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TABLE OF CONTENTS

1. EXECUTIVE SUMMARY .................................................................................................... 4

1.1. Introduction ...................................................................................................................... 4

1.1.1. Toxicity ......................................................................................................................... 4

1.1.2. Exposure ....................................................................................................................... 5

1.2. Results Summary.............................................................................................................. 6

2. PROBLEM FORMULATION ............................................................................................... 7

2.1. Approach .......................................................................................................................... 7

2.1.1. Air Dispersion Modeling .............................................................................................. 8

2.1.1.1. Prior Consideration of Available Air Dispersion Models......................................... 8

2.1.1.2. PERFUM Modeling Approach ................................................................................. 9

2.2. Stressors of Concern....................................................................................................... 10

2.2.1. Use Characterization................................................................................................... 12

2.2.1.1. Agricultural Use ...................................................................................................... 12

2.2.1.1.1. Application Rate Information .............................................................................. 14

2.2.1.1.2. Acres Treated Information .................................................................................. 14

2.2.1.2. Non-Agricultural Use.............................................................................................. 15

2.3. Hazard Characterization ................................................................................................. 15

2.3.1. Mode of Action and Adverse Outcomes .................................................................... 15

2.3.2. Human Incidents: Summary of Respiratory Effects ................................................... 18

2.3.3. Dose Response and Endpoint Selection ..................................................................... 21

2.3.3.1. Acute Inhalation Exposure ...................................................................................... 21

2.3.3.2. Acute Toxicity Adjustment Factor for Chlorpyrifos-oxon ..................................... 22

2.4. Exposure Characterization ............................................................................................. 23

2.4.1. Field Volatility Data ................................................................................................... 23

2.4.1.1. Alfalfa (MRID 48883201)7 ..................................................................................... 24

2.4.1.1.1. Study Summary ................................................................................................... 24

2.4.1.1.2. Flux Rates ............................................................................................................ 24

2.4.1.2. Potato (MRID 48998801)11..................................................................................... 26

2.4.1.2.1. Study Summary ................................................................................................... 26

2.4.1.2.2. Flux Rates ............................................................................................................ 27

2.4.2. Conclusions and Modeling Inputs .............................................................................. 27

3. RISK ASSESMENT ............................................................................................................. 31

3.1. Buffer Distributions........................................................................................................ 31

3.1.1. Alfalfa ......................................................................................................................... 31

3.1.1.1. Chlorpyrifos (Parent Only) ..................................................................................... 31

3.1.1.2. Total Toxic Chlorpyrifos Residues ......................................................................... 35

3.1.2. Potato .......................................................................................................................... 37

3.1.2.1. Chlorpyrifos (Parent Only) ..................................................................................... 37

3.2. Buffer Durations ............................................................................................................. 37

3.2.1. Alfalfa ......................................................................................................................... 37

3.2.1.1. Chlorpyrifos (Parent Only) ..................................................................................... 37

3.3. Risk Estimations ............................................................................................................. 39

4. ISSUES FOR CONSIDERATION ....................................................................................... 47

4.1. Air Monitoring Data ....................................................................................................... 47

4.2. Uncertainties ................................................................................................................... 47
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4.2.1. Hazard Assessment ..................................................................................................... 47

4.2.2. Application Rate Scaling ............................................................................................ 49

4.2.3. Crop to Crop Variation (Flux and Dispersion) ........................................................... 49

4.2.4. Formulation ................................................................................................................ 51

4.2.5. Application Method .................................................................................................... 53

4.2.6. Weather ....................................................................................................................... 54

4.2.7. Degradation in Air ...................................................................................................... 54

5. CONCLUSIONS................................................................................................................... 55

APPENDIX A: LIST OF ABBREVIATIONS AND ACRONYMS 

APPENDIX B: CHLORPYRIFOS USAGE DATA

APPENDIX C: FLUX CALCULATIONS 

APPENDIX D: CURRENT CHLORPYRIFOS BUFFER RESTRICTIONS 

ATTACHMENT 1: CHLORPYRIFOS MASTER USE SUMMARY DOCUMENT 

ATTACHMENT 2: PERFUM MODELING FILES 

 
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1. EXECUTIVE SUMMARY 

1.1. Introduction 

This assessment supplements the June 2011 Chlorpyrifos Preliminary Human Health Risk 

Assessment for Registration Review (HHRA)1

 and the recent spray drift assessment2 by 

evaluating the potential risk to bystanders, from exposure to vapor phase chlorpyrifos and 

chlorpyrifos-oxon emitted from treated fields following the application of chlorpyrifos. 

For the purposes of this analysis bystanders are those who live and/or work in proximity to 

treated fields, including children. Pesticides emitted from treated fields can travel to non-target 

areas which, depending on concentrations, could present a risk of concern. Bystander exposure 

from chlorpyrifos and chlorpyrifos-oxon emitted from treated fields depends on two main 

factors: 1) the rate at which these chemicals are emitted (described as the off-gassing, emission 

or flux) off treated surfaces such as crops and 2) how those vapors disperse in the air over and 

around the treated field. 

This assessment employs approaches EPA has used previously to assess inhalation exposures to 

fumigant pesticides3,4and is consistent with the recommendations of the December 2009 Federal 

Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel (SAP)5 meeting on the 

scientific issues associated with field volatilization of conventional (semi-volatile) pesticides. 

1.1.1. Toxicity

This evaluation of bystander risk from chlorpyrifos volatilization focuses only on the inhalation 

route of exposure. This volatilization assessment is based on an acute inhalation toxicity study6

using aerosolized chlorpyrifos which measured lung, plasma, red blood cell (RBC), and brain 

 

1

 Drew, D.; Britton, W.; Soderber, D.; Negrón-Encarnación, I.; Christensen, C.; Lowit, A.; Irwin, W.; Doherty, J.; 

Smegal, D. U.S. EPA Office of Chemical Safety and Pollution Prevention, Chlorpyrifos: Preliminary Human 

Health Risk Assessment for Registration Review, June 30, 2011 PC Code 059101 DP Barcode: 388070; EPAHQ-OPP-2008-0850-0025 2

 Dawson, J.; Bohaty, R.; Mallampalli, N. U.S. EPA Office of Chemical Safety and Pollution Prevention, 

Chlorpyrifos—Evaluation of the Potential Risks from Spray Drift and the Impact of Potential Risk Reduction 

Measures, June 20, 2012 PC Code 059101, DP Barcode 399483 and 399485; EPA-HQ-OPP-2008-0850-0105 3

 U.S. EPA 2004d. FIFRA Science Advisory Panel Meeting Minutes - Fumigant Bystander Exposure Model 

Review: Probabilistic Exposure and Risk Model for Fumigants (PERFUM) Using Iodomethane as a Case Study. 

Available at http://www.epa.gov/scipoly/sap/meetings/2004/august1/august2425minutes.pdf, also refer to the 

following for additional information http://www.epa.gov/oppsrrd1/reregistration/soil_fumigants/ 4

 The assessments can be found in the dockets for each fumigant. Four of which are provided here chloropicrin - 

EPA-HQ-OPP-2007-0350; dazomet - EPA-HQ-OPP-2005-0128; metam sodium/potassium - EPA-HQ-OPP-2005-

0125; and methyl bromide - EPA-HQ-OPP-2005-0123 5

 U.S. EPA 2009. FIFRA Science Advisory Panel Meeting Minutes - Scientific Issues Associated with Field 

Volatilization of Conventional Pesticides. Available at 

http://www.epa.gov/scipoly/sap/meetings/2009/december/120309meetingminutes.pdf 6 EPA MRID 48139303: Acute Inhalation Exposure of Adult Crl:CD(SD) rates to particulate chlorpyrifos 

aerosols: Kinetics of Concentration-Dependent Cholinesterase (CHE) Inhibition in Red Blood Cells, Plasma, 

Brain and Lung; Authors: J. A. Hotchkiss, S. M. Krieger, K. A. Brzak, and D. L. Rick; Sponsor: Dow 

AgroSciences LLC, 9330 Zionsville Road Indianapolis, IN 46268-1054 
5 of 70 

cholinesterase (ChE) inhibition. Risk estimates were calculated for both lung and RBC ChE 

inhibition based on a 6-hour exposure duration and used quantitatively to estimate risk. The point 

of departures (PODs) for lung and RBC were calculated using the EPA’s Reference 

concentration method for calculating human equivalent concentrations (HECs), which accounts 

for physiological differences between animals and humans. 

1.1.2. Exposure

Dow AgroSciences (DAS) recently submitted a field volatility study7

 as part of the data call-in 

requirements for the registration review of chlorpyrifos. This study measured both vapor phase 

chlorpyrifos and chlorpyrifos-oxon, a transformation product, in air samples following an 

application of a low VOC (volatile organic compounds or volatile organic chemicals) 

formulation8,9,10 of chlorpyrifos to alfalfa. Approximately 30% of the applied chlorpyrifos was 

emitted from the treated field in the first 24 hours (28% considering chlorpyrifos only; 30% 

considering chlorpyrifos and chlorpyrifos-oxon combined). The flux profile for chlorpyrifos is 

similar to those generally observed for fumigants in that there is a peak emission shortly after 

application during the warmer part of the day. The study measured chlorpyrifos for a period of 

72 hours following application. 

This assessment also incorporates a field volatility study published in the open literature; 

conducted with application of a non-low VOC formulation of chlorpyrifos to potatoes.11 Since 

the raw data for this study could not be obtained, the flux rates could not be independently 

verified by EPA and, thus, evaluation of experimental details and associated data quality review 

of this study is not as rigorous as that associated with the alfalfa study. The open literature study 

only measured parent chlorpyrifos and did not measure concentrations of chlorpyrifos-oxon. The 

results from this study are presented in this assessment to provide another line of evidence of the 

potential volatility of chlorpyrifos, as demonstrated in the registrant submitted study, and to help 

describe the potential variability in chlorpyrifos flux rates due to different study conditions (e.g., 

 

7 EPA MRID 48883201: Direct Flux Measurement of Chlorpyrifos and Chlorpyrifos-Oxon Emissions Following 

Applications of Lorsban Advanced Insecticide to Alfalfa; Authors: Aaron Rotondaro and Patrick Havens; Sponsor: 

Dow AgroSciences LLC, 9330 Zionsville Road Indianapolis, IN 46268-1054, 2012. 8

 California's Department of Pesticide Regulation (Cal DPR) defines a low VOC pesticide formulation when the 

total emission potential (see footnote 9) is 25% or less (see footnote 10). The emission rate corresponds to total 

VOC emissions and not specially one component of the formulation (i.e., the active ingredient). EPA does not 

currently define low VOC pesticide formulations. 9

 Emission potential is based on Thermogravimetric Analysis; Oros, D., Spurlock, F. California Department of 

Pesticide Regulation, ESTIMATING PESTICIDE PRODUCT VOLATILE ORGANIC COMPOUND OZONE 

REACTIVITY. PART 1: SPECIATING TGA -BASED VOLATILE ORGANIC COMPUND EMISSIONS 

USING CONFIDENTIAL STATEMENTS OF FORMULA, January 27, 2011 

http://www.cdpr.ca.gov/docs/emon/pubs/ehapreps/analysis_memos/2286_segawa.pdf 10 Proposed regulation can be found at: http://www.cdpr.ca.gov/docs/legbills/rulepkgs/12-001/text.pdf 11 EPA MRID 48998801: Volatilization of the Pesticides Chlorpyrifos and Fenpropimorph from a Potato Crop; 

Authors: Minze Leistra, Johan H. Smelt, J. Hilbrand Weststrate, Frederik VanDenBerg, and Rene Aalderink; 

Sponsor: This work was carried out within the framework of the EU APECOP project Effective Approaches for 

Assessing the Predicted Environmental Concentrations of Pesticides (QLK4-CT-1999-01338) and of Research 

Program 416, Pesticides and the Environment, of the Dutch Ministry of Agriculture, Nature and Food Quality; 

Citation: Leistra, M; Smelt, J. H.; Weststrate, J. H.; Van Den Berg, F; Aalderink, R. Environ. Sci. Technol. 2006,

40, 96-102. 
6 of 70 

crop canopy, formulation, and weather). While the absolute flux for chlorpyrifos observed in this 

study is higher than the alfalfa study the flux profiles12 are similar in both studies. 

The two field volatility studies suggest that volatilization of chlorpyrifos and/or chlorpyrifosoxon from treated crops is a pathway of dissipation in the environment that may result in 

bystander exposure to vapor phase chlorpyrifos and chlorpyrifos-oxon. These studies were 

conducted at rates much lower than the current maximum single broadcast application; however, 

based on usage data, the rates used in these studies are consistent with typical average single 

application rates. In order to evaluate potential risks under varied conditions, offsite 

concentrations of chlorpyrifos and total toxic chlorpyrifos residues (chlorpyrifos and 

chlorpyrifos-oxon) were estimated using PERFUM and the flux rates derived from the two field 

volatility studies. This approach is consistent with what has been done for fumigant pesticides4

and with the recommendations of the 20043 and 20095 SAP reviews. 

1.2. Results Summary 

Volatilization as a pathway of exposure was examined to quantify the potential risk estimates 

associated with bystander exposure to vapor phase chlorpyrifos and chlorpyrifos-oxon near a 

treated field. This was done by comparing peak 6 hour chlorpyrifos and total toxic chlorpyrifos 

vapor [chlorpyrifos and chlorpyrifos-oxon (expressed in chlorpyrifos toxicity equivalents)] 

concentrations estimated at various distances away from a treated field with endpoints based on 

lung and RBC ChE inhibition. The results indicate that offsite concentrations may exceed the 

target concentration established for lung ChE inhibition for many currently registered uses at 

distances away from the field edge. Concentrations are not expected to exceed the RBC ChE 

inhibition target concentration under all the conditions that were evaluated in this assessment. 

Depending on which percentile of exposure and the field size considered, buffers zones13

estimated to ensure concentrations of chlorpyrifos (only) are at or below the lung ChE target 

concentration range from 0 to greater than 4000 ft away from the perimeter of a treated field. 

Higher application rates and/or large field sizes lead to higher exposure and, therefore, large 

buffers. The spray drift buffers for protection of bystanders in sensitive sites14 currently required 

on chlorpyrifos labels range from 0 to 100 feet depending on the application method. 

Consideration of vapor phase chlorpyrifos-oxon in addition to chlorpyrifos (only) increases the 

estimated buffer distances needed to ensure air concentrations are below the target concentration 

than those presented for chlorpyrifos only. 

In addition to estimating buffer distances based on the peak air concentrations, a buffer duration 

analysis was completed by examining air concentrations over several days. This analysis is based 

 

12 A flux profile is the emissions from a treated field over a defined period of time (i.e., an hourly time series of flux 

estimates during a period of measurement following application). 13 In the context of presenting modeling results the term "buffer zone" does not refer to any regulatory decision 

pertaining to risk mitigation for chlorpyrifos. It refers to the distances determined based on a target concentration 

defined by the HEC adjusted by the uncertainty factor. 14 Buffers are around sensitive sites (a circle drawn around the sensitive site with a radius equal to the buffer 

distance) and do not correspond to buffers around a given field (a circle drawn around a treated field with a radius 

equal to the buffer distance). 
7 of 70 

on whole field buffers as compared to maximum buffers. Depending on which percentile of 

exposure and the field size evaluated, the estimated buffers may need to remain for several 

hours. For example, buffers would need to be in place for at least 12 hours for l pound of active 

ingredient per acre (lb a.i./A) applications and for at least 36 hours for 6 lb a.i./A applications 

when considering the 95% percentile exposure concentration. 

2. PROBLEM FORMULATION 

Problem formulation provides a strategic framework in the risk assessment. In this case, it 

describes the history of EPA’s air modeling approach and strategy used in this assessment to 

estimate potential risk due to volatilization of chlorpyrifos and its oxon following application in 

an agricultural setting. In addition, to identifying the chlorpyrifos application variables for the 

assessment, the problem formulation also outlines relevant physical-chemical properties, 

transformation products, usage data, hazard endpoints and exposure data needed for the 

assessment. 

2.1. Approach 

Pesticide volatilization can potentially impact those who are in proximity to treated fields 

following application events, as depicted in Figure 1. The example in Figure 1 is an exposure 

pattern that is reasonably expected to occur with an emission event, given that farmers, residents 

(e.g., homeowners, renters, visitors, etc.) and the general public may frequent areas adjacent to 

treated fields. To assess the potential risk from this exposure pathway, a residential/bystander 

assessment based on exposure to vapor phase residues at various distances away from a treated 

field was conducted. In order to evaluate the volatilization potential and associated risks, an 

approach based on dispersion modeling coupled with techniques used to evaluate inhalation 

exposure was utilized. In this case, dispersion modeling was completed for a variety of 

application scenarios including different field sizes, application rates, and metrological 

conditions. 

Figure 1. Conceptual Model of the Exposure Pathway Associated with Volatilization 
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2.1.1. Air Dispersion Modeling 

2.1.1.1. Prior Consideration of Available Air Dispersion Models

Air dispersion modeling uses mathematical formulas to characterize how atmospheric processes 

will disperse a pollutant emitted by a source. For the fumigants, the EPA used dispersion models 

to estimate the downwind concentration emitted from an area source such as a treated field 

consistent with the EPA’s air model development and implementation methods.15 The EPA 

considered three air dispersion models for use in the soil fumigant risk assessments16. These 

models consisted of two “Gaussian Plume” models (ISCST3 model and AERMOD) and one 

“Gaussian Puff” model (CALPUFF). All three of these models are currently listed or have 

previously been listed in Appendix W of 40 CFR Part 51 (Guideline on Air Quality Models) 

which contains the EPA’s guidance on the general regulatory applicability of various air quality 

dispersion models.17 EPA selected the ISCST3 model, which in 2004 was the agency’s 

recommended regulatory air model, for use in the fumigant assessments.16 

After a modeling approach based on the use of ISCST3 was selected, a series of preliminary risk 

assessments were completed using ISCST3.18 The findings indicated significant risk mitigation 

measures might be needed (e.g., large buffer distances) but there was the potential for 

refinements to refine risk estimates. The key refinement was incorporating actual weather data as 

a basis for predicting risks instead of using constant atmospheric conditions. In essence, this 

refinement allowed for distributional consideration of changing weather conditions rather than 

the ISCST3 deterministic approach to weather which was based on wind speed, direction and 

atmospheric stability being constant. 

The regulated community developed three models that incorporated actual weather data: 

Probabilistic Exposure and Risk model for Fumigants (PERFUM) 19, the Fumigant Emissions 

Modeling System (FEMS)20, and the Soil Fumigant Exposure Assessment System (SOFEA)21, 

were submitted to the EPA by the regulated community. All of these models are essentially preand post-processors for ISCST3 (and CALPUFF in the case of FEMS) that incorporate the 

ability to complete distributional and/or probabilistic analyses. Each of these models was 

reviewed by the 2004 SAP.3

 The SAP concluded that each of the three models could provide 

scientifically defensible estimates of the bystander exposures and risks associated with soil 

fumigation practices and also suggested modifications and additional data that could further 

refine risk estimates (for specifics see the final SAP report3

). Many of the SAP’s recommended 

modifications have been made to these models since that time. After the SAP reviews, the EPA 

 

15 See http://www.epa.gov/scram001/ for more details. 16 See http://www.epa.gov/oppsrrd1/reregistration/soil_fumigants/soil-fum-reg-backgrnd.html for more details 17 http://www.epa.gov/ttn/scram/guidance/guide/appw_05.pdf 

18 There was a regulatory precedent for using ISCST3 as it had previously been used by EPA for air permitting, as 

well as to define buffer zones for methyl bromide in California. 19 Reiss, R. and Griffin, J. 2006. User’s Guide for the Probabilistic Exposure and Risk Model for FUMigants 

(PERFUM), Version 2. Arysta LifeScience North America Corporation. Cary, North Carolina. 20 Fumigant Exposure Modeling System (FEMS), Background Document: Fumigant Emissions Modeling System, 

Sullivan, Hlinka, and Holdsworth, July, 2004 21 SOil Fumigant Exposure Assessment System (SOFEA), SOFEA (User's and Programmer's Guide), van 

Wesenbeeck and Cryer, Copyright 2004 
9 of 70 

selected PERFUM as the probabilistic air model for use in the fumigant assessments. Other 

modeling analyses, using systems such as FEMS, SOFEA, AERMOD or CALPUFF, would also 

be considered should these models be developed and submitted to the EPA for consideration, 

since these models are also considered appropriate for regulatory purposes. 

The 2009 SAP5

 discussed in limited fashion, as it was not part of their official charge, that the 

current dispersion models (mentioned above) have not been validated for non-fumigant 

pesticides. The EPA acknowledges that use of dispersion models was briefly discussed at the 

2009 SAP5 but no significant discussions were held on this particular issue. The SAP concluded 

that the concept of coupling a fate and transport model, to predict the flux of a chemical, with a 

dispersion model, to estimate air concentrations at different distances from the field is a sound 

approach. 

In order for a model to be included as an EPA recommended air model (part of Appendix W), 

the model must go through an extensive peer review and testing process. This peer review 

process ensures that models are acceptable to be used for a variety of sources like point sources 

(e.g., a stack on a building) and area sources (e.g., a treated field), as well as for a variety of 

pollutants (i.e., volatile organic compounds, semi-volatile organic compounds, and particulate 

matter). While this testing process did not explicitly consider pesticides, except for one case 

where a fumigant was released from a field (i.e., the Prairie Grass Study22), it included semivolatile organic compounds. Most pesticides fall under this classification given physicalchemical characteristics such as vapor pressure. Based on the EPA’s model peer review process 

and the use of air dispersion modeling for the fumigants risk assessments over the last ten years, 

the use of dispersion modeling for semi-volatile pesticides is considered a valid and scientifically 

defensible methodology. 

2.1.1.2. PERFUM Modeling Approach

Before a PERFUM analysis can be performed, hazard and exposure inputs need to be defined. 

