorpyrifos 

was gradual. 
Page 62 of 107

Because all the sampling sites are located in California, the average DFR values for each day at 

each site were used to plot a regression for all the sites combined. Predicted DFR values

calculated from the regression of the individual and combined sites are presented in Appendix

D/Table 4. The result of statistical analysis for the individual and combined citrus DFR data 

from the 3 California sites after airblast application is presented in Table 2.4.2.2b below.

Table 2.4.2.2b. Citrus DFR Dissipation After Airblast Application (MRID 43062701)

Lorsban® 4EC (Emulsifiable Concentrate)

CA - Site No. 2 CA - Site No. 5 CA - Site No. 6 Combined CA Sites 2, 5, 

& 6

r

2

0.58 0.55 0.61 0.67

Slope

(Ln DFR vs. t) -0.13 -0.14 -0.15 -0.15

T1/2 (days) 5.27 4.83 4.56 4.76

Notes: The nominal application rate was 6 lb ai/A.

MRID 44748101 (Apples, Almonds, Pecans)

This DFR study (MRID 44748101) examined the residues of chlorpyrifos on apple, almond, and 

pecan trees. Three test sites were established for each crop in the major crop production areas (1 

apple trial each in CA, NY and WA; 3 almond trials in CA; 1 pecan trial each in GA, LA and 

TX). One treatment plot for the application of Lorsban® 50W (wettable powder) was 

established at all trial sites. For pecans, an additional treatment plot was established for 

application of Lorsban® 4E (emulsifiable concentrate). The test products were applied as two 

airblast spray applications, each at a nominal rate of 1.5 lb ai/A for apple trees and at 2.0 lb ai/A 

for almond and pecan trees. The re-treatment intervals were 7 days for apple trees, 15 days for 

almond trees, and 7 days for pecan trees. Applications were made in a spray volume rate of 

approximately 89-109 gallons per acre. At each sampling interval a sample was taken from the 

control plot and triplicate samples were taken from the treated plot using a Birkenstrand leaf 

punch. Leaf samples were collected at nominal sampling intervals of prior to each application, at 

days 0, 2, and 5 days after the first application, and at 0, 1, 2, 4, 7, 14, 21, 28 and 35 days after 

the second application. Samples were analyzed for residues of chlorpyrifos. Concurrent 

recoveries fell within the acceptable range of 70 - 120 percent. The LOD and LOQ were 

reported as 0.001 μg/cm2and 0.003 μg/cm2respectively. The overall average recoveries of field 

fortified samples for apple, almond, and pecan leaves ranged from 73 to 77 percent for the 5 

μg/sample level and from 79 to 87 percent for the 50 μg/sample level. DFR values were 

calculated by adjusting the residues for site-specific field fortification recoveries when the 

recoveries were less than 90%. The adjustment was made using the average recovery for the 

fortification level closest to the residue level. 

Actual average DFR values are provided in Appendix D/ Table 5 which accompanies this 

document. Two of the almond sites (CA2 and CA3) were located in California and have similar 

dissipation rates. Therefore, average daily DFR values were combined for those two sites and 
Page 63 of 107

the regression was performed on the combined DFR values. The remaining almond site (CA4) 

was also located in California, but had much slower rate of dissipation from the other two 

almond sites. Since this may be reflect of different climatological conditions (i.e., more arid 

climate), the DFR values from this site were not combined with the other two almond sites. The 

predicted values calculated from the regressions are provided in Appendix D/Table 6. The result 

of statistical analysis for the individual apple and pecan crop sites and the individual and 

combined citrus DFR data from the 3 California sites after airblast application is presented in 

Table 2.4.2.2c below.

Table 2.4.2.2c. Apple, Almond and Pecan DFR Dissipation After Airblast Application 

(44748101)

Lorsban® WP

(Wettable Powder)

Lorsban® 4EC

(Emulsifiable 

Concentrate)

Apple Almonds Pecans Pecans

CA NY WA CA2 & CA3 CA4 GA LA TX GA LA TX

r

2

0.77 0.93 0.80 0.82 0.6047 0.99 0.80 0.97 0.65 0.80 0.95

Slope

(Ln DFR 

vs. t)

-0.31 -0.61 -0.52 -0.33 -

0.0873 -0.37 -0.25 -1.70 -0.96 -0.24 -0.61

T1/2 (days) 2.22 1.13 1.35 2.09 7.94 1.87 2.78 0.41 0.72 2.93 2.25

Notes: The nominal application rate was 1.5 lb ai/A for apple trees (total of 3 lb ai/A) and 2.0 lb ai/A for almond 

and pecan trees (total of 4 lb ai/A).

MRID 44748102 (Cotton, Sugarbeet, Sweet Corn)

This DFR study (MRID 44748102) examined the residues of chlorpyrifos on cotton, sugar beet, 

and sweet corn after application of either Lorsban® 4E (emulsifiable concentrate), Lock-on 

(emulsifiable concentrate), or Lorsban® 15G (granular). Lorsban® 4E trials were conducted on 

all crops. Lock-on trials were conducted on cotton only and Lorsban® 15G trials were 

conducted on sweet corn only. Three test sites were established for each crop for each 

formulation tested. Cotton trials were conducted in California, Texas, Mississippi (Lorsban® 4E 

only) and Arizona (Lock-on only). Sugar beet trials were conducted in California, Oregon, and 

Minnesota. Sweet corn trials were conducted in Oregon, Minnesota, and Illinois. Each test site 

received two broadcast foliar applications of a test product spaced five days apart. The 

application rate used for Lorsban® 4E on all crops was 1 lb ai/A. Lock-on was applied to cotton 

at 0.5 lb ai/A and Lorsban® 15G was applied to sweet corn at 1 lb ai/A. At each sampling 

interval a sample was taken from the control plot and triplicate samples were taken from the 

treated plot using a Birkenstrand leaf punch. Leaf samples were collected at sampling intervals 

of prior to the first application, immediately following the application (within 1-2 hours or as 

soon as the spray had completely dried or dust had settled on the foliage), and at 1, 2, and 4 days 

after the first application. Leaf punch samples were also collected immediately following the 

second application and at 1, 2, 4, 7, 14, 21, and 28 days after the second (final) application. 

Samples were analyzed for residues of chlorpyrifos. Concurrent recoveries fell within the 

acceptable range of 70 - 120 percent. The LOD and LOQ were reported as 0.001 μg/cm2and 

0.003 μg/cm2respectively. Field fortifications were collected on three sampling events at each 
Page 64 of 107

test. Average field fortification recoveries (not corrected for laboratory recovery), ranged from 

82% to 117%. DFR values were calculated by adjusting the residues for site-specific field 

fortification recoveries when the recoveries were less than 90%. The adjustment was made using 

the average recovery for the fortification level closest to the residue level. 

Actual average DFR values are provided in Appendix D/Table 7 which accompanies this 

document. Predicted values calculated from the regressions are provided in Appendix D/Table 

8. A regression was not conducted for the granular application on sweet corn at the Minnesota 

site because all DFR values were <LOQ. The result of statistical analysis for the individual 

cotton, sugar beet and sweet corn crop sites after ground application is presented in Table 

2.4.2.2d below.

Table 2.4.2.2d. Cotton, Sugarbeet and Sweet Corn DFR Dissipation After Ground Application (MRID 

44748102)

Cotton Sugar beet Sweet Corn

Lorsban® 4E 

(Emulsifiable 

Concentrate)

Lock-On (Emulsifiable 

Concentrate)

Lorsban® 4E 

(Emulsifiable 

Concentrate)

Lorsban® 4E 

(Emulsifiable 

Concentrate)

Lorsban® 15G

(Granular)

CA TX MS CA AZ TX CA OR MN IL MN OR IL OR

r

2

0.82 0.69 0.87 0.65 0.767 0.539 0.74 0.36 0.83 0.69 0.83 0.82 0.440 0.645

Slope

(Ln 

DFR 

vs. t)

-0.40 -0.91 -3.21 -0.29 -1.458 -0.170 -0.35 -0.61 -0.43 -0.95 -0.78 -1.33 -0.498 -1.014

T1/2

(days) 1.73 0.76 0.22 2.36 0.475 4.073 1.97 1.13 1.62 0.73 0.89 0.52 1.392 0.684

Notes: Application rates were 1.0 lb ai/A for Lorsban® 4E and Lock-On (total of 2 lb ai/A) and 0.5 lb ia/A for 

Lorsban® 15G (total of 1 lb ai/A).

MRID 44829601 (Turf) 

This TTR study (MRID 44829601) examined the residues of chlorpyrifos on turf after 

application of Dursban®® Pro (an emulsifiable concentrate), Dursban®® 50W (wettable 

powder), Dursban® 2.5G (granular) and Dursban® 1F (flowable granular). Three test sites were 

established in California, Indiana, and Mississippi, with one test plot for each test substance. 

Single applications of the test products were made to the treatment plots using tractor mounted 

boom sprayers for the liquid applications and either a tractor mounted air spreader or tractor 

mounted spreader for the granular applications. The liquid applications were made at a nominal 

application ate of 4 lb ai/A and the granular applications were made at a nominal application rate 

of 2 lb ai/A. Actual application rates, determined by analyzing filter paper anchored to the test 

plots for liquid applications and placing plastic pans in the test plots for granular applications,

ranged from 95% to 100% for the Dursban® Pro application, 97% to 102% for the Dursban®

50W application, 91% to 95% for the Dursban® 2.5G application and 77% to 86% for the 

Dursban® 1F application. Duplicate samples were collected using the California roller technique 

at the following intervals: one day prior to the application, just after the application (-2.5 hours 

after application), and days 1, 2, 4, and 7 after application. Samples were analyzed for residues 
Page 65 of 107

of chlorpyrifos. The overall average concurrent recovery was 98%±3%. The LOD and LOQ 

were reported as 0.001 μg/cm2and 0.003 μg/cm2respectively. The field study samples did not 

require correction for field fortification recovery. Average field fortification recoveries 

(corrected for concurrent recovery), at each of three fortification levels were greater than 90% at 

all sites. 

Actual average TTR values (corrected for concurrent recovery) are provided in Appendix 

D/Table 9 and predicted values calculated from the regressions are provided in Appendix 

D/Table 10, both which accompany this document. Regression analysis could not be conducted 

for the trials with Dursban® 2.5G because all residues were <LOQ and with Dursban® 1F trials 

in Indiana and Mississippi because there were not enough data points above the LOQ. The result 

of statistical analysis for the individual turf sites after ground application is presented in Table 

2.4.2.2e below.

Table 2.4.2.2e. Turf TTR Dissipation After Ground Application (44829601)

Predicted DFR (μg/cm2)

Dursban® Pro

(Emulsifiable Concentrate)

Dursban® 50W

(Wettable Powder)

Dursban®

1F

(Granular)

CA IN MS CA IN MS CA

r

2

0.94 0.92 1.00 0.851 0.99 0.98 0.84

Slope

(Ln DFR vs. t) -0.86 -1.44 -2.79 -1.486 -1.24 -2.09 -0.712

T1/2 (days) 0.81 0.48 0.25 0.467 0.56 0.33 0.97

Notes: Application rates were a nominal 4 lb ai/A for liquid applications (Dursban® Pro and Dursban® 50W ) and 

2.0 lb ai/A for granular applications (Dursban® 2.5G and Dursban® 1F).

MRID 46722701 (Greenhouse Ornamental - Total Release Aerosol)

This DFR study (MRID 46722701) examined the residues of chlorpyrifos on smooth leaf 

(Philadendron) and hairy leaf (African Violet) ornamentals in greenhouses located in Columbia, 

Missouri after three applications of Duraplex TRTM, a total release aerosol product containing 

8.0% chlorpyrifos and 1.6% cyfluthrin. The applications were made at a rate of 2.0 oz 

formulated product per 1,500 ft2(0.008 lb ai/1,000 ft2), using 7 ± 1-day intervals. The 

applications were made according to label recommendations by placing a single aerosol canister 

in the greenhouse and releasing the entire contents at each application. Treated and control leaf 

punch samples were collected the afternoon of the day prior to each application, four hours after 

each application, one day after application numbers 1 and 2, eight hours after the last (third) 

application, and on days 1, 2, 3, 5, 7, 10, 14, 21, 28, and 35 days after the last (third) application. 

Concurrent laboratory recoveries of smooth leaf samples ranged from 93.3% to 153% with an 

overall mean recovery of 116% ± 20.4%. Fortification recoveries for hairy leaf samples ranged 

from 93.9% to 151% with an overall mean recovery of 103% ± 13.5%.The overall mean field 

fortification recoveries for the smooth leaf and hairy leaf ornamentals were 96% and 87%, 

respectively. Only the raw residue values for hairy leaf ornamentals required correction for an 

overall field fortification recovery <90%. 
Page 66 of 107

Actual average DFR values are provided in Appendix D/Table 11 and predicted values 

calculated from the regressions are provided in Appendix D/Table 12 for hairy ornamentals, both 

which accompany this document. For both plant types, dislodgeable foliar residues increased 

initially (to a maximum on 2 days after the last application for the smooth leaf ornamentals and 8 

hours after the last application for the hairy leaf ornamentals) and then declined to a relatively 

steady state until the end of the monitoring period. The average residues on the hairy leaf 

ornamentals, however, showed a slight increase at the end of the monitoring period. The 

residues for both plants were very similar, with the hairy ornamental exhibiting slightly higher 

DFR values. Therefore, only the hairy ornamental DFR data was using for risk assessment 

purposes. The result of statistical analysis for the hairy ornamental after total release aerosol 

application is presented in Table 2.4.2.2f below.

Table 2.4.2.2f. Ornamental (Hairy) DFR Dissipation After Total Release Aerosol 

Application (MRID 46722701)

Duraplex TRTM (Total Release Aerosol)

r

2

0.649

Slope

(Ln DFR vs. t) -0.127

T1/2 (days) 5.47

Notes: Application rate was a nominal 0.008 lb ai/1,000 ft2

MRID 46722702 (Greenhouse Ornamental - Microencapsulated)

This DFR study (MRID 46722702) examined the residues of chlorpyrifos on smooth leaf 

(Philodendron) and hairy leaf (African Violet) ornamentals in greenhouses located in Columbia, 

Missouri after three applications of DuraGuard METM insecticide, a microencapsulated 

formulation containing 20% (w/w) chlorpyrifos as the active ingredient. The applications were 

made at a rate of 0.5 oz. of DuraGuard METM per gallon of water at a rate of 100 gallons per acre 

(0.626 lbs ai per acre), using 14 ± 1-day intervals. The applications were made using a portable 

gasoline powered sprayer at a height of approximately 24 inches above the canopy. Leaf disk 

samples were collected from the untreated control ornamentals and the treated ornamentals on 

the following schedule: immediately before each application, immediately after the spray had 

dried following each application, one day after application number 1 and 2, and on 0.17 (4 

hours), 0.33 (8 hours), 1, 2, 3, 5, 7, 10, 14, 21, 28, and 35 days after the third application. For 

smooth leaf ornamentals, laboratory recoveries ranged from 73 to 134% with an overall mean 

recovery of 97 ± 19%. For hairy leaf ornamentals, laboratory recoveries ranged from 77 to 

113% with an overall mean recovery of 92 ± 11%. The initial set of field fortification samples 

resulted in poor recoveries (<70%) due to problems associated with the fortification technique. 

