UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

MEMORANDUM 

DATE: 6/25114 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

SUBJECT: Chlorpyrifos: Reevaluation of the Potential Risks from Volatilization in 

Consideration of Chlorpyrifos Parent and Oxon Vapor Inhalation Toxicity Studies 

PC Code: 059101 

MRID No.: 49119501,49210101 

Petition No.: NA 

Risk Assessment Type: Single Chemical; 

Volatilization 

TXRNo.: NA 

FROM if kit~ Wade Bri tt~~-~ , Inaustrial Hygienist 

William A. Irwin, PhD, DABT, Toxicologist 

Risk Assessment Branch V 

Hr~!Ah,Effects Qi~(HED; 7509P) 

IJ)~q_.~ 

THROUGH: Michael Metzger, Branch Chief 

Risk Assessment Branch VI VII 

Health Effects Division (7509P) 

TO: Kevin Costello, Branch Chief 

DP Barcode: D4171 05 

Registration No.: NA 

Regulatory Action: Registration Review 

Case No.: NA 

CAS No.: 2921-88-2 

Jeffrey L. Dawson, Chemist 

Anna B. Lowit, Ph.D., Senior c entist ... _ 

Elizabeth Mendez, Ph.D. Senior Scientist --~~-----· Immediate Office 

Health Effects Division (HED; 7509P) 

Joel Wolf, Chemical Review Manager 

Risk Management and Implementation Branch 2 

Pesticide Re-Evaluation Division (PRD; 7508P) 

The purpose of this memorandum is to reevaluate the potential human health risks from 

bystander exposure to the volatilization of chlorpyrifos and chlorpyrifos-oxon emitted from 

treated fields following the application of chlorpyrifos. The information and conclusions 

contained in this memo update and supersede those in the January 2013 Preliminary Evaluation 
Page 2 of 7

of the Potential Risks from Volatilization1assessment. Since the preliminary volatilization 

assessment was released for public comment in February 2013, new chlorpyrifos and 

chlorpyrifos-oxon inhalation toxicity data generated using saturated vapor phase administration 

were submitted to the Agency. The results of these studies have significantly changed how EPA 

considers the hazard to chlorpyrifos and chlorpyrifos oxon vapor. Based on the new data, there 

are no human health risks of concern anticipated for volatilization exposure to either chlorpyrifos 

or chlorpyrifos-oxon. 

Volatilization Evaluation

Chlorpyrifos is currently undergoing registration review, EPA’s periodic reevaluation of all 

registered pesticides. As part of registration review, the chlorpyrifos preliminary Human Health 

Risk Assessment (HHRA) was released for public comment in July 2011.2 In the preliminary 

HHRA, potential risks to bystanders from spray drift and exposure from volatilization were

identified as possible concerns. The potential risks from spray drift and the impact of potential 

risk reduction measures were assessed in a July 2012 memorandum3. Spray drift is the 

movement of aerosols and volatile components away from the treated area during the application 

process. To increase protection for children and other bystanders, chlorpyrifos technical 

registrants voluntarily agreed to lower application rates and other spray drift mitigation 

measures4. As of December 2012, spray drift mitigation measures and use restrictions appear on 

all chlorpyrifos agricultural product labels. 

In January 2013, a preliminary assessment of the potential risks from volatilization was 

conducted. The assessment evaluated the potential risks to bystanders, or those who live and/or 

work in proximity to treated fields, from inhalation exposure to vapor phase chlorpyrifos and 

chlorpyrifos-oxon emitted from fields following application of chlorpyrifos. The results of the 

January 2013 assessment indicated that offsite concentrations of chlorpyrifos and chlorpyrifosoxon may exceed the target concentration based on the toxicological endpoints used at that time. 

Specifically, for the January 2013 evaluation, points of departure (PoDs) were derived from an 

acute inhalation toxicity study5using aerosolized chlorpyrifos which measured lung, plasma, 

RBC, and brain cholinesterase (ChE) inhibition; volatilization exposure was assessed only with 

use of the RBC and lung ChE data. 

The January 2013 evaluation identified risks from off-field exposure but one significant area of 

uncertainty described in the preliminary assessment was the use of the aerosolized chlorpyrifos 

 

1 R. Bohaty, C. Peck, A. Lowit, W. Britton, N. Mallampalli, and A. Grube. Chlorpyrifos: Preliminary Evaluation of 

the Potential Risks from Volatilization. 1/31/2013. U.S. EPA Office of Chemical Safety and Pollution Prevention. 

D399484, D400781. 

2 Available at http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2008-0850-0025. 

3

J. Dawson, W. Britton, R. Bohaty, N. Mallampalli, and A. Grube. Chlorpyrifos: Evaluation of the Potential Risks 

from Spray Drift and the Impact of Potential Risk Reduction Measures. 7/13/12. U.S. EPA Office of Chemical 

Safety and Pollution Prevention. D399483, D399485.