The hazard inputs used in this assessment are discussed in detail in Section 2.3. Exposure inputs 

in the form of a flux profile [the flux or rate of pesticide emissions from the treated fields per 

unit area per unit time (μg/m2

/sec)] was determined from two different field volatility studies 

discussed further in Section 2.4. Numerous factors can influence flux rates, such as the 

application rate, the treated surface (e.g., foliage or soil), field structure (e.g., canopy type and 

shape), application practices, and atmospheric conditions such as temperature and humidity. A 

discussion of these factors is provided in Section 4. In the end, PERFUM compares a target 

concentration derived from the HEC and the uncertainty factor23 to off field air concentrations 

estimated based on flux rates surrounding a treated field in order to estimate buffer zones13, if 

needed. 

 

22 United States Air Force. 1958. Project Prairie Grass, A Field Program in Diffusion, Volume I. Geophysics 

Research Directorate, Air Force Cambridge research Center, Air Research and Development Command 

Geophysical Research Papers No. 59, Document Number AD152572, July 1958 23 The uncertainty factor is this case is a value developed to account for uncertainty associated with animal to human 

extrapolation of toxicity data, as well the potential sensitivity difference between humans within a given 

population (e.g., sensitivity subpopulations ). See Section 2.3 for information on how the target concentration 

used in this assessment was derived. 
10 of 70 

Buffer zones13 and risk estimates are presented in Section 3 below for several different potential 

application scenarios (e.g., different meteorological conditions, field sizes, and application rates) 

for an array of percentiles of exposure for whole field and maximum buffers. A summary of the 

results is provided in Section 4. Note: PERFUM does not produce buffer zones greater than 4724 

ft (1440 m), thus, buffer zones for cases where the 4724 ft (1440 m) limit is reached may be very 

large. These cases are indicated with a “>4724 ft”. 

 

2.2. Stressors of Concern 

Chlorpyrifos (O,O-diethyl o-(3,5,6-trichloro-2-pyridyl phosphorothioate; CAS 2921-88-2) is a 

broad spectrum organophosphate (OP) insecticide. It is widely used in agriculture and has been 

measured in various air monitoring programs. Chemical identification and select physical 

chemical properties of chlorpyrifos are provided in Table 1. 

Table 1. Chemical Identification and Select Physical-Chemical Properties of Chlorpyrifos 

and Chlorpyrifos-oxon

Parameter Chlorpyrifos Chlorpyrifos-oxon 

Chemical Name O,O-diethyl O-(3,5,6-trichloro-2-pyridyl 

phosphorothioate 

O,O-diethyl O-3,5,6-trichloropyridin-2-yl 

phosphate

Chemical 

Abstracts Service 

(CAS) Registry 

Number 

2921-88-2 5598-15-2 

Empirical 

Formula C20H17F5N2O2 C9H11Cl3NO4P 

USEPA Pesticide 

Code (PC #) 059101 -- 

Smiles Notation S=P(OC1=NC(=C(C=C1Cl)Cl)Cl)(OCC)OCC O=P(Oc1nc(c(cc1Cl)Cl)Cl)(OCC)OCC 

Molecular Weight 350.57 g/mol 334.52 g/mol 

Vapor Pressure 

(25 °C) 1.87x10-5 torr 6.65x10-6 torra 

Water Solubility 

(20 °C) 1.4 mg/L 26.0 mg/La 

Henry’s Law 

Constant 6.2 x 10-6 atm - m3

/mol 5.5 x 10-9 atm - m3/mola 

Log Kow 4.7 2.89 mg/La 

Saturated Vapor 

Concentration 

(25 °C) 

489 μg/m3 b 

353 μg/m3 c 120 μg/m3 d

a. EPI Suite estimated value. 

b. See footnote 24

c. Calculated: n/V=P/RT; P=1.87x10-5 torr x (1 atm/760 torr) =2.46x10-8 atm @ 25 °C; R= 0.0821 L atm/ mol K; T= 

25 °C= 298 K; 2.46 x10-8 atm/ (0.0821 L atm/ mol K*298 K) = 1.0 x 10-9 mol/L x (350.57 g/mol) x 106 μg/g = 

0.353 μg/L* 1L/10-3m3) = 353 μg/m3

d.Calculated: n/V=P/RT; P=6.65x10-6 torr x (1 atm/760 torr) =2.46x10-8 atm @ 25 °C; R= 0.0821 L atm/ mol K; T= 

25 °C= 298 K; 2.46 x10-8 atm/ (0.0821 L atm/ mol K*298 K) = 3.6 x10-10 mol/L x (334.52 g/mol) x 106 μg/g = 

0.120 μg/L* 1L/10-3m3

) = 120 μg/m3

 

24 Reported by Dow AgroSciences on December 11, 2012 in a presentation (Bartels, M., Cleveland, C., Hotchkiss, 

J., Juberg, D. Perspectives on Risk Assessment Elements for Chlorpyrifos) to EPA. 
11 of 70 

Based on available environmental fate data submitted to EPA, chlorpyrifos is expected to 

undergo several chemical transformations in the environment as shown in Figure 2. 

O P O N

Cl Cl

Cl

S

O

O P O N

Cl Cl

Cl

O

O

HO N

Cl Cl

Cl

Chlorpyrifos Chlorpyrifos-oxon

3,5,6-trichloro-2-pyridinol (TCP)

H3CO N

Cl Cl

Cl

3,5,6-trichloro-2-methoxypyridine (TMP)

Figure 2. Environmental Transformation of Chlorpyrifos 

Chlorpyrifos-oxon, a transformation product, is considered as a residue of concern which has 

greater potency for cholinesterase inhibition than chlorpyrifos. There are also air monitoring data 

that include both chlorpyrifos and chlorpyrifos-oxon detections supporting the potential exposure 

to both chemicals as a result of emission from a treated field and subsequent transport offsite.25 It 

is unclear if the concentrations observed in the majority of air monitoring studies reported in the 

open literature are due to spray or vapor phase drift of chlorpyrifos and chlorpyrifos-oxon; 

however, the alfalfa field volatility study discussed in detail below indicate that it is likely that 

both vapor phase chlorpyrifos and chlorpyrifos-oxon vapors are present simultaneously 

downwind of a treated field. Therefore, in addition, to exposure to chlorpyrifos itself the 

potential exposure to both chlorpyrifos and chlorpyrifos-oxon at the same time is also explored 

in this assessment. 

In a recently submitted air photolysis study26 chlorpyrifos was reported to undergo indirect and 

direct photolysis [t1/2 = 2 h (indirect) and 6 h (direct)]. These results have not been verified by 

EPA as the raw data submitted to the EPA are not adequate; however, the results obtained for 

indirect photolysis are consistent with the Estimation Program Interface (EPI) Suite27

estimations.28 EPI Suite is commonly used by EPA to estimate physical-chemical parameters 

when data are unavailable. This study confirms the formation of chlorpyrifos-oxon via 

photolysis. Chlorpyrifos-oxon was reported to undergo indirect and direct photolysis [t1/2 = 11 h 

(indirect) and 6 h (direct)]. The EPI Suite estimated indirect photolysis was about half29 that 

 

25 Glotfelty, D. E.; Majewski, M. S.; Selber, J. N. Distribution of Several Organophosphorus Insecticides and Their 

Oxygen Analogues in a Foggy Atmosphere. Environ. Sci. Technol., 1990, 24 (3), 353-357. 26 EPA MRID 48789701: Gas-Phase Photolysis and Photo-oxidation of Chlorpyrifos and Chlorpyrifos oxon; 

Authors: Amalia Munoz; Sponsor: Dow AgroSciences European Development Centre, 3 Milton Park, Abington, 

Oxon, OX14 4RN 27 http://www.epa.gov/opptintr/exposure/pubs/episuite.htm; The version used in this assessment is 4.00. 28 t1/2 = 3 h (indirect) 29 t1/2 = 7 h (indirect) 
12 of 70 

calculated by the study authors. The physical-chemical properties of chlorpyrifos-oxon are 

provided in Table 1. 

2.2.1. Use Characterization 

There are several different types of chlorpyrifos formulations currently registered including: 

liquid, dry flowable, microencapsulated, and granular products. Registered use sites include food 

crops such as fruit and nut trees, many types of fruits and vegetables, and grain crops; and nonfood crops such as forage, golf course turf, industrial sites, greenhouse and nursery production, 

sod farms, and wood products. This section includes a summary of chlorpyrifos use, including: 

an evaluation of yearly chlorpyrifos percent crop treated (PCT), application rate information, and 

total acreage treated annually for all agricultural crops treated with chlorpyrifos. 

A complete list of chlorpyrifos use sites and application rates are provided in ATTACHMENT 

1. Broadcast application rates currently permitted on chlorpyrifos labels range from 0.5 to 6 lb 

a.i./A. 

2.2.1.1. Agricultural Use 

Based on private market research data30, approximately 8 million pounds of chlorpyrifos are used 

annually in agriculture. Total chlorpyrifos usage by crop varies widely with the average PCT in 

the survey of years 2006-2010, unless otherwise noted –as low as 1% for several crops and as 

high as 62 % (for apples). The five crops with the highest PCT are apples (62%), broccoli (53%), 

walnuts (46%), onions (45%), and cauliflower (41%) as shown in Table 2. These five crops 

account for approximately thirteen percent of the total pounds of chlorpyrifos applied each year. 

Crops that have low total chlorpyrifos usage (EPA defined this as less than 20,000 pounds used, 

on average) but show relatively large area treated (EPA defined this as greater than 25% PCT) 

include: strawberries, asparagus, and cauliflower. 

Table 2. Typical Use Data for Chlorpyrifos (all formulations combined; national level data) 

Crop

Average 

Percent 

Crop 

Treated 

(PCT)

Average 

Pounds 

Applied per 

Year

Average 

Single 

Application 

Rate

Upperbound Rate 

(Lbs. a.i./A-single app.)

Soybeans 6 1,800,000 0.41 0.5 (90%), 0.75 (95%) 

Corn 1 1,000,000 0.9 1.25 (84%), 1.5 (97%) 

Oranges 24 630,000 2.5 5.75 (81%), 6.0 (100%) 

Alfalfa 3 450,000 0.56 0.75 (79%), 1.0 (100%) 

Almonds 24 440,000 1.86 2.0 (97%) 

Wheat, Winter 2 400,000 0.42 0.5 (93%) 

Apples 62 390,000 1.5 1.75 (52%), 2.0 (99.9%) 

Walnuts 46 350,000 1.85 2.0 (99%) 

 

30 GfK Kynetec Database 
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Pecans 28 260,000 0.9 1.25 (91%) 

Peanuts 7 160,000 1.72 2.0 (97%) 

Sugar Beets 11 130,000 0.8 1.5 (88%) 

Wheat, Spring 2 120,000 0.32 0.25 (65%), 0.5 (99%) 

Cotton 2 100,000 0.3 0.75 (77%), 1.0 (100%) 

Sweet Corn 11 100,000 0.96 1.25 (83%), 1.5 (94%) 

Broccoli 53 100,000 1.36 2.0 (78%), 2.25 (100%) 

Lemons 39 96,000 3.5 5.0 (88%) 

Tobacco 13 90,000 1.96 3.0 (90%) 

Cherries 37 84,000 1.5 2.0 (99.6%) 

Sunflowers 6 66,000 0.46 0.5 (88%), 0.75 (94%) 

Onions 45 65,000 0.9 3 1.0 (70%), 1.25 (99.8%) 3 

Wine Grapesc 5 60,000 1.96 2.42 

Peachesb 26 56,000 1.34 2 1.75 (69%), 2.0 (98%)2 

Grapefruit 22 48,000 2.1 2.75 (90%) 

Table Grapesc 13 27,000 1.88 2.43 

Plums/Prunes 11 24,000 1.73 2.0 (100%) 

Pears 17 20,000 1.85 2.0 (100%) 

Cauliflower 41 17,000 1.09 1.25 (89%), 1.5 (97%) 

Asparagus 39 16,000 0.99 1.0 (95%) 

Strawberries 28 15,000 0.97 1.0 (99%) 

Dry Beans/Peasa 1 13,000 0.48 0.5 (95%) 

Cabbage 14 12,000 1.16 1.25 (86%), 1.5 (92%) 

Sorghum (Milo) a 0.4 11,000 0.42 0.5 (80%), 0.75 (95%) 

Hazelnutsa 10 4,800 1.27 1.5 (77%), 2.0 (100%) 

Beansa 1 1,800 0.77 0.75 (84%), 1.0 (100%) 

Peppersa 2 1,800 0.81 0.75 (77%), 1.0 (94%) 

Pumpkinsa 1 1,700 0.93 1.25 (100%) 

Peasa 1 1,400 0.98 1.0 (98%) 

Squasha 1 820 1.11 1.25 (88%), 1.5 (91%) 

Cucumbersa 1 550 0.76 1.0 (100%) 

Source: Proprietary Data 2006-2010. Table is sorted to show crops with highest to lowest total chlorpyrifos 

applied per year 

a. Data are not robust (total sample size of <50 across 5 years and this crop not surveyed in USDA NASS) 

b. Peaches: Higher values were reported in NASS. For a conservative value, EPA recommends: Rate: 1.6 

lb a.i./A, Number of Applications: 1.4; 90th percentile rate: 2.6 lb a.i./A (USDA NASS 2009)

c. Updated Typical Use Data for Chlorpyrifos Used on Grapes in California (Average 2006 and 2009); 

Source: USDA NASS (2006, 2009) Agricultural Chemical Usage Fruit Summary 
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Chlorpyrifos use is highest on corn and soybean in terms of average pounds total chlorpyrifos 

applied per year, accounting for approximately thirty-nine percent of the total pounds of 

chlorpyrifos applied each year. Corn use is over 1,000,000 pounds per year while soybean use is 

nearly 2,000,000 pounds per year for the period surveyed. Chlorpyrifos use on orchard and trellis 

crops is about thirty-three percent of the total chlorpyrifos use per year. 

2.2.1.1.1. Application Rate Information 

Table 2 also provides use information such as the application rate (average single application 

rates and upper-bound single application rates) for national-level total chlorpyrifos use (all 

formulations and all application methods) on use sites where information was available from 

2006-2010.31 Seventeen crops (out of a total of 38 crops surveyed) show average application 

rates that are greater than 1 lb a.i./A. These crops are apples, almonds, broccoli, cabbage, 

cauliflower, cherries, grapefruit, hazelnuts, lemons, oranges, peaches, peanuts, pears, 

plums/prunes, squash, tobacco, and walnuts. The majority of growers surveyed reported use of 

single application rates of 2 lb a.i./A or less. For many of the crops included in Table 2 the 

upper-bound single application rate is equal to the maximum single application rate currently 

permitted on the label. 

Additional analysis of the data by formulation type is presented in APPENDIX B. 

2.2.1.1.2. Acres Treated Information

Data on the size of fields treated per application of chlorpyrifos helps define current use 

practices. However, with the exception of the detailed pesticide use recording database 

maintained by the California Department of Pesticide Regulation (CDPR), EPA is not aware of 

data sources that would provide this kind of information for chlorpyrifos (or any other pesticide). 

Results of EPA’s analysis of relevant CDPR data are presented in APPENDIX B (Table B2). 

These data describe chlorpyrifos usage in California, and may not reliably apply to other parts of 

the United States as different pest pressures and crop production practices are likely altering the 

relative use of chlorpyrifos. Additional discussion of the scope of these data is included in 

APPENDIX B. 

Using the CDPR data, EPA determined that the area treated per application per day varies widely 

across use sites. In general, it appears that ground-based treatments covered larger acres treated 

per application per day than aerial treatments at the 90th percentile range. While the factors 

contributing to this result were not investigated, EPA speculates such factors as cost of 

application, efficacy, and accessibility of application equipment may influence the use of ground 

over aerial application methods. Although maximum area treated in some crops (e.g., walnuts, 

almonds, asparagus, grapes, alfalfa) can be several hundred acres, these are often single data 

points (i.e., only one grower/applicator was reporting this amount treated). The 50th and 90th 

percentile columns depicted in Table B2 are probably more descriptive for typical use than the 

 

31 Stebbins, K. U.S. EPA Office of Chemical Safety and Pollution Prevention, Chlorpyrifos: Additional Typical Use 

Data for Chlorpyrifos, January 11, 2012 PC Code 059101 
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maximum reported number of acres treated per day. The percentile columns show the amount of 

acres treated per application per day at or below which 50 or 90 percent of applicator reports 

occur in the CDPR database (within each crop/product/application method grouping). 

 

2.2.1.2. Non-Agricultural Use 

Chlorpyrifos is used in settings that are not considered an agricultural use per se including 

agricultural farm premises, nurseries and plantations (e.g., ornamental, non-bearing fruit tree, 

and Christmas tree), golf course turf, recreational areas, rights-of-way, and utility areas. 

However, usage data in these areas are not surveyed by the proprietary market databases that 

EPA uses to gather pesticide use information. In addition, use information for these sites is not 

collected by the CDPR in a form that allows reliable interpretation for risk assessment purposes. 

Therefore, EPA has no verifiable description of application rates or area treated in these uses. 

This remains an uncertainty. 

2.3. Hazard Characterization 

2.3.1. Mode of Action and Adverse Outcomes 

Chlorpyrifos, like other OPs, binds to and phosphorylates the enzyme, acetylcholinesterase 

(AChE), in both the central (brain) and peripheral nervous systems leading to accumulation of 

acetylcholine and, ultimately, to clinical signs of toxicity. For OPs, AChE inhibition is most 

often the most sensitive dose response data for use in assessing human health risk. For 

chlorpyrifos, the EPA concluded in the 2011 preliminary HHRA, that ChE inhibition was the 

most sensitive dose response data for use in deriving a POD for all durations, routes of exposure, 

and lifestages. 

EPA typically uses data on AChE inhibition data, the initiating event in the mode of action, for 

endpoint selection in human health risk assessment since protecting against inhibition of AChE 

thereby protects against potential acute neurotoxicity. EPA’s ChE policy32 describes the manner 

in which ChE data are used in human health risk assessment. Typically, experimental laboratory 

studies measure brain (central) and blood (plasma and red blood cell) ChE. Blood measures do 

not represent the target tissue, per se, but are instead used as surrogate measures for peripheral 

nervous tissue in studies with laboratory animals when peripheral data are missing or for 

potential peripheral and/or central toxicity in humans. In addition, RBC measures represent 

AChE, whereas plasma measures are approximately 50% butyryl-ChE (BuChE). Thus, RBC 

AChE data may provide a better representation of the inhibition in target tissues than plasma 

ChE data. 

There are many experimental toxicology studies which measure RBC and plasma ChEs. In 

general, measurements of AChE or ChE inhibition in peripheral tissues (e.g., liver, diaphragm, 

heart, lung, etc.) are rare. Data for chlorpyrifos are unique in that measures of some peripheral 

tissue are available in some experimental toxicity studies. For example, heart ChE was measured 

 

32 USEPA (2000) Office of Pesticide Programs, US Environmental Protection Agency, Washington DC 20460. 

August 18, 2000 Office of Pesticide Programs Science Policy of The Use of Data on Cholinesterase Inhibition for 

Risk Assessments of Organophosphorous and Carbamate Pesticides.
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in the chlorpyrifos developmental neurotoxicity study33,34, an associated companion study, in 

addition to an acute oral study by Dittenber.35 Liver ChE was measured by Lassiter36, Hunter37, 

and Moser and Padilla38. Padilla39 (measured diaphragm ChE) and Marable40 (measured 

diaphragm, left atrium, and quadriceps ChE) evaluated chronic exposure in adult animals where 

blood, brain and peripheral tissue ChE inhibition were at steady-state. Lung ChE was measured 

in the special acute inhalation study being used to assess the risk from volatilization. Other data 

on lung ChE come from oral studies. Specifically, in an oral study by Carr41 heart, lung, and 

skeletal muscle ChE were evaluated in post-natal pups dosed using a unique design that included 

every other day dosing and different durations and doses across different aged pups. Given the 

unique design of the Carr study, it is difficult to assess sensitivity of different tissues. Richard 

and Chambers42 evaluated lung ChE in rat fetuses following oral exposure to the dams and 

showed that lung ChE was similar at the lowest dose on PND1 to brain and RBC AChE but lung 

ChE was more inhibited in PND1 pups than brain and RBC AChE at the higher doses (ChE 

inhibition was not measured in dams).

With respect to clinical signs of neurotoxicity, in experimental toxicity studies with chlorpyrifos 

acute, oral toxicity can be associated with signs typical of cholinergic overstimulation, such as 

hypothermia, salivation, lacrimation, diarrhea, incoordination, tremors, and fasciculations. In 

addition, altered motor function was one of the more sensitive endpoints evaluated in these 

studies.43,38,44,45 

 

33 EPA MRID 44648101: Effects of Chlorpyrifos Administered Via Gavage to CD Rats During Gestation and 

Lactation on Plasma, Erythrocyte, Heart and Brain Cholinesterase and Analytical Determination of Chlorpyrifos 

and Metabolites. Authors: Mattsson J. L., Maurissen J. P., Spencer, P. J., Brzak K. A., Zablotny C.L.; Sponsor: 

The Dow Chemical Co. for Dow AgroSciences, August 31, 1998. 34 Mattsson, J. L., Maurissen, J. P., Nolan, R. J., and Brzak, K. A. Lack of differential sensitivity to cholinesterase 

inhibition in fetuses and neonates compared to dams treated perinatally with chlorpyrifos. Toxicol Sci, 2000, 53, 

438-46. 35 EPA MRID 44273901: Chlorpyrifos: Evaluation of Single Oral Doses on Cholinesterase and Neurotoxic 

Esterase Inhibition in F344 Rats. Author: Dittenber, D. A., Sponsor: Dow Chemical Co. Study No. 960036. 

