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Note to Docket # EPA-HQ-OPP-2008-0850

Publication Date: February 6, 2013

Chlorpyrifos Volatility Assessment: Guide to Commenters

The purpose of this note is to announce that the Environmental Protection Agency (EPA) is 

opening the public comment period for the Chlorpyrifos Preliminary Evaluation of the Potential 

Risks from Volatilization. The public is invited to comment on EPA’s volatilization assessment. 

EPA will carefully consider all comments, as well as any additional information or data provided 

in a timely manner, prior to issuing a final assessment. The chlorpyrifos volatilization assessment 

can be found at www.regulations.gov, in docket EPA-HQ-OPP-2008-0850. The public comment 

period is 30 days from the date of the Federal Register Notice announcing the availability of the 

volatilization assessment.

The chlorpyrifos volatility assessment evaluates the potential exposure to vapor phase 

chlorpyrifos and chlorpyrifos-oxon emitted from treated fields. Two different field volatility 

studies suggest that volatilization of chlorpyrifos and/or chlorpyrifos-oxon from treated crops is a 

route 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. 

As part of this draft risk assessment, EPA has also determined an acceptable concentration for 

exposure to volatilized residues of chlorpyrifos and chlorpyrifos-oxon based on an acute 

inhalation toxicity study conducted with aerosolized chlorpyrifos. Using that study EPA 

determined that humans should not experience any adverse effects from exposures that are less 

than 8.3 μg/m3

. This level is the target concentration. EPA has concerns about potential risks for 

bystanders when the estimated concentration for bystanders is above the target concentration. 

The approaches used to evaluate the volatilization of chlorpyrifos and chlorpyrifos-oxon and the 

associated risks resulting from exposure to the volatilized residues are consistent with the 

methodology used in assessments conducted in the past for fumigant pesticides. As discussed in 

the Preliminary Evaluation of the Potential Risks from Volatilization, while there are a number 

of uncertainties and assumptions associated with this assessment, available data indicate that 

bystander exposure to vapor phase chlorpyrifos and chlorpyrifos-oxon is possible, and can occur 

at concentrations that may cause adverse effects. Generally, application rates greater than 1 lb 

a.i./A in large fields results in concentrations of concern at significant distances offsite.

If the final assessment estimates risks similar to those presented in this preliminary evaluation, 

EPA anticipates that it may be necessary to take actions to reduce bystander risks. There are 

several risk assessment inputs and assumptions that if further refined could reduce exposure that 

would result in lower risk estimates. Although no final decisions regarding risk mitigation have 

been made, one of the possible risk mitigation measures could be to require buffer zones between 

application sites and areas where bystanders might be present. Additional information explaining 

how buffer zones could be designed appears after the list of questions on which EPA is seeking 
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comment. During this public comment period, stakeholders are specifically asked to provide 

information and data on the following questions that will assist EPA in completing a refined 

assessment.

Questions regarding assessing volatility

1. What crop related factors (e.g., canopy height, leaf area index, leaf type, crop density, 

surface roughness) should be considered when measuring pesticide flux rates and 

interpreting (e.g., scaling flux rates or modifying flux rates from one crop to another) the 

results? What are the potential impacts of such factors?

2. What field conditions (e.g., degree of foliage, leaf type and shape, crop canopy, etc.) 

should be considered when measuring pesticide flux rates and interpreting the results?

3. What are the best methods for conducting a flux study so that it can be used to represent a 

broad spectrum of agricultural uses? Are approaches available for extrapolating flux 

results from one crop to another crop or from one region to another?

4. The correlation of pesticide application parameters (i.e., application rate) to the flux rate 

is assumed to be linear (i.e., as the application rate increases, the flux rate increases 

directly). What data are available to suggest otherwise or are there other application 

parameters that should be considered? If data are available, what are those parameters? 

When is it appropriate to scale the flux rate up or down based on application rates?

5. What other assumptions and uncertainties or sources of variability should be considered 

when measuring pesticide flux rates, interpreting flux data, and extrapolating results (e.g., 

stable meteorological conditions/calm wind)?