4 R. Keigwin. Spray Drift Mitigation Decision for Chlorpyrifos (059101). 7/2012. U.S. EPA Office of Chemical 

Safety and Pollution Prevention. EPA-HQ-OPP-2008-0850-0103.

5 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.
Page 3 of 7

inhalation toxicity study -- as opposed to chlorpyrifos vapor -- for evaluation of lung ChE

resulting from field volatilization. Because field volatilization is the production and release of 

vapor into the atmosphere after sprays have settled on treated soils and plant canopies, it is well 

accepted that the vapor, rather than the aerosol, is the relevant form for evaluation of bystander 

volatilization exposures. However, EPA lacked chlorpyrifos vapor data at the time it conducted 

the preliminary volatilization assessment in 2013. Following the release of the preliminary 

volatilization assessment, Dow AgroSciences LLC conducted two, high quality nose-only vapor 

phase inhalation studies for both chlorpyrifos and chlorpyrifos-oxon6to address this uncertainty. 

In both studies, female rats were administered a saturated vapor, meaning that the test subjects 

received the highest possible concentration of chlorpyrifos or chlorpyrifos-oxon which can 

saturate the air in a closed system. At these saturated concentrations, no statistically significant 

inhibition of ChE activity was measured in RBC, plasma, lung, or brain at any time after the sixhour exposure period in either study. Under actual field conditions, indications are that 

exposures to vapor phase chlorpyrifos and its oxon would be much lower as discussed in the 

January 2013 preliminary volatilization assessment. Summaries of the two vapor studies are 

included as an attachment to this document. 

EPA’s approach to human health risk assessment follows the four step paradigm established by the 

NAS (NRC, 1983). These steps include hazard identification, dose-response assessment, exposure 

assessment, and risk characterization, where risk is a function of both hazard and exposure. In the 

case of vapor exposure to chlorpyrifos or its oxon from volatilization, the new studies show no 

effects at saturation concentration in laboratory animals. In other words, no response related to 

toxicity to chlorpyrifos or its oxon were observed at the highest achievable concentration --

concentrations that likely far exceed those available to bystanders near treated fields. Accordingly, 

this finding leads to the conclusion that there is no hazard to chlorpyrifos or chlorpyrifos oxon vapor 

up to the highest possible concentration in the air. As such, if there is no hazard to the vapor for 

these pesticides, there is no risk. By extension, if there is no risk to chlorpyrifos or chlorpyrifos 

oxon, there is no need to perform quantitative risk estimates for off-field exposure. Although, in any 

risk assessment there is some uncertainty associated with extrapolating from laboratory animals to 

humans, the architecture of the lung is similar between rats and humans7and, therefore, the Agency 

has confidence is this conclusion. 

 

The preliminary volatilization assessment was released for public comment on February 2013 

and closed May 2013. During this period comments were received regarding the exposure 

methodologies employed, and the uncertainties and assumptions in the preliminary volatilization 

assessment8. The Dow studies have made clear that volatilization of chlorpyrifos does not 

present a risk of ChE inhibition from inhalation of chlorpyrifos vapor, and none of the comments 

 

6 W. Irwin. Review of Nose-Only Inhalation of Chlorpyrifos Vapor: Limited Toxicokinetics and Determination of 

Time-Dependent Effects on Plasma, Red Blood Cell, Brain and Lung Cholinesterase Activity in Femal CD(SD): Crl 

Rats. U.S. EPA Office of Chemical Safety and Pollution Prevention. 6/25/14. D411959. TXR# 0056694. EPA 

MRID# 49119501.

W. Irwin. Review of Nose-Only Inhalation of Chlorpyrifos-Oxon Vapor: Limited Toxicokinetics and Determination 

of Time-Dependent Effects on Plasma, Red Blood Cell, Brain, and Lung Cholinesterase Activity in Female 

CD(SD):Crl Rats. U.S. EPA Office of Chemical Safety and Pollution Prevention. 6/25/14. D415447. TXR# 

0056869. EPA MRID# 49210101.

7 W. Hofmann, L. Koblinger, T.B. Martonen. Structural Differences Between Human and Rat Lungs: Implications 

for Monte Carlo Modeling of Aerosol Deposition. Health Physics [1989, 57 Suppl. 1:41-6; Discussion 46-7]. 