March 13, 1997. 36 Lassiter T. L., Padilla S., Mortensen, S.R., Chanda, S.M., Moser, V.C., Barone, S . Gestational Exposure to 

Chlorpyrifos: Apparent Protection of the Fetus? Toxicol. Applied Pharmacol. 1998a, 152, 56-65 37 Hunter, D. L., Lassiter, T. L., and Padilla, S. Gestational Exposure to Chlorpyrifos: Comparative Distribution of 

Trichloropyridinol in the Fetus and Dam. Toxicol Applied Pharmacol. 1999, 158, 16-23. 38 Moser, V. C., Padilla, S. Age- and Gender-related Differences in the Time Course of Behavioral and Biochemical 

Effects Produced by Oral Chlorpyrifos in Rats. Toxicol Applied Pharmacol. 1998, 149, 107-19. 39 Padilla, S., Marshall, R. S., Hunter, D. L., Oxendine, S., Moser, V. C., Southerland, S. B., Mailman, R. B. 

Neurochemical Effects of Chronic Dietary and Repeated High-level Acute Exposure to Chlorpyrifos in Rats. 

Toxicol Sci. 2005, 88, 161-71. 40 Marable, B.R., Maurissen, J. P., Mattsson, J. L., Billington, R. Differential Sensitivity of Blood, Peripheral, and 

Central Cholinesterases in Beagle Dogs Following Dietary Exposure to Chlorpyrifos. Regul. Toxicol. Pharmacol.

2007, 47, 240-8. 41 Carr, R. L., Chambers, H. W., Guarisco, J. A., Richardson, J. R., Tang, J., Chambers, J. E. Effects of Repeated 

Oral Postnatal Exposure to Chlorpyrifos on Open-field Behavior in Juvenile Rats. Toxicol. Sci. 2001, 59, 260-7. 42 Richardson, J., Chambers, J. Effects of Repeated Oral Postnatal Exposure to Chlorpyrifos on Cholinergic 

Neurochemistry in Developing Rats. Toxicol. Sci. 2003, 84, 352-59. 43 Mattsson, J. L., Wilmer, J. W., Shankar, M. R., Berdasco, N. M., Crissman,J. W., Maurissen, J.P., and Bond, D. 

M. Single-dose and 13-week Repeated-dose Neurotoxicity Screening Studies of Chlorpyrifos Insecticide. Food 

Chem. Toxicol. 1996, 34, 393-405. 
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With respect to the respiratory tract, although there are three chlorpyrifos inhalation studies, 

none measured apical outcomes relevant to pulmonary function. As such there are no data in 

experimental animals evaluating the potential effects of lung ChE inhibition on pulmonary 

function following inhalation chlorpyrifos exposure. Tracheobronchial glands receive 

parasympathetic innervation; AChE inhibitors can stimulate secretion of tracheobronchial fluid, 

in addition, to bronchorestriction and stimulation of chemoreceptors in the carotid and aortic 

bodies.46 

Volatile nerve agents are highly potent OP chemicals that inhibit AChE, as do the OP pesticides. 

In humans, OP nerve agents cause paralysis of respiratory muscles (e.g., diaphragm, abdominal, 

thoracic), airway secretions, edema and bronchoconstriction47. Experimental toxicology studies 

with laboratory animals exposed to OP war agents are consistent with these effects on the 

peripheral nervous system. For example, inhalation exposure to OP war agents in baboons 

induces cardiac arrhythmias, apnea, hypoxia, and respiratory disturbances.48 There are some 

experimental studies in laboratory animals where lung, bronchoalveolar lavage fluid and/or 

diaphragm ChE has been measured following exposure to war agents (e.g., VX, sarin, 

soman).47,49,50,51 

EPA acknowledges that information on war agents needs to be interpreted with caution in the 

context of the toxicological profile for chlorpyrifos. Although, there are rat three inhalation 

studies6,52,53 available with chlorpyrifos; none of these chlorpyrifos studies specially evaluated 

effects relevant to pulmonary function. Thus, the relationship between measures of ChE 

inhibition in the lung and pulmonary effects is not known. The war agent studies aid in 

 44 Nostrandt, A. C., Padilla, S., Moser, V. C. The Relationship of Oral Chlorpyrifos Effects on Behavior, 

Cholinesterase Inhibition, and Muscarinic Receptor Density in Rat. Pharmacal. Biochem. Behav. 1997, 58, 15-23. 45 Moser, V. C., Chanda, S.M., Mortensen, S.R., Padilla, S. Age- and Gender-Related Differences in Sensitivity to 

Chlorpyrifos in the Rat Reflect Developmental Profiles of Esterase Activities. Toxicol. Sci. 1998,46, 211-222. 46 Taylor, P. Cholinergic Agonists in Goodman and Gillman’s The Pharmacological Basis of Therapeutics. 1985

Seventh Edition. 47 Graham, J.R., Wright, B.S., Rezk, P. E., Gordon, R. K., Sciuto, A. M., Nambiar, M. P. Butyrylcholinesterase in 

Guinea Pig Lung Lavage: A Novel Biomarker to Assess Lung Injury Following Inhalation Exposure to Nerve 

Agent VX. Inhal Toxicol., 2006, 18, 493-500. 48 Anzueto, A., deLemos, R.A., Seidenfeld, J., Moore, G., Hamil, H., Johnson, D., Jenkinson, S.G.,1990. Acute 

inhalation toxicity of soman and sarin in baboons. Fundam. Appl. Toxicol. 14, 676–687. 49 Che, M.M., Song, J., Oguntayo, S., Doctor, B.P., Rezk, P., Perkins, M.W., Sciuto, A.M., Nambiar, M.P. 

Treatment with Endotracheal Therapeutics After Sarin Microinstillation Inhalation Exposure Increases Blood 

Cholinesterase Levels in Guinea Pigs. Toxicol. Mech. Methods. 2012, 22, 250-9. 50 Perkins, M.W., Pierre, Z., Rezk, P., Song, J., Oguntayo, S., Sciuto, A. M., Doctor, B. P., Nambiar, M. P. Acute 

Changes in Pulmonary Function Following Microinstillation Inhalation Exposure to Soman in Nonatropenized 

Guinea Pigs. Int. J. Toxicol. 2011, 30, 348-57. 51 Perkins, M.W., Pierre, Z., Rezk, P., Sabnekar, P., Kabra, K., Chanda, S., Oguntayo, S., Sciuto, A.M., Doctor, B. 

P., Nambiar, M. P. Acute Respiratory Toxicity Following Inhalation Exposure to Soman in Guinea Pigs. Toxicol. 

Appl. Pharmacol. 2010, 245, 171-8. 52 EPA MRID 40013901 and 40166501: Chlorpyrifos: 13-Week Nose-only Vapor Inhalation Exposure Study in 

Fischer 344 Rats Laboratory Project Id: HET K-044793-077. Authors: Corley, R.; Landry, T.; Calhoun, L. et 

al.; Sponsor: Dow Chemical USA 1986 53 EPA MRID 40908401: A Thirteen Week Nose-Only Inhalation Toxicity Study of Chlorpyrifos Technical 

(Pyrinex) in the Rat: Project No. 88-8058. Authors: Newton, P.; Prepared by Bio/dynamics, Inc.; 1988 
18 of 70 

considering the human relevance and toxicological adversity associated with changes in lung 

ChE and potential cholinergic effects on the respiratory system. The qualitative findings of the 

war agent studies provide a starting point for evaluating human incident reports for the kinds of 

potential respiratory effects which may result from exposure to high levels of chlorpyrifos. 

2.3.2. Human Incidents: Summary of Respiratory Effects

EPA considers human incident data in order to assist in better defining and characterizing the 

risk of pesticides/pesticide products. It is important to recognize, however, that reports of adverse 

health effects allegedly due to a specific pesticide exposure are largely self-reported and 

therefore, generally speaking, neither exposure to a pesticide nor reported symptoms (nor the 

connection between the two) is validated. Therefore, only rarely can causation be determined or 

definitively identified based on incident data. However, incident information can provide 

important feedback to the EPA regarding the nature, circumstances, frequency, and severity of 

pesticide exposure events. Human incident data, considered jointly with the human health risk 

assessment, can assist the Agency in determining the risk pesticides/pesticide products may or 

may not pose and can help characterize that risk in a qualitative manner. 

Following the chlorpyrifos incident report summarized in the 2011 HHRA, chlorpyrifos 

incidents from the OPP Incident Data System (IDS), the National Pesticide Information Center 

(NPIC), National Institute for Occupational Safety and Health’s (NIOSH) Sentinel Event 

Notification System for Occupational Risk (SENSOR-Pesticides), and the California Pesticide 

Illness Surveillance Program (CA PISP) were further reviewed for an association with 

respiratory effects (such as wheezing, chest tightening, and difficulty breathing). Respiratory 

effects are consistent with what is seen in animal studies exposed to chlorpyrifos by inhalation. 

The incidents reviewed involved only the single active ingredient chlorpyrifos because incidents 

involving only one pesticide are considered to provide more certain information on the suspected 

substance that may be causing adverse effects. EPA’s review of chlorpyrifos incidents showed an 

association between chlorpyrifos incidents and respiratory effects across all the databases 

reviewed. These incidents occurred due to both agriculture related use of chlorpyrifos and nonagriculture related use of chlorpyrifos. To focus the scope of the review in a manner relevant to 

this volatilization assessment and to be consistent with the current use pattern, EPA focused the 

review on incidents associated with drift, including volatilization, occurring post 2002. 

In general, it should be noted that it is difficult to distinguish between incidents associated with 

drift and those associated with volatilization because information may be limited on the timing of 

exposure relative to the application event in each of the incident databases. In SENSOR, for 

example, drift is defined as “...the movement of pesticides away from the treatment site. The 

pesticide spray, mist, fumes, or odor are carried from the target site by air.” As such, there is no 

clear distinction between cases exposed from volatilization versus spray drift when analyzing the 

SENSOR-Pesticides data. This is generally true of the other incident databases as well. 

Incidents associated with chlorpyrifos from each of OPP’s four databases are described below in 

additional detail: 
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OPP’s Main Incident Data System: For OPP’s Main Incident Data System (IDS), from January 

1, 2002 to February 1, 2011, there were 141 incidents in which chlorpyrifos was the only active 

ingredient in the suspected substance.54 Of these 141 incidents, 52 (37%) incidents reported 

respiratory effects including—but not limited to—dyspnea, throat irritation, irritated breathing, 

difficulty breathing, coughing/choking, and nasal congestion. These incidents were mostly 

categorized as HCs (Human Moderates); however, there was one fatality55 and 4 incidents 

classified as major severity. Due to the limited and varying exposure scenario details available, 

it is not possible to distinguish between incidents occurring due to drift and those occurring due 

to volatilization. 

National Pesticide Information Center: From 2002 to 2010, National Pesticide Information 

Center (NPIC) reported 88 cases in which chlorpyrifos was the only active ingredient in the 

suspected substance.56 Of these 88 cases involving chlorpyrifos, 20 (23%) had respiratory 

symptoms such as difficulty breathing, coughing, runny nose, and tightness in chest. Ten 

incidents occurred due to drift exposure; however, only 4 of these drift incidents reported 

respiratory effects. 

California Pesticide Illness Surveillance Program: For California Pesticide Illness 

Surveillance Program (CA PISP) from 2002 to 2009, there were 100 cases in which chlorpyrifos 

was the only active ingredient in the suspected substance.57 Of these 100 cases, 56 (56%) 

reported respiratory effects, such as shortness of breath, difficulty breathing, coughing, chest 

tightness, throat irritation, respiratory discomfort. Sixty one of the chlorpyrifos incidents are 

associated with drift or volatilization. Of these 61 incidents associated with drift or volatilization, 

39 (45%) reported respiratory effects. While the available data are insufficient to directly link 

respiratory effects to chlorpyrifos volatilization exposure, this may be because bystanders are 

less likely to associate respiratory symptoms with volatilization than they are, for example, with 

an obvious spray drift incident. A specific drift example involves an incident that occurred in 

2007 in Tulare County, California. Twenty six vineyard workers reported effects from the 

drift/volatilization from a chlorpyrifos application to almonds in an adjacent field. Twelve of 

these workers reported respiratory effects. These respiratory effects include difficulty breathing, 

shortness of breath, cough, sore throat, chest tightness. 

NIOSH Sentinel Event Notification System for Occupational Risk-Pesticides: 

For NIOSH SENSOR-Pesticides from 2002 to 2009, there were 204 cases in which chlorpyrifos 

was the only active ingredient in the suspected substance.58 Specifically: 

 

54 There were 106 additional incident reported to IDS for chlorpyrifos that involved multiple chemicals. 

55 The death incident is described in the database as follows, “A 58 year old male was reportedly exposed to a ULV 

product five to six times. Each time he was taken to the emergency room with difficulty breathing and an upset 

stomach. He was diagnosed with and treated for COPD. The last time he was exposed to the ULV product, he fell 

while taking a shower after the exposure. He was subsequently hospitalized and died about 10 days later. He also 

suffered from peeling skin.” 56 There were 90 additional incidents reported to NPIC for chlorpyrifos that involved multiple chemicals. 

57 There were 133 additional incidents reported to CA PISP for chlorpyrifos that involved multiple chemicals. 58 SENSOR-Pesticides 2002-2009 has a total of 409 cases involving Chlorpyrifos. Of these, 204 cases involve a 

single Active Ingredient (pc code=059101). For the purposes of this analysis, only the 204 cases involving a 

single AI will be included. 
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 Of the 204 cases, 91 cases (45%) reported one or more respiratory symptoms. 

 Of the 91 cases with respiratory symptoms, 77 were low in severity, 11 were moderate in 

severity and 3 were high in severity.

 Of the 91 cases with respiratory symptoms, the most commonly reported respiratory 

effect was upper respiratory pain (n=44), followed by cough (n=27) and 

dyspnea/shortness of breath (n=26). 

 42 of the 204 cases (21%) were attributable to drift, including volatilization59 and of these 

42 drift cases, 20 (49%) reported one or more respiratory symptoms. 

 There are 20 cases in SENSOR-Pesticides that are both attributable to drift and have a 

respiratory symptom. Six of these 20 cases involved bystander exposures to drift; 13 

were work-related (exposure while conducting field work or applying pesticide), and one 

involved a residential application. 

 Of the 42 chlorpyrifos drift cases, 12 cases (29%) involved a cited violation. These 12 

cases all stem from one event in New Mexico (event ID # NM00001) where a certified 

applicator was cited for a drift event that resulted in these 12 case reports. Further, of the 

42 chlorpyrifos drift cases, 20 cases have label use information, of which 19 cases did not 

follow the label and one case did follow the label. Twelve of the 19 cases that did not 

follow the label were in fact the same cases from New Mexico (event ID # NM00001). 

SENSOR-Pesticides collects information on the activity that was performed at the time of the 

exposure. Of the 91 cases that involved a respiratory effect: 24 cases were applying the pesticide, 

37 cases were conducting routine work activity (includes field residue), and 15 cases were 

conducting routine indoor living activities60 (other activities were unknown or included a small 

number of cases and are not included here). 

In summary, 45% of the chlorpyrifos cases reported in SENSOR-Pesticides from 2002-2009 had 

respiratory symptoms. It is important to note that in SENSOR-Pesticides, respiratory effects are 

the most commonly reported symptom with 45% of all cases in the database reporting a 

respiratory symptom. 

Conclusion 

EPA’s review of chlorpyrifos incidents from IDS, NPIC, PISP, and SENSOR-Pesticides 

databases show numerous instances in which respiratory effects were seen following both 

occupational and residential chlorpyrifos exposures. While the available data are insufficient to 

directly link respiratory effects to chlorpyrifos volatilization exposure, the data suggest that 

 

59 The SENSOR-Pesticides program defines drift as “...the movement of pesticides away from the treatment site. 

The pesticide spray, mist, fumes, or odor are carried from the target site by air.” There is no clear distinction 

between cases exposed from volatilization versus spray drift when analyzing the SENSOR-Pesticides data. 

60 Residential uses of chlorpyrifos are no longer allowed. 
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inhalation exposure to some forms of chlorpyrifos (vapor or aerosol) may result in respiratory 

health effects. The reported respiratory symptoms appear to be consistent with cholinergic 

stimulation of the respiratory tract. Specifically, symptoms commonly reported include difficulty 

breathing, nasal congestion coughing, and chest tightening which are qualitatively consistent 

with the anticipated cholinergic effects reported by Taylor46 and by studies of OP war agents. 

2.3.3. Dose Response and Endpoint Selection 

2.3.3.1. Acute Inhalation Exposure

As discussed in detail in the 2011 preliminary HHRA, the agency did a thorough review of the 

scientific literature and determined that ChE inhibition provides the most robust dose response 

data for deriving PODs. Moreover, EPA determined that route-specific studies were most 

relevant to avoid uncertainties associated with route to route extrapolation. The evaluation of 

residential bystander risk from chlorpyrifos volatilization is focused only on the inhalation route 

of exposure, and for short durations. 

For chlorpyrifos, there is a special acute inhalation toxicity study6

 available that can be used in 

the assessment of volatilization risk. In the special acute inhalation study, female rats were 

exposed by nose only to atmospheric concentrations of up to 53.9 mg/m3

 of particulate 

chlorpyrifos for 6 hours and allowed an additional 72 hours to recover. The mass median 

aerodynamic diameter (MMAD)/geometric standard deviations for these exposure levels were 

determined to be 1.93/1.58, 1.86/1.61, 1.79/1.59 and 1.9/1.51 microns, respectively. Plasma, 

RBC, brain, and lung ChE were measured. 

The EPA uses the Reference concentration (RfC) methodology in extrapolating animal PODs to 

human equivalent concentrations (for details of the RfC calculations, see the 2011 HHRA). In 

the 2011 risk assessment, the POD was an HEC calculated based on 24 hour exposure duration 

which was appropriate for evaluating ambient air exposure. In this volatilization assessment, a 6 

hour exposure duration was selected for the purpose of assessing residential bystander risk from 

volatilization to avoid the need for an exposure duration adjustment and to better match the offfield volatility data (samples were collected every six hours in the alfalfa field volatility study; 

see Section 3.4). 

Consistent and significant lung and plasma ChE inhibition were noted at the lowest 

concentration tested of 3.7 mg/m3

, which is a lowest-observed-adverse-effect-level (LOAEL). 

RBC and brain ChE inhibition were noted at ≥ 12.9 mg/m3

 and 53.9 mg/m3, respectively, 

indicating less sensitivity than lung and plasma ChE inhibition following acute inhalation 

exposures. A no-observed-adverse-effect-level (NOAEL) was not established. A benchmark 

dose (BMD) analysis was attempted with the lung ChE data but did not provide high confidence 

results (for details, see 2011 HHRA). A 6-hour HEC was calculated to be 2.5 mg/m3

 based on 

the LOAEL of 3.7 mg/m3

. 

For the RBC ChE data, a benchmark response (BMR) level of 10% was used to calculate a 

benchmark response level of 10% (BMDL10). The BMD10 is the estimated dose where ChE is 

inhibited by 10% compared to background. The BMDL10 is the lower confidence bound on the 
22 of 70 

BMD10. Extensive analyses conducted as a part of the OP cumulative risk assessment61 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. BMD analysis of the RBC ChE data resulted in a BMDL10 = 4.3 

mg/m3. Using the RfC methodology, a 6-hour HEC was calculated to be 15 mg/m3. 

The inhalation endpoints and PODs used for this analysis are summarized in Table 3. 

Table 3. Toxicological Concentrations, Human Equivalent Concentrations, Endpoints and 

Points of Departure for the Chlorpyrifos Residential Bystander Assessment from 

Volatilization

Exposure Scenario Point of Departure (mg/kg/day) Study and Toxicological 

Effects 

Acute Inhalation (Lung) 

Inhalation LOAEL = 3.7 mg/m3 

HEC = 2.5 mg/m3

UFA = 3x UFH = 10x 

UFDB = 10x (LOAEL to NOAEL 

extrapolation 

Residential LOC for MOE = 300 

Lung ChE Inhibition 

Special 6 hour acute inhalation study 

(MRID 48139303). (Aerosol) 

- 47% lung ChE inhibition 

(LOAEL = 3.7 mg/m3

,

 NOAEL not 

established) 

Acute Inhalation (RBC) 

BMDL10 = 4.3 mg/m3 

HEC = 15 mg/m3

UFA = 3x UFH = 10x 

Residential LOC for MOE = 30 

RBC ChE Inhibition 

Special 6 hour acute inhalation study 

(MRID 48139303). (Aerosol) 

- 10 % lung ChE inhibition 

(BMDL10 = 4.3 mg/m3

) 

lowest observe affect level (LOAEL); no observe affect level (NOAEL); human equivalent concentration (HEC); 

uncertainty factor for animal to human extrapolation (UFA); uncertainty factor for with human variability to account 

for sensitivity subpopulations (UFH); uncertainty factor database (UFDB); level of concern (LOC); margin or 

exposure (MOE); cholinesterase (ChE); red blood cell (RBC); benchmark response level of 10% (BMDL10) 

2.3.3.2. Acute Toxicity Adjustment Factor for Chlorpyrifos-oxon

There is potential for bystander inhalation exposure to volatilized chlorpyrifos-oxon, a 

transformation product of chlorpyrifos. EPA developed a toxicity adjustment factor (TAF) to 

estimate the potency of chlorpyrifos-oxon relative to chlorpyrifos. An acute study evaluating 

ChE inhibition from inhalation exposure to the oxon is not available. As such, EPA must use the 

oral studies as a reasonable approximation. 

In order to determine a TAF for chlorpyrifos-oxon, BMD modeling of available oxon data for 

acute oral dosing studies was conducted as part of the preliminary risk assessment.1 The 

complete BMD analysis is described in the 2011 preliminary HHRA; only summary information 

is provided here. 