Questions regarding assessing hazard characterization/identification

6. Please provide comment on the interpretation of the special acute inhalation study1 for 

use in the human health risk assessment. As part of your comments, consider the findings 

related to cholinesterase (ChE) inhibition in lung, plasma, red blood cells, and brain. 

Specific to lung ChE, please provide information relevant to interpreting ChE inhibition

in the lung and its relationship to adverse effects on pulmonary function.

7. Please provide comment on the strengths and limitations associated with use of an 

inhalation study conducted with aerosolized chlorpyrifos to assess the toxicity of 

chlorpyrifos in the vapor phase.

Questions regarding additional information on chlorpyrifos use

8. Please provide information regarding application rates and acres treated per application 

per day (i.e. field size treated per day), specific to your growing region and pest spectrum

9. What application methods do you most frequently/typically use?

10. What formulation types do you most frequently use?

11. What is the application timing (date of first application and application intervals) by crop 

– national, state, and county?

a. When during the growing season do chlorpyrifos applications predominately 

occur in tree and vine crops?

 1 MRID 48139303, available at http://www.epa.gov/opp00001/chem_search/cleared_reviews/csr_PC-059101_20-

May-11_a.pdf.
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After reviewing comments received during the public comment period, EPA may issue a revised 

volatilization assessment, explain any changes from the preliminary volatilization assessment, 

respond to comments, and evaluate the need for any regulatory action.

Useful Information

A. What is a Buffer Zone?

Buffer zones are areas around a treated field that are established to manage the risks from acute 

inhalation exposure to bystanders. A buffer zone is the distance between the application site 

(i.e., edge of field) and the bystanders that allows airborne residues to disperse before reaching 

the bystanders. The greater the distance (i.e. buffer zone), the greater the potential for residues to 

disperse, thus decreasing the likelihood that air concentrations where bystanders are located will 

cause acute adverse effects. EPA’s goal is to determine distances, or buffer zones, which will 

adequately protect bystanders from acute exposures. The distance or size of the buffer zones 

would be based on factors such as application rates and field size. Air monitoring data, incident 

reports, and air dispersion modeling based on these factors can be used to help determine 

appropriate buffer zone distances. 

B. What is a Buffer Zone Period?

Buffer zones, if required, would begin at the start of a chlorpyrifos application and remain in 

effect for a specified period of time after the application is complete. The duration of the buffer 

zone requirement would depend upon when off-site emissions have declined to a point where 

bystanders are no longer at risk. This duration is known as the buffer zone period.

C. What Type of Buffer Zone Might Be Required?

If buffer zones are required, due to the length of time that the vapor phase of chlorpyrifos 

exceeds the target concentration, the buffer zones would likely need to remain in place for some 

period of time (e.g. 6, 12, 24, etc hours) after application. A buffer zone for some set period of 

time would allow for the dispersion of chlorpyrifos vapor such that the likelihood of bystanders 

experiencing acute adverse effects diminishes because it would allow the vapor concentration to 

drop below the target concentration. 

EPA also believes that if buffer zones are required, the buffers would likely need to encompass 

the entire field. That is, whatever buffer distance is determined appropriate to manage the risks 

of acute inhalation exposure to bystanders, would be the distance from the edge of the 

application area around the entire perimeter of the field equally in all directions. This approach is 

consistent with the approach used and presented in the recent fumigant risk assessments.2

Buffers around the entire perimeter would likely be necessary because due to the length of time a 

buffer zone may need to be maintained (e.g. hours not just at the time of application) makes it 

difficult to determine when a person may utilize the area surrounding the application site. In 

 2 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.
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addition, the variability of meteorological conditions makes it hard to predict where peak 

concentrations will occur around the application site, thus necessitating the need for buffers 

around the entire perimeter. EPA’s decision regarding the appropriate buffer distance is informed 

by the “Whole Field Buffer” and “Maximum Buffer” distributions.3 The “Whole Field Buffer” is

estimated based on an array of results for the entire perimeter of a field. The “Maximum Buffer”

is the largest buffer estimated from any given direction away from the field.

 3 For more information regarding “Whole Field Buffer” and “Maximum Buffer” see one of the fumigant risk 

assessments such as the “Chloropicrin: Third Revision of the HED Human Health Risk Assessment” available at 

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2007-0350-0398. The discussion begins on page 

38.