8 All public comments and related documentation are available in docket EPA-HQ-OPP-2008-0850. 
Page 4 of 7

received included data or other information that would suggest that an appropriate vapor study 

does not represent the best means for assessing the potential for inhalation risk from volatilized 

chlorpyrifos. While a revised volatilization assessment is no longer needed, the EPA intends to

review and respond to comments in a subsequent memorandum.
Page 5 of 7

Attachment – Vapor Inhalation Study Summaries

Citation: Hotchkiss, JA, Nose-Only Inhalation of Chlorpyrifos Vapor: Limited Toxicokinetics 

and Determination of Time-Dependent Effects on Plasma, Red Blood Cell, Brain and Lung 

Cholinesterase Activity in Femal CD(SD): Crl RATS (2013), MRID 49119501, Unpublished.

Sponsor: Dow AgroSciences LLC, 9330 Zionsville Road, Indianapolis, Indiana 46268

Summary: 

In a special acute inhalation toxicity study (MRID 49119501), chlorpyrifos 97.6% a.i., (CPF, Lot 

# 7299412; TSN100759) was administered as a saturated vapor to 40 non-fasted Crl:CD Sprague 

Dawley strain female rats by nose-only inhalation exposure at a time-weighted concentration of 

17.7 ppb (0.254 mg/m3) for 6 hours in comparison to control rats with 0 ppb. Rats were 

sacrificed immediately (0 hr) and at 2, 4, 6, or 12 hours after the end of exposure (n=8/exposure 

group/sacrifice time). Blood and tissues were isolated and processed to determine cholinesterase 

(ChE) activity in plasma, red blood cells (RBC), brain, and lung tissue. Whole blood samples 

from n=4 rats in each experimental group were analyzed to determine the concentration of 

chlorpyrifos (CPF), oxon and trichloropyrinol (TCP). An additional group of unexposed female 

control rats (n=8), which experienced the same laboratory and exposure tube acclimation and 

animal husbandry activities as the control and CPF exposure group animals, were sacrificed at 

the start of the air and CPF exposures. The oxon content in the chlorpyrifos was <0.1% and 1.4% 

in the test atmosphere.

No clinical effects indicative of cholinesterase inhibition were noted during the six-hour 

exposure period. In-life observations noted post-exposure were limited to soiling in four rats 

exposed to CPF and all rats appeared normal by test day 2. There were no decreases in ChE 

activity in any tissue which were significantly significant. 

In the red blood cell samples, the ChE activity at all time points was comparable to control 

activity, with none having statistical significance. The ChE activities for the 17.7 ppb samples 0, 

2, 4, 6 and 12 hours after exposure were 105.7%, 98.6%, 93.3%, 103.3%, and 110.5% of the 0 

ppb control group, respectively.

In the plasma samples, the ChE activity for the 17.7 ppb samples 0, 2, 4, 6 and 12 hours after 

exposure were 94.9%, 106.3%, 85.8%, 105.4%, and 96.9% of the 0 ppb control group, 

respectively, with none having statistical significance. A slight decrease in ChE activity was 

measured at 4 hours post dose, however, it was not statistically significant and the trend was not 

consistent with the 2-hour and 6-hour results.

In the lung samples, the ChE activity for the 17.7 ppb samples 0, 2, 4, 6 and 12 hours after 

exposure were 88.6%, 94.8%, 88.8%, 93.8%, and 96.6% of the 0 ppb control group, respectively, 

with none having statistical significance. The 0 and 4-hour results had activity below 90%, 

however, this trend was not consistent with the 2, 6 and 12-hour findings.
Page 6 of 7

In the brain samples, statistically significant ChE inhibition was not observed at any time point 

for the 17.7 ppb samples ChE activity at 0, 2, 4, 6 and 12 hours after exposure was 104.1%, 

98.3%, 99.7%, 106.1%, and 101.7% of the 0 ppb control group. The 6-hour sample was the only 

time point with a statistically significant result, however, it was for an increase in ChE activity, 

not inhibition.

For chlorpyrifos, the blood levels ranged from the lower limit of quantitation (LLQ) to 0.335 

pg/g during the time period after exposure. For TCP, the blood levels ranged from 12.4 to 90.4 

pg/g during the time period after exposure. No oxon greater than or equal to the lower limit of 

quantitation (LLQ; 0.00015 nmole/g blood) was measured in the blood of any control 

(unexposed or 0 ppb-exposed) or CPF-exposed rat at any time point.

The purpose of the study was to determine pharmacokinetics and ChE inhibition of the RBC, 

plasma, brain and lung of rats exposed to a single dose of saturated vapor of chlorpyrifos. This 

purpose was achieved and no meaningful ChE inhibition was observed at the saturation 

concentration. 

This study is classified as acceptable/non-guideline, however, since it was a special study, it does 

not fulfill any guideline requirement.

Citation: Hotchkiss, JA, Nose-Only Inhalation of Chlorpyrifos-Oxon Vapor: Limited 

Toxicokinetics and Determination of Time-Dependent Effects on Plasma, Red Blood Cell, Brain, 

and Lung Cholinesterase Activity in Female CD(SD):Crl Rats. MRID 49210101, Unpublished. 