 

61 U.S. Environmental Protection Agency. Revised Organophosphorous Pesticide Cumulative Risk Assessment; 

June 10, 2002. Available at: http://www.epa.gov/pesticides/cumulative/rra-op/. 
23 of 70 

There is a high quality comparative ChE study for the oxon and parent, chlorpyrifos using the 

oral route of exposure.62 The comparative ChE study evaluates effects of brain and blood ChE in 

juvenile and adult rats following acute exposures and following 11 repeated exposures. Only the 

results of the acute study are relevant for the acute TAF and thus only the results of the acute 

study are discussed here. The most sensitive endpoint from the acute comparative ChE study is 

RBC ChE inhibition for both chlorpyrifos and chlorpyrifos-oxon. 

When deriving PODs, the agency uses the BMDL, not the BMD, since the BMDL accounts for 

variability of the data. When making comparisons among chemicals, the BMD provides the more 

appropriate point of comparison across studies. In the case of this assessment, the BMD10

provides the basis for determining the relative toxicity of the chlorpyrifos oxon compared to 

chlorpyrifos. A toxicity factor for chlorpyrifos-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 comparative ChE study). The 

toxicity factors may be used in assessments where exposures to chlorpyrifos and chlorpyrifosoxon are to be combined (i.e., when exposure to both chemicals is expected to occur 

simultaneously). 

2.4. Exposure Characterization 

2.4.1. Field Volatility Data 

EPA reviewed and incorporated the results from two field volatility studies (applied to alfalfa 

and potato) conducted with chlorpyrifos. While chlorpyrifos is not currently registered for use on 

potatoes in the United States, potatoes are thought to be reasonable representative, for the 

purposes of extrapolation of field volatility data, of other field crops treated with chlorpyrifos 

and the results of potato field volatility study provide useful information on the potential 

volatility of chlorpyrifos under field conditions. As previously mentioned, the majority of 

chlorpyrifos is applied to field crops (67% of the total chlorpyrifos applied per year) including 

alfalfa (6% of the total chlorpyrifos applied per year). 

These studies were conducted at rates much lower than the current maximum single broadcast 

application rate; however, based on usage data the rates used in these studies are consistent with 

typical average single application rates for these crops. These two studies are summarized in the 

sections below and were used to estimate flux rates (i.e., rate of volatilization) off treated crops. 

Additional study details are provided in the data evaluation record (DER) for each of the 

respective studies.7,11

 

 

62 Marty and Andrus 2010, MRID No.: 48139301 TXR No. 0055409 
24 of 70 

2.4.1.1. Alfalfa (MRID 48883201)7

2.4.1.1.1. Study Summary

The concentration of chlorpyrifos, chlorpyrifos-oxon, and 3,5-6 tricloropyridinol (TCP)63 in air 

were measured following application of Lorsban Advanced Insecticide64 (EPA Reg. No. 62719-

591; a liquid, low VOC formulation containing 3.66 lb/gal chlorpyrifos as the active ingredient), 

to a 7.59 acre alfalfa field (a crop height of 10 inches) via ground broadcast application at an 

application rate of 0.877 lb a.i./A. The study was conducted in Los Baños, California. The 

application was made on September 9, 2011, between 9:10 – 10:40 am. 

Air sampling was conducted for three days during and after the application. Air samples were 

collected from eight off-field monitoring stations placed uniformly around each field (30 m from 

the edge of the treated area) during the first sampling period, which included the application 

event. The sampling height of the off-field monitors was 1.5 m. After the application, an on-site 

profile monitoring station was set up in the center of the field to measure concentrations at 

heights of 0.15, 0.33, 0.55, 0.90, and 1.5 m. The on-field air samples were taken at 

approximately 6 hour intervals from time 0 (after the application) through 24 hours and at 12 

hour intervals from time 24 to 72 hours. The 12 hour samples coincided approximately with 

sunrise and sunset and were centered at 07:00 and 19:00 hours when possible. On-site 

meteorological conditions were monitored continuously at the site. 

Flux rates for chlorpyrifos and total toxic chlorpyrifos residues (TTCR; chlorpyrifos and 

chlorpyrifos-oxon) in μg/m2

-s were calculated using the Indirect Flux Method (IFM) for the first 

period when the application was in progress (Sampling Period 1) and both the Integrated 

Horizontal Flux (IHF) method and the Aerodynamic (AD) method for the other sampling 

periods. A brief description of these methods is provided in APPENDIX C. Approximately 28% 

(chlorpyrifos) and 30% (TTCR) of the applied active ingredient was observed to volatilize within 

the first 24 hours. For dispersion modeling and subsequent buffer calculations, TTCR is defined 

in terms of toxic parent equivalents—the sum of the chlorpyrifos concentration and the oxon 

concentration that has been corrected to parent equivalents using a toxicity adjustment factor 

(TAF; discussed in detail in Section 2.3) of 12.65

This study is classified as acceptable and is used quantitatively in this assessment. 

2.4.1.1.2. Flux Rates 

EPA calculated flux rates for chlorpyrifos (parent only) and TTCR (chlorpyrifos and 

chlorpyrifos-oxon) in μg/m2-s for the first period (e.g., during application) (Sampling Period 1) 

employing the IFM. Flux rates for the subsequent sampling periods (thru day 3) were calculated 

using both the IHF method and the AD flux method. Flux rates were calculated accounting for 

field fortifications (FF) as well as storage stability and are reported in the DER. Table 4 contains 

the flux rates for chlorpyrifos (parent only) calculated using both methods (AD and IHF) 

 

63 TCP is not considered as a residue of concern. 

64 Marketed as a low VOC chlorpyrifos formulation. 

65 Email from Wade Britton (OPP/HED) to Rochelle Bohaty (OPP/EFED) September 10, 2012. 
25 of 70 

incorporating data for both FF and storage stability for all sampling periods and based on the 

application rate of 0.877 lb a.i./A used in the study. 

Table 4. Estimated Flux Rates for Chlorpyrifos Using AD and IHF Methods for the Alfalfa 

Study 

Period Sample Start 

Times 

Sample End 

Times 

Duration 

(hh:mm) 

Hours Post 

Application 

at Sample 

Collection 

AD 

Method 

IHF 

Method* 

Estimated 

Flux FF 

(μg/m2-s) 

Estimated 

Flux FF 

(μg/m2-s) 

2 9/9/2011 10:45 9/9/2011 13:00 2.25 2.25 0.824 0.591 

3 9/9/2011 13:00 9/9/2011 19:00 6.00 8.25 0.260 0.759 

4 9/9/2011 19:00 9/10/2011 1:00 6.00 14.25 0.038 0.171 

5 9/10/2011 1:00 9/10/2011 7:00 6.00 20.25 0.026 0.054 

6 9/10/2011 7:00 9/10/2011 19:00 12.00 32.25 0.081 0.167 

7 9/10/2011 19:00 9/11/2011 7:00 12.00 42.25 0.012 0.021 

8 9/11/2011 7:00 9/11/2011 19:00 12.00 54.25 0.020 0.033 

9 9/11/2011 19:00 9/12/2011 7:00 12.00 66.25 0.007 0.006 

10 9/12/2011 7:00 9/12/2011 19:00 12.00 78.25 0.018 0.012 

11 9/12/2011 19:00 9/13/2011 7:00 12.00 90.25 0.002 0.005 

Field Fortification (FF) 

*Flux profile used in assessment for quantitative assessment (i.e., used in PERFUM modeling). 

Data provided in this table are based on the application rate used in the study (0.877 lb a.i./A). 

The results for chlorpyrifos (parent only) for the AD method compared well against the values 

calculated by the study authors; however, the results for the IHF method were higher than those 

calculated by the study author. A review of the study author’s calculations indicated that the IHF 

analysis was truncated at 150 cm, which resulted in lower flux rate estimates. The IHF method 

provided slightly more conservative flux profiles; therefore, the flux rates from this method were 

used in the PERFUM analyses. 

EPA examined the flux rate values for oxon only; however, attempts at conducting a regression 

analysis generated poor fitting results. The study authors also completed this analysis and their 

results are consistent with EPA’s analysis. It is unclear if the chlorpyrifos-oxon concentrations 

are a result of chlorpyrifos-oxon flux off the field or from transformation of chlorpyrifos in the 

air or on the tube.66 In any case, it suggests the potential for exposure to chlorpyrifos oxon. In 

order to account for the formation and potential increased toxicity from exposure to chlorpyrifosoxon, EPA used a total toxic residue approach which combines chlorpyrifos and chlorpyrifosoxon residues. Initial attempts at conducting a regression analysis for the TTCR generated poor 

fitting results. In general, the oxon concentrations did not follow a decreasing trend with height 

as did the chlorpyrifos concentrations, resulting in poor fitting results. In an effort to use the 

available data, a conversion factor was developed for each sampling period, which took the total 

mass of oxon for all four air samplers and divided this total by the total mass of chlorpyrifos for 

 

66 The study authors conducted an additional analysis and developed a correction factor to adjust the chlorpyrifosoxon concentrations to account for chlorpyrifos-oxon formation on the sample tubes during the study. 
26 of 70 

all four air samplers. The mass of chlorpyrifos at each height was then multiplied by this 

conversion factor to estimate the mass of oxon at the same height. 

Table 5 contains the TTCR flux profile developed for all sampling periods for the alfalfa study. 

Table 5. Estimated Flux Rates for Total Toxic Chlorpyrifos Residues (Chlorpyrifos and 

Chlorpyrifos-oxon) Using AD and IHF Methods for the Alfalfa Study 

Sampling 

Period 

Sample Start 

Times 

Sample End 

Times 

Duration 

(hh:mm) 

Hours Post 

Application 

at Sample 

Collection 

AD 

Method 

IHF 

Method* 

Estimate

d Flux 

FF 

(μg/m2-

s) 

Estimated 

Flux FF 

(μg/m2-s) 

2 9/9/2011 10:45 9/9/2011 13:00 2.25 2.25 0.83 0.59 

3 9/9/2011 13:00 9/9/2011 19:00 6.00 8.25 0.27 0.78 

4 9/9/2011 19:00 9/10/2011 1:00 6.00 14.25 0.038 0.17 

5 9/10/2011 1:00 9/10/2011 7:00 6.00 20.25 0.026 0.051 

6 9/10/2011 7:00 9/10/2011 19:00 12.00 32.25 0.088 0.18 

7 9/10/2011 19:00 9/11/2011 7:00 12.00 42.25 0.012 0.024 

8 9/11/2011 7:00 9/11/2011 19:00 12.00 54.25 0.021 0.033 

9 9/11/2011 19:00 9/12/2011 7:00 12.00 66.25 0.007 0.006 

10 9/12/2011 7:00 9/12/2011 19:00 12.00 78.25 0.020 0.013 

11 9/12/2011 19:00 9/13/2011 7:00 12.00 90.25 0.002 0.004 

Field Fortification (FF) 

*Flux profile used in assessment 

Data provided in this table are based on the application rate used in the study (0.877 lb a.i./A). 

Flux profile is based on mass only and does not take into account the difference in toxicity between 

chlorpyrifos and chlorpyrifos-oxon. 

2.4.1.2. Potato (MRID 48998801)11

2.4.1.2.1. Study Summary

The rate of volatilization of chlorpyrifos only was measured following a 0.61 lb a.i./A ground 

broadcast application of Dursban EC, an emulsifiable concentration, non-low VOC formulation 

containing 4.0 lb a.i./gal chlorpyrifos as the active ingredient [the currently marketed equivalent 

is Dursban 4E (EPA Reg. No.62719-220)], to a potato field (crop height of 22 inches). This 

study did not measure chlorpyrifos-oxon concentrations. The study was conducted in the 

Netherlands on June 25, 2002, with the application beginning at 11:30 am. 

Air concentration samples were collected beginning at 12:35 pm and ending the following day at 

1:37 pm. Samples were collected at 1.0, 1.6, and 1.9 m above the top of ridges on the soil surface 

at the center of a potato field. Wind speed and temperature data were collected at various heights 

using equipment positioned at the center of the field during the sample collection period. 

Flux rates for chlorpyrifos in μg/m2

-s were calculated using the AD method. Approximately 71% 

of the applied chlorpyrifos was estimated to volatilize within 24 hours following application 

assuming continuous flux. Sampling did not occur at night; therefore, in order to develop a 24 
27 of 70 

hour flux profile. EPA developed a flux rate for the missing sampling periods by averaging the 

flux rate prior to and after the time period when sample collection did not occur. 

This study is classified as supplemental and is used for characterization purposes in this 

assessment. 

2.4.1.2.2. Flux Rates 

EPA developed a flux profile (shown in Table 6) for chlorpyrifos in mg/m2

-hr using the flux 

rates calculated by the study authors using the AD method for the first 24 hours following 

application of chlorpyrifos at 0.61 lb a.i./A to potatoes. During the overnight hours 

(approximately 21:00 to 10:00) sample collection did not occur. In order to develop a 24 hour 

flux profile, the average flux rate for the sampling period immediately before and immediately 

after the overnight hours (Periods 4 and 5 in Table 6) were averaged and assumed to represent 

the flux that occurred during the overnight period. The raw data for this study have not been 

made available to EPA so the flux rates could not be independently verified by EPA. However, 

the results from this study are presented here to provide another line of evidence to characterize 

the observations in the registrant submitted study to help describe the potential variability in flux 

rates due to difference in study conditions (e.g., crop canopy, formulation, and weather). 

Table 6. Estimated Flux Rates for Chlorpyrifos AD Method For the Potato Study 

Sampling 

Period 

Sample Start 

Times 

Sample End 

Times 

Duration 

(hh:mm) 

AD Method 

Estimated Flux 

(μg/m2-s) 

1 6/25/2002 12:35 6/25/2002 13:56 1:21 1.87 

2 6/25/2002 14:01 6/25/2002 15:34 1:33 2.17 

3 6/25/2002 15:42 6/25/2002 17:26 1:44 0.82 

4 6/25/2002 19:20 6/25/2002 20:44 1:24 0.17 

5 6/26/2002 09:54 6/26/2002 11:32 1:38 0.16 

6 6/26/2002 11:42 6/26/2002 13:37 1:55 0.08 

Data provided in this table are based on the application rate used in the study 0.61 lb a.i./A. 

2.4.2. Conclusions and Modeling Inputs

In general, the flux trends estimated by EPA and those by the study author for the alfalfa study 

are consistent. In addition, although the magnitudes of the peak flux rate are different, the results 

of the potato study are consistent with the alfalfa study (Figure 3). 
28 of 70 

a) b) 

Figure 3. 24 Hour Flux Profile Generated Based on the Two Available Field Volatility 

Studies for Chlorpyrifos (Parent Only); a) the flux rate compared to hours post application and b) the 

flux rate compared to the time of day. NOTE: Flux profiles are scaled up from the actual application rates of 0.877 

lb a.i./A (alfalfa) and 0.61 lb a.i./A (potato) to 6 lb a.i./A (current the highest single maximum broadcast application 

rate registered). 

The difference in the peak flux values estimated between the two studies may be the result of 

several different factors including: the product formulation used, the crop treated (alfalfa 

compared to potato), environmental conditions (temperature, humidity, etc.), differences in plant 

foliage and canopy structure, or a combination of these and other factors. In addition, the 

difference in the peak values obtained may also be the result of the difference in sample 

collection periods which could have led to an averaging out of the peak flux based on the longer 

sampling period used in the alfalfa study as compared to the potato study. Figure 3 shows the 

flux profile for both studies for the first 24 hours following chlorpyrifos application based on the 

calculated flux rates. 

The flux profile for chlorpyrifos is similar to those observed for fumigants in that there is a peak 

emission shortly after application (Figure 3a) and during the warmer part of the day (Figure 

3b). The peak is followed by a much lower emission rate that generally decreases over time. 

While the potato study does not extend beyond 24 hours post application, the alfalfa study shows 

a general decrease in emission rates over the next two days. However, a secondary “peak” flux 

was observed on day two of the alfalfa field volatility study. This observed increase in the flux 

rate following the overnight hours is likely due to an increase in temperature. 

For PERFUM modeling the flux rates from the field volatility studies were adjusted to an 

application rate of 6 lb a.i./A (currently, the highest single maximum broadcast application rate 

registered). In the past, flux rates and subsequent modeling were completed by scaling down 

results for studies conducted at higher application rates; however, in this case, the available 

studies were conducted at application rates [0.877 lb a.i./A (alfalfa); 0.61 lb a.i./A (potato)] 

which is below the maximum labeled single broadcast application rate (6 lb a.i./A for citrus). The 

single application rate used in the alfalfa study is lower than the currently labeled maximum 

single application rate for alfalfa (1 lb a.i./A). 
29 of 70 

The flux profiles used in PERFUM model runs are presented in Table 7 (alfalfa) and Table 8

(potato). To account for the increased toxicity of the oxon (alfalfa study) compared to the parent 

chlorpyrifos, the observed oxon concentration was adjusted by a TAF of 12 to generate an 

exposure concentration in units of parent equivalent toxicity67. The adjusted concentration value 

([oxon]*TAF) was added to chlorpyrifos (parent only) concentration to develop a TTCR flux 

profile for PERFUM analyses (shown in Table 7). 

Table 7. Flux Profile Used in PERFUM Analysis Generated from the Alfalfa Study 

Hour of the 

Day 

Chlorpyrifos (only)a

μg/m2/sec

Total Toxic Chlorpyrifos Residuesa,b

μg (parent toxicity equivalent 

units)/m2/sec

Day 1 Day 2 Day 3 Day 1 Day 2 Day 3 

1 1.17 0.14 0.04 1.35 0.32 0.05 

2 0.37 0.14 0.04 0.44 0.32 0.05 

3 0.37 0.14 0.04 0.44 0.32 0.05 

4 0.37 0.14 0.04 0.44 0.32 0.05 

5 0.37 0.14 0.04 0.44 0.32 0.05 

6 0.37 0.14 0.04 0.44 0.32 0.05 

7 0.37 0.14 0.04 0.44 0.32 0.05 

8 1.14 0.23 0.04 2.63 0.39 0.26 

9 1.14 0.23 0.23 2.63 0.39 0.26 

10 1.50 1.14 0.23 3.14 2.63 0.39 

11 1.50 1.14 0.23 3.14 2.63 0.39 

12 4.04 1.14 0.23 5.41 2.63 0.39 

13 4.04 1.14 0.23 5.41 2.63 0.39 

14 5.19 1.14 0.23 8.68 2.63 0.39 

15 5.19 1.14 0.23 8.68 2.63 0.39 

16 5.19 1.14 0.23 8.68 2.63 0.39 

17 5.19 1.14 0.23 8.68 2.63 0.39 

18 5.19 1.14 0.23 8.68 2.63 0.39 

19 5.19 1.14 0.23 8.68 2.63 0.39 

20 1.17 0.14 0.04 1.35 0.32 0.05 

21 1.17 0.14 0.04 1.35 0.32 0.05 

21 1.17 0.14 0.04 1.35 0.32 0.05 

22 1.17 0.14 0.04 1.35 0.32 0.05 

23 1.17 0.14 0.04 1.35 0.32 0.05 

24 1.17 0.14 0.04 1.35 0.32 0.05 

 

67 Parent equivalent toxicity is not representative of the mass balance. 
30 of 70 

Hour of the 

Day 

Chlorpyrifos (only)a

μg/m2/sec

Total Toxic Chlorpyrifos Residuesa,b

μg (parent toxicity equivalent 

units)/m2/sec

Day 1 Day 2 Day 3 Day 1 Day 2 Day 3 

a. Flux profile has been scaled up from an application rate of 0.877 lb a.i./A to 6 lb a.i./A (current 

maximum single broadcast application rate) and are based on the flux profile that considers FF 

samples. 

b.To account for the increased toxicity of the oxon compared to the parent chlorpyrifos, the 

observed oxon concentration was adjusted by a toxicity adjustment factor (TAF) of 12 to 

generate an exposure concentration in units of parent equivalent toxicity67. The adjusted 

concentration value ([oxon]*TAF) was added to chlorpyrifos (parent only) concentration to 

develop a TTCR flux profile. These values do not directly correlate to 12x the values presented 

in Table 5. 

Table 8. Flux Profile Used in PERFUM Analysis Generated from the Potato Study 

Hour 

Chlorpyrifos (only)a

μg/m2/sec

Day 1 

1 1.58 

2 1.58 

3 1.58 

4 1.58 

5 1.58 

6 1.58 

7 1.58 

8 1.58 

9 1.58 

10 1.58 

11 1.58 

12 1.58 

13 18.67 

14 18.67 

15 21.67 

16 21.67 

17 8.22 

18 8.22 

19 8.22 

20 1.68 

21 1.68 

21 1.58 

22 1.58 

23 1.58 

24 1.58 
31 of 70 

Hour 

Chlorpyrifos (only)a

μg/m2/sec

Day 1 

a.Flux profile has been scaled up from 

an application rate of 0.61 lb a.i./A to 

6 lb a.i./A (current maximum single 

broadcast application rate). 

3. RISK ASSESMENT 

Chlorpyrifos can be used under a variety of agricultural and environmental conditions. In order 

to understand the impact such varied conditions can have on exposure (and subsequent risk 

assessments), analyses were completed using PERFUM to examine the impact of varied 

application rates, field sizes, and meteorological data (see bullets below). These are the same 

parameters which have been used as the basis for other similar analyses completed for soil 

fumigants by EPA. For brevity only a subset of these analyses are presented in the sections 

below. Example PERFUM output files are provided in ATTACHMENT 2. The results of this 

analysis are presented in the sections below. Note: the PERFUM analyses do not provide insight 

into the potential impact of topography, field layout crop canopy, and formulation, on air 

concentrations. These variables are further discussed in Section 4.