Subsequently, control samples were fortified again and analyzed after 15 and 190 days to assess 

storage stability. Since only two samples at one concentration were analyzed, it is uncertain if the 

recoveries obtained from the samples are representative of the field and storage conditions; 

therefore, the field data were not corrected for the recovery results.

Actual average DFR values are provided in Appendix D/Table 13 which accompanies this 

document. Chlorpyrifos persisted on both smooth and hairy leaf ornamentals with no rapid 
Page 67 of 107

decline prior to the completion of the monitoring period 35 days following the third application. 

A regression analysis was not performed because there was no substantial dissipation of 

chlorpyrifos residue.

2.4.3 Use of Post-application DFR Data

HED considered all DFR and TTR data as presented in the above summaries and determined the 

approaches most appropriate for its use. DFR data for an individual crop was applied to that 

specific crop, as well as to crops in the same crop grouping (e.g., cauliflower data was used for 

cauliflower and all other cole crops). For other crops in which no crop-specific or crop groupspecific data are available, the DFR data for the crop deemed the closest match were used as 

surrogates to calculate potential exposure (e.g., cauliflower data were also used for strawberries, 

cranberries, and leafy vegetables). Additionally, whenever possible, a label use was assessed 

using DFR data for the same formulation type. Granular, microencapsulated, and aerosol uses 

were only assessed using DFR data for the same formulation type. For crops with DFR data for 

both liquid concentrates and wettable powder (enclosed in water soluble packets) formulations, 

DFR data which resulted in the most conservative risk estimate were used unless one of the 

formulations was not registered for that crop. For crops where water dispersible granule 

(enclosed in water soluble packets) formulations are registered, DFR data from the liquid 

concentrates and wettable powder formulations were used as surrogates. If DFR data from 

multiple within-State field trial sites for an individual crop were not significantly different, these 

were combined. If determined significantly different, the most conservative of the within-State 

field trial sites were used for risk analysis (e.g., CA almond DFR study). 

DFR data from multiple states for the same crop were kept separate because these were 

considered to represent unique climatologic regions of the U.S. HED identified growing regions 

for chlorpyrifos use sites/crops by referencing the EPA Residue Chemistry Test Guidelines, 

OPPTS 860.1500, Crop Field Trials (1996), specifically, “Table 6 – Regional Distribution of 

Crop Production” and “Attachment 10, Border Definitions of Regions.” HED used a cut point of 

10% or greater crop production by region to determine those areas of the US where chlorpyrifos 

use is likely. This regional information was used in conjunction with the multi-state DFR data to 

recommend a best fit for the assessment of occupational post-application exposure and risk. For 

example, the sweet corn DFR study (MRID 44748102) had trial sites in Illinois, Minnesota and 

Oregon. This data source was selected as a close match for the assessment of post-application 

exposure from chlorpyrifos use on field corn and corn grown from seed. However, field corn 

and corn grown from seed are grown mostly in Region 5 of the US (which includes Michigan, 

Indiana, Illinois, Wisconsin, Minnesota, Iowa and parts of Ohio, West Virginia, Kentucky, 

Missouri, Kansas, Nebraska, South Dakota and North Dakota). Because the DFR data includes 

trials from the region, Illinois and Minnesota, these were recommended and used for assessment 

purposes, whereas the Oregon data was not identified for use. In the instance that state-specific 

DFR data did not result in a regional match, the most climatologically representative data source 

was selected. Further, if multiple DFR data sources selected for a region result in differing risk 

estimates, it is recommended that the more conservative of them be selected for the purpose of 

risk mitigation. Attachment 10, Border Definitions of Regions has been included in Appendix D 

(which accompanies this document) for reference. It includes a description of all states and areas 

within states included for US regional analysis. 
Page 68 of 107

No formulation-specific data is available for the assessment of emulsifiable concentrate (EC) 

greenhouse (nursery) uses of chlorpyrifos. Two DFR studies were submitted to the Agency by 

the registrant for microencapsulated and total release fogger formulation use in greenhouses. 

These studies result in MOE estimates below those estimated using DFR data measured from 

outdoor chlorpyrifos use and consequently, a significant increase in REIs. Due to the lack of 

DFR data for the EC formulation in greenhouses, HED has recommended the use of the total 

release fogger formulation greenhouse DFR study as a surrogate for all liquid formulation 

applications. HED determined that the microencapsulated formulation was not appropriate for 

use because of its time release characteristic. 

The DFR values from the submitted studies were adjusted proportionately to reflect the label 

application rates. The new DFR values are calculated as follows:

study application rate(lbsai/A)

Study DFR ( g / cm ) x crop application rate(lbsai/A) Adjusted DFR ( g / cm )=

2

2

The DFR data were extrapolated to the currently labeled crops as follows:

Cauliflower (MRID 42974501):

Berry: Low

Vegetable: Head and Stem Brassica

Vegetable: Leafy

Citrus (MRID 43062701):

Tree Fruit: Evergreen

Ornamental/Nurseries 

Vine/Trellis

Apple (MRID 44748101):

Tree Fruit: Deciduous (Pome and Stone Fruit)

Forestry

Ornamentals/Nurseries

Almond and Pecan (MRID 44748101):

Tree Nuts

Cotton (MRID 44748102):

Field and Row Crops: Low to Medium (Fiber, Legume Vegetable, Non-grass Animal Feeds) 

Vegetable: Fruiting
Page 69 of 107

Stalk and Stem Vegetable

Sugarbeet (MRID 44748102):

Field and Row Crops: Low to Medium

Ornamentals (Outdoor Only)

Vegetable: Root and Tuber

Stalk and Stem Vegetable

Sweet Corn – Emulsifiable Concentrate and Granular Formulation (MRID 44748102):

Field and Row Crops: Tall

Turf (MRID 44829601):

Turf

Greenhouse Ornamentals – Total Release Aerosol Formulation (MRID 46722701):

Greenhouse Use (All Liquid Formulations)

Greenhouse Ornamentals – Microencapsulated Formulation (MRID 46722702):

Greenhouse Use (Microencapsulated Formulation Only)

2.4.4 Occupational Post-application Exposure and Risk Summary

HED has assessed short- and intermediate term occupational post-application dermal exposure 

and risk for any crops which reentry into an area previously treated with chlorpyrifos is 

anticipated. The assessment was completed using 7 chemical-specific registrant submitted DFR 

studies. The studies, which encompass the use of five different formulations and twelve different 

crops, have been extrapolated to other groups based on the nature of the crop and application 

method and used to calculate risks for post-application workers in every region of the county. 

The results of the post-application exposure and risk assessment are summarized in Table 2.4.4 

below. A full presentation of post-application exposure and risk including estimates calculated 

for low, medium and high contact activities and resulting REIs reference Appendix E which

accompanies this document.

The MOEs estimated for liquid spray and granular formulation reentry are not of concern to 

HED (i.e., an MOE ≥ 100) in the range of 0 to 4 days for lower to medium exposure activities 

and 0 to 8 days for high exposure activities, with the greater majority falling between 1 to 4 days 

when all exposure activities are considered. HED also estimated the MOEs for reentry into 

microencapsulated and total release fogger formulation treated greenhouses. These estimates

range from 0 to > 35 days after treatment (the completion of the monitoring period) for all 

exposure activities considered. 
Page 70 of 107

Table 2.4.4. Occupational Post-application Exposure and Risk Summary 

Crop Group Crop App. Rate 

(lbs ai/A)

DFR Data 

Source

DFR 

Study Location

Growing Region Estimated REI Range (days)

(Dermal LOC = 100)

Berry: Low Strawberry 1.0 MRID 42974501 

(cauliflower WP) AZ 3, 5, 10 and 12 0 - 2

Cranberry 1.5 1, 5, 10 and 12 0 - 4

Field and Row 

Crops: Low to 

Medium

Clover (Grown for 

Seed) 1.9 MRID 44748102 

(sugar beet EC)

MN 5 1

OR 11 1

Perennial Grass Seed 

Crops 1.0 MRID 44748102 

(sugar beet EC)

MN 5 1

OR 11 1

Alfalfa

1.0

MRID 44748102 

(cotton EC) TX 5 and 7 1

Cotton

1.0

MRID 44748102 

(cotton EC)

CA 10 0 – 3

MS 4 0 – 1

TX 6 and 8 0 – 1

Peppermint/

Spearmint 2.0 MRID 44748102 

(sugar beet EC)

MN 5 0 – 1

OR 11 0 – 1

Wheat 1.0 MRID 44748102 

(sugar beet EC)

CA 8 1

MN 5 and 7 1

Soybean 1.0 MRID 44748102 

(cotton EC) MS 4 and 5 0 – 1

Sugar Beet

1.0 MRID 44748102 

(sugar beet EC)

CA 10 0 – 1

MN 5 0 – 1

OR 11 0 – 1

Field and Row 

Crops: Tall

Corn: Sweet

1.5

MRID 44748102 

(sweet corn EC)

IL 1 and 5 0 – 3

MN 1 and 5 0 – 3

OR 11 0 – 2

Corn: Sweet

1.0

MRID 44748102 

(sweet corn EC)

IL 1 and 5 0 – 3

MN 1 and 5 0 – 3

OR 11 0 – 2

Corn: Field, Including 

Grown for Seed 1.5

MRID 44748102 

(sweet corn EC)

IL 5 0 – 3

MN 5 0 – 3

Corn: Field, Including 

Grown for Seed 1.0

MRID 44748102 

(sweet corn EC)

IL 5 0 – 3

MN 5 0 – 3
Page 71 of 107

Table 2.4.4. Occupational Post-application Exposure and Risk Summary 

Crop Group Crop App. Rate 

(lbs ai/A)

DFR Data 

Source

DFR 

Study Location

Growing Region Estimated REI Range (days)

(Dermal LOC = 100)

Sorghum

1.0

MRID 44748102 

(sweet corn EC)

IL 5, 6, 7 and 8 0 – 1

MN 5, 6, 7 and 8 0 – 1

Sunflowers 1.5 MRID 44748102 

(sweet corn EC)

IL 5 and 7 1

MN 5 and 7 1

Tree Fruit: 

Deciduous

Apple 

(Dormant and Delayed 

Dormant)

2.0

MRID 44748101 

(apple WP)

CA 10 0 – 1

WA 11 0 – 2

NY 1, 2, 5 0 – 2

Cherry (Sweet)

(Dormant and Delayed 

Dormant)

2.0 MRID 44748101 

(apple WP)

CA 10 0 – 1

WA 11 0 – 2

NY 5 0 – 2

Cherry (Sour)

(Dormant and Delayed 

Dormant)

2.0 MRID 44748101 

(apple WP) NY 1 and 5 0 – 2

Peaches

(Dormant and Delayed 

Dormant)

2.0 MRID 44748101 

(apple WP)

CA 10 0 – 1

NY 2 0 – 2

Pears

(Dormant and Delayed 

Dormant)

2.0 MRID 44748101 

(apple WP)

CA 10 0 – 1

WA 11 0 – 2

Nectarines, Plums, 

Prunes

(Dormant and Delayed 

Dormant)

2.0 MRID 44748101 

(apple WP) CA 10 0 – 1

Apples

1.5

MRID 44748101 

(apple WP)

CA 10 0 – 1

WA 11 0 – 4

NY 1, 2, 5 0 – 4

Cherries (Sour) 1.5 MRID 44748101 

(apple WP) NY 1 and 5 1 – 4

Peaches 

(Post-harvest) 3.0 MRID 44748101 

(apple WP)

CA 10 0 – 1

NY 2 0 – 2

Pears

(Post-harvest) 2.0 MRID 44748101 

(apple WP)

CA 10 0 – 1

WA 11 0 – 2

Tree Fruit: 

Evergreen

Conifer Trees and 

Christmas Tree 1.0 MRID 43062701 (citrus EC) CA Any 0 – 1
Page 72 of 107

Table 2.4.4. Occupational Post-application Exposure and Risk Summary 

Crop Group Crop App. Rate 

(lbs ai/A)

DFR Data 

Source

DFR 

Study Location

Growing Region Estimated REI Range (days)

(Dermal LOC = 100)

Plantations

Citrus 6.0

(CA and 

AZ)

MRID 43062701 

(citrus EC) CA 3 and 10 0 – 2

3.5 MRID 43062701 

(citrus EC) CA 3 and 10 0 – 1

Forestry

Cottonwood/ Poplar 

Trees Grown for Pulp

(Dormant)

2.0 MRID 44748101 

(apple WP)

WA 11 2

NY 1 and 7 2

Deciduous Trees 

(Plantations and Seed 

Orchards)

1.0 MRID 44748101 

(apple WP)

CA 10 1

WA 11 1

NY 1 and 7 1

Tree Nuts

Almonds 2.0 MRID 44748101 

(almond WP)

CA

(arid) 10 1

Almonds

(Dormant and Delayed 

Dormant) 2.0 MRID 44748101 

(almond WP)

CA

(arid) 10 1

Filberts

2.0

MRID 44748101 

(pecan EC) GA 12 0

Pecans MRID 44748101 

(pecan EC)

GA 2 0

LA 6 0

TX 8 0

Walnuts MRID 44748101 

(pecan EC) TX 10 0

Filberts (Dormant and 

Delayed Dormant) 

2.0

MRID 44748101 

(pecan EC) GA 12 0

Walnuts (Dormant and 

Delayed Dormant) MRID 44748101 

(pecan EC) TX 10 0

Ornamentals/ 

Nurseries 

(Outdoor Only)

Deciduous Trees in 

Nurseries and 

Orchards Except 

Apples (Dormant and 

2.0 MRID 44748101 

(apple WP)

CA 10 0

WA 11 1

NY 1, 2, 5 1
Page 73 of 107

Table 2.4.4. Occupational Post-application Exposure and Risk Summary 

Crop Group Crop App. Rate 

(lbs ai/A)

DFR Data 

Source

DFR 

Study Location

Growing Region Estimated REI Range (days)

(Dermal LOC = 100)

Delayed Dormant)

Non-bearing Apple 

Trees 

Ornamentals/ 

Nurseries 

(Outdoor Only)