Sponsor: Dow AgroSciences LLC, 9330 Zionsville Road, Indianapolis, Indiana 46268

Summary:

In a special acute inhalation toxicity study (MRID 49210101), chlorpyrifos oxon (100% purity) 

was administered as a saturated vapor to 48 female CD(SD):Crl rats for six hours via nose-only 

exposure. This study was designed to assess the effect of an acute six-hour nose-only inhalation 

exposure to a saturated vapor concentration of chlorpyrifos oxon (oxon) on cholinesterase (ChE) 

activity in red blood cells (RBC), plasma, lung, and brain and to determine the blood 

concentrations of oxon and the metabolite 3,5,6-trichloro-2-pyridinol (TCP) in oxon-exposed 

rats. Female CD(SD):Crl rats were exposed six consecutive hours to filtered air (control) or a 

time-weighted average concentration of 35.3 μg/m3(2.58 ppb) oxon vapors using a flow-past 

nose-only inhalation exposure system. Rats were sacrificed immediately (0 hr) and 1, 2, 4, 8, or 

24 hours after the end of exposure (n=8/exposure group/sacrifice time). Blood and tissues were 

isolated and processed to determine ChE activity in RBC, plasma, lung, and brain tissue. Oxon 

and TCP concentrations were determined in whole blood samples from four rats in each 

experimental group/sacrifice time. 

During or after oxon exposure, no clinical signs of toxicity were noted in oxon-exposed rats at 

any time. There were no statistically significant decreases in ChE activity in any tissue 
Page 7 of 7

monitored. The only statistically significant changes in cholinesterase (ChE) activity were in the 

RBC and brain, however, increases of ChE activity occurred.

In the red blood cell samples, the ChE values for the 35.3 μg/m3samples at 0, 1, 2, 4, 8 and 24 

hours after exposure were 93.6%, 110.6%, 113.9%, 116.5%, 102.2%, and 111.9%, respectively,

of the 0 μg/m3control group, with none having both statistical significance and an activity 

decrease greater than 10%. The RBC ChE activity at 4 hours was statistically significant, 

however it was an increase in activity.

In the plasma samples, the ChE values for the 35.3 μg/m3samples at 0, 1, 2, 4, 8 and 24 hours 

after exposure were 103.0%, 107.5%, 101.5%, 136.7%, 101.0%, and 127.6%, respectively, of the 

0 μg/m3control group, with none having statistical significance nor a decrease in ChE activity.

In the lung samples, the ChE values for the 35.3 μg/m3samples at 0, 1, 2, 4, 8 and 24 hours after 

exposure were 93.7%, 108.3%, 118.6%, 109.1%, 101.7% and 91.6%, respectively, of the 0 

μg/m3control group, with none having statistical significance nor a decrease in activity greater 

than 10%.

In the brain samples, the ChE values for the 35.3 μg/m3samples at 0, 1, 2, 4, 8 and 24 hours after 

exposure were 100.5%, 106.2%, 102.0%, 100.9%, 101.2%, and 101.0%, respectively, of the 0 

μg/m3control group, with none having both statistical significance and an activity decrease. The 

brain ChE activity at 1 hour was statistically significant, however it was an increase in activity.

At any time after exposure, no oxon was measured in the blood (lower-limit of quantitation; LLQ 

= 0.118 ng/g blood), however, blood TCP levels > LLQ (2.44 ng/g blood) were measured at the 

end of exposure and through eight hours post-exposure. No statistically significant inhibition of 

ChE activity by oxon was measured in RBC, plasma, lung, or brain at any time after exposure. 

The presence of TCP in the blood of oxon-exposed rats confirms that inhaled oxon vapor is 

absorbed by the respiratory tract, however, the oxon is rapidly metabolized to TCP and the oxon 

is not systemically bioavailable at blood levels ≥ the LLQ of 0.118 ng/g blood (3.53x10-4 

nmole/g blood). The six-hour No Observed Effect Concentration (NOEC) for inhaled oxon vapor 

is > 35 μg oxon/m3air, based on the absence of statistically-significant cholinesterase inhibition 

in RBC, plasma, brain, or lung (the portal-of-entry tissue). The results of this study suggest that 

there is no cholinesterase-based hazard from inhalation of a saturated vapor concentration (35.3 

μg/m3) of chlorpyrifos oxon.

The purpose of the study was to determine pharmacokinetics and ChE inhibition of the RBC, 

plasma, brain and lung of rats exposed to a single dose of saturated vapor of chlorpyrifos. This 

purpose was achieved and no meaningful ChE inhibition was observed at the saturation 

concentration. 

This study is classified as acceptable/non-guideline, however, since it was a special study, it does 

not fulfill any guideline requirement.