 Application rates: 6, 5, 4, 3, 2.3, 2, 1, 0.5, and 0.1 lb a.i./A 

 Field size: 1, 5, 10, 20, 40, 60, 80, 100, and 120 acres 

 Meteorological data: Bakersfield, California; Bradenton, Florida; Flint, Michigan; 

Tallahassee, Florida; Ventura, California; and Yakima, Washington 

3.1. Buffer Distributions 

3.1.1. Alfalfa 

3.1.1.1.Chlorpyrifos (Parent Only) 

Table 9 presents the PERFUM results based on lung ChE inhibition and the flux profile derived 

from the alfalfa study. The results are provided for chlorpyrifos (parent only) at an application 

rate of 1 lb a.i./A. An application rate of 1 lb a.i./A as presented in Table 9 is close to the actual 

application rate made in the alfalfa study (0.877 lb a.i./A) and many applications for various 

crops (e.g., alfalfa, asparagus, bean, sweet corn, cotton, onions, peas, soybean, strawberries, and 

wheat) are made at approximately 1 lb a.i./A or less. The results from three of the six 

meteorological datasets (i.e., Ventura, California, Bradenton, Florida, and Flint, Michigan) used 

in model simulations are presented as these three meteorological datasets are thought to 

reasonably represent the range of environmental conditions across the United States. 

 
32 of 70 

Table 9. Comparison of Results For Chlorpyrifos (parent only) PERFUM Buffer 

Distributions Based On Alfalfa Studya, b 

Percentiles 

Meteorological Station Location 

Ventura, CA Bradenton, FL Flint, MI 

Max 

Bufferc 

(ft) 

Whole 

Field 

Bufferd 

(ft) 

Max 

Bufferc 

(ft) 

Whole 

Field 

Bufferd 

(ft) 

Max 

Bufferc 

(ft) 

Whole 

Field 

Bufferd 

(ft) 

10 A, 1 lb a.i/A 

50 16 - 33 - - - 

75 16 - 148 - 16 - 

90 66 16 262 33 16 - 

95 148 16 377 98 49 16 

99 377 82 607 246 148 16 

20 A, 1 lb a.i/A 

50 16 - 98 - - - 

75 33 - 246 16 16 - 

90 164 16 427 82 66 16 

95 262 16 607 164 131 16 

99 623 180 1001 394 279 82 

60 A, 1 lb a.i/A 

50 16 - 262 - 16 - 

75 148 16 558 16 82 - 

90 410 33 935 213 230 16 

95 623 131 1312 377 344 66 

99 1345 443 2100 804 640 262 

120 A, 1 lb a.i/A 

50 82 - 443 - 16 - 

75 279 16 902 66 180 - 

90 689 98 1542 361 410 49 

95 1050 230 2165 591 607 148 

99 2215 722 3510 1263 1198 459 

a.Lung: 6 h HEC 2,500 μg/m3, UF=300, target concentration 8.3 μg/m3

b.Registrant study conducted on alfalfa using Lorsban Advanced 

c.Max buffers are the largest buffer estimated from any given direction away from the field. 

d.Whole field buffers are estimated based on an array of results for the entire perimeter of a 

field. 

The results presented in Table 9 show how buffer distances (whole field68 and maximum69

buffers) change in relation to field size, percentile of exposure, and meteorological conditions. 

Buffer distances are calculated based on the distance at which the target concentration [i.e., 

toxicity point of departure (HEC for lung ChE inhibition) divided by the uncertainty factor] of 

8.3 μg/m3

 is reached. Depending on the percentile of exposure and the field size considered, the 

maximum buffers needed to be at or below the target concentration range from 0 to greater than 

3500 ft. Spray drift buffers to protect bystanders in sensitive sites70 required on chlorpyrifos 

 

68 Whole field buffers are estimated based on an array of results for the entire perimeter of a field. 69 Max buffers are the largest buffer estimated from any given direction away from the field. 70 Buffers are around sensitive sites (a circle drawn around the sensitive site and do not correspond to buffers around 

a given field (a circle drawn around a treated field). 
33 of 70 

labels range from 0-100 feet depending on the application method. The buffers currently required 

on all chlorpyrifos labels are summarized in APPENDIX D. 

Considering an application scenario similar to that reported in the alfalfa study—1 lb a.i./A 

(actual: 0.877 lb a.i./A) chlorpyrifos application made to a 10 A (actual: 7.59 A) alfalfa field in 

Ventura, California (actual: Los Baños, California) —a 16 foot whole field buffer and a 148 foot 

maximum buffer were estimated by PERFUM to ensure that 95% of the time bystanders offsite 

would not be exposed to concentrations higher than the target concentration. If, however, a 

similar application where to occur in an area with meteorological conditions similar to 

Bradenton, Florida71 PERFUM estimates that a 98 foot whole field buffer and a 377 foot 

maximum buffer are needed to ensure that 95% of the time bystanders are not exposed to air 

concentrations higher than the target concentration. When considering a larger treatment area, for 

example, a 120 A alfalfa field similar to the study site.72 PERFUM estimates that a 230 foot 

whole field buffer and a 1050 foot maximum buffer are needed to ensure that 95% of the time 

bystanders are not exposed to air concentrations higher than the target concentration. 

Although not presented in detail in this assessment, similar analyses using a target concentration 

of 497 μg/m3 derived based on the RBC ChE inhibition endpoint, indicate that currently labeled 

buffers (APPENDIX D) would result in bystander exposure to offsite concentrations that are 

less than the target concentration. The target concentration based on the RBC ChE inhibition 

endpoint is approximately equal to the saturated vapor concentration (489 μg/m3) of 

chlorpyrifos.Error! Bookmark not defined. This suggests that in the environment, due to 

mixing and dispersion, chlorpyrifos vapor concentrations are not likely to occur near the RBC 

ChE inhibition target concentration except under extreme conditions (e.g., inversion). 

Table 10 presents the buffer distributions for a range of applications rates. Again, these results 

are based on the lung ChE inhibition endpoint and the alfalfa flux profile. Model run results 

reported in Table 10 are for chlorpyrifos (only) using the Bradenton, Florida meteorological 

data. Generally this meteorological dataset results in the largest estimated buffers of all six 

meteorological stations. Examination of the buffer distributions resulting from the PERFUM 

runs for all the meteorological datasets show results similar to those presented in Table 10. This 

analysis indicates that higher application rates lead to larger estimated buffers as expected. For 

example, when considering a same chlorpyrifos application scenario to that presented above for 

alfalfa in Bradenton, Florida71, but using a higher application rate (i.e., turf at 4 lb a.i./A—the 

current maximum single application rate) the estimated buffer is approximately seven times 

higher. For chlorpyrifos (only), a 673 foot whole field buffer and a 2215 foot maximum buffer 

would be needed to ensure that 95% of the time the air concentration that bystanders are exposed 

is not higher than the lung ChE inhibition target concentration. 

 

71 1 lb a.i./A (actual: 0.877 lb a.i./A) chlorpyrifos application made to a 10 A (actual: 7.59 A) alfalfa field in 

Bradenton, Florida (actual: Los Baños, California) 72 1 lb a.i./A (actual: 0.877 lb a.i./A) chlorpyrifos application made to a 120 A (actual: 7.59 A) alfalfa field in 

Ventura, California (actual: Los Baños, California) 
34 of 70 

Table 10. Comparison of Results For Chlorpyrifos (parent only) PERFUM Buffer Distributions Based On Ratesa,b 

Percentiles 

Bradenton, FL 

0.5 lb a.i./A 1 lb a.i./A 2 lb a.i./A 4 lb a.i./A 

Max 

Bufferc 

(ft) 

Whole 

Field 

Bufferd 

(ft) 

Max 

Bufferc 

(ft)

Whole 

Field 

Bufferd 

(ft)

Max 

Bufferc 

(ft)

Whole 

Field 

Bufferd 

(ft)

Max 

Bufferc 

(ft)

Whole 

Field 

Bufferd 

(ft)

10 A 

50 - - 33 - 328 - 886 - 75 16 - 148 - 541 33 1280 148 90 16 - 262 33 820 213 1804 459 95 16 16 377 98 1033 328 2215 673 99 131 16 607 246 1476 623 2871 1230 

20 A 

50 - - 98 3 525 - 1362 - 75 16 - 246 16 837 98 1969 246 90 16 - 427 82 1296 328 2822 689 95 98 16 607 164 1624 509 3510 1017 99 262 66 1001 394 2264 935 >4724 1870 

60 A 

50 16 - 262 - 1066 - 2740 16 75 33 - 558 16 1722 213 4183 492 90 180 16 935 213 2674 640 >4724 1296 95 279 33 1312 377 3461 984 >4724 1936 99 640 213 2100 804 >4724 1821 >4724 3707 

120 A 

50 16 - 443 - 1706 16 4511 66 75 115 - 902 66 2789 344 >4724 755 90 328 16 1542 361 4446 984 >4724 1919 95 509 98 2165 591 >4724 1493 >4724 2936 99 1099 361 3510 1263 >4724 2838 >4724 >4724 

a. Registrant study conducted on alfalfa using Lorsban Advanced b. Lung: 6 h HEC 2,500 μg/m3, UF=300, target concentration 8.3 μg/m3c. Max buffers are the largest buffer estimated from any given direction away from the field. 

d. Whole field buffers are estimated based on an array of results for the entire perimeter of a field e. Note: PERFUM does not produce buffer zones greater than 4724 ft (1440 m), thus, buffer zones for cases where the 47

reached may be very large. These cases are indicated with a “>4724 ft ”. 
35 of 70 

3.1.1.2. Total Toxic Chlorpyrifos Residues 

To account for the potential exposure to both chlorpyrifos and chlorpyrifos-oxon simultaneously, 

PERFUM was also used to estimate buffer distances based on exposure to TTCR. This technique 

incorporates a toxicity adjustment factor that accounts for the potential difference in toxicity 

between the two chemicals (discussed in Section 2.3). Table 11 presents the PERFUM results, 

again based on the toxicity endpoint derived from lung ChE inhibition and the flux profile 

derived from the alfalfa study, for both chlorpyrifos (parent only) and TTCR [in parent 

equivalents (i.e., chlorpyrifos + chlorpyrifos-oxon*TAF)]. 

Table 11. Comparison of Results For Chlorpyrifos (parent only) and Total Toxic 

Chlorpyrifos Residues PERFUM Buffer Distributionsa

Percentiles 

Bradenton, FL 

Chlorpyrifos 

(parent only) 

TTCR (Chlorpyrifos + 

Oxon*TAF) 

Alfalfab Potatoc Alfalfab 

Max 

Bufferd 

(ft) 

Whole 

Field 

Buffere 

(ft) 

Max 

Bufferd 

(ft)

Whole 

Field 

Buffere 

(ft)

Max 

Bufferd 

(ft)

Whole 

Field 

Buffere 

(ft)

10 A , 1 lb a.i/A 

50 33 - 394 - 213 - 

75 148 - 558 82 394 16 

90 262 33 738 262 623 148 

95 377 98 902 377 804 246 

99 607 246 1280 623 1165 492 

20 A, 1 lb a.i/A 

50 98 - 607 - 361 - 

75 246 16 837 148 623 49 

90 427 82 1132 394 984 246 

95 607 164 1362 558 1263 377 

99 1001 394 1919 919 1821 755 

60 A, 1 lb a.i/A 

50 262 - 1132 16 738 - 

75 558 16 1624 295 1296 148 

90 935 213 2198 738 2034 492 

95 1312 377 2690 1033 2641 771 

99 2100 804 3904 1722 3953 1460 

120 A, 1 lb a.i/A 

50 443 - 1690 33 1181 - 

75 902 66 2510 459 2067 246 

90 1542 361 3494 1083 3379 755 

95 2165 591 4364 1526 4380 1165 

99 3510 1263 >4724 2625 >4724 2264 

a. Lung: 6 h HEC 2,500 μg/m3, UF=300, target concentration 8.3 μg/m3

b. Registrant study conducted on alfalfa using Lorsban Advanced 

c. Open literature study conducted on potato using Dursban 

d. Max buffers are the largest buffer estimated from any given direction away from the field. 

e. Whole field buffers are estimated based on an array of results for the entire perimeter of a 

field 
36 of 70 

Based on exposure to the total toxic residues (expressed in parent equivalents), the modeling 

results indicate that larger buffers are necessary to ensure air concentrations that bystanders 

could be exposed to are lower than the target concentration estimated, compared to those 

estimated for chlorpyrifos (parent only). When considering chlorpyrifos (parent only) in the 

application scenario (10A, 1 lb a.i./A) presented above for Bradenton, Florida71 a 98 foot whole 

field buffer and a 377 foot maximum buffer are needed for the 95th% percentile of exposure. 

However, when the concentrations of chlorpyrifos (parent only) and chlorpyrifos-oxon are 

combined and accounted for, the increased toxicity of chlorpyrifos-oxon are taken into account, 

the buffer estimates are larger—246 feet (whole field buffer) and 804 feet (maximum buffer) for 

the 95th percentile of exposure. 

While the flux profiles are derived from concentrations measured on the field of application, 

PERFUM modeling does not account for the potential transformation of chlorpyrifos and 

chlorpyrifos-oxon as these chemicals move offsite (flux study sampling occurs on the application 

site). Potential transformation pathways in air for chlorpyrifos and chlorpyrifos-oxon include 

direct and indirect air photolysis. To evaluate the impact of air photolysis on chlorpyrifos air 

concentrations, EPA ran simulations for a 20 acre square field using AERMOD73 with and 

without the half-life term of 7200 seconds, or 2 hours (based on indirect photolysis), in the 

control pathway. All other input parameters (e.g., field size, meteorological conditions, receptor 

locations, etc) remained the same for both simulations. In both cases, the maximum 1-hour 

chlorpyrifos concentration occurred at the same receptor location (near the edge of the corner of 

the field) and meteorological conditions. The chlorpyrifos air concentration incorporating air 

photolysis was approximately 7% lower than the air concentration, which did not account for air 

photolysis. Therefore, the concentration of chlorpyrifos is not expected to markedly change and 

little impact on the estimated buffer distances for chlorpyrifos (parent only) generated using 

PERFUM and a 6-hour endpoint is expected, if air photolysis is considered. 

This analysis reinforces the hypothesis that chlorpyrifos can be converted to chlorpyrifos-oxon in 

the air via direct and indirect photolysis, and demonstrates that even at low concentrations (≤7%) 

formation of chlorpyrifos-oxon can have an appreciable impact on the estimated buffers 

distances for TTCR due to the expected increased toxicity (12x) of chlorpyrifos-oxon compared 

to chlorpyrifos. The formation of chlorpyrifos-oxon in the air over a treated field is expected to 

be captured in the flux profile for TTCR derived from the alfalfa study. 

 

 

73 AERMOD is a steady-state plume model that incorporates air dispersion based on planetary boundary layer 

turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both 

simple and complex terrain. Developed by the American Meteorological Society/Environmental Protection 

Agency Regulatory Model Improvement Committee (AERMIC), AERMOD became EPA’s preferred air 

dispersion model in 2005, replacing the Industrial Source Complex (ISC) model. AERMOD uses a simple 

runstream setup file, which contains information on the selected modeling options, as well as source location and 

parameter data, receptor locations, meteorological data file specifications, and output options. Unlike ISC, 

AERMOD requires two types of meteorological data files that are provided by the AERMET meteorological 

preprocessor program. One file consists of surface scalar parameters, and the other file consists of vertical 

profiles of meteorological data. 
37 of 70 

3.1.2. Potato

3.1.2.1.Chlorpyrifos (Parent Only) 

In addition to the buffer distribution analysis considering chlorpyrifos (parent only) and TTCR 

based on the alfalfa study, Table 11 also includes buffer distributions based on the flux profile 

for chlorpyrifos (parent only) derived from the potato study. This table shows that both whole 

and maximum field buffers calculated using PERFUM are larger based on the potato flux study 

as compared to the alfalfa study. For example, in the application scenario presented above for 

alfalfa in Bradenton, Florida,71 PERFUM estimates that based on the potato flux profile for 

chlorpyrifos (parent only) a 98 foot whole field buffer and a 377 foot maximum buffer are 

needed to ensure that 95% of the time the concentration bystanders may be exposed to is less 

than the target concentration based on the alfalfa study. For the potato study the estimated 

buffers are 377 ft (whole field buffer) and 902 ft (maximum buffer) related to the same scenario. 

The difference in the flux profile and the results of the dispersion modeling (buffer size) between 

the two studies may be the result of different experimental conditions between the two studies 

including: the crop treated (alfalfa compared to potato), meteorological conditions (temperature, 

humidity, etc.), the product formulation used (low VOC compared to a non-low VOC 

formulation), or a combination of these and other factors. In addition, some of the variation in 

the estimated buffer distances may be an artifact of the study design given the differences in 

sample collection periods [6 h (alfalfa) versus 1.5 h (potato)]. This could have led to an 

averaging out of the peak flux due to the longer sampling periods used in the alfalfa study as 

compared to the potato study. The aforementioned issues are further discussed Section 3.5. 

3.2. Buffer Durations 

This assessment is based on peak offsite exposure concentrations estimated using PERFUM 

modeling based on flux profiles derived from two available field volatility studies. An analysis 

on the potential duration that a buffer would need to be in place to ensure bystanders are not 

exposed to chlorpyrifos and chlorpyrifos-oxon concentrations higher than the target 

concentration is presented below. The shape of flux profiles was an important consideration 

included in the soil fumigant assessments conducted in the past (e.g., how long significant 

emissions occur after application). 

3.2.1. Alfalfa 

3.2.1.1.Chlorpyrifos (Parent Only) 

Based on the six hour exposure duration, six hour toxicity endpoint for lung, and the 95th

percentile exposure, a 50 foot buffer would need to be in place for at least 12 hours for a 10A 

field treated at 1 lb a.i./A. A 300 foot buffer would need to be in place for 18 hours for the same 

size field treated at 6 lb a.i/A, in order to ensure that bystanders are not exposed to 

concentrations of chlorpyrifos (parent only) higher than the target concentration. A graphical 
38 of 70 

depiction of this analysis is provided in Figure 4. The same analysis was conducted for 20, 60, 

and 120 acre field sizes. Results for 1 lb a.i./A and 6 lb a.i./A are presented for all the field sizes 

assessed in terms of risk estimations in Section 4. The results indicate the peak concentration of 

chlorpyrifos (parent only) estimated offsite occurs on the day of application; however, on the day 

following application, there is an increase in the estimated flux concentrations likely due to an 

increase in temperature following the overnight hours. For some application scenarios, 

concentrations exceed the target concentration on the second day even when concentrations 

observed for the 6 hours sampling period prior to this secondary peak concentrations are lower 

than the target concentration. For some scenarios large buffers (e.g., 300 ft) may need to be 

inplace for a shorter duration (e.g., 12 hr) than a smaller buffer (e.g., 200 ft; 18hr); however, 

since the buffer duration for the smaller buffer may be longer, the longer duration may be 

considered for the larger buffers. 

Figure 4. Buffer Duration Analysis for Various Buffer Distances for a 10 Acre Field 

Considering Chlorpyrifos (parent only) using Bradenton, Florida Meteorological Data; 

Registrant conducted field volatility study on alfalfa using Lorsban Advanced; Lung: 6 h HEC 
39 of 70 

2,500 μg/m3

, UF=300 (i.e., MOE =300), target concentration 8.3 μg/m3; a. actual distance: 230 ft 

(70 m) 

3.3. Risk Estimations 

The risk estimates associated with bystander inhalation of vapor phase chlorpyrifos and 

chlorpyrifos-oxon (specific to the alfalfa application scenario) were calculated based on 

estimated air concentrations (based on an average concentration around the entire field similar to 

the whole field buffer concept) at various buffer distances away from a chlorpyrifos treated field 

for a range of percentiles of exposure and application rates. This provides an additional, 

conceptual approach of evaluating the potential for bystander risks. The results for chlorpyrifos 

(parent only) are presented in Table 12. The toxicological endpoint used in this analysis is lung 

ChE inhibition (6 h HEC 2,500 μg/m3). The total applicable uncertainty factor is 300, so a 

margin of exposure (MOE) < 300, with cells highlighted in red, would be of concern. 