Non-bearing Citrus, 

Tree Nut and Cherry 4.0 MRID 43062701 (citrus EC) CA 2, 3, 6, 8, 10, 12 0

Non-bearing Peach 

and Nectarine Trees 3.0 MRID 44748101 (apple WP)

CA 10 1

NY 2 1

Conifers in Nurseries 1.0 MRID 43062701 

(citrus EC) CA Any 0

Field and Row 

Crops: Low to 

Medium 

(Outdoor Only)

Ornamentals 2.0 MRID 44748102 

(sugar beet EC)

CA Any 0 – 5

MN Any 0 – 6

OR Any 0 – 2

Vegetable: Root 

and Tuber

Carrot (Grown for 

Seed) 0.94 MRID 44748102 (sugar beet EC)

CA 10 0 – 1

MN 3 and 5 0 – 1

Radish (Grown for 

Seed) 0.94 MRID 44748102 (sugar beet EC) MN 3 and 5 0 – 1

Vegetable: 

Fruiting Pepper 1.0 MRID 44748102 (cotton EC)

CA 9 and 10 0 – 2

MS 2 and 3 0 – 1

TX 8 0 – 1

Vegetable: 

Head and Stem 

Brassica

Broccoli, Brussel 

Sprout and 

Cauliflower 1.0

MRID 42974501 

(cauliflower WP) AZ 10

0 – 8

Cabbage MRID 42974501 

(cauliflower WP) AZ 1, 2 and 5

Vegetable: 

Leafy

Bok Choy 1.0 MRID 42974501 

(cauliflower WP) AZ 3 and 10

0 – 4

Collards, Kale, 

Kohlrabi 1.0 MRID 42974501 (cauliflower WP) AZ 2

Stalk and Stem: 

Vegetable

Asparagus

1.0

MRID 44748102 

(sugar beet EC)

CA 10 0 – 1

MN 5 0 – 1

OR 11 0 – 1

Non-bearing Pineapple

1.9

MRID 44748102 

(cotton EC) MS 13 0 – 1

Vine/ Trellis Grapes (Dormant and 2.0 MRID 43062701 CA 10 0
Page 74 of 107

Table 2.4.4. Occupational Post-application Exposure and Risk Summary 

Crop Group Crop App. Rate 

(lbs ai/A)

DFR Data 

Source

DFR 

Study Location

Growing Region Estimated REI Range (days)

(Dermal LOC = 100)

Delayed Dormant)

Grapes (Post-harvest 

and Prior to Budbreak)

(citrus EC)

Turf

Turf for Sod and Seed

4.0

MRID 44829601 

(turf EC and WP)

CA 10 1

IN 5 1

MS 2 and 6 1

Turf for Golf Course

1.0

MRID 44829601 

(turf EC and WP)

CA 10 0

IN 5 0

MS 2 and 6 0

Granular Applications

Field and Row 

Crops: Low to 

Medium

Soybeans 1.0 MRID 44748102 

(sweet corn G) IL 4 and 5 0

Sugar Beet 2.0 MRID 44748102 

(sweet corn G)

IL 5 0

OR 10 and 11 0 – 1

Peanuts 4.0 MRID 44748102 

(sweet corn G) IL 2 and 6 0 – 1

Field and Row 

Crops: Tall

Corn: Sweet 1.0 MRID 44748102 

(sweet corn G)

IL 1 and 5 0

OR 11 0

Corn: Field and 

Grown for Seed 1.0 MRID 44748102 (sweet corn G) IL 5 0

Nursery

Woody Ornamentals 

(In Container and 

Field Grown)

6.0

MRID 44748102 

(sweet corn G)

IL

Any 0

OR

Turf

Turf for Sod or Seed

1.0

MRID 44829601 

(turf G and 

fertilizer)

CA Any

0

Golf Course

0

Microencapsulated Formulation Application

Greenhouse 

(Microencap. 

Formulations)

Commercial 

Ornamentals, 

Greenhouse 

Production: Bedding 

Plants, Cut Flowers, 

Flowering Hanging 

Baskets, Potted 

1.4

MRID 46722702 

(smooth 

ornamentals ME)

MO Any 0 to > 35
Page 75 of 107

Table 2.4.4. Occupational Post-application Exposure and Risk Summary 

Crop Group Crop App. Rate 

(lbs ai/A)

DFR Data 

Source

DFR 

Study Location

Growing Region Estimated REI Range (days)

(Dermal LOC = 100)

Flowers, Ornamentals, 

Trees and Shrubs 

Total Release Fogger Application

Greenhouse 

(Total Release 

Fogger and.

Liquid 

Concentrate 

Formulations)

Ornamentals 2

MRID 46722701 

(hairy 

ornamentals ME)

MO Any 18 to > 30

Commercial 

Ornamentals, 

Greenhouse 

Production: Bedding 

Plants, Cut Flowers, 

Flowering Hanging 

Baskets, Potted 

Flowers, Ornamentals, 

Trees and Shrubs 

0.01 lb ai/

fogger/

3,000 sq ft

0.15 lb ai/A

MRID 46722701 

(hairy 

ornamentals ME)

MO Any 2 – 22
Page 76 of 107

2.4.5 Summary of Exposure Data Gaps

No new data gaps have been identified for the assessment of occupational handler exposure/risk 

in addition to those identified in the Registration Eligibility Decision (RED) for chlorpyrifos 

(finalized 7/31/06; IRED issued 2/2002). These data requirements were addressed in a 90-day 

progress report submitted (August 28, 2003) by the technical registrant, Dow AgroSciences. The 

following data requirements were addressed which pertain to occupational post-application 

exposure to chlorpyrifos:

Exposure data for reentry into treated areas with soil incorporated/directed applications.

In the 2003, 90-day progress report, Dow requested a waiver because, “reentry into (e.g., corn 

field) after soil incorporation is not expected to be a source of exposure to chlorpyrifos. Most 

soil incorporation use refers to the incorporation of chlorpyrifos into the soil under or adjacent to 

the seed as they are planted. Thus, a soil incorporated application, due to its placement, does not 

present a route of exposure, either dermal or inhalation. Added to the lack of a route of exposure 

to the incorporated material, is the lack of reentry activities for a crop immediately after planting 

and soil incorporation. Based on these two conditions, the physical barrier afforded by the soil 

covering the application and the lack of reentry associated with a freshly planted field, Dow 

requested that a waiver of the requirement for soil residue dissipation data and reentry exposure 

data for reentry into soil incorporated treated areas be granted.” HED has determined that data 

are no longer required for this potential route of exposure. 

Use pattern information (i.e., timing of application relative to post-application activities, 

greenhouse DFR, and biological monitoring data to develop transfer coefficients for 

various greenhouse/nursery activities).

Dislodgeable foliar residue data were submitted by Dow AgroSciences, Hicks, S. (2005) 

Dissipation of the Dislodgeable Foliar Residue of Chlorpyrifos After Application of Duraplex TR

(EPA Reg. No. 499-405) Insecticide to Smooth Leaf and Hairy Leaf Ornamentals in a 

Greenhouse. Project Number: 48796. Unpublished study prepared by Analytical Bio-Chemistry 

Labs., Inc. 110 p. (MRID 46722701) and Hicks, S. (2005) Dissipation of the Dislodgeable Foliar 

Residues of Chlorpyrifos After Application of Duraguard ME (EPA Reg. No. 499-367) 

Insecticide to Smooth Leaf and Hairy Leaf Ornamentals in a Greenhouse. Project Number: 

48797. Unpublished study prepared by Analytical Bio-Chemistry Labs., Inc. 109 p. (MRID 

46722702). HED has reviewed these data and determined that the data are acceptable for 

quantitative use purposes and, therefore, the requirement is satisfied. 

Neither the data addressing the timing of application relative to post-application activities nor the 

biological monitoring data required for chlorpyrifos greenhouse/nursery ornamental use have 

been submitted; however, HED has determined that biological monitoring data are no longer 

required. In the 2003, 90-day response DowAgrosciences responded that the “Agricultural 

Reentry Exposure Taskforce (ARETF) is in the process of developing transfer coefficients for 

greenhouse activities and has previously developed TCs for nursery activities.” These data have 

been received and therefore the requirement has been satisfied. 
Page 77 of 107

2.4.6 Occupational Post-application Characterization

The occupational post-application exposure and risk assessment for chlorpyrifos is based upon 

an array of calculations completed for 15 different crop groups. These unique crop groupings are 

defined essentially based on the nature of the crop where a work activity would take place. 

Within each of these groupings, ranges of transfer coefficients were considered to reflect 

differences in exposures that would be associated with the variety of cultural practices required 

to produce the crop/product. Transfer coefficients are used “generically” to allow for estimation 

of exposure for any pesticide active ingredient using estimates for exposure time and the 

concentration of residue the workers will contact that is specific to the pesticide of interest. The 

Agency has adopted a method of clustering groups, crop growth stages, and post-application 

activities into groups that are expected to result in comparable exposure. Estimates of 

chlorpyrifos post-application exposure were completed over subsequent days after application to 

reflect residue dissipation over time in the environment and to allow for a more informed risk 

management decision. 

The exposure data used in the chlorpyrifos post-application exposure and risk assessment 

represent the best data and approaches that are currently available. The latest HED transfer 

coefficients have been used to complete the assessment, as referenced from the Science Advisory 

Council for Exposure (ExpoSAC) Policy Number 3.2: Agricultural Transfer Coefficients 

(5/5/11). Most of the transfer coefficient values in Policy 3.2 are based on the work of the 

Agricultural Reentry Task Force (ARTF). The choice of post-application activities studied by 

the ARTF, as well as the subsequent assignment of transfer coefficients derived from these 

studies to non-monitored post-application activities was developed with input from both the 

ARTF and HED staff Agency and reviewed by the FIFRA SAP in 2008. It is possible that there 

are exposure scenarios not addressed by HED either due to the lack of appropriate exposure data 

or because the transfer coefficient model is not appropriate or little or no foliar contact is 

associated with a specific activity. Furthermore, unlike the vast majority of crop-activity 

combinations listed under the Transfer Coefficient Table in Policy 3.2, some common 

agricultural activities do not follow the standard “foliar-based” transfer coefficient methodology. 

This should not be interpreted to mean that there is no potential exposure from these activities 

but rather that “foliar-based” transfer coefficients are not applicable to evaluate worker exposure. 

For example, the crop-activity combinations of mechanical windrowing, mechanical sweeping 

and dormant hand pruning for the “nut tree” crop grouping are standard cultural practices; 

however, do not follow the standard transfer coefficient methodology. 

HED completed the assessment of occupational post-application exposure and risk to 

chlorpyrifos using 7 chemical-specific DFR studies submitted by the registrant in support of the 

re-registration of chlorpyrifos. The studies, which encompass the use of five different 

formulations and twelve different crops, have been extrapolated to other groups based on the 

nature of the crop and application method and used to calculate risks for post-application 

workers in every region of the county. It is standard practice for the Agency to use these kinds 

of studies in this manner. It is possible that the use of the 7 chemical specific DFR studies to 

represent all crops and regions within the country could lead to results that do not reflect actual 

use practices and conditions in some parts of the country. Furthermore, the extrapolation of DFR 

data from one crop may not represent precisely the dissipation of another. For example, DFR 
Page 78 of 107

data which measured the dissipation of chlorpyrifos from cotton after application of an 

emulsifiable concentrate were used to represent dissipation of chlorpyrifos from soybeans which 

like cotton, are classified in the low/medium field row crop grouping. HED assumes that residue 

dissipation monitored in available studies approximate residues from like crops, but the extent 

that these residues might be an under- or over-estimate is unknown. Finally, DFR data for 

several crops were conducted in multiple states reflective of the regions of the country where the 

crops are typically grown and chlorpyrifos is used. HED has presented all state-specific DFR 

data for each crop under the assumption that these data accurately reflect dissipation anticipated 

in the different regions of the country (e.g., the subtropical Southeastern U.S. and the semi-arid 

climate of the Central Valley of California). HED has considered available use and usage 

information in development of the occupational post-application assessment and has refined the 

use of available region-specific DFR data to those areas of the U.S. where chlorpyrifos usage 

occurs. 

In summary, the Agency believes that the risk values presented in this post-application 

assessment represent the highest quality results that could be produced given the exposure, use, 

and toxicology data that are available. Risk managers and other interested parties should 

consider the quality of individual inputs when interpreting the results and make decisions 

accordingly. It is difficult to determine where on a distribution the values which have been 

calculated fall because the distributional data for exposure, residue dissipation and many other 

parameters are unrefined. The Agency does believe, however, that the risks represent 

conservative estimates of exposure because maximum application rates are used to define residue 

levels upon which the risk calculations are based and many maximum application rates exceed 

what is assumed to be typical. Additionally, risk estimates are thought to be conservative even 

when measures of central tendency are used because values that would be considered to be in the 

lower percentile aspect of any input parameter have not been used in the calculations.

3.0 Residential Exposures and Risks

A residential risk assessment for registered homeowner uses was performed in 2000. The risk 

estimates of concern resulted in an agreement for the registrants to voluntarily cancel all 

chlorpyrifos homeowner uses, except a granular ant mound use and roach bait in child-resistant 

packaging. A phase out was also developed for termite treatment products (post- and preconstruction). To date, all homeowner use product registrations have been cancelled, except for 

ant and roach bait products. Additionally, residential/recreational uses remain for aerial and 

ground-based fogger ULV mosquitocide and for golf course turf applications. 

It has been determined there is the potential for post-application exposure from entering areas 

previously treated with chlorpyrifos. Post-application exposure is anticipated from from adults 

playing golf on chlorpyrifos-treated courses and from the inhalation of airborne chlorpyrifos or 

the contact with residues that have deposited on residential turf following aerial and groundbased ULV mosquitocide use. HED assumes that golfers are exposed over short- and 

intermediate-terms of duration, while a short-term duration only is assumed for dermal and 

incidental oral exposure from the mosquitocide use. Acute inhalation exposure is assumed from 

the aerial and ground-based mosquitocide use. There is the potential for residential applicator 

exposure to occur from the handling of the roach bait product; however, exposure is anticipated 
Page 79 of 107

to be negligible. The ant mound use can only be applied professionally. Residential postapplication exposure is not anticipated from either the ant mound or roach bait products.

In addition to the assessment of residential post-application exposure to chlorpyrifos, HED has 

assessed inhalation exposure resulting from field volatilization of chlorpyrifos. This residential 

bystander assessment assumes acute and short-/ intermediate-term durations of exposure. Due to 

the unique nature of this assessment, it is presented separately in Section 3.3. 

3.1 Residential Handler Exposures and Risks 

The Agency uses the term “handlers” to describe those individuals who are involved in the 

pesticide application process. The Agency believes that there are distinct tasks related to 

applications and that exposures can vary depending on the specifics of each task. Job 

requirements (e.g., the amount of chemical to be used in an application), the kinds of equipment 

used, and the target being treated can cause exposure levels to differ in a manner specific to each 

application event. 

Based upon review of all chlorpyrifos registered uses, only the roach bait products can be applied 

by an applicator in a residential setting; however, exposure/risk from application of the roach 

bait products was not quantitatively assessed because HED expects handler exposure to be 

negligible. The roach bait product is designed such that the active ingredient is contained within 

the bait station, therefore, limiting contact with the active ingredient in the product. 

3.2 Residential Post-application Exposures and Risks

The Agency uses the term “post-application” to describe exposures to individuals that occur as a 

result of being in an environment that has been previously treated with a pesticide. Chlorpyrifos 

can be used in areas frequented by the general population including ant mounds, golf courses,

and as an aerial and ground-based ULV mosquitocide applied by a public agency made in the 

vicinity of residential areas. As a result, individuals could be exposed during the treatment 

(spray drift) or by entering these areas if they have been previously treated. HED has assessed 

residential post-application exposure and risk from the golf course and ULV mosquitocide uses. 

Additionally, acute inhalation (spray drift) was assessed for the ULV mosquitocide uses. 

Exposure from residential ant mound treatment (applied by professional only) was not assessed 

quantitatively because contact with the mound is not anticipated. 

3.2.1 Data and Assumptions for Residential Handler Exposure Scenarios

A series of assumptions and exposure factors served as the basis for completing the residential 

post-application risk assessments. The assumptions and factors used in the risk calculations, as 

detailed below, are consistent with current HED policy for completing residential exposure 

assessments (i.e., Draft SOPs for Residential Exposure Assessment). In addition to these factors, 

HED has used turf transferable residue (TTR) data from a chemical-specific turf study. 

The assumptions and factors used in this assessment include:
Page 80 of 107

HED assumes the maximum application rates allowed by labels in its risk assessments;

the tiered approach for personal protection using increasing levels of PPE is not used in 

residential risk assessments;

residential post-application assessments are based on the assumption that individuals are 

wearing shorts, short-sleeved shirts, socks, and shoes;

the adverse effects for the short- and intermediate-term dermal and inhalation endpoints 

are based on studies where the effects were observed in males and females, therefore the 

average body weight of an adult (i.e., 70 kg) is used to complete the risk assessment;

3 year old children are expected to weigh 15 kilograms (representing an average weight 

from years one to six);

HED assumes that golfers can be exposed to chlorpyrifos over an extended duration 

(short- and intermediate-term) based on the potential for frequent play, while a brief 

duration of exposure (short-term) is assumed for exposure from public health 

mosquitocide due to the intermittent nature of this application. Inhalation exposure from 

the mosquitocide use is assumed to be an acute event. 

Exposures to adults golfing have been assessed using the latest HED standard operating 

procedures for assessment of the turf exposure scenario including: 

the TTR at Day 0 from the chlorpyrifos-specific turf transferable residue study (MRID 

44829601) was used to assess exposure from golfing. The study was conducted with an 

emulsifiable concentrate, granular and wettable powder formulation. HED used the 

emulsifiable concentrate and granular data; however, the wettable powder data were not 

because no wettable powder formulations are currently registered. The study was 

conducted at 3 state sites, California, Indiana and Mississippi, using the emulsifiable 

concentrate and wettable powder formulations and the granular formulation in California 

only. The liquid applications were made at a nominal application rate of 4 lb ai/A and the 

granular applications were made at a nominal application rate of 2 lb ai/A. Exposure was 

estimated by normalizing Day 0 TTR measures from study application rates to the current 

maximum application rate allowable by the label. Results have been presented using the 

multi-state data as a representation of distinct climatologic regions;

the dermal transfer coefficients used are those presented in HED‟s Residential Standard 

Operating Procedures (SOPs; draft 1997);

high contact lawn activites: 14,500 cm2/hr for adults and 5,200 cm2/hr for children, 

golfing: 3,400 cm2/hr for adults;

the duration of adult exposure on a golf course is assumed to be 4 hours;

Children, who are 12 years of age or older, are likely to represent the vast majority of the 
Page 81 of 107

youth that play golf on a routine basis. Risk assessments for children (i.e., less than 12 

years old) are more difficult to complete because of the increased uncertainties associated 

with extrapolation of adult dermal exposure data, and because of the increased likelihood 

of other behaviors that might contribute to exposure, such as mouthing contaminated 

hands or golf balls. Therefore, the risk associated with children in a golfing scenario is 

based on the surface area to body weight ratio (SA/BW) for male children 5 years of age 

(i.e., the difference is larger for males compared to female making the value more 

protective) calculated by using the 95th percentile body surface area and the 50th 

percentile for body weight. This skewed SA/BW for children was compared to that of 

the average adult, and found to be approximately 70 percent greater. Based on this 

parameter alone, the child‟s exposure could be almost twice that of the adult golfer, 

however, it should be noted that a child is not expected to use the golf course for the 

same length of time as an adult. While an adult is likely to play a full round of golf (i.e., 

18 holes), which takes approximately 4 hours, a child would probably only spend about 2 

hours (i.e., the 75th percentile for time spent playing on grass by children aged 1-4 years 

and 5-11 years) on the course. Thus, the child‟s shorter duration on the golf course 

offsets the higher SA/BW, and therefore, the child golfer‟s exposure is likely to be 

similar to that of the adult golfer. [Note: The values used to calculate SW/BW and 

estimate time spent playing on grass were obtained from the EPA Exposure Factors 

Handbook (1997)]. 

ULV Mosquito Control Applications:

In common agricultural aerial application events, the objective is to deliver the chemical in a 

manner that minimizes spray drift and deposits the material as quickly as possible to the treated 

surface. Several engineering aspects are focused on achieving these kinds of results including: 

the use of larger droplets (that fall and deposit faster); the use of a low aircraft release height; and 

meteorological condition restrictions for application (e.g., avoidance of thermal inversion 

conditions).

The objectives of wide area treatments for pests with public health implications such as 

mosquitoes are distinctly different from common aerial agricultural applications. Wide area 

treatments are generally used for mosquito adulticide chemicals where the efficacy of the 

compound is dependent upon contacting the mosquito in-flight. As a result, the engineering 

aspects of mosquito adulticide applications are antithetical to common aerial agricultural 

applications. In fact, the objective is to maximize the potential for spray drift by increasing the 

release height, using (in most cases) Ultra Low Volume (ULV) techniques, and using much 

smaller droplets that are less likely to quickly fall and deposit on surfaces.

HED anticipates the following exposure scenarios from ULV mosquito application in residential 

settings:

Dermal exposure from residues deposited on turf (adult and child)

Incidental oral ingestion of residues deposited on lawns from hand-to-mouth transfer 

(child)

Inhalation exposure from airborne residue (adult and child)
Page 82 of 107

It should be noted that inhalation exposure usually does not factor significantly into residential 

post-application risk. However, due to the use of chlorpyrifos aerial and ground ULV mosquito 

control applications, a risk assessment was performed. The assessment of residential postapplication inhalation was conducted under the assumption that people are present in the 

residential setting during the actual ULV application (ground and aerial). Likewise, inhalation is 

anticipated to be an acute event. In contrast, dermal and incidental oral exposures from ULV 

applications (due to the subsequent settling of airborne residues on turf) are anticipated to be 

short-term in duration because the potential for exposure extends beyond the application event. 

For these reasons, residential inhalation estimates from ULV application have been presented, 

but not aggregated with the other routes of exposure assessed. While the assessment of postapplication inhalation from the mosquitocide use has been included under the residential postapplication Section, it is likely more accurately characterized as spray drift. 

Chemical-specific data for mosquito uses are not available. Therefore the equations and 

assumptions for these scenarios were taken from the Draft SOPs for Residential Exposure 

Assessment. The assumptions are presented below and the equations in Section 3.2.2. In 

addition to the use of the Residential SOPs, the unique nature of the mosquito control uses 

requires additional information to determine the deposition rate of chlorpyrifos (i.e., the amount 

deposited on residential turf). 

The following information was used to determine the deposition rates for ground-based foggers 

and aerial applications and airborne concentration after aerial applications.

Ground-based Foggers:

In the study conducted by Moore et al., [Downwind Drift and Deposition of Malathion on 

Human Targets From Ground Ultra-Low Volume Mosquito Sprays: J.C. Moore, J.C. Dukes, J.R. 

Clark, J. Malone, C.F. Hallmon, and P.G. Hester; Journal of the American Mosquito Control 

Association; Vol. 9, No. 2 (June, 1993)] both human exposure and deposition was quantified 

over 5 separate application events. A 91 percent formulation of malathion was applied in April 

and May of 1989 in the early evening (a time of day for relative atmospheric stability). A Leco 

HD ULV cold aerosol generator (Lowndes Engineering Company, Valdosta Georgia) was used 

to make each application. The application parameters included a fluid flow rate of 4.3 fluid 

ounces per minute, a vehicle ground speed of 10 mph, and a nominal application rate of 0.05 lb 

ai/acre (i.e., equates to a deposition rate of 0.51 μg/cm2). Deposition was monitored at three 

locations downwind from the treatment area (i.e., 15.2 m, 30.4 m, and 91.2 m). For the events 

considered in the deposition calculations, “average amounts of malathion deposited on ground 

level at 15.2, 30.4, and 91.2 m were not significantly different.” The percentage of the 

application rate reported to have deposited ranged from 1 to 14 percent. The mean deposition 

value for all measurements was 4.3 percent (n=35, CV=98).

In the study conducted by Tietze et al., [Mass Recovery of Malathion in Simulated Open Field 

Mosquito Adulticide Tests: N.S. Tietze, P.G. Hester, and K.R. Shaffer; Archives of 

Environmental Contamination and Toxicology; 26: 473-477 (1994)] only deposition was 

quantified over 6 separate application events (i.e., one event was not included in deposition 
Page 83 of 107

calculations “due to negative air stability”). The application parameters were similar to that used 

by Moore et al. A 95 percent formulation of malathion was applied from May to August of 

1993. A Leco 1600 ULV cold aerosol generator (Lowndes Engineering Company, Valdosta 

Georgia) was also used to make each application. The application parameters included a fluid 

flow rate of 4.3 fluid ounces per minute, a vehicle ground speed of 10 mph, and a nominal 

application rate of 0.057 lb ai/acre (i.e., equates to a deposition rate of 0.58 μg/cm2). Deposition 

was monitored at four locations downwind from the treatment area (i.e., 5 m, 25 m, 100 m and 

500 m). For the events considered in the deposition calculations, “malathion mass deposited 

differed significantly between the 500 m site and the three closer sites (df = 3; F-value = 3.42; 

P<0.05).” The percentage of the application rate reported to have deposited (not including 500 

m samples which were much less) ranged up to 5.8 percent. The mean deposition value for all 

measurements was 3.8 percent.

After considering the data that are available in the Tietze et al. and Moore et al. papers, an offtarget deposition rate of 5 percent of the application rate was used by HED to evaluate groundbased ULV applications (i.e., 5 percent of application rate is the deposition rate of which 20 

percent is assumed to be available for dislodging). A value slightly higher than the mean values 

for both studies was selected because of the variability in the data and the limited number of data 

points. It should be noted that this value is also consistent with the draft modeling assessment 

for ground-ULV approaches completed by S.T. Perry and W.B. Petersen of EPA‟s Office of 

Research and Development (i.e., within a factor of 5). Perry and Petersen used “the INPUFF 

Lagrangian puff model” as the basis for their assessment (Petersen and Lavdas, 1986: INPUFF 

2.0 - A Multiple Source Gaussian Puff Dispersion Algorithm, User’s Guide, EPA/600/8-86/024). 

Depending on the scenario selected from this document, deposition rates ranged from 

approximately 2.5 percent deposition 450 m downwind to 15 to 20 percent deposition 

immediately adjacent to the treatment zone.

Aerial Application:

Data similar to that for ground applications discussed above were not available for the aerial 

deposition. Therefore, in order to calculate deposition from aerial ULV applications, HED used 

AgDRIFT (V 2.01) which is the model that was developed as a result of the efforts of the Spray 

Drift Task Force (SDTF). The SDTF is a coalition of 38 pesticide registrants whose primary 

objectives were to develop a comprehensive database of off-target drift information in support of 

pesticide registrations and an appropriate model system. This model was selected based on the 

consensus of several experts in the spray drift area because it represents the current state-of-theart. It is important to note that no proprietary SDTF data were used in the completion of this 

assessment. The following inputs were used as the basis of the AgDRIFT calculations:

AgDRIFT Model Tier: 3

Droplet Size Distribution: Dv0.1 = 20.58 μm; Dv0.5 = 47.79 μm; Dv0.9 = 97.95 μm; and 

<141 μm = 98.44 percent. [Note: The droplet distribution was developed based on the 

chlorpyrifos ULV mosquito labeling. No proprietary SDTF data were used in the 

completion of this assessment.]
Page 84 of 107

Spray Material: User-defined option (oil option). Inputs include: nonvolatile rate 0.24 lb 

per acre, specific gravity 0.92, spray rate 0.05 gallons/acre, active ingredient application 

rate (0.01 lb ai/acre), and evaporation rate (0 μm

2

/deg C/sec). [No proprietary SDTF 

physical property data were used in the completion of this assessment.]

Aircraft: User-defined option (fixed-wing option). Inputs include: Air Tractor AT-502, 

wingspan: 50 ft (semispan 25 ft), typical application airspeed: 155 mph, weight: 6,660

pounds, planform area: 300 ft2, propeller RPM: 2,200, propeller radius: 4.4 feet, engine 

vertical distance: 0 feet, and engine forward distance: 15.25 feet. [Note: AT-502-specific 

inputs were obtained from the AgDRIFT aircraft library.]

Nozzles: User-defined option. Inputs include number of nozzles: 3, vertical distance of 

nozzles from wing: 0 feet, horizontal distance from wing: 0, -17.5 (2) feet, and horizontal 

distance limit: 76.3 percent.

Meteorology: Inputs were not changed from Tier 3 recommendations of wind speed: 2 

mph, wind direction: -90 degrees (perpendicular to flight path), temperature: 86 F, and 

relative humidity: 90 percent.

Control: Inputs were altered from the Tier 3 recommendations. The parameters that 

were used included a spray release height of 300 feet, 20 spray lines (aircraft passes) in 

each application event, a swath width of 499 feet, and a swath displacement based on the 

aircraft centerline.

Advanced Settings: Inputs were not changed from Tier 3 recommendations of wind 

speed height (2 meters), maximum compute time (600 seconds), maximum downwind 

distance (795 meters), vortex decay rate (0.56 m/s), aircraft drag coefficient (0.1), 

propeller efficiency (0.8), and ambient pressure (1013 mb).