The buffer duration analysis presented in Section 3.2 is presented in terms of risk estimations for 

all field sizes assessed for 1 lb a.i./A and 6 lb a.i./A in Table 13. These results are for 

chlorpyrifos (parent only), are specific to the alfalfa application scenario and are based on whole 

field buffers. This buffer duration analysis indicates that buffers need to be in place for at least 

12 hours for l lb a.i./A application and for at least 36 hours for 6 lb a.i./A application when 

considering the 95% percentile exposure concentration. This analysis is based on estimated air 

concentrations (based on an average concentration around the entire field similar to the whole 

field buffer concept) at various buffer distances away from a chlorpyrifos treated field. 
40 of 70 

Table 12. MOEs Comparison for Chlorpyrifos (parent only) Based on Various PERFUM Buffers a,b 

Bradenton, FL 

Scenario 50 ft (15 m)c 100 ft (30 m)c 200 ft (61 m)c 

Application 

Rate (lb a.i./A) Percentile Field Size (A) [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

0.5 

50 

10 0 15000 0 -- 0 -- 

20 0 7500 0 15000 0 -- 60 1 3000 1 5000 0 -- 

120 1 2143 1 3000 0 7500 

75 

10 2 1250 2 1667 1 3750 

20 3 938 2 1250 1 2143 60 4 652 3 789 2 1250 

120 5 556 4 652 3 938 

90 

10 4 652 3 789 3 1250 

20 5 517 4 612 3 938 60 6 390 6 455 4 612 

120 8 333 7 375 5 484 

95 

10 5 500 4 600 3 938 

20 6 405 5 484 4 682 60 8 313 7 353 5 469 

120 9 268 9 294 7 375 

99 

10 14 174 2 200 9 288 

20 12 211 11 234 8 300 60 11 221 10 242 8 313 

120 13 188 12 205 10 254 

1 

50 

10 0 - 0 - 0 -- 20 1 3750 0 7500 0 -- 60 2 1500 1 2500 0 -- 

120 2 1071 2 1500 1 3750 

75 

10 4 625 3 833 1 1875 20 5 469 4 625 2 1071 60 8 326 6 395 4 625 

120 9 278 8 326 5 469 

90 

10 8 326 6 395 4 625 20 10 259 8 306 5 469 60 13 195 11 227 8 306 

120 15 167 13 188 10 242 

95 10 10 250 8 300 5 469 
41 of 70 

Bradenton, FL 

Scenario 50 ft (15 m)c 100 ft (30 m)c 200 ft (61 m)c 

Application 

Rate (lb a.i./A) Percentile Field Size (A) [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

20 12 203 10 242 7 341 60 16 156 14 176 11 234 

120 19 134 17 147 13 188 

99 

10 29 87 25 100 17 144 20 24 106 21 117 17 150 60 23 110 21 121 16 156 

120 27 94 24 103 20 127 

2 

50 

10 1 3750 0 -- 0 -- 

20 1 1875 1 3750 0 -- 60 3 750 2 1250 0 -- 

120 5 536 3 750 1 1875 

75 

10 8 313 6 417 3 938 

20 11 234 8 313 5 536 60 15 163 13 197 8 313 

120 18 139 15 163 11 234 

90 

10 15 163 13 197 8 313 

20 19 129 16 153 11 234 60 26 97 22 114 16 153 

120 30 83 27 94 21 121 

95 

10 20 125 17 150 11 234 

20 25 101 21 121 15 170 60 32 78 28 88 21 117 

120 37 67 34 74 27 94 

99 

10 9 87 25 100 17 144 

20 35 72 31 82 23 110 60 45 55 41 60 32 78 

120 53 47 49 51 39 64 

4 

50 

10 1 1875 0 -- 0 -- 20 3 938 1 1875 0 -- 60 7 375 4 625 0 -- 

120 9 268 7 375 3 938 

75 

10 16 156 12 208 5 469 20 21 117 16 156 9 268 60 31 82 25 99 16 156 
42 of 70 

Bradenton, FL 

Scenario 50 ft (15 m)c 100 ft (30 m)c 200 ft (61 m)c 

Application 

Rate (lb a.i./A) Percentile Field Size (A) [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

120 36 69 31 82 21 117 

90 

10 31 82 25 99 16 156 20 39 65 33 77 21 117 60 51 49 44 57 33 77 

120 60 42 53 47 41 60 

95 

10 40 63 33 75 21 117 20 49 51 41 60 29 85 60 64 39 57 44 43 59 

120 75 33 68 37 53 47 

99 

10 57 44 50 50 35 72 20 69 36 61 41 45 55 60 91 28 83 30 64 39 

120 107 23 97 26 79 32 

6 

50 

10 2 1250 0 -- 0 -- 

20 4 625 2 1250 0 -- 60 10 250 6 417 0 -- 

120 14 179 10 250 4 625 

75 

10 24 104 18 139 8 313 

20 32 78 24 104 14 179 60 46 54 38 66 24 104 

120 54 46 46 54 32 78 

90 

10 46 54 38 66 24 104 

20 58 43 49 51 32 78 60 77 32 66 38 49 51 

120 90 28 80 31 62 40 

95 

10 60 42 50 50 32 78 

20 74 34 62 40 44 57 60 96 26 85 29 64 39 

120 112 22 102 25 80 31 

99 

10 86 29 78 33 52 48 

20 104 24 92 27 68 37 60 136 18 124 20 96 26 

120 218 11 204 12 170 15 

a. Registrant study conducted on alfalfa using Lorsban Advanced 
43 of 70 

Bradenton, FL 

Scenario 50 ft (15 m)c 100 ft (30 m)c 200 ft (61 m)c 

Application 

Rate (lb a.i./A) Percentile Field Size (A) [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

b. Lung: 6 h HEC 2,500 μg/m3, UF=300 (i.e., MOE =300), target concentration 8.3 μg/m3

c. 50 ft (15 m)c reported for Ring; 100 ft (30 m) reported for Ring 6; 200 ft (61 m) reported for Ring 8 [actual distance: 230 ft (70 m

10 

Table 13. Buffer Duration Comparison for Chlorpyrifos (parent only) Based on Various PERFUM Buffer

Bradenton, FL 

Scenario 50 ft (15 m)d 100 ft (30 m)d 200 ft (61 m)d 

Application 

Rate (lb a.i./A) 

Field 

Size 

(A) 

Hours Posttreatment [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

1 

10 

6 4 577 3 750 2 1250 

12 10 250 8 300 5 469 18 4 682 3 750 2 1071 24 2 1500 1 1875 1 2500 30 1 1875 1 2500 1 3750 36 3 938 2 1250 1 1875 42 0 7500 0 7500 0 7500 48 0 7500 0 7500 0 7500 54 0 7500 0 7500 0 -- 60 0 7500 0 7500 0 7500 66 0 -- 0 -- 0 -- 72 0 -- 0 -- 0 -- 

20 

6 5 500 4 625 3 938 

12 12 203 10 242 7 341 18 5 536 4 577 3 833 24 2 1250 2 1500 1 2500 30 2 1500 1 1875 1 3750 36 3 833 3 938 2 1250 42 1 3750 1 3750 0 7500 48 1 3750 0 7500 0 7500 54 0 7500 0 7500 0 -- 60 0 7500 0 7500 0 7500 66 0 -- 0 -- 0 -- 72 0 -- 0 -- 0 -- 
44 of 70 

Bradenton, FL 

Scenario 50 ft (15 m)d 100 ft (30 m)d 200 ft (61 m)d 

Application 

Rate (lb a.i./A) 

Field 

Size 

(A) 

Hours Posttreatment [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

60 

6 6 395 5 469 4 682 

12 16 156 14 176 11 234 18 6 395 6 417 5 536 24 3 938 2 1071 2 1500 30 2 1250 2 1500 1 2500 36 4 625 4 682 3 938 42 1 2500 1 2500 1 3750 48 1 3750 1 3750 0 7500 54 0 7500 0 7500 0 7500 60 1 3750 1 3750 0 7500 66 0 7500 0 7500 0 -- 72 0 -- 0 -- 0 -- 

120 

6 7 341 6 395 5 536 

12 19 134 17 147 13 188 

18 8 326 7 341 6 417 24 3 833 3 938 2 1250 30 2 1071 2 1250 1 1875 36 5 500 4 577 3 750 42 1 2500 1 2500 1 2500 48 1 2500 1 3750 1 3750 54 0 7500 0 7500 0 7500 60 1 3750 1 3750 1 3750 66 0 7500 0 7500 0 7500 72 0 -- 0 -- 0 -- 

6 10 

6 26 96 20 125 12 208 

12 60 42 50 50 32 78 18 22 114 20 125 14 179 24 10 250 8 313 6 417 30 8 313 6 417 4 625 36 16 156 12 208 8 313 42 2 1250 2 1250 2 1250 48 2 1250 2 1250 2 1250 54 2 1250 2 1250 0 -- 
45 of 70 

Bradenton, FL 

Scenario 50 ft (15 m)d 100 ft (30 m)d 200 ft (61 m)d 

Application 

Rate (lb a.i./A) 

Field 

Size 

(A) 

Hours Posttreatment [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

60 2 1250 2 1250 2 1250 66 0 -- 0 -- 0 -- 72 0 -- 0 -- 0 -- 

20 

6 30 83 24 104 16 156 

12 74 34 62 40 44 57 18 28 89 26 96 18 139 24 12 208 10 250 6 417 30 10 250 8 313 4 625 36 18 139 16 156 12 208 42 4 625 4 625 2 1250 48 4 625 2 1250 2 1250 54 2 1250 2 1250 0 -- 60 2 1250 2 1250 2 1250 66 0 -- 0 -- 0 -- 72 0 -- 0 -- 0 -- 

60 

6 38 66 32 78 22 114 

12 96 26 85 29 64 39 18 38 66 36 69 28 89 24 16 156 14 179 10 250 30 12 208 10 250 6 417 36 24 104 22 114 16 156 42 6 417 6 417 4 625 48 4 625 4 625 2 1250 54 2 1250 2 1250 2 1250 60 4 625 4 625 2 1250 66 2 1250 2 1250 0 -- 72 2 1250 2 1250 0 -- 

120 

6 44 57 38 66 28 89 

12 112 22 102 25 80 31 18 46 54 44 57 36 69 24 18 139 16 156 12 208 30 14 179 12 208 8 313 36 30 83 26 96 20 125 
46 of 70 

Bradenton, FL 

Scenario 50 ft (15 m)d 100 ft (30 m)d 200 ft (61 m)d 

Application 

Rate (lb a.i./A) 

Field 

Size 

(A) 

Hours Posttreatment [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE [Conc. (μg/m3)] MOE 

42 6 417 6 417 6 417 48 6 417 4 625 4 625 54 2 1250 2 1250 2 1250 60 4 625 4 625 4 625 66 2 1250 2 1250 2 1250 72 2 1250 2 1250 2 1250 

a. Registrant study conducted on alfalfa using Lorsban Advanced b.Lung: 6 h HEC 2,500 μg/m3, UF=300 (i.e., MOE =300), target concentration 8.3 μg/m3

c. For brevity, the results are presented for the 95% percentile of exposure. 

d. 50 ft (15 m)c reported for Ring; 100 ft (30 m) reported for Ring 6; 200 ft (61 m) reported for Ring 8 [actual distance: 230 ft (70 m]; 

10 
47 of 70 

4. ISSUES FOR CONSIDERATION 

4.1. Air Monitoring Data 

There are 15 available chlorpyrifos air monitoring studies which were summarized in the 

preliminary HHRA.1 These include two application site studies done in Tulare and Lompoc 

Counties, California by the California Air Resources Board (CARB), and 13 ambient air studies 

in which the particular source of chlorpyrifos within the impacted areas was not identified. These 

ambient air studies were conducted in the North Central and Yakima Valleys of Washington by 

the University of Washington Department of Environmental and Occupational Health Sciences. 

The remaining11 studies were conducted by Pesticide Action Network North America 

(PANNA), two in Cowiche and Tieton, Washington, and nine in Lindsay, California. 

The risk associated with air concentrations measured in these studies was assessed as part of the 

2011 preliminary HHRA which also provides more detail concerning the conduct of the studies.1 

Risks of some degree were identified in both the ambient and application site monitoring data. 

Risks associated with ambient monitoring do not directly relate to the type of analysis presented 

herein because the monitoring data may represent multiple chlorpyrifos applications but the data 

suggest chlorpyrifos is found in air samples which may be the result of volatilization off a treated 

field. The risks identified for application sites monitoring studies are directly related to this 

analysis and are consistent with the findings of this assessment. These data coupled with the 

alfalfa and potato flux studies further indicate that chlorpyrifos volatilize from treated crops after 

applications at levels which could lead to bystander exposure at levels of concern. 

4.2. Uncertainties 

4.2.1. Hazard Assessment

The special, acute inhalation study evaluated measures of the central (brain) and peripheral 

nervous system (e.g., pulmonary tissues) and the blood measures. This study also includes time 

course data to establish the time to peak effect and time to recovery for ChE inhibition in each 

tissue. As noted in EPA’s ChE policy, measures of peripheral nervous tissue are preferred over 

surrogate measures; specifically, use of lung ChE alleviates the uncertainties associated with use 

of surrogate tissue (i.e., RBC AChE) which are not innervated. Measurements of lung ChE 

provide a direct measure relevant to the route of interest and thus protecting against lung ChE 

should thereby protect against cholinergic effects in the respiratory system. Because there are no 

measures of pulmonary function in the acute inhalation study, the relationship between measures 

of lung ChE inhibition and respiratory effects is not known. Thus, there is uncertainty associated 

with relating the degree of lung ChE inhibition with adverse apical outcomes (e.g., pulmonary 

function). However, human incident reporting is consistent with the anticipated respiratory 

effects associated with cholinergic stimulation. The incident reported in the databases suggest 

difficulty breathing, nasal congestion, coughing, and chest tightening which are qualitatively 

consistent with the anticipated cholinergic effects reported by Taylor46 and by studies of OP war 

agents.48,49,50,51 While the available human incident data are insufficient to directly link 

respiratory effects to chlorpyrifos volatilization exposure, this information qualitatively supports 
48 of 70 

the lung ChE effects reported in the acute inhalation study in rats as being relevant for human 

health risk assessment. 

In addition to the lung ChE POD, the EPA is also estimating volatilization risk using the RBC 

ChE endpoint. Despite the uncertainties associated with surrogate measures, the EPA is taking 

this approach because 1) the agency has significant experience with the blood endpoints in risk 

assessment and thus provides a historical comparison point(s); 2) the RBC AChE inhibition data 

can be used as a surrogate for other peripheral tissues (e.g., digestive tract, heart) for which such 

inhalation data are not available; 3) PODs for other routes (i.e., dermal, oral) are being derived 

from RBC AChE data; and 4) the acute TAF is derived from RBC AChE data. 

There are areas of uncertainty associated with the hazard characterization/identification in the 

volatilization assessment such as the toxicity adjustment factor derived from oral data and the 

nature of the exposure (aerosol) in the acute inhalation study used to assess volatilization 

exposure (vapor). The tissue dosimetry appears to be different between oral and inhalation 

exposure. With oral exposure, the liver primarily metabolizes chlorpyrifos prior to circulation to 

the remainder of the body. In contrast, from inhalation exposure, chlorpyrifos enters the blood 

stream directly. Given the different patterns of tissue dosimetry between oral and inhalation, 

there is uncertainty in the extent to which the oral toxicity adjustment factor is predictive of 

inhalation exposure. Nonetheless, the comparative ChE oral study is the best available source of 

dose response data comparing the parent and its transformation product and thus provides a 

source of data to develop a toxicity adjustment factor. If additional inhalation data for 

chlorpyrifos-oxon become available in the future, the EPA will, if appropriate, reconsider the 

adjustment factor. 

The special acute inhalation study involves aerosolized chlorpyrifos with relatively small 

particles (MMAD/geometric standard deviations:1.93/1.58, 1.86/1.61, 1.79/1.59 and 1.9/1.51 

microns). There are two subchronic inhalation studies which involved exposure to vapor phase 

chlorpyrifos. Toxicity studies based on vapor exposure are believed to be more relevant for 

assessing risks of volatilization exposure. The vapor studies showed no biologically relevant 

findings up to 287 μg/m3 (albeit, lung ChE, the most sensitive endpoint from the aerosol study 

was not measured). In contrast, the aerosol study shows close to 50% lung ChE inhibition at the 

lowest concentration of 3.7 mg/m3 (370 μg/m3). Given the high quality design and conduct of the 

aerosol study, that the particle size was < 2 microns, and that the aerosol study involves acute 

exposure which is more relevant to volatilization, EPA has chosen to use the special acute 

inhalation study for deriving a POD for this volatilization assessment. 

In the preliminary HHRA, the EPA determined that for acute oral exposures, RBC AChE data from 

post-natal day 11 (PND11) represented the most sensitive life stage. With respect to this 

assessment, the special acute inhalation study involved testing of only adult female rats. As 

such, there is some uncertainty as to whether or not juvenile rats may be more sensitive than 

adults to inhalation exposure of chlorpyrifos. It is noteworthy that the degree of sensitivity noted 

in the oral CCA study for PND11 rats compared to adult rats is only 2-fold. There are two 

literature studies which evaluated lung ChE in fetuses exposed to chlorpyrifos during gestation39 or 

rat pups directly dosed postnally but neither provides useful information about the relative sensitivity 

between post-natal pups and adults with respect to lung ChE inhibition.41 Given the small (2-fold) 
49 of 70 

difference in sensitivity between PND11 and adult rats in the CCA study, the lack of inhalation data 

in rats pups does not contribute large uncertainty to this volatility assessment.

Despite the noted uncertainties related to aerosolized chlorpyrifos vs. vapor chlorpyrifos and the 

lack of inhalation data in post-natal pups, given the robust study design and conduct of the 

special acute inhalation study, EPA has confidence in the hazard assessment for chlorpyrifos 

presented in this assessment. 

 

4.2.2. Application Rate Scaling

The flux studies available for consideration in this assessment were conducted using application 

rates [0.877 lb a.i/A (alfalfa) and 0.61 lb a.i./A (potato)] and are much lower than the maximum 

single broadcast application rate of 6 lb a.i./A for citrus currently permitted on chlorpyrifos 

products. In addition to citrus, several other uses also have maximum single application rates 

higher than those used in the available flux studies including: mint (2.0 lb a.i./A), turf (4.0 lb 

a.i./A) and alfalfa (1 lb a.i./A). 

In past assessments (e.g., conducted for the soil fumigants), maximum application rates were 

used in the flux studies and, for mitigation purposes, EPA assumed that there was a linear 

correlation between pesticide application rate and the flux rate (i.e., as the application rate 

decreased, the flux rate should decrease directly). This assumption was justified, as one would 

not expect less material to leave the field on a per unit basis if the application rate were lowered. 

There is also regulatory precedent for this type of approach in human health risk assessment 

(e.g., scaling of dislodgeable foliar residue data to calculate Restricted Entry Intervals). There is 

some additional uncertainty, however, with scaling up based on the application rate; it is unclear 

as to how the diffusion of the material into the air would be impacted by adding more material to 

the foliar surface or by increasing the layer of active ingredient on the foliar surface. Thus, the 

magnitude of the resulting buffers could be higher than expected based on the current approach 

(i.e., a linear scale up based on the application rate). 

4.2.3. Crop to Crop Variation (Flux and Dispersion) 

The difference in flux rates and subsequent dispersion of vapor phase chlorpyrifos and 

chlorpyrifos-oxon residues from different crops is unknown. There are several crop related 

factors including canopy shape and height, leaf area index, leaf type, crop density, surface 

roughness length, and possibly others factors that may influence the flux rate of a given pesticide 

on a particular treated crop and the dispersion of the pesticide in and around the treated area. 

Therefore, the extrapolation of the flux rates measured in both the alfalfa and potato field 

volatility studies to other crops is an uncertainty. 

There is some uncertainty in using PERFUM for dispersion modeling of semi-volatile pesticides 

applied to elevated crops such as orchards and trellis crops. At this time, EPA has not evaluated 

if this uncertainty is due to the flux estimates not being representative of emissions from these 

types of crops, or is a result of insufficient model parameterization. The use of PERFUM to 

model dispersion associated with an application to crops was an issue discussed by the 2009 

SAP5

 as there are no validated methods available to explore this uncertainty and very limited 

monitoring data are available to do a comparison. As a follow-up to the 2009 SAP, and in 
50 of 70 

addition to searching the open scientific literature, EPA has engaged government, academic, and 

industrial scientists either directly or at various scientific conferences to gain a better 

understanding of the various crop related factors and how such factors may impact pesticide 

volatilization from vegetative surfaces. To date, EPA is not aware of research that addresses this 

area of uncertainty. 

Nevertheless, for the particular scenarios used in this assessment a validation study was 

conducted that closely resembles the scenario considered in this assessment for chlorpyrifos 

treated field crops with strongly supports the approach used in this analysis. This study is 

referred to as the Prairie Grass study22 that was used to support the ground level release 

algorithms in the ISCST model is the underlying PERFUM engine. This study was conducted on 

an uncultivated field covered with natural prairie grasses using a volatile pesticide with field 

conditions similar to the available flux studies conducted on alfalfa and potato. 

To provide some characterization of the PERFUM model for an elevated source, EPA looked at 

monitoring data (offsite concentrations) collected following a chlorpyrifos citrus application and 

compared the measured concentrations to the model estimated results, based on the flux derived 

from the alfalfa flux study. The measured and estimated concentrations are in good agreement as 

far as magnitude [30 μg/m3 (peak observed); 20-40 μg/m3 (peak estimated)]. An explanation of 

this analysis is provided below. 

In June of 1996, the CARB conducted an air monitoring study around a 60-acre orange orchard 

that had been treated with chlorpyrifos at a rate of 6 lb a.i./A. The maximum 6-hour measured air 

concentration for chlorpyrifos was approximately 28 μg/m3 measured 57 feet from the edge of 

the field. In order to characterize the magnitude of air concentration around an orchard (i.e., to 

see if the levels from the model were approximately the same as those observed in the CARB 

monitoring study), EPA used PERFUM and the flux rates from the alfalfa study. EPA ran 

PERFUM in the greenhouse mode to simulate an elevated source. The source had an area of 60 

acres and a release height of 4 m. The flux profile was the same one developed from the alfalfa 

study for chlorpyrifos (parent only), scaled for a 6 lb a.i./A application rate. Ventura, California 

meteorological data for June 1995-1999 were used. The maximum 6-hour concentration 60 feet 

from the edge of the field was 24 ug/m3. Although PERFUM has yet to be validated for semivolatile emissions from an orchard and chlorpyrifos flux data are not available for an orchard, the 

modeling simulations are in reasonable agreement with the 28 μg/m3 observed in the CARB 

study. 

EPA is aware of one literature article74 that describes a method for adjusting flux rates to develop 

an estimated flux profile for orchard crops taking into account the leaf surface area and orchard 

tree density. Using the method reported in the literature article, flux rates would be much higher 

and as a result, EPA expects that the buffer zones would need to be even larger than the 

estimated buffers reported in this assessment to ensure bystanders are not exposed to 

concentrations higher than the target concentration. It is important to note that the method 

 

74 Woodrow, J. E.; Seiber, J. N. Correlation Techniques for Estimating Pesticide Volatilization Flux and Downwind 

Concentrations, Environ. Sci. Technol. 1997, 31, 523-529. 
51 of 70 

described in this article has not been validated and it is uncertain if this is a suitable method for 

scaling flux profiles to account for volatilization of pesticides from orchard canopies. 

In summary, there is uncertainty related to the emissions of pesticides applied to orchard and 

trellis crops. This includes the methods that can be used for quantify emissions from treated 

orchard and trellis crops. Also, there are ongoing discussions in the scientific community on the 

driving factors for emission from treated crops (e.g., alfalfa) and how these factors may affect 

flux rates for other crops such as orchard and trellis crops. The disparate findings between the 

two analysis (CARB monitoring comparison and the leaf area index scale up method) attempted 

by EPA also underscore this uncertainty. Given these considerations, at this time EPA believes it 

is appropriate to acknowledge the uncertainties and note that these factors should be considered 

when developing a risk management strategy for orchard and trellis crops. 