AgDRIFT is capable of producing a variety of useful outputs. The key for HED in this 

assessment was to determine from the model what percentage of the application volume 

remained aloft and what percentage of the resulting droplets deposited on the surfaces in the 

treatment area as well as downwind from the treatment area. AgDRIFT is generally intended to 

calculate deposition rates in areas that are downwind from the treatment area (i.e., presented 

from the border of the treatment area to areas of interest downwind). HED has used the values at 

the border of the treatment area to represent the deposition rate within the treated area. The 

results indicate that there is approximately 25% deposition from the edge of the treatment area 

and 2.5% deposition 2,000 feet downwind. Figure 3.2.1a below shows the AgDRIFT output for 

chlorpyrifos deposition on turf. Based upon the transport of treatment at 300 ft, the airborne 

concentration of chlorpyrifos at the breathing height of adults and children is approximately 0.4 

ng/L (0.0004 mg/m3). Figure 3.2.1b below shows the AgDRIFT output for the concentration of 

chlorpyrifos in air from aerial treatment. 
Page 85 of 107

Figure 3.2.1a: Estimated Chlorpyrifos Deposition from Aerial Treatment of

Mosquitocide at 300 feet Using AgDRIFT

Figure 3.2.1b: Estimated Chlorpyrifos Air Concentration (Breathing Height 

of Adults and Children) from Aerial Treatment of Mosquitocide at 300 feet 

Using AgDRIFT
Page 86 of 107

The general assumptions and factors used to assess ULV mosquitocide dermal and incidental 

oral exposure include:

the chlorpyrifos ULV mosquitocide labels specify a maximum application rate of 0.01 lb 

ai/A;

hand-to-mouth exposures are based on a frequency of 20 events/hour and a surface area 

per event of 20 cm2representing the palmar surfaces of three fingers;

saliva extraction efficiency is 50 percent meaning that every time the hand goes in the 

mouth approximately 1⁄2 of the residues on the hand are removed;

object-to-mouth exposures are based on a 25 cm2surface area;

the dermal transfer coefficients used are those presented in HED‟s Residential Standard 

Operating Procedures (SOPs; draft 1997);

high contact lawn activites: 14,500 cm2/hr for adults and 5,200 cm2/hr for children;

durations of exposure (short-term) on turf for adults and children is assumed to be 2

hours per day;

child dermal and incidental oral exposures are combined to represent an overall risk from 

exposure to turf containing residue from mosquitocide applications.

The general assumptions and factors used to assess ULV mosquitocide inhalation exposure 

include:

the entire active ingredient applied by ground to a 1 acre area is airborne and available to 

be inhaled by a child or adult.

the chlorpyrifos ULV mosquitocide labels specify a maximum application rate of 0.01 lb 

ai/A;

dilution of airborne concentration of 1 to 100 (i.e., 1 percent (0.01) of product released is 

available for exposure from ULV ground application; and

estimated airborne concentrations of chlorpyrifos from aerial and ground ULV 

applications have been compared to the acute inhalation HEC. 

Residential Post-Application Exposure Studies:

The exposure data used to estimate exposure from golfing on chlorpyrifos treated turf is based 

upon the following chlorpyrifos TTR exposure study:
Page 87 of 107

MRID 44829601 (Turf) 

This TTR study (MRID 44829601) examined the residues of chlorpyrifos on turf after 

application of Dursban® Pro (an emulsifiable concentrate), Dursban® 50W (wettable powder), 

Dursban® 2.5G (granular) and Dursban® 1F (flowable granular). Three test sites were 

established in California, Indiana, and Mississippi, with one test plot for each test substance. 

Single applications of the test products were made to the treatment plots using tractor mounted 

boom sprayers for the liquid applications and either a tractor mounted air spreader or tractor 

mounted spreader for the granular applications. The liquid applications were made at a nominal 

application ate of 4 lb ai/A and the granular applications were made at a nominal application rate 

of 2 lb ai/A. Actual application rates, determined by analyzing filter paper anchored to the test 

plots for liquid applications and placing plastic pans in the test plots for granular applications,

ranged from 95% to 100% for the Dursban® Pro application, 97% to 102% for the Dursban®

50W application, 91% to 95% for the Dursban® 2.5G application and 77% to 86% for the 

Dursban® 1F application. Duplicate samples were collected using the California roller 

technique at the following intervals: one day prior to the application, just after the application (-

2.5 hours after application), and days 1, 2, 4, and 7 after application. Samples were analyzed for 

residues of chlorpyrifos. The overall average concurrent recovery was 98%±3%. The LOD and 

LOQ were reported as 0.001 μg/cm2and 0.003 μg/cm2respectively. The field study samples did 

not require correction for field fortification recovery. Average field fortification recoveries 

(corrected for concurrent recovery), at each of three fortification levels were greater than 90% at 

all sites. 

Actual average DFR values (corrected for concurrent recovery) are provided in Table 3.2.1a. 

Table 3.2.1a. Summary of Actual Average DFR Values on Turf After One Broadcast Foliar 

Application of Chlorpyrifos

Sampling 

Interval 

(Days)

Actual Average TTR (μg/cm2)

Dursban® Pro

(Emulsifiable Concentrate)

Dursban® 50W

(Wettable Powder)

Dursban® 2.5G

(Granular)

Dursban® 1F

(Granular)

CA IN MS CA IN MS CA IN MS CA IN MS

After 1st (Only) Application

0 0.124 0.09 0.146 0.234 0.657 0.351 <LOQ <LOQ <LOQ 0.052 <LOQ <LOQ

1 0.039 0.011 0.009 0.027 0.155 0.032 <LOQ <LOQ <LOQ 0.02 <LOQ <LOQ

2 0.01 0.005 <LOQ 0.012 0.039 0.006 <LOQ <LOQ <LOQ 0.011 0.003 0.01

4 0.004 <LOQ <LOQ <LOQ 0.005 <LOQ <LOQ <LOQ <LOQ 0.004 0.003 <LOQ

7 <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ <LOQ

Notes:

LOQ = 0.003 μg/cm

2

Residues were corrected for concurrent recoveries.

Application rates were a nominal 4 lb ai/A for liquid applications (Dursban® Pro and Dursban® 50W ) and 2.0 lb 

ai/A for granular applications (Dursban® 2.5G and Dursban® 1F).

Residential Post-application Exposure and Risk Estimates 

Risks were calculated using the Margin of Exposure (MOE) approach, which is a ratio of the 

body burden to the toxicological endpoint of concern. Exposures were calculated by considering 

the potential sources of exposure then calculating dermal, inhalation and non-dietary ingestion 
Page 88 of 107

exposures. Short-term dermal (adults and children) and incidental oral (children only) exposures 

to turf following aerial and ground-based ULV mosquito treatments and adults golfing on treated

turf were calculated. In addition, acute inhalation exposures to adults and children were 

estimated from aerial and ground ULV mosquitocide applications. Exposure and risks from 

residential post-application uses were calculated in the same manner as described in Section 

2.2.1. 

A total uncertainty factor of 100X is appropriate for dermal and incidental oral exposures [10X 

for interspecies extrapolation, 10X for intraspecies variation and a 1X FQPA factor].

For acute inhalation exposures, a total uncertainty factor of 300X was applied [3X for 

interspecies extrapolation (reduced from 10X because RfC methodology used which takes into 

consideration the pharmacokinetic differences between animals and humans), 10X for 

intraspecies variation, and a 10X FQPA database uncertainty factor (for extrapolation from a 

LOAEL to a NOAEL). 

Dermal Exposure from Turf (Adults and Children) and Golfing (Adults)

The approach used to calculate the dermal exposures that are attributable to exposure from 

contacting turf with chlorpyrifos deposited from aerial and ground-based ULV mosquito 

applications and from golfing on treated turf:

ADD = (TTRt * ET * TC * DA * CF1) / BW

Where:

ADD = average daily dose (mg/kg/day);

TTRt = turf transferable residue on day "0" (μg/cm2);

ET = exposure time (2 hr/day adults and child on turf, 4 hr/day golfing);

TC = transfer coefficient (14,500 cm2/hr for adults and 5,200 cm2/hr for children on 

turf, 3,400 cm2for adults golfing);

DA = dermal absorption factor;

CF1 = weight unit conversion factor to convert μg units to mg for the daily exposure 

(0.001 mg/μg); and

BW = body weight (70 kg for adults and 15 kg for children).

and, 

TTRt = AR * F * CF2 * CF3

Where:

AR = application rate (lb ai/acre) x percentage deposited (0.01 lb ai/A)

F = fraction of ai available on the foliage as dislodgeable residue (9.5% aerial, 

5.0% ground applications)

t = post-application day on which exposure is being assessed (day 0)

CF2 = weight unit conversion factor to convert the lbs ai in the application rate to 

μg for the DFR value (4.54E+8 μg/lb)

CF3 = area unit conversion factor to convert the surface area units (ft2) in the 
Page 89 of 107

application rate to cm2for the DFR value (2.47E-8 acre/cm2if the 

application rate is per acre)

Note: TTR is calculated using the above equation only for the ULV mosquitocide application 

using deposition percentages for aerial and ground applications. TTR is defined for golfing 

exposure using measures from chemical-specific TTR data. 

Incidental Oral Exposure from Turf (Children)

The approach used to calculate the incidental oral exposures that are attributable to hand-tomouth behavior on treated turf is:

ADD = (TTRt * SA * FQ * ET * SE * CF1) / BW

Where:

ADD = average daily dose (mg/kg/day);

TTRt = turf transferable residue on day "0" (μg/cm2);

SA = surface area of the hands (20 cm2/event);

FQ = frequency of hand-to-mouth activity (20 events/hr);

ET = exposure time (2 hr/day);

SE = extraction by saliva (50%);

CF1 = weight unit conversion factor to convert μg units in the TTR value to mg 

for the daily exposure (0.001 mg/μg); and

BW = body weight (15 kg).

Inhalation Exposure (Spray Drift) from Aerial ULV Mosquitocide (300 ft. release height):

The airborne concentration of chlorpyrifos, 0.0004 mg/m3from aerial ULV mosquitocide 

treatment was estimated using the AgDRIFT model. This concentration was compared directly 

to the acute inhalation HEC to estimate exposure/risk for the exposure scenario.

Inhalation Exposure (Spray Drift) from Ground ULV Mosquitocide:

AC = AR * CF1 * CF2 * CF3 * D

Where: 

 AC = airborne concentration (mg/m3);

 AR = application rate (0.01 lb ai/acre);

CF1 = conversion factor to convert lb ai/A to lb ai/ft2 (1 A/43,500 ft2);

Expressed as an airborne concentration: lb ai/ ft2 = lb ai/ ft3

CF2 = conversion factor to convert lb ai/ft2 to lb ai/m2(35.3 ft3/ 1m3); 
Page 90 of 107

CF3 = conversion factor to convert lb ai/m3 to mg/m3 (454,000 mg/lb)

D = dilution factor (0.01)

The airborne concentration of chlorpyrifos was estimated using the above formula. As was done 

for aerial mosquitocide application, this concentration was compared directly to the acute 

inhalation HEC to estimate exposure/risk for the exposure scenario. 

3.2.3 Residential Post-application Risk Summary

HED has assessed residential post-application exposures to chlorpyrifos. Short-term dermal and 

and incidental oral (children only) exposures to turf following aerial and ground based ULV 

mosquito treatments are presented in Table 3.2.3a and 3.2.3b, respectively. Acute inhalation 

exposure following aerial and ground ULV application (spray drift) for adults and children is 

presented in Table 3.2.3c and 3.2.3d, respectively. Short- and intermediate-term dermal 

exposure/risk to adults resulting from playing golf is presented in Table 3.2.3e.

Estimated short-term adult and child dermal and child incidental oral exposure to turf following 

aerial and ground mosquito treatments are not concern to HED (i.e., an MOE > 100). Combined 

child exposure estimates (dermal and incidental oral) to turf following aerial mosquito treatment 

are of concern to HED; however, combined estimates following ground treatment are not of 

concern. Acute adult and child inhalation exposure following aerial mosquito treatment is not of 

concern (i.e., an MOE > 300), but is of concern following ground treatment. 

Adult dermal exposure from golfing is not concern to HED (i.e., an MOE > 100) using any TTR 

region-specific data for the emulsifiable concentrate formulation at the 1.0 lb ai/A, or 0.25 

application rates. 

Table 3.2.3a Adult and Child Short-term Risks (MOEs) from Residential Post-application 

Exposure to Turf Following Aerial ULV Mosquito Treatments At 300 Foot Spray Release 

Height 

Children 3 to < 6

Dermal 260

Incidental Oral 130

Combined Exposure 88

Table 3.2.3b Adult and Child Short-term Risks (MOEs) from Residential Post-application 

Exposure to Turf Following Ground-based ULV Mosquito Treatments

Children 3 to < 6

Dermal 1,300

Incidental Oral 670

Combined Exposure 440
Page 91 of 107

Table 3.2.3c Adult and Child Acute Risk (MOEs) from Residential Post-application 

Inhalation Exposure Following Aerial ULV Mosquito Treatments At 300 Foot Spray 

Release Height 

Children 3 to < 6

Inhalation 1,600

Table 3.2.3d Adult and Child Acute Risk (MOEs) from Residential Post-application 

Inhalation Exposure Following Ground ULV Mosquito Treatments

Children 3 to < 6

Inhalation 17

Table 3.2.3e Adult Estimated Short- and Intermediate-term Risk (MOEs) from Postapplication Golfing Exposure to Chlorpyrifos Treated Golf Course Turf (MRID 44829601)

Application Rate - 1.0 lb ai/A

State Emulsifiable Concn. Granular

CA 830 960

IN 1,200 NA

MS 710 NA

Application Rate – 0.25 lb ai/A

State Emulsifiable Concn. Granular

CA 3,300 3,800

IN 4,600 NA

MS 2,800 NA

3.2.4 Residential Post-application Exposure and Risk Characterization

Chlorpyrifos occupational use results in the potential for exposure to the general population in 

residential settings. Adults can be exposed through golfing on chlorpyrifos treated turf and 

adults and children can be exposed from chlorpyrifos deposition on turf after aerial or groundbased ULV mosquitocide applications. HED has relied upon the Draft SOPs for Residential 

Exposure Assessment for all scenarios assessed. The data used in the chlorpyrifos residential 

post-application exposure assessment represent the best exposure data and approaches that are 

currently available. To the extent possible, HED has used chlorpyrifos-specific data such as the 

TTR data used for assessment of exposure to treated golf course turf. Chemical-specific data for 

aerial and ground based ULV mosquito uses are not available. For ground based ULV 

application, HED used data from studies conducted to measure off site deposition from these 

applications. Data similar to that for ground applications were not available to determine aerial 

deposition. In order to calculate chlorpyrifos deposition on turf from aerial ULV applications, 

HED used the AgDRIFT (V 2.01) model. Once the deposition input was identified, HED used 

the high-end equations and assumptions from the Draft SOPs for Residential Exposure 

Assessment to assess turf exposure (dermal and incidental oral) and inhalation exposure from 

aerial and ground based mosquito applications and inhalation exposure from aerial application. 

Although the SOPs were initially developed for direct turf applications, the models are used in 
Page 92 of 107

this assessment to determine if there is a potential concern using a conservative, screening level 

approach. 

HED believes that the values presented in this assessment represent the highest quality results 

that could be produced based on the currently available post-application exposure data. Readers 

of this document should consider the quality of individual inputs when interpreting the results 

and make decisions accordingly. It is difficult to ascertain where, on a distribution, the 

calculated values fall because the distributional data for exposure, residue dissipation and many 

other parameters are unrefined. HED does believe, however, that the risks represent 

conservative estimates of exposure because maximum application rates are used to define residue 

levels upon which the calculations are based. Additionally, estimates are thought to be 

conservative even when measures of central tendency (e.g., most transfer coefficients are thought 

to be central tendency) are used because values that would be considered to be in the lower 

percentile aspect of any input parameter have not been used in the calculations.

3.3 Residential Bystander Exposure and Risk 

Recently, the Agency has begun exploring the development of an approach for assessing 

inhalation exposure resulting from the field volatilization of conventional pesticides. The 

Agency has sought expert advice and input on these issues from its Federal Insecticide, 

Fungicide, and Rodenticide Act Scientific Advisory Panel in December 2009. More information 

on pesticide volatilization can be found on the Agency‟s website at 

http://www.epa.gov/opp00001/about/intheworks/volatilization.htm. 

The Agency has developed a preliminary bystander volatilization inhalation exposure assessment 

for chlorpyrifos using currently available inhalation toxicity and air monitoring data. There are 

15 available chlorpyrifos air monitoring studies. These include:

2 application site studies done in Tulare and Lompoc Counties, CA by the California Air 

Resource Board (CARB), and 

13 ambient air studies

o 2 conducted in North Central and Yakima Valley, OR by the University of 

Washington Department of Environmental and Occupational Health Sciences; and 

o 11 conducted by Pesticide Action Network North America (PANNA), two in 

Cowiche and Tieton, WA, and nine in Lindsay, CA.

Application site air monitoring refers to the collection of air samples around the edges of a 

treated field during and after a pesticide application. Samples are generally collected for short 

intervals (e.g., < 8 hours), for at least the first day or two after application with subsequent 

samples increasing in duration. In this type of study, it is typically known when an application 

occurred, the equipment used for the application, and the application rate. Application site 

monitoring data represents an exposure to vapors at or near the field edge resulting from an 

application.

Ambient air monitoring typically is focused on characterizing the airborne pesticide levels within 

a localized airshed or community structure of some definition (e.g., city, township, or 
Page 93 of 107

municipality). This type of monitoring effort also can be focused on capturing chronic 

background levels or other temporal characteristics of interest such as focusing on seasonal 

pesticide use patterns. Typically, samples are taken for 24 consecutive hours and collected at the 

same site over an extended period of time (e.g., several weeks or months). In contrast to 

application site air monitoring, information on the precise timing and location of pesticide 

applications are rarely collected in ambient air monitoring studies. However, this does not mean 

that an application did not occur near an ambient sampler during the monitoring period

The chlorpyrifos bystander volatilization inhalation exposure assessment compares the 

maximum air concentration detected in each of the monitoring studies to the acute HEC for 

residential bystanders. This comparison was done to represent a potential resident who lives 

next to a treated field and may be exposed to the peak concentration of chlorpyrifos volatilizing 

off the field over a 24-hour period. In addition, the arithmetic mean chlorpyrifos air 

concentration from each study was compared to the short-/intermediate-term HEC for residential 

bystanders. This comparison was done to represent a potential seasonal exposure. Table 3.3.1

provides chlorpyrifos volatilization MOE calculations for each site. None of the air 

concentrations results in acute risks of concern; however, there were a couple of average air 

concentrations from some sites that resulted in short-/intermediate-term risks of concern.

Chlorpyrifos Air Monitoring Study Summaries:

Application Site

Washington Department of Health (WA DOH) – Organophosphorus Pesticide Air Monitoring 

Project – Application Site

3 sample types were employed (1 application site and 2 ambient) to determine levels of 

chlorpyrifos and chlorpyrifos oxon in residential communities in 2 regions, North Central 

and the Yakima Valley

Near-field “perimeter” samples were conducted to determine air concentrations at the 

edge of an application site

A 4 day sampling schedule was used with samples collected on the pre-spray day, on the 

spray day and post-spray days 1 and 2.

Sampling occurred at two locations: North Central Perimeter Site and Yakima Valley 

Perimeter Site.

North Central Permiter Site (April 6 to April 10, 2008):

o Located in a 4.02 acre study block at the end of a young vertical trellis apple 

orchard.

o Sampling stations were located 25 ft out from the perimeter of the block (outside 

row of trees).

o A total of 1.5 lbs chlorpyrifos was applied using airblast equipment. 

o The maximum chlorpyrifos air concentration measured was 1,145 ng/m3and the 

average was 153 ng/m3 (air concentration measure for chlorpyrifos include the

addition of oxon measures as described in Section 4.0).

Yakima Valley Perimeter Site (March 31 to April 5, 2008):

o Located in a 5.02 acre study block in a mature traditional apple orchard situated 

near a mix of tree fruit blocks.
Page 94 of 107

o Sampling stations were located 20 ft out from the perimeter of the block (outside 

row of trees).

o A total of 10 lbs chlorpyrifos was applied using airblast equipment. 

o The maximum chlorpyrifos air concentration measured was 1,002 ng/m3and the 

average was 294 ng/m3 (air concentration measure for chlorpyrifos include the 

addition of oxon measures as described in Section 4.0).

Tulare County, CA CARB report from 1998 – Application Site Monitoring

Application site monitoring was conducted during June 1996 before, during, and for 72 

hours after an application to an orange grove in Tulare County.

Chlorpyrifos was applied via airblast at the rate of 6 lbs ai/A. 

Samplers were generally placed on each side of the field, at a distance of about 10 to 20 

yards from the edge of the field.

The sampling schedule is as follows: 

o A minimum 1-hour background sample within 24 hours prior to application.

o A combined sample consisting of sampling during and 1 hour after application. 

o A 2-hour sample from 1 to 3 hours after application. 

o A 4-hour sample from 3 to 7 hours after application. 

o Two consecutive 8-hour samples from 7 to 15 and 15 to 23 hours after 

application.

o Two consecutive 24-hour samples starting at the end of the second 8-hour sample.

o The maximum positive chlorpyrifos detection over the sampling interval was 

27,700 ng/m3and the average chlorpyrifos air concentration detected was 6,448 

ng/ m

3

.

o All of the application samples, including background samples, were found to be 

above the LOQ of 0.20 ug per sample for both chlorpyrifos and chlorpyrifos oxon 

(equivalent to an air concentration of 7 ng/m3 for a 24-hour sample).

Ambient Monitoring

Washington Department of Health (WA DOH) – Organophosphorus Pesticide Air Monitoring 

Project - Ambient Air Monitoring

3 sample types were employed (1 application site and 2 ambient) to determine levels of 

chlorpyrifos and chlorpyrifos oxon in residential communities in 2 regions, North Central 

and the Yakima Valley.

“Ambient community” data were collected from 4 samplers outside different houses in 

residential communities.

Near-field “receptor" collection sites were located in areas believed to receive relatively 

high exposure during airblast applications to tree fruit.

North Central Receptor and Ambient Sites (41 days starting March 24, 2008):

o Samples were collected over 31 days (due to snow the first 10 days) for 24 hours 

at a time.

o No orchards were within 1,000 meters of ambient sampling sites.

o Every receptor site was in close proximity to (bordered by) a conventional apple 

orchard, ranging from 9 to 276 feet away from the nearest orchards.

o The maximum chlorpyrifos air concentration measured at the ambient site was 21 
Page 95 of 107

ng/m3and the average was 7 ng/m3. The maximum measured at the receptor sites

was 607 ng/m3and the average was 33 ng/m3(air concentration measure for 

chlorpyrifos include the addition of oxon measures as described in Section 4.0).

Yakima Valley Receptor and Ambient Sites (March 7 to April11, 2008):

o Sampling period was 24 hours with the exception of one 48 hour sample.

o No orchards were within 1,000 meters of ambient sampling sites.

o Every receptor site was near conventional apple orchards, ranging from 72 to 

1,145 feet away from the nearest orchards.

o The maximum chlorpyrifos air concentration measured at ambient sites was 30

ng/m3and the average was 9 ng/m3. The maximum measured at the receptor sites 

was 243 ng/m3and the average was 30 ng/m3(air concentration measure for 

chlorpyrifos include the addition of oxon measures as described in Section 4.0).

Lindsay, CA PANNA Drift Catcher report from 2005 – Ambient Air Monitoring

Collected data from 5 samplers in 2004 (July 13 to August 2) and 4 samplers in 2005 

(June 13 to July 22).

The samplers ranged from 50 to 400 feet away from orange groves.

Monitoring was conducted to coincide with summer use of chlorpyrifos as an insecticide 

on oranges for the control of lepidopterous pests and scale.

It is unknown when applications occurred during the monitoring period.

80% of samples were found to be above the LOQ of 30 ng chlorpyrifos per sample 

(equivalent to an air concentration of 6 ng/m3 for a 24-hour sample).

Yakima Valley, WA PANNA Drift Catcher report from 2006 – Ambient Air Monitoring

Collected data from a sampler located in Tieton, WA in 2005 (April 1 to 21) and from a 

sampler located in Cowiche, WA in 2006 (April 3 to 23).

The Cowiche sampler was set up 57 feet from the orchard and the Tieton sampler was set 

up 46 feet from the orchard.

Monitoring was conducted to coincide with spring use of chlorpyrifos as an insecticide 

on apples, pears and cherries for the control of coddling moth, leaf roller and other pomefruit pests.

It is unknown when applications occurred during the monitoring period.

All 21 samples at both locations were found to be above the LOQ of 20 ng chlorpyrifos 

per sample (equivalent to an air concentration of 7 ng/m3 for a 24-hour sample).

Lompoc, CA CARB report from 2003 – Ambient Air Monitoring

Collected data from four samplers located around Lompoc, CA (May 31 to August 3, 

2000).

Monitoring was conducted around Lompoc because agricultural fields dominate the area 

between Lompoc and the coast. The five major crops or crop groups grown in this area 

include cole crops (broccoli, cabbage, and cauliflower), lettuce, dried beans, celery, and 

flowers.

It is unknown when applications occurred during the monitoring period.

34.2% of samples were found to have at least trace amounts of chlorpyrifos. Only seven 

samples were found to be above the estimated quantitation limit.
Page 96 of 107

Tulare County, CA CARB report from 1998 – Ambient Air Monitoring

Collected data from four samplers located in Tulare County, CA (May 28 to June 28, 

1996).

Monitoring was conducted in Tulare County, CA because orange groves are predominant 

in this area.

It is unknown when applications occurred during the monitoring period.

74% of samples were found to be above the LOQ of 200 ng chlorpyrifos per sample 

(equivalent to an air concentration of 9.4 ng/m3 for a 24-hour sample).

The maximum positive chlorpyrifos detection over the sampling interval was 815 ng/m3

and the average detected was 53 ng/m3. 

3.3.1 Residential Bystander Exposure and Risk 

HED has assessed residential bystander exposure to chlorpyrifos based on the available ambient 

and application site air monitoring data. The chlorpyrifos bystander volatilization inhalation 

exposure assessment includes both acute and short-/intermediate-term exposure scenarios. The 

acute scenario compares the maximum air concentration detected in the monitoring studies to the 

acute HEC. The short-/intermediate-term scenario compares the arithmetic mean chlorpyrifos air 

concentration from several monitoring studies to the short -term HEC. 

Of the 24 acute ambient air concentrations assessed, 4 result in a risk of concern to HED (i.e., an 

MOE < 300). No short-/intermediate-term ambient data assessed result in a risk of concern. Of 

the 5 acute application site air concentrations assessed, 3 resulted in a risk of concern. Of the 5 

short- and intermediate-term application site air concentrations assessed, 4 resulted in risks of 

concern (i.e., an MOE < 30).

Table 3.3.1: Chlorpyrifos Preliminary Volatilization MOE Analysis for Residential 

Bystanders

Study

Year 

of 

Study

Sampler/Site Location

Maximum 

Air 

Concentration 

(ng/m3)

Arithmetic 

Mean Air 

Concentration 

(ng/m3)

Acute 

MOEsa

(LOC is 

an MOE 

= 300)

Short- / 

Int.-term 

MOEsb

(LOC is 

an MOE 

= 30)

Ambient Air Data

Washington DOH 2008

North Central District

Ambient 21 7 29,000 850

North Central District

Receptor 607 33 1,000 180

Yakima Valley Ambient 30 9 21,000 620

Yakima Valley Receptor 243 30 2,500 190

Lompoc County, 

CA (CARB) 2003

Central 8.3 1.5 19,000 3,800

Northwest 8.4 0.84 19,000 6,800

Southwest 6.8 0.78 24,000 7,400

West 17 2.3 9,400 2,500

Tulare, CA 

(CARB) 1996

Air Resource Board 39 10 16,000 590

Jefferson Elementary 

School 432 94 1,400 61

Kaweah School 412 70 1,500 82
Page 97 of 107

Table 3.3.1: Chlorpyrifos Preliminary Volatilization MOE Analysis for Residential 

Bystanders

Study Year 

of 

Study

Sampler/Site Location Maximum 

Air 

Concentration 

(ng/m3)

Arithmetic 

Mean Air 

Concentration 

(ng/m3)

Acute 

MOEsa

(LOC is 

an MOE 

= 300)

Short- / 

Int.-term 

MOEsb

(LOC is 

an MOE 

= 30)

Sunnyside Union 

Elementary School 815 52 760 110

University of CA, 

Lindcove Field Station 168 39 3,700 150

Cowiche, WA 

(PANNA) 2006 462 155 350 37

Tieton, WA

(PANNA) 2005 475 182 340 32

Lindsay, CA

(PANNA) 2004 Blue House 137 54 1,200 110

Lindsay, CA

(PANNA) 2004 Green House 720 120 220 50

Lindsay, CA

(PANNA) 2004 Orange House 1,340 190 120 30

Lindsay, CA

(PANNA) 2004 Purple House 180 48 900 120

Lindsay, CA

(PANNA) 2004 Red House 90 43 1,800 130

Lindsay, CA

(PANNA) 2005 Blue House 421 107 380 54

Lindsay, CA

(PANNA)

2005 Green House 1,119 177 140 32

Lindsay, CA

(PANNA)

2005 Orange House 561 188 290 31

Lindsay, CA

(PANNA)

2005 Purple House 515 123 310 47

Application Site Data

Washington DOH 2008

North Central District 

Perimeter Site 1145 153 540 37

Yakima Valley Perimeter 

Site 1,002 294 620 20

Tulare, CA 

(CARB) 1996

North 27,700 7,706 22 1

East 14,700 5,974 42 1

South 25,400 5,664 24 1

a. Acute MOE = Acute HEC (620,000 ng/m3) / Study maximum air concentration (ng/m3). LOC = 300

b. Short-term MOE = Short-term HEC (5,700 ng/m3) / Study arithmethic mean air concentration (ng/m3). LOC = 30

3.3.2 Residential Bystander Exposure and Risk Characterization

Some of the limitations and considerations that have been identified that should be considered in 

the interpretation of these results include:

Most of the data utilized in this preliminary assessment are 24-hour air samples. When 

this data is used, an assumption is made that an individual is exposed to the same air 

concentration for 24-hours every day. However, this is not always the case as real world 

time-activity data indicate that many parts of the population move from site to site on a 

daily basis (e.g., go to work and back).
Page 98 of 107

This assessment is only representative of outdoor concentrations (i.e., the exposure and 

risk estimates assume an individual is outdoors all the time). It does not take into account 

potential effects of air conditioning systems and similar air filtration systems which could 

potentially reduce air concentrations of chlorpyrifos indoors. The Agency believes that 

indoor concentrations will be at worst equivalent to outdoor concentrations and may 

potentially be lower.