4.2.4. Formulation

Two formulations of chlorpyrifos were used to generate the available emissions data (Lorsban 

Advance and Dursban EC). There are marked differences in the emission rates for each. This 

could be based on a variety of factors as discussed above such as field conditions, crop, weather, 

formulation, etc. It is unclear how the volatility of chlorpyrifos may differ between formulations 

(i.e., between one low VOC formulation and another or a low VOC formulation, and a non-low 

VOC formulation) as the low VOC designation is solely based on a reduction in the total amount 

of mass loss from a given formulation including components such as solvents, and is not specific 

to the active ingredient. Exploration of the potential formulation effect on volatility following 

application was done using CDPR’s online VOC Emissions Calculator.75 This tool was 

developed based on Thermogravimetric analysis (TGA) data submitted to the State of California. 

TGA allows for the comparison of one formulation to another by comparing the amount of total 

material that can be displaced as a result of heating. Examination of several different 

formulations suggests that non-low VOC formulations may result in volatilization that is twice as 

high as low-VOC formulations on a total mass basis (see Figure 5). These results are for total 

mass—that is the total amount of volatility organic chemicals—and is not necessarily reflective 

of one component of the formulation (i.e., the active ingredient). In general, the low VOC 

formulations developed for chlorpyrifos replace volatile solvents with water to reduce the 

volatility of the overall formulation. In the case of the Lorsban Advanced formulation, it is 

unclear if the change in formulation reduces the volatility of chlorpyrifos or simply reduces the 

amount of total VOC emitted. 

 

75 http://apps.cdpr.ca.gov/voc-calculator/start.cfm; accessed September 19, 2012 
52 of 70 

Figure 5. Volatility Comparison for Various Chlorpyrifos Formulated Products 

Chlorpyrifos emissions presented in Figure 5 were calculated based on the percentage included 

in the product formulation and may not reflect actual emissions concentrations. In addition, this 

analysis was based on an application rate of 1 gallon formulated product/acre applied to a 20 acre 

field. Since the studies used in the development of the VOC Emissions Calculator75 are not 

chemical specific (i.e., the studies did not specially measure the amount of chlorpyrifos emitted), 

the formulation effect on the volatilization rate of chlorpyrifos is unclear. 

A report76 recently submitted by Dow AgroSciences to EPA indicates that under laboratory 

conditions the volatility of chlorpyrifos does not vary between liquid formulations except when 

the active ingredient is encapsulated. This study was done by applying aqueous dilutions of 

various chlorpyrifos formulations to an inert sand surface. The samples were stored in an 

uncapped vial in a 54 °C (129 °F) oven. At selected time points, chlorpyrifos was extracted from 

the sand samples and quantified. Figure 6 shows the results from this study in terms of 

remaining chlorpyrifos—that is the amount of chlorpyrifos that was extracted from the sand. 

 

76 EPA MRID 49005301: Lab Volatility Study of selected Chlorpyrifos Products; Author: Dennis Wujek Study 

Sponsor: Dow AgroSciences LLC, 9330 Zionsville Road Indianapolis, IN 46268-1054 
53 of 70 

Figure 6. Laboratory Volatility Comparison for Various Chlorpyrifos Formulations 

Encapsulation (pyrinex), as expected, was shown to reduce the volatility of chlorpyrifos. 

However, this study did not specifically measure the amount of chlorpyrifos that volatilized, just 

the amount remaining in the sand, assuming volatilization was the only pathway of dissipation. It 

is also unclear how the conditions in this study can be extrapolated to environmental conditions 

[e.g., cooler temperatures, higher or lower humidity, surface properties (crop surfaces)] and used 

quantitatively for risk estimation. 

Formulations that include the encapsulation of chlorpyrifos either in a microcapsule or within a 

granular (as long as the microcapsule is not allowed to rest in an application solution where the 

active ingredient can diffuse into solution or granular is not dissolved prior to application) are 

likely to result in lower concentrations of chlorpyrifos and chlorpyrifos-oxon off field as 

chlorpyrifos would be expected to be released slowly over time. At this time, EPA does not have 

any data to conduct an assessment on volatilization of chlorpyrifos and chlorpyrifos-oxon 

following the application of microencapsulated or granular formulations. In order to address this 

uncertainty, additional studies would be necessary to evaluate flux rates following applications 

of microencapsulated and granular chlorpyrifos formulations. 

4.2.5. Application Method 

Different applications methods may result in different surface coverage of chlorpyrifos on crops, 

and may impact volatilization of chlorpyrifos and chlorpyrifos-oxon from treated crops. 

Additional data would be needed that explores the surface coverage of chlorpyrifos using 

different application methods and the subsequent volatilization. 

Applications made directly to soil surfaces will likely result in reduced chlorpyrifos 

volatilization, as chlorpyrifos is expected to sorb to soil surfaces. Furthermore, incorporation of 
54 of 70 

liquid formulations of chlorpyrifos should also reduce volatility; however, the magnitude of the 

reduction is unknown. To address this uncertainty, a flux study conducted on a bare-soil plot 

could be completed with and without soil incorporation. 

4.2.6. Weather 

In addition to temperature, environmental factors such as wind speed, air turbulence, humidity, 

and rainfall may influence the amount of chlorpyrifos and chlorpyrifos-oxon observed in air 

samples. While the temperature throughout the alfalfa flux study may have been high, higher 

humidity may increase the amount of chlorpyrifos and chlorpyrifos-oxon observed in air 

samples. It is uncertain how the results obtained from the alfalfa flux study conducted in the San 

Joaquin Valley of California, compare to the results that may be obtained for other sites in the 

United States. The same uncertainty can be said for extrapolation of the potato study conducted 

in the Netherlands to represent areas within the United States. While the weather was slightly 

different (lower average temperature and higher winds reported in the potato study as compared 

to the alfalfa study) in both of these studies, the studies were conducted during the typical 

growing reason and, therefore, reasonably represent the potential volatilization of chlorpyrifos 

and chlorpyrifos-oxon under those specific conditions. 

To address this uncertainty, when limited studies were available for fumigant risk assessments, 

PERFUM modeling was conducted using all six of the available meteorological stations and the 

largest buffers calculated for all the scenarios were reported for risk assessment purposes. In 

addition, for some soil fumigants, this uncertainty was addressed by submission of additional 

field volatility data. For chlorpyrifos, additional studies permit an empirical evaluation of flux 

rate variability under different environmental conditions in different regions of the country. 

4.2.7. Degradation in Air 

In an air photolysis study77, chlorpyrifos was observed to convert to chlorpyrifos-oxon. 

PERFUM modeling results presented in this assessment do not account for the potential for 

degradation to occur during dispersion as the modeling results are based on flux profiles derived 

from air concentrations measured on the field of applications. Based on a tier I analysis using 

AERMOD, the rate of direct and indirect photolysis for chlorpyrifos is not likely to impact the 

buffers estimated for chlorpyrifos (parent only). Conversion of chlorpyrifos to chlorpyrifos-oxon 

may result in the need for larger buffers due to the increased toxicity of chlorpyrifos-oxon; 

however, EPA’s current methods for dispersion modeling of pesticides do not permit the 

consideration of the rate of formation and decline of transformation products. Nevertheless, the 

total toxic residue approach utilized in this assessment is thought to reasonably capture the 

potential exposure to both chlorpyrifos and chlorpyrifos-oxon simultaneously. 

 

 

77 EPA MRID 48789701: Gas-phase Photolysis and Photo-oxidation of Chlorpyrifos and Chlorpyrifos-oxon; 

Authors: Amalia Munoz; Sponsor: Dow AgroSciences, European Development Centre, 3 Milton Park, Abingdon, 

Oxon, OX144RN 
55 of 70 

5. CONCLUSIONS 

In summary, two different field volatility studies suggest that volatilization of chlorpyrifos 

and/or chlorpyrifos-oxon from treated crops is a pathway of dissipation in the environment that 

may result in bystander exposure to vapor phase chlorpyrifos and chlorpyrifos-oxon. Based on 

these two studies as well as monitoring data bystander exposures were examined to quantify the 

potential risks associated with vapor phase chlorpyrifos and chlorpyrifos-oxon near a treated 

field. 

The approaches used to evaluate the volatilization of chlorpyrifos and chlorpyrifos-oxon and 

associated risks are consistent with assessments conducted in the past for fumigant pesticides and 

have been vetted through a public peer review process.3,5 While there are a number of 

uncertainties associated with this assessment, as discussed in the previous section, available data 

indicate that bystander exposure to vapor phase chlorpyrifos and chlorpyrifos-oxon is possible, 

and can occur at concentrations higher than the concentration of concern established in this 

assessment for lung. Generally application rates greater than 1 lb a.i./A and large field sizes 

results in concentrations that are higher than the target concentration at distances ranging from 0 

to greater than 4724 feet depending on the percentile of exposure. The uncertainties which have 

been identified should also be considered in the interpretation of this assessment and any 

subsequent regulatory actions based on its findings. 

Further refinements may require additional research that could be focused on eliminating or 

reducing the uncertainties associated with this analysis and its findings. The design of any such 

research should be discussed with the Agency. Given the current available information and the 

state of the science concerning volatilization of pesticides, this preliminary risk assessment 

indicates risks of concern are exceeded for bystanders. While the risk to bystanders in terms of 

the distance from treated fields over time, the information currently evaluated indicates that the 

risks are likely offsite regardless of how the stated uncertainties are interpreted. 

 
56 of 70 

APPENDIX A 

LIST OF ABBREVIATIONS AND ACRONYMS 

AChE acetylcholinesterase 

AD Aerodynamic 

AERMIC American Meteorological Society/Environmental Protection Agency Regulatory Model 

Improvement Committee 

BMD benchmark dose 

BMDL10 benchmark response level of 10% 

BMR benchmark response 

BuChE butyryl-ChE 

CA PISP California Pesticide Illness Surveillance Program 

CARB California Air Resources Board 

CAS Chemical Abstract Service 

CDPR California Department of Pesticide Regulation 

ChE cholinesterase 

DAS Dow AgroSciences 

DER data evaluation record 

EC emulsifiable concentration 

EPA Environmental Protection Agency 

FEMS Fumigant Emissions Modeling System 

FF field fortifications 

HECs/HEC Human Equivalent Concentrations/ Human Equivalent Concentration 

HHRA Chlorpyrifos Preliminary Human Health Risk Assessment for Registration Review 

IDS OPP Incident Data System 

IFM Indirect Flux Method 

IHF Integrated Horizontal Flux 

ISC Industrial Source Complex 

ISCST3 Industrial Source Complex-short term 

lb a.i./A Pound active ingredient per acre 

LOAEL lowest-observed-adverse-effect-level 

LOC level of concern 

MMAD Mass Median Aerodynamic Diameter 

MOE margin or exposure 

NIOSH National Institute for Occupational Safety and Health 

NOAEL no-observed-adverse-effect-level 

NPIC National Pesticide Information Center 

OP organophosphate 

OPP Office of Pesticide Programs 

PANNA Pesticide Action Network North America 

PCT chlorpyrifos percent crop treated 

PERFUM Probabilistic Exposure and Risk model for Fumigants 

PODs/POD point of departures/ point of departure 

RBC red blood cell 

SAP Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel 

SENSOR-Pesticides NIOSH’s Sentinel Event Notification System for Occupational Risk 

SOFEA Soil Fumigant Exposure Assessment System 

TAF toxicity adjustment factor 

TCP 3,5-6 tricloropyridinol 

TTCR total toxic chlorpyrifos residues 

UF uncertainty factor 
57 of 70 

UFA uncertainty factor for animal to human extrapolation 

UFDB uncertainty factor database 

UFH uncertainty factor for with human variability to account for sensitivity subpopulations 

VOC volatile organic compounds or volatile organic chemicals 

 
58 of 70 

APPENDIX B 

CHLORPYRIFOS USE BY FORMULATION 

The discussion below, along with the data presented, is excerpted from a supporting 

memorandum developed by OPP’s Biological and Economic Analysis Division (BEAD). This 

paper78 will also be included in the chlorpyrifos docket. 

Chlorpyrifos Application Rate Information

Table B.1. presents chlorpyrifos application rates with information for low VOC formulations 

broken out separately from other formulations. The low VOC products identified in the usage 

data presented are Lorsban AdvancedTM and Chlorpyrifos 4E AGTM. For about one-half of the 

non-low VOC formulations the number of observations was large enough that the rate 

information should be quite reliable. However, for most of the low VOC formulations the 

number of observations was much lower. The average and 90th percentile single application 

rates for the non-low VOC and low VOC formulations are generally quite close. Although not 

noted in the tables, for some crops and formulations the 90th percentile was equal to the 100th 

percentile (maximum). 

Table B.1. Chlorpyrifos Single Application Rate Data for Low VOC and Non-low VOC 

Formulationsa

 

Crop Formulation

Application Rate (lb a.i./A)

Average 

Single 

Application 

Rate 

(Non-low 

VOC 

Productsb) 

90th 

Percentile 

Single 

Application 

(Non-low 

VOC 

Products)

Average 

Single 

Application 

(Low VOC 

Productsc)

90th 

percentile 

Single 

Application 

(Low VOC 

Products)

Ratiod of Low 

VOC to Nonlow VOC 

Products 

Applications

Oranges Liquid 2.6 6 2.3 5.7 0.28 

Lemons Liquid 3.4 5 3.4 5.7 0.28 

Grapes, Table Liquid 2.9 4 1.6 2.2 0.14 

Grapefruit Liquid 2.1 3 1.6 2.4 0.37 

Broccoli Liquid 1.5 2.3 1.9 2.2 0.28 

Broccoli Granular 1.5 2.1 . . . 

Cabbage Granular 1.2 2.1 . . . 

Almonds Liquid 1.9 2 1.8 2 0.22 

Apples Liquid 1.6 2 1.4 2 0.12 

Cherries Liquid 1.8 2 1.7 2 0.11 

Grapes, Raisin Liquid 2 2 1.6 2.2 0.18 

Grapes, Wine Liquid 2 2 1.8 1.9 0.38 

 

78 Mallampalli, N. A. Grube, and J. Becker. “Information on Application Rates and Acres Treated per Application 

per Day for Selected Chlorpyrifos Formulations”. USEPA/OCSPP/OPP/BEAD memorandum. 
59 of 70 

Peaches Liquid 1.3 2 2.1 3 0.04 

Peanuts Granular 1.8 2 . . 

Pears Liquid 1.8 2 2 2 0.09 

Plums/Prunes Liquid 1.9 2 1.4 2 0.1 

Sugar Beets Granular 1.3 2 . . 

Tobacco Liquid 1.9 2 2 2 0.01 

Walnuts Liquid 1.9 2 1.8 2 0.15 

Apples Dry Flowable 1 1.6 . . . 

Apples Wettable 1 1.5 . . . 

Hazelnuts Liquid 1.1 1.5 1.9 1.9 0.07 

Cauliflower Granular 1.2 1.4 . . . 

Corn Granular 1.1 1.4 . . . 

Sweet Corn Granular 1.2 1.4 . . . 

Cherries Dry Flowable 0.9 1.2 . . . 

Onions Granular 0.9 1.1 . . . 

Pumpkins Granular 0.7 1.1 . . . 

Alfalfa Liquid 0.5 1 0.6 0.9 0.03 

Asparagus Liquid 0.9 1 0.9 1 0.26 

Beans, Green Liquid 0.9 1 0.9 1 1.21 

Broccoli Dry Flowable 0.9 1 . . 

Broccoli Wettable 1 1 . . 

Cabbage Liquid 1 1 1.6 2.2 0.27 

Cauliflower Dry Flowable 1 1 . . . 

Cauliflower Liquid 1 1 1.1 1.9 0.17 

Corn Liquid 0.6 1 0.4 0.5 0.09 

Cotton Liquid 0.7 1 0.9 1 0.13 

Onions Liquid 0.9 1 0.9 1 0.13 

Oranges Granular 0.8 1 . . . 

Pecans Liquid 0.9 1 0.9 1 0.08 

Pumpkins Liquid 0.9 1 . . 

Squash Liquid 0.8 1 1 1 0.05 

Strawberries Liquid 1 1 0.9 1 0.1 

Sugar Beets Liquid 0.5 1 0.5 0.6 0.09 

Sweet Corn Liquid 0.9 1 0.9 1 0.22 

Soybeans Dry Flowable 0.4 0.7 . . . 

Sorghum Liquid 0.4 0.6 . . . 

Dry Beans/Peas Liquid 0.5 0.5 0.8 1 0.17 

Soybeans Liquid 0.4 0.5 0.3 0.5 0.07 

Sunflowers Liquid 0.4 0.5 0.5 0.6 0.05 

Wheat, Spring Liquid 0.3 0.5 0.3 0.5 0.23 

Wheat, Winter Liquid 0.4 0.5 0.3 0.5 0.03 
60 of 70 

a. The table is arranged to show data for the 90th percentile application rate for non-low VOC products in descending order.

b. Examples include: Lorsban 75WG (EPA Reg. No. 62719-301), Lorsban 4E (EPA Reg. No. 62719-220) 

c. Examples include: Lorsban AdvancedTM (EPA Reg. No. 62719-591), Chlorpyrifos 4E AGTM (EPA Reg. No. 19713-520); 

The “Ratio” column provides a comparison of the number of applications of “low VOC” products compared to the number 

of applications of “traditional” products. 

Crops and formulations with very low numbers of observations (less than a total of 10 observations over the five year period) 

are not included. 

“.” Entry indicates no data for use of any low VOC product. Note that low VOC products are all liquid formulations. 

The ratio of the number of low VOC product applications to the number of traditional 

chlorpyrifos product applications in each crop were calculated when data were available. The 

number of applications of each type of product was summed across the five year period before 

the ratios were computed. A ratio of one means equal numbers of grower observations for both 

formulation types; a ratio of less than one indicates higher use of non-low VOC formulations, 

while a ratio greater than one indicates higher use of low VOC products. 

Scope and Data Limitations

For many crops and formulations, there were a limited number or reported users over the survey 

period, which leads to uncertainty with how representative the usage data are for the respective 

crop/formulation combinations. For chlorpyrifos used on only a small percentage of a crop, the 

sampling procedure may lead to no use being reported, simply because none of the users 

happened to be among those surveyed. 

Crop/formulation combinations with very low numbers of observations (less than a total of 10 

observations over the five year period) were excluded. Crops where data on some (but not all) 

formulations were excluded due to low sample size include: alfalfa, almonds, apples, apricots, 

asparagus, green beans, cabbage cantaloupes, cauliflower, celery, cherries, corn, cotton, 

grapefruit, lemons, lettuce, onions, oranges, peaches, peanuts, green peas, pecans, peppers, 

pistachios, potatoes, sorghum, soybeans, squash, strawberries, sunflowers, sweet corn, tobacco, 

tomatoes, and walnuts. 

There are also some labeled crops for which pesticide use data are not available in the pesticide 

usage database, and these crops are, therefore, not included in this analysis. These crops include 

Brussels sprouts, collards, cranberries, figs, kale, kohlrabi, kumquats, limes, nectarines, radishes, 

rutabagas, sweet potatoes, tangelos, tangerines, and turnips. 

Based on private pesticide marketing survey data, over the five year period from 2007 to 2011 

twenty nine different chlorpyrifos formulated products were used on agricultural sites. For 

individual crop/year combinations, anywhere from one to eight different products were reported 

as having been used. Liquid formulations accounted for about 90% of total national use. One 

Lorsban 4ETM, accounted for more than one-half (approximately 4,000,000 pounds) of all 

chlorpyrifos use on agricultural crops. Lorsban products also accounted for most granular use of 

chlorpyrifos. Application rates for chlorpyrifos varied significantly among crops. Rates for the 

citrus crops and grapes were higher than for any other crops. Field crops tended to have the 

lowest average application and 90th percentile application rates. 
61 of 70 

According to the available usage data, two “low VOC” products show substantial use in the 

agricultural crops surveyed. These products are Lorsban AdvancedTM (approximately 400,000 

pounds of chlorpyrifos applied across all crops) and Chlorpyrifos 4E AGTM (approximately 

200,000 pounds of chlorpyrifos applied across all crops). Together, these products accounted for 

about 10% of the liquid formulation pounds of chlorpyrifos applied and about 9% of the total 

pounds of chlorpyrifos applied nationally, as described in the proprietary database. 

For most crops, the low VOC formulations account for a relatively small percentage of the liquid 

chlorpyrifos applications. This would be expected since the low VOC formulations account for 

about ten percent of all liquid chlorpyrifos formulations when measured in pounds of 

chlorpyrifos applied. The exception is green beans, where more than half the applications were 

with the low VOC formulations. About 30% of the applications to wine grapes, broccoli, lemons 

and grapefruit were made with the low VOC formulations. 

Chlorpyrifos Area Treated per Application per Day Information

Data and Scope 

Since 1990 California has required the full reporting of annual agricultural pesticide use. Under 

this program, all agricultural pesticide use is reported monthly to county agricultural 

commissioners, who in turn, report the data to the CDPR. These data are accessible from 

CDPR’s website and provide the most detailed information available on the use of agricultural 

pesticides in California. No other state has comparable information on pesticide use. 

Data files for 2006 through 2010 were downloaded from CDPR’s website (CDPR, 2012). These 

data were then extracted and imported into a SAS dataset. Supplemental data on crop sites, 

county, chemical, product, and formulation were added to this dataset. An analytical chlorpyrifos 

dataset was created from this comprehensive dataset by keeping only data records that met all of 

the following criteria: active ingredient is chlorpyrifos, record is not a CDPR identified error, 

area treated is in units of acres, record is an individual application (i.e., not a monthly summary), 

application was made to an agricultural site. Using these criteria, a total of 60,910 observations 

are summarized in Table B.2 below. 