All of the data used for this analysis have been generated in California and Washington; 

however, chlorpyrifos is used in many regions throughout the country. Therefore, the 

results based on the limited available air monitoring data were used to represent the rest

of the country due to a lack of adequate information for any other region. It is unclear 

what potential impacts this extrapolation might have on the risk assessment. Factors such 

as meteorology and cultural practices may impact the overall amounts of chlorpyrifos that 

volatilize from a treated field as well as the rate at which it volatilizes.

As part of the December 2009 SAP, the Agency presented their analysis of several 

models that could be used as screening tools to predict the air concentration and

volatilization flux based on intrinsic properties and transport behaviors of pesticides. 

These models would allow the Agency to better represent the potential volatilization of 

semi-volatile pesticides across various regions of the country and thus would provide 

refinement to this assessment over using straight air monitoring data. The SAP provided 

a number of comments regarding the Agency‟s model analysis, including the 

recommendation to evaluate some additional models. The Agency is currently in the 

process of evaluating the SAP‟s comments. As appropriate, the Agency will revise the 

modeling approach presented to the SAP for determining the rate of volatilization (flux) 

for semi-volatile pesticides and for estimating air concentrations of applied pesticides in 

the atmosphere under varying environmental conditions. After any policies or procedures 

are put into place, the Agency may revisit the chlorpyrifos residential bystander exposure 

and risk assessment.

At this point in time, HED is not including the residential bystander estimated exposure 

in the chlorpyrifos human health risk assessment aggregate due to the limitations 

surrounding this preliminary assessment.

3.4 Spray Drift

Spray drift is a potential source of exposure to residents nearby to spraying operations. This is 

particularly the case with aerial application, but, to a lesser extent, could also be a potential 

source of exposure from the ground application method employed for chlorpyrifos. The Agency 

has been working with the Spray Drift Task Force, EPA Regional Offices and State Lead 

Agencies for pesticide regulation and other parties to develop the best spray drift management 

practices (see the Agency‟s Spray Drift website for more information at 

http://www.epa.gov/opp00001/factsheets/spraydrift.htm). On a chemical by chemical basis, the 

Agency evaluates the need for interim mitigation measures for aerial applications for placement 

on labels/labeling. The Agency has completed its evaluation of the new database submitted by 

the Spray Drift Task Force, a membership of U.S. pesticide registrants, and is developing a 
Page 99 of 107

policy on how to appropriately apply the data and the AgDRIFT computer model to its risk 

assessments for pesticides applied by air, orchard airblast and ground hydraulic methods. After 

the policy is in place, the Agency may seek further refinements in spray drift management 

practices to reduce off-target drift with specific products with significant risks associated with 

drift.

A quantitative residential post-application (acute) inhalation exposure (spray drift) assessment 

was conducted for ground and aerial ULV mosquitocide applied by a public agency made in the 

vicinity of residential areas (Section 3.2 above). HED used AgDRIFT (V 2.01), which is the 

model that was developed as a result of the efforts of the Spray Drift Task Force (SDTF), to 

determine residue deposition and the airborne concentration of chlorpyrifos anticipated from 

aerial product application. Based upon this assessment, it was determined that adult and child 

inhalation exposures following aerial mosquito treatments are not of concern to HED (i.e., an 

MOE > 300). Inhalation exposure from ground based ULV treatment was assessed by assuming 

that the entire active ingredient applied to a 1 acre area is airborne and available to be inhaled by 

a child or adult. The assessment of adult and child acute inhalation from ground mosquito 

treatment resulted in a risk estimate of concern to HED.

4.0 Evaluation of Chlorpyrifos Oxon 

Chlorpyrifos is activated by desulfuration, reacting in bioactivation to the more toxic and potent 

acetyl cholinesterase (AChE) inhibitor, chlorpyrifos oxon. The oxon is highly unstable due to 

rapid deactivation through hydrolytic cleavage by a process called dearylation which releases 

TCP. HED has considered the exposure potential for occupational and residential exposure to 

chlorpyrifos oxon. Workers reentering an environment previously treated with chlorpyrifos 

(occupational post-application) and the general population residing near chlorpyrifos application 

sites (bystanders) could potentially be exposed to the oxon as chlorpyrifos is metabolized. 

Dermal exposure to the oxon could occur through contact with chlorpyrifos treated surfaces and 

inhalation exposure through airborne oxon. However, the likelihood of exposure to the oxon is 

slight due to its rapid deactivation to TCP. In an effort to further explore the potential for oxon 

exposure, HED has researched and reviewed all available information sources. Based upon this 

review, HED intends to require additional studies to address uncertainties regarding the 

formation of chlorpyrifos oxon in the air post-application and its formation and decay in 

greenhouses. 

No occupational exposure studies (handler, post-application or DFR) were identified that sought 

to measure the presence of the oxon. Chlorpyrifos and chlorpyrifos oxon were measured in 

several air monitoring studies (2 application site and 3 ambient air monitoring) conducted to 

assess inhalation exposure from pesticide volatilization. Study summaries and measured oxon 

residues are described below in Section 4.1. A search of open literature resulted in 4 

metabolism studies which measured whole fruit and leaf surface residue of chlorpyrifos and 

chlorpyrifos oxon. HED typically uses DFR studies, as was done in Section 2.4 of this 

document, to assess occupational post-application exposure to handlers based on the contact and 

transfer of residues from leaf surfaces. 3 of the 4 metabolic studies identified measured leaf 

surface residues in addition to whole fruit/tissue samples. While the fourth study, Diminution of 

Chlorpyrifos and Chlorpyrifos Oxon in Tomatoes and Green Beans Grown in Greenhouses, 
Page 100 of 107

measured only whole fruit resides, it has been determined to be useful as resource for 

characterization of the oxon in an indoor environment. A summary of the studies and results are 

described below in Section 4.1. HED‟s interpretation and application of the studies is described 

in Section 4.2.

4.1 Oxon Study Summaries

Air Monitoring Data: 

Application Site

Washington Department of Health (WA DOH) – Organophosphorus Pesticide Air Monitoring 

Project 

3 sample types were employed (1 application site and 2 ambient) to determine levels of 

chlorpyrifos and chlorpyrifos oxon in residential communities in 2 regions, North Central 

and the Yakima Valley

Near-field “perimeter” samples were conducted to determine air concentrations at the 

edge of an application site

A 4 day sampling schedule was used with samples collected on the pre-spray day, on the 

spray day and post-spray days 1 and 2.

Sampling occurred at two locations: North Central Perimeter Site and Yakima Valley 

Perimeter Site.

North Central Permiter Site (April 6 to April 10, 2008):

o Located in a 4.02 acre study block at the end of a young vertical trellis apple 

orchard.

o Sampling stations were located 25 ft out from the perimeter of the block (outside 

row of trees).

o A total of 1.5 lbs chlorpyrifos was applied using airblast equipment. 

o The maximum chlorpyrifos oxon concentration measured was 61.3 ng/m3

and the average was 13.5 ng/m3.

Yakima Valley Perimeter Site (March 31 to April 5, 2008):

o Located in a 5.02 acre study block in a mature traditional apple orchard situated 

near a mix of tree fruit blocks.

o Sampling stations were located 20 ft out from the perimeter of the block (outside 

row of trees).

o A total of 10 lbs chlorpyrifos was applied using airblast equipment. 

o The maximum chlorpyrifos oxon concentration measured was 124 ng/m3and 

the average was 27 ng/m3.

Tulare County, CA CARB report from 1998 – Application Site Monitoring

Application site monitoring was conducted during June 1996 before, during, and for 72 

hours after an application to an orange grove in Tulare County.

Chlorpyrifos was applied via airblast at the rate of 6 lbs ai/A. 

Samplers were generally placed on each side of the field, at a distance of about 10 to 20 

yards from the edge of the field.

The sampling schedule is as follows: 
Page 101 of 107

o A minimum 1-hour background sample within 24 hours prior to application.

o A combined sample consisting of sampling during and 1 hour after application. 

o A 2-hour sample from 1 to 3 hours after application. 

o A 4-hour sample from 3 to 7 hours after application. 

o Two consecutive 8-hour samples from 7 to 15 and 15 to 23 hours after 

application.

o Two consecutive 24-hour samples starting at the end of the second 8-hour sample.

o The maximum positive oxon detection was 1,850 ng/m3 over the sampling 

period and the average chlorpyrifos air concentration detected was 703 

ng/m3.

o All of the application samples, including background samples, were found to be 

above the LOQ of 0.20 ug per sample for both chlorpyrifos and chlorpyrifos oxon 

(equivalent to an air concentration of 7 ng/m3 for a 24-hour sample).

Ambient

Washington Department of Health (WA DOH) – Organophosphorus Pesticide Air Monitoring 

Project 

3 sample types were employed (1 application site and 2 ambient) to determine levels of 

chlorpyrifos and chlorpyrifos oxon in residential communities in 2 regions, North Central 

and the Yakima Valley.

“Ambient community” data were collected from 4 samplers outside different houses in 

residential communities. 

Near-field “receptor" collection sites were located in areas believed to receive relatively 

high exposure during airblast applications to tree fruit.

North Central Receptor and Ambient Sites (41 days starting March 24, 2008):

o Samples were collected over 31 days (due to snow the first 10 days) for 24 hours 

at a time.

o No orchards were within 1,000 meters of ambient sampling sites.

o Every receptor site was in close proximity to (bordered by) a conventional apple 

orchard, ranging from 9 to 276 feet away from the nearest orchards.

o The maximum chlorpyrifos oxon concentration measured at ambient the 

sites was 6.2 ng/m3and the average was 2.2 ng/m3. The maximum measured 

at the receptor sites was 110 ng/m3and the average was 7.1 ng/m3. 

Yakima Valley Receptor and Ambient Sites (March 7 to April11, 2008):

o Sampling period was 24 hours with the exception of one 48 hour sample.

o No orchards were within 1,000 meters of ambient sampling sites.

o Every receptor site was near conventional apple orchards, ranging from 72 to 

1,145 feet away from the nearest orchards.

o The maximum chlorpyrifos oxon concentration measured at the ambient site 

was 9.9 ng/m3and the average was 3.6 ng/m3. The maximum measured at 

the receptor sites was 21 ng/m3and the average was 4.0 ng/m3.

Lindsay, CA PANNA Drift Catcher report from 2005 – Ambient Air Monitoring

Collected data from 5 samplers in 2004 (July 13 to August 2) and 4 samplers in 2005 

(June 13 to July 22).
Page 102 of 107

The samplers ranged from 50 to 400 feet away from orange groves.

Monitoring was conducted to coincide with summer use of chlorpyrifos as an insecticide 

on oranges for the control of lepidopterous pests and scale.

It is unknown when applications occurred during the monitoring period.

Chlorpyrifos oxon was not detected in any of the samples.

Yakima Valley, WA PANNA Drift Catcher report from 2006 – Ambient Air Monitoring

Collected data from a sampler located in Tieton, WA in 2005 (April 1 to 21) and from a 

sampler located in Cowiche, WA in 2006 (April 3 to 23).

The Cowiche sampler was set up 57 feet from the orchard and the Tieton sampler was set 

up 46 feet from the orchard.

Monitoring was conducted to coincide with spring use of chlorpyrifos as an insecticide 

on apples, pears and cherries for the control of coddling moth, leaf roller and other pomefruit pests.

It is unknown when applications occurred during the monitoring period.

All 21 samples at both locations were found to be above the LOQ of 20 ng chlorpyrifos 

per sample (equivalent to an air concentration of 7 ng/m3 for a 24-hour sample).

Chlorpyrifos oxon was not detected in any of the samples.

Lompoc, CA CARB report from 2003 – Ambient Air Monitoring

Collected data from four samplers located around Lompoc, CA (May 31 to August 3, 

2000).

Monitoring was conducted around Lompoc because agricultural fields dominate the area 

between Lompoc and the coast. The five major crops or crop groups grown in this area 

include cole crops (broccoli, cabbage, and cauliflower), lettuce, dried beans, celery, and 

flowers.

It is unknown when applications occurred during the monitoring period.

11.4% of samples were found to have at least trace amounts of the chlorpyrifos oxon. 

Only one sample was found to be above the estimated quantitation limit.

Tulare County, CA CARB report from 1998 – Ambient Air Monitoring

Collected data from four samplers located in Tulare County, CA (May 28 to June 28, 

1996).

Monitoring was conducted in Tulare County, CA because orange groves are predominant 

in this area.

It is unknown when applications occurred during the monitoring period.

70% of samples were found to be above the LOQ of 200 ng chlorpyrifos oxon per sample 

(equivalent to an air concentration of 9.4 ng/m3 for a 24-hour sample).

The maximum positive chlorpyrifos oxon detection over the sampling interval was 230 

ng/m3and the average detected was 32 ng/m3. 

Fruit Metabolism Study Oxon Data

Chlorpyrifos Applied to California Citrus: Residue Levels on Foliage and on and in Fruit. 