There are some limitations that should be considered when interpreting these data. Caution 

should be used when extrapolating these results to areas outside of California, as different pest 

pressures, crop production practices, etc. in these areas will likely alter the relative use of 

chlorpyrifos. BEAD also notes here that the data are shown as reported by individual county 

staff. Some counties sometimes enter crop type specifically, while others may use generic crop 

codes (e.g., the one for “grapes” instead of “grapes, wine”). Furthermore, CDPR does not have 

crop codes for some types of crops (e.g., there is no specific code for “table grapes”). These 

limitations are reflected in the table below. 

Table B.2. Acres Treated with Chlorpyrifos per Application per Daya, b 

Cropc Application Product Sample Acres treated per application per day
62 of 70 

Methodd Appliede Size Minimum 50th 

Percentile

90th 

Percentile Maximum

Corn (human 

consumption) Air Liquid 7 13 30 160 160 

Avocado Ground Liquid 10 1 6 158 300 

Almonds Air Liquid 267 4 51 156 640 

Grapes (wine) Ground Liquid 1117 0 32 146 595 

Sorghum 

(Forage/Fodder) Air Liquid 13 36 70 140 160 

Sorghum (Milo) Air Liquid 11 10 59 130 160 

Sugarbeets Air Liquid 19 18 70 128 146 

Almond Ground Liquid 1394 1 36 120 626 

Grapes (wine) Air Liquid 3 14 27 116 116 

Cotton Air Liquid 300 4 47 114 316 

Asparagus Air Liquid 171 1 18 110 235 

Wheat (unspecified 

type) Air Liquid 97 5 45 102 216 

Lemons Air Liquid 15 5 28 100 100 

Wheat (forage/fodder) Air Liquid 426 4 62 100 185 

Alfalfa Air Liquid 11735 1 50 96 518 

Alfalfa Other Liquid 33 2 40 95 101 

Tomatoes (processing) Air Liquid 3 58 76 93 93 

Walnuts Air Liquid 745 1 34 93 430 

Sugarbeets Ground Liquid 2 59 76 92 92 

Corn (forage/fodder) Air Liquid 1539 1 39 91 399 

Corn (forage/fodder) Ground Liquid 582 1 38 85 308 

Sorghum 

(Forage/Fodder) Ground Liquid 6 10 55 85 85 

Strawberries Ground Liquid 558 1 28 84 220 

Prunes (dried plums) Ground Liquid 40 2 34 82 120 

Alfalfa Ground Liquid 1846 0 37 80 217 

Corn (forage/fodder) Other Liquid 28 20 47 80 100 

Figs Air Liquid 1 80 80 80 80 

Sunflowers Air Liquid 35 2 25 78 170 

Corn (grain) Air Liquid 2 38 57 77 77 

Sunflowers Other Liquid 4 14 58 75 75 

Wheat (forage/fodder) Ground Liquid 11 38 70 75 113 

Cotton Ground Liquid 82 3 38 74 160 

Grapes (unspecified 

type) Ground Liquid 237 1 25 73 322 

Asparagus Ground Liquid 141 1 30 69 586 

Walnuts Ground Liquid 3271 1 20 65 517 

Citrus (unspecified 

type) Ground Liquid 32 2 7 60 240 

Mint Ground Liquid 25 2 25 60 160 

Beans (dried) Air Liquid 2 25 40 55 55 

Lemons Ground Liquid 1402 0 14 54 145 
63 of 70 

Cropc Application 

Methodd

Product 

Appliede

Sample 

Size

Acres treated per application per day

Minimum 50th 

Percentile

90th 

Percentile Maximum

Almond Other Liquid 9 6 20 53 53 

Tomatoes Air Liquid 2 38 46 53 53 

Orchard floor 

(unspecified crop) Ground Liquid 1 46 46 46 46 

Figs Ground Liquid 1 40 40 40 40 

Oranges Ground Liquid 3152 0 15 40 240 

Oranges Other Liquid 28 1 19 40 42 

Pecans Ground Liquid 33 2 19 40 42 

Grapes (wine) Other Liquid 14 4 14 39 41 

Plums Ground Liquid 325 1 10 38 140 

Sweet potatoes Ground Liquid 10 8 19 38 38 

Beans (dried) Ground Liquid 27 5 19 37 43 

Brussels sprouts Ground Liquid 363 1 12 35 52 

Lemons Other Liquid 2 15 25 35 35 

Turf/Sod Other Liquid 6 7 27 35 35 

Oranges Air Liquid 3 9 10 33 33 

Onions (dry) Ground Liquid 30 1 15 32 45 

Peaches Air Liquid 3 8 10 30 30 

Pomelo Ground Liquid 14 2 5 30 30 

Walnuts Other Liquid 5 10 18 30 30 

Turf/Sod Ground Liquid 23 5 17 27 60 

Grapes (unspecified 

type) Other Liquid 2 10 18 25 25 

Sudangrass Air Liquid 1 25 25 25 25 

Tangelo Ground Liquid 87 1 10 25 80 

Strawberries Air Liquid 1 24 24 24 24 

Corn (human 

consumption) Ground Liquid 35 4 10 23 39 

Tangerines Ground Liquid 166 1 10 23 73 

Apples Ground Liquid 164 1 5 21 72 

Peaches Ground Liquid 235 2 8 21 46 

Beans (unspecified 

type) Ground Liquid 1 20 20 20 20 

Broccoli Ground Liquid 3899 0 11 20 162 

Nectarines Ground Liquid 181 1 9 20 47 

Pears Ground Liquid 11 1 2 20 35 

Turnips Ground Liquid 13 8 10 20 25 

Broccoli Other Liquid 22 5 10 19 36 

Sunflowers Ground Liquid 3 8 10 18 18 

Cauliflower Ground Liquid 1125 1 10 16 36 

Peas Ground Liquid 33 3 10 16 19 

Beans (succulent) Ground Liquid 12 4 10 15 18 

Cherries Ground Liquid 34 1 5 15 20 

Tangerines Other Liquid 1 15 15 15 15 
64 of 70 

Cropc Application 

Methodd

Product 

Appliede

Sample 

Size

Acres treated per application per day

Minimum 50th 

Percentile

90th 

Percentile Maximum

Broccoli Air Liquid 6 5 7 14 14 

Cabbage Ground Liquid 381 1 6 14 27 

Cauliflower Air Liquid 1 13 13 13 13 

Collards Ground Liquid 10 5 9 12 13 

Grapefruit Other Liquid 1 12 12 12 12 

Kale Ground Liquid 70 0 2 12 15 

Pecans Air Liquid 2 11 11 11 11 

Beans (succulent) Air Liquid 1 10 10 10 10 

Grapefruit Ground Liquid 109 0 5 10 46 

Lettuce (leaf) Ground Liquid 1 10 10 10 10 

Limes Ground Liquid 3 1 2 6 6 

Rappini Ground Liquid 103 1 5 6 9 

Bok Choy (cabbage) Air Liquid 1 3 3 3 3 

Bok Choy Ground Liquid 249 0 1 3 5 

Cabbage, Chinese 

(Nappa) Ground Liquid 187 1 2 3 5 

Kumquats Ground Liquid 1 2 2 2 2 

Radishes Ground Liquid 191 1 1 2 3 

Grapes (wine) Ground Low VOC 367 3 42 212 560 

Sorghum (Milo) Air Low VOC 9 38 76 115 115 

Prunes (dried plums) Ground Low VOC 8 7 25 114 114 

Corn (forage/fodder) Ground Low VOC 33 2 50 110 230 

Citrus (unspecified 

type) Ground Low VOC 14 1 10 102 150 

Alfalfa Ground Low VOC 73 3 40 100 150 

Lemons Air Low VOC 2 5 53 100 100 

Alfalfa Air Low VOC 473 1 52 97 319 

Almond Ground Low VOC 231 3 20 90 470 

Corn (forage/fodder) Air Low VOC 210 3 38 84 262 

Grapes (unspecified 

type) Ground Low VOC 68 3 40 80 301 

Cotton Air Low VOC 6 10 30 76 76 

Walnuts Air Low VOC 90 5 21 76 163 

Walnuts Ground Low VOC 430 2 20 76 200 

Wheat (forage/fodder) Air Low VOC 3 25 65 66 66 

Strawberries Ground Low VOC 70 0 28 60 324 

Almond Air Low VOC 2 8 31 54 54 

Asparagus Air Low VOC 47 1 21 53 125 

Asparagus Ground Low VOC 62 4 24 53 471 

Sunflowers Air Low VOC 3 10 10 50 50 

Turf/Sod Ground Low VOC 7 10 30 50 50 

Lemons Ground Low VOC 229 1 10 45 126 

Oranges Air Low VOC 2 10 28 45 45 
65 of 70 

Cropc Application 

Methodd

Product 

Appliede

Sample 

Size

Acres treated per application per day

Minimum 50th 

Percentile

90th 

Percentile Maximum

Apples Ground Low VOC 21 1 18 43 80 

Brussels sprouts Ground Low VOC 42 2 8 40 52 

Corn (human 

consumption) Ground Low VOC 5 2 8 40 40 

Oranges Ground Low VOC 758 1 17 40 248 

Tangerines Ground Low VOC 61 2 12 40 157 

Peas Ground Low VOC 7 7 12 38 38 

Cotton Ground Low VOC 1 36 36 36 36 

Sweet potatoes Ground Low VOC 4 10 20 34 34 

Almond Other Low VOC 1 32 32 32 32 

Beans (succulent) Ground Low VOC 56 3 20 31 50 

Tangelo Ground Low VOC 26 5 10 31 42 

Peaches Ground Low VOC 16 2 10 24 40 

Beans (dried) Ground Low VOC 16 10 14 21 25 

Broccoli Other Low VOC 35 3 15 20 25 

Grapes (unspecified 

type) Other Low VOC 1 20 20 20 20 

Grapefruit Ground Low VOC 27 2 6 20 26 

Onions (dry) Ground Low VOC 1 20 20 20 20 

Pecans Ground Low VOC 8 12 19 20 20 

Strawberries Air Low VOC 3 5 7 18 18 

Broccoli Ground Low VOC 1061 0 10 17 29 

Cauliflower Ground Low VOC 145 1 10 17 25 

Plums Ground Low VOC 13 2 7 17 30 

Pomelo Ground Low VOC 10 1 6 16 20 

Broccoli Air Low VOC 3 12 12 13 13 

Cauliflower Other Low VOC 2 5 8 11 11 

Nectarines Ground Low VOC 9 2 5 11 11 

Oranges Other Low VOC 2 10 10 10 10 

Radishes Ground Low VOC 65 3 5 7 9 

Cabbage Ground Low VOC 94 1 4 5 13 

Sunflowers Ground Low VOC 1 5 5 5 5 

Bok Choy Ground Low VOC 38 1 2 3 3 

Cabbage, Chinese 

(Nappa) Ground Low VOC 12 1 1 2 2 

Kale Ground Low VOC 6 1 2 2 2 

Kale Other Low VOC 1 2 2 2 2 

Citrus (unspecified 

type) Ground Granular 35 15 80 240 240 

Corn (forage/fodder) Ground Granular 121 6 40 89 192 

Walnuts Ground Granular 10 5 24 82 99 

Lemons Ground Granular 16 1 5 80 85 

Strawberries Ground Granular 7 4 14 71 71 

Tangerines Ground Granular 24 1 14 69 88 
66 of 70 

Cropc Application 

Methodd

Product 

Appliede

Sample 

Size

Acres treated per application per day

Minimum 50th 

Percentile

90th 

Percentile Maximum

Sweet potatoes Ground Granular 93 4 19 60 104 

Oranges Ground Granular 198 0 19 58 240 

Tangerines Other Granular 1 48 48 48 48 

Broccoli Other Granular 15 1 10 40 40 

Cauliflower Other Granular 18 8 15 40 40 

Corn (human 

consumption) Ground Granular 119 2 16 40 48 

Onions (dry) Ground Granular 39 3 24 40 180 

Cauliflower Air Granular 6 4 9 35 35 

Broccoli Air Granular 119 1 11 28 45 

Broccoli Ground Granular 9847 0 10 19 161 

Cabbage Ground Granular 1012 1 8 19 53 

Cauliflower Ground Granular 3542 0 10 16 276 

Mustard Ground Granular 13 6 14 15 16 

Brussels sprouts Ground Granular 412 1 8 14 44 

Collards Ground Granular 27 3 9 14 16 

Rappini Ground Granular 56 1 5 12 20 

Grapefruit Ground Granular 38 0 3 10 23 

Oranges Other Granular 1 10 10 10 10 

Turf/Sod Ground Granular 1 10 10 10 10 

Peas Ground Granular 3 6 7 9 9 

Cabbage Other Granular 2 8 8 8 8 

Lettuce (leaf) Ground Granular 3 3 4 8 8 

Cabbage Air Granular 3 5 6 7 7 

Canola (rapeseed) Ground Granular 4 4 6 7 7 

Cabbage, Chinese 

(Nappa) Ground Granular 1471 0 4 6 15 

Kale Ground Granular 137 0 1 4 10 

Bok Choy Ground Granular 751 0 2 3 8 

Alfalfa Ground Granular 1 2 2 2 2 

Radishes Ground Granular 81 0 0 1 24 

Turnips Ground Granular 85 0 0 1 2 

a. Data are for use in the years 2006 through 2010. The table shows data grouped first by “product applied” and within 

these groups, sorted by the 90th percentile application rate (in descending order--high to low].

b. “Product” is referred to as “form” in the CDPR database. 

c. Crop names are shown as reported in the CDPR database (see the “Data and Scope’ section above for further discussion). 

d. “Other” application methods may include one or more of the following, as described by CDPR: chemigation, paints, dips, 

or other non-tractor ground-based methods. 

e. “Liquid” products correspond to the “regular” – non-low VOC – products 

 
67 of 70 

APPENDIX C 

FLUX CALCULATIONS 

1. Flux determination using the Indirect Method (IM): 

The indirect method, commonly referred to as the “back calculation” method, was the technique 

employed for estimating flux rates from fields during application. In the indirect method, air 

samples are collected at various locations outside the boundaries of a treated field. Meteorological 

conditions, including air temperature, wind speed, and wind direction, are also collected for the 

duration of the sampling event. The dimensions and orientation of the treated field, the location of 

the samplers, and the meteorological information is used in combination with the ISCST3 

dispersion model (Version 02035) and a unit flux rate of 0.001 μg/m2•s to estimate 

concentrations at the sampler locations. Since there is a linear relationship between flux and the 

concentration at a given location, the results from the ISC model runs are compared to those 

concentrations actually measured and a regression is performed, using the modeled values along 

the x-axis and the measured values along the y-axis. If the linear regression does not result in a 

statistically significant relationship, the regression may be rerun forcing the intercept through the 

origin, or the ratio of averages between the monitored to modeled concentrations may be 

computed, removing the spatial relationship of the concentrations. The indirect method flux back 

calculation procedure is described in detail in Johnson et al., 1999. 

2. Flux determination using the integrated horizontal flux (IHF) method:

The integrated horizontal flux method, also referred to as the “mass balance” method, was the 

technique employed for estimating flux rates from fields following application. In the integrated 

horizontal flux method, a mast is erected in the middle of the treated field and concentration 

samples are typically collected at four or five different heights, ranging from approximately 0.5 to 

5 feet. Likewise, wind speed data are collected at a variety of heights. A log-linear regression is 

performed relating the natural logarithm of the sample height to the air concentration and wind 

speed following the log law relationships for the atmospheric boundary layer. These relationships 

are then incorporated into an equation to estimate flux. The methods to estimate flux and related 

equations are presented in Majewski et al., 1990. The equation for estimating flux using the 

integrated horizontal flux method is the following expression: 

  

Z p

Z

cudz

x

P

0

1

where P is the volatile flux in units of g/m2·s, c is the average pesticide residue concentration in 

units of g/m3

 at height Z in units of meters, u is the wind speed in units of m/s at height Z, x is 

the fetch of the air trajectory blowing across the field in units of meters, Z0 is the aerodynamic 

surface roughness length in units of meters, Zp is the height of the plume top in units of meters, 

and dz is the depth of an incremental layer in units of meters. Following trapezoidal integration, 

the above equation is simplified as follows (Yates, 1996): 

    

ZP

Z

A Ln z B C Ln z D dz

x

P

0

( * ( ) ) * ( * ( ) ) 1
68 of 70 

where A is the slope of the wind speed regression line by ln(z), B is the intercept of the wind 

speed regression line by ln(z), C is the slope of the concentration regression by ln(z), D is the 

intercept of the concentration regression by ln(z), z is the height above ground level. Zp can be 

determined from the following equation: 

  





 



   C

D Z P

0.1

exp

The minimum fetch requirement of 20 meters for this method to be valid was satisfied at all 

times. The raw wind speed and wind direction data at varying heights were averaged for the 

concentration sampling durations. The wind direction values were used to calculate a fetch at 

every minute, which then was averaged for the concentration sampling periods. Finally, the 

period averaged wind speed values were used along with the natural logarithm of the height 

measurements to calculate regression coefficients (slope, intercept, and r2

) for each period. 

Period averaged meteorological parameters for each field and the meteorological regression 

coefficients are shown in Appendix B. 

3. Flux determination using the Aerodynamic Method (AD): 

The aerodynamic method, also referred to as the “flux-gradient” method, was the technique 

employed for estimating flux rates from fields following application. In the aerodynamic method, 

a mast is erected in the middle of the treated field and concentration samples are typically 

collected at four or five different heights, ranging from 0.5 to 10 feet. Likewise, temperature and 

wind speed data are collected at a variety of heights. A log-linear regression is performed relating 

the natural logarithm of the sample height to the concentration, temperature, and wind speed. 

These relationships are then incorporated into an equation to estimate flux. These methods to 

estimate flux and related equations are presented in Majewski et al., 1990. The equation for 

estimating flux using the aerodynamic method is Thornthwaite-Holzman Equation which is 

shown in the following expression: 

 

2

1

2

2

[ln ]

( )( )

















  

z

z

k c u P

 m p

where P is the flux in units of g/m2·s, k is the von Karman’s constant (dimensionless ~0.4), Δc

is the vertical gradient pesticide residue concentration in air in units of g/m3

 between heights ztop

and zbottom in units of meters, Δū is the vertical gradient wind speed in units of m/s between 

heights ztop and zbottom, and  m and  p are the momentum and vapor stability correction terms 

respectively. Following the conditions expected in the neutrally stable internal boundary layer 

characterized by an absence of convective (buoyant) mixing but mechanical mixing due to wind 

shear and frictional drag, a log-linear regression is performed relating the natural logarithm of the 

sample height to the concentration, temperature, and wind speed. The adjusted values of the 

concentration, temperature, and wind speed from this regression is incorporated into the equation 

above to arrive at the following equation, which is ultimate used to compute the flux. 

2

2

ln

(0.42) ( )( )

















   

bottom

top

m p

ztop zbottom ztop zbottom

z

z

c c u u

Flux

 
69 of 70 

where  m and  p are internal boundary layer (IBL) stability correction terms determined 

according to the following conditions based on the calculation of the Richardson number, Ri: 

Equation x3

 2 273.16

2

(9.8)( )( )

ztop zbottom

ztop zbottom

top bottom ztop zbottom

i

u u

T T

z z T T

R

  





























 

  

where Tztop and Tzbottom are the regressed temperatures at the top and bottom of the vertical profile 

in units of °C. 

if Ri >0 (for Stagnant/Stable IBL) 

 0.33  m  116Ri and 0.4 0.885(1 34 )  p   Ri

if Ri <0 (for Convective/Unstable IBL) 

  0.33 1 16   m   Ri and 0.4 0.885(1 22 )

  p   Ri

The minimum fetch requirement that the fetch is 100 times the highest height of the air sampler 

for this method to be valid was not satisfied at all times. The raw wind speed and temperature 

data at varying heights were averaged for the concentration sampling durations. The period 

averaged wind speed and temperature values were used along with the natural logarithm of the 

height measurements to calculate regression coefficients (slope, intercept, and r2

) for each period. 

Meteorological parameters for each averaging period and the regression coefficients are shown in 

Appendix D. The aerodynamic method used to estimate flux and related equations are presented 

in Majewski et al., 1990. 

 
70 of 70 

APPENDIX D 

CURRENT CHLORPYRIFOS BUFFERS RESTRICTIONS 

Table D.1. Chlorpyrifos Buffer Distances for Aquatic Sites

Application Method Required Buffer Zone 

(feet) 

Groundboom 25 

Chemigation 25 

Airblast 50 

Aerial (fixed wing or helicopter) 150 

Table D.2. Chlorpyrifos Human Health Spray Drift Buffer Zones for Sensitive Sites

Application rate (lb ai/A) Nozzle Droplet Type Required Setback (Buffer Zones) (feet) 

Aerial Airblast Ground 

>0.5 - 1 coarse or very coarse 10 10 10 

>0.5 - 1 medium 25 10 10 

>1 - 2 coarse or very coarse 50 10 10 

>1 - 2 medium 80 10 10 

>2 - 3 coarse or very coarse 80a 10 10 

>2 - 3 medium 100a 10 10 

>3 - 4 medium or coarse NAb 25 10 

>4 medium or coarse NA 50 10 

a.Aerial application of greater than 2 lb ai/A is only permitted for Asian Citrus Psylla control, up to 2.3 lb 

ai/A.

b. NA is not allowed.

Table D.3. Granular Applications Chlorpyrifos Buffer Zones for Sensitive Sites 

Application Rate 

(lb a.i./A) 

Required Setback 

(Buffer Zones) (feet) 

Aerial Grounda

>0.5 - 1 25 10 

>1 - 2 Not allowed 10 

>2 - 3 Not allowed 10 

>3 - 4 Not allowed 10 

>4 Not allowed 10 

a. The required buffer zones for ground application apply 

to applications made via spreaders.