Iwata, Y., et al. 1983. American Chemical Society.
Page 103 of 107

Lorsban® applied to California orange (Navel and Valencia) and grapefruit trees. Residue data 

collected to assist in setting worker reentry intervals and legal fruit tolerances. Dissipation data 

for chlorpyrifos and its oxygen analogue (oxon) on citrus foliage was determined. Both 

dissipated rapidly, with the maximum oxon level found was 0.033 μg/cm2 in a 3-day sample. 

Application rates ranged from 5.0 to 10. lbs ai/ A for all trees treated. 

August 1979: Maximum temperature on the day of application was 32.2 C and for 23 days postapplication the daily maximum air temperature was above 35.0 C. Between 7 and 18 days postapplication, the temperature was at or above 37.8 C. Chlorpyrifos residues dissipated rapidly 

under these conditions. 

Chlorpyrifos Residue: For the 5.0 lb ai/A residues were 0.013 and 0.005 μg/cm2 after 4 and 10 

days, respectively. At 10.0 lb ai/A residues were 0.031, 0.012 and 0.006 μg/cm2 at 4, 10, and 17 

days post-application. At 10.0 lb ai/A with low volume equipment residues were 0.080, 0.21, 

0.015 and 0.008 μg/cm2 at 4, 10, 17 and 31 days, respectively. Due to rapid dissipation, no 

meaningful dissipation curve line could be described. 

Oxon: No oxon above the 0.01 μg/cm2 level sought was present due to the very rapid 

disappearance of the parent and likely any oxon formed. 

August 1981 - The maximum air temperature was, except for 1 day, at or above 33.3 C for at 

least 16 days post-application and 9 of these days had temperatures between 37.2 and 43.3 C. 

total rainfall over the entire period was 5.9 mm and maximum rainfall over a 24 hour period was 

3.3 mm. 

Chlorpyrifos Residue: Orange Trees - For 10 lbs ai/A residues were approximately 0.27, 0.028 

and 0.008 μg/cm2 after 3, 10 and 17 days post-application, respectively. For 10 lbs ai/A applied 

with low-volume equipment residues were 0.70, 0.033 and 0.015 μg/cm2, respectively. 

Oxon: Orange Trees – A trace amount of oxon was (0.014 μg/cm2) was found only in the 3-day 

sample. One field sample was collected for each of the three replicate field plots on each 

sampling date for each application rate. The 3-day sample was approximately 19X below 

chlorpyrifos measure at the high volume application and 50X below low volume on the 

same day.

Chlorpyrifos Residue: Grapefruit Trees - For 5 lbs ai/A low volume application residues were 

approximately 0.40, 0.025 and 0.0070 after days 3, 10 and 17, respectively. For 10 lbs ai/A 

applied with low volume equipment residues were approximately 0.45, 0.90, 0.020 after days 4, 

10 and 17, respectively.

Oxon: On day 3 oxon residues were 0.028 and 0.0050 for the 5 and 10 lb ai/ A applications, 

respectively. On day 10 residues were below LOQ and 0.013 for the 5 and 10 lb ai applications. 

Day 3 oxon samples were 14X and 90X below chlorpyrifos measured for the 5 and 10 lb 

applications, respectively, and below LOQ and 69X below the chlorpyrifos 10 lb 

application measure for the same days. 
Page 104 of 107

Diminution of Chlorpyrifos and Chlopyrifos Oxon in Tomatoes and Green Beans Grown in 

Greenhouses. J. I. Martinez, et al. 1998. Journal of Agricultural and Food Chemistry. 

The study reported the results obtained in different field trials to establish the influence of 

different growing conditions and application rates in the presence of chlorpyrifos and oxon 

applied on two different commodities. Experiments were conducted in two types of greenhouses 

in Almeria, Spain: 1) a flat roof greenhouse of polyethylene 2) an asymmetric greenhouse with a 

window on the roof. Green beans and tomatoes were grown in 0.5 hectare (1.2 acre) plots 

incorporating 30,000 and 10,000 plants, respectively. Dursban® was applied at rates of 0.9 lb 

ai/A (half dose) and 1.5 lb ai/ A (full dose). The treatments were carried out in the spring of 

1996 and winter 1997. Samples were collected at random at 0, 0.5, 1, 2, 3, 4, 5, 8 and 15 days 

after application. Each sample was chopped and divided into four subsamples which were stored 

until extraction. Measures were reported in units mg ai/kg sample weight. Chlorpyrifos residue 

was measured for green beans and tomatoes in winter (full dose), winter (half dose), spring (full 

dose) and spring (half dose). Corresponding chlorpyrifos oxon residue was measured in the 

spring (full dose) sampling period only. For the purpose of this summary review, only spring 

full dose measures are presented for chlorpyrifos and oxon for comparison. 

Chlorpyrifos: 

Flat roof greenhouse green beans - Residue measures approximately 1.70, 1.65, 1.55, 1.30, 1.25, 

0.90, 0.80, 0.60 and 0.20 mg/kg at 0, 0.5, 1, 2, 3, 4, 5, 8 and 15 days after application. 

Asymmetrical roof greenhouse green beans – Residue measures approximately 1.70, 1.68, 1.60, 

1.40, 1.15, 0.90, 0.75, 0.40 and 0.13 at 0, 0.5, 1, 2, 3, 4, 5, 8 and 15 days after application.

Flat roof greenhouse tomatoes - Residue measures approximately 1.70, 1.60, 1.20, 1.10, 0.80, 

0.70, 0.60, 0.35 and 0.15 at 0, 0.5, 1, 2, 3, 4, 5, 8 and 15 days after application.

Chlorpyrifos Oxon: 

Flat roof greenhouse green beans - Residue measures approximately 0.125 (14X below 

chlorpyrifos measured), 0.17 (10X), 0.15 (10X), 0.13 (10X), 0.20 (6X), 0.13 (7X), 0.13 (6X), 

0.0 and 0.0 at 0, 0.5, 1, 2, 3, 4, 5, 8 and 15 days after application.

Asymmetrical roof greenhouse green beans – Residue measures approximately 0.125 (14X), 0.17 

(10X), 0.15 (11X), 0.13 (11X), 0.20 (6X), 0.13 (7X), 0.13 (6X), 0.0 and 0.0 at 0, 0.5, 1, 2, 3, 4, 5, 

8 and 15 days after application.

Flat roof greenhouse tomatoes - Residue measures approximately 0.125 (14X), 0.17 (9X), 0.15 

(8X), 0.13 (8X), 0.20 (4X), 0.13 (5X), 0.13 (5X), 0.0 and 0.0 at 0, 0.5, 1, 2, 3, 4, 5, 8 and 15 days 

after application.
Page 105 of 107

Persistence, Penetration, and Surface Availability of Chlorpyrifos, Its Oxon, and 3,5,6-

Trichloro-2-pyridinol in Elm Bark. Heng Jin and G. R. Barrie Webster. 1997. Journal of 

Agricultural and Food Chemistry.

The persistence, penetration and bark surface contact availability of chlorpyrifos and its main 

degradation products, the oxon and 3,5,6-trichloro-2-pyridinol, in elm bark were investigated 

after chlorpyrifos had been applied to the basal 1 m of elm tree trunks at different application 

times and sites. Experiments were carried out in two sites in Canada using Dursban® applied by 

backpack sprayer. Samples of elm bark including cambium layer were collected at time 0 (1 

hour after), 4, 11, 32, 62, 279, 341, 427, 627, and 791 days after application on August 15, 1991 

and 0, 4, 14, 30, 74, 141, 364, 433, and 532 days after application on May 7, 1992. The 

availability of bark surface residues of chlorpyrifos was determined by collecting pressing glass 

fiber filter paper against the chlorpyrifos treated bark surface for 1 minute. One sample was 

taken from one of the three trees in a plot (8 plots total) at each sampling time. No measure of 

the oxon or pyridinol was collected for the surface residue portion of the study. The initial 

measure of surface chlorpyrifos was 1.9 mg/m3and dropped rapidly to 0.23 mg/m3by the 4 day 

sample. Wood tissue and bark disk samples were also collected to determine the residues present 

and degradation of these residues in elm bark, cambium and wood tissues. Oxon residues were 

not detected at any time from samples taken from elm bark at any experimental site or 

application time.

The Persistence and Degradation of Chlorthalonil and Chlorpyrifos in a Cranberry Bog. 

Putnam, R., et al. 2003. Journal of Food and Agricultural Chemistry. 

The effect of a spray-tank adjuvant on the persistence, distribution, and degradation of two 

pesticides, chlorthalonil and chlorpyrifos was studied in a commercial cranberry bog. Pesticides 

were applied according to label instructions to cranberry plants in parallel plot studies. 

Dislodgeable foliar and whole fruit residues of both pesticides and several degradation products 

were assessed over a growing season. Residues were also assessed in soil samples collected at 

fruit harvest. Four experimental plots on an established cranberry bog in Plymouth, MA were 

used for backpack applications of Bravo (chlorthalonil) and Lorsban®® (chlorpyrifos) with and 

without adjuvant. Adjuvant increased both fruit and foliar residues but did not significantly alter 

the dissipation rate or metabolism of either pesticide. Chlorpyrifos applications were made at a 

rate of 2.0 L/ha. Chlorpyrifos applications in the presence of an adjuvant were made at 0.5 L/ha. 

Foliage samples were initially collected at 1 hour post-application, followed by 3, 7, 13, and 28 

days post-application using pre-weighed cotton cheesecloth. Chlorpyrifos was detected in fruit at 

harvest (62 days post-application), but no metabolites were found. Chlorpyrifos-oxon and 3,5,6-

trichloro-2-pyridinol, however, were detected in earlier fruit samples and in foliage and soil 

samples. 

Applications of Lorsban®® in the presence and in the absence of adjuvant resulted in initial (2 

hr) dislodgeable foliar chlorpyrifos residues of 125.4 and 52.5 mg/kg, respectively. The 

dissipation of dislodgeable foliar residues also followed first-order kinetics (r2= 0.893 and 

0.913) and displayed a more rapid dissipation from cranberry leaves, with estimated half-lives of 

3.3 and 3.8 days in the presence an in the absence of adjuvant, respectively. After 3 days, 33.5 
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and 19.1% of the initial residues remained, and after 15 days, 3.0 and 4.9% of the initial residues 

remained, in the presence and absence of the adjuvant. Small amounts of chlorpyrifos oxon 

were initially detected (< 7 μg/kg), but residues did not accumulate. 

4.2 Interpretation and Application of Oxon Studies

HED has reviewed the available oxon studies and considered the potential for occupational postapplication and residential bystander exposure and risk. Exposure of the general population 

residing near chlorpyrifos application sites (bystanders) is possible, but the assessment of 

exposure and risk is characterized by uncertainty. Based upon review of the available air 

monitoring studies which measured the oxon, all application location sites (3) and 4 of the 6 

ambient monitoring sites had measurable quantities. A comparison of average chlorpyrifos and 

chlorpyrifos oxon measured (ng/m3) for each site ranged from 4 to 11X below the parent for 

application air monitoring sites and 2 to 8X for ambient sites (2 had no measurable quantity and 

another only had one measure above the level of detection). However, Washington Department 

of Health‟s Final Report on the Organophosphorus Pesticide Air Monitoring Project, which 

summarizes the result of chlorpyrifos and oxon air monitoring exposure data summarized 

previously, states the following: “Most samples collected in this study had measurable amounts 

of either chlorpyrifos or chlorpyrifos-oxon or azinphos-methyl-oxon. Previous studies in 

California have indicated that the oxygen analogs can be produced artificially by the sampling 

process. For example, a chlorpyrifos molecule captured in the air sampling tube can be 

converted to chlorpyrifos-oxon during the sampling period.” In such cases, WA DOH elected to 

combine the concentration of the parent and the oxon to produce a total pesticide concentration 

for each sample. Furthermore, WA DOH conducted both a laboratory study and a field study to 

determine the extent to which the oxygen analog of chlorpyrifos could be formed artificially 

during sampling. Preliminary analysis of these studies suggested that some of the oxon 

measured in their 2008 samples was present in the air sample, and because the oxon is more 

toxic than the parent it was determined that, “additional research would be conducted to fully 

characterize potential formation of oxygen analogs of OP pesticides in the air where OP 

pesticides are commonly applied.” No further publications from WA DOH regarding the topic 

were identified. 

HED acknowledges the presence of the oxon in the air monitoring studies; however, it is not 

known what the fraction of the oxon residue measured in the studies is present in the 

environment to be inhaled, or formed in the sampling media. The oxon has been determined by 

HED to be between 11.9 (acute) and 18X (chronic) times more toxic than the parent. If HED 

were to assume 100% formation of the oxon in the outdoor environment and compare the ratios 

of oxon to chlorpyrifos measured for each site (as summarized in the previous paragraph), the 

potential would exist for risk estimates to be of greater concern to HED (i.e., a lower MOE) than 

the parent for a few of the exposure sites monitored. In order to present a quantification of 

exposure and risk to the general population residing near chlorpyrifos application sites, 

additional information is required to address these uncertainties. Specifically, HED needs to 

know whether the oxon concentration measured in the outdoor environment from air monitoring 

is due entirely to formation in sampling media or if the formation is fractional. If the formation 

is determined to be fractional, HED needs to know the percent of that measured in the sampling 

media which represents an amount that could be inhaled by the general population. 
Page 107 of 107

Dermal exposure to the oxon on foliar surfaces from reentry into an outdoor environment 

previously treated with chlorpyrifos is not anticipated and, therefore, has not been assessed. The 

3 fruit metabolism studies which measured leaf surface residue of the oxon in outdoor 

environments were either well below the parent (14-90X), not detectable, or detected at a level 

just above the level of detection (LOD). The potential for exposure to the oxon is further 

minimized due to rapid deactivation of the oxon to TCP. HED‟s review of the fruit metabolism 

tomato and green bean greenhouse study is not as definitive. HED is concerned, based on study 

results, that the formation of the oxon may be greater and its deactivation slower in greenhouses 

when compared to the outdoor environment. The study results indicate that oxon is from 9 to 

14X less than the parent from fruit analyzed on the day of application in flat and asymmetric roof 

greenhouses. The proportion of oxon to parent is less for all days which measurable levels were 

observed (all but 8 and 15 days after application). The oxon was detected until day 5 with levels 

between 5 and 6X below that of the parent. It should be noted that residues of chlorpyrifos and 

oxon were measured from analysis of whole fruit samples and HED typically assesses 

occupational post-application exposure and risk based upon the potential for transfer from 

external, leaf surface residues. The whole fruit samples which include surface residues, as well 

as residues which may have been contained within the fruit sample likely represent a 

conservative measure of available residue because within-fruit contact is not anticipated. In 

order address these uncertainties and more accurately address the risk potential for exposure 

from occupational reentry into greenhouses treated with chlorpyrifos, HED requires a study to 

measure chlorpyrifos and oxon residues on leaf surfaces following treatment with a liquid 

formulation of chlorpyrifos in greenhouses.