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September 17, 2014

Spray Drift and Volatilization: Issues to Navigate Carefully as EPA Develops Registration Review Decisions

Bergeson & Campbell, P.C.

Spray drift and volatilization issues increasingly are significant issues in pesticide product risk assessments. Earlier this year, the U.S. Environmental Protection Agency (EPA) issued drafts of key guidance documents, which focused on issues that were key in the chlorpyrifos petition response, and more recently, at least one registration review decision that reflects current and still evolving EPA policy on spray drift and volatilization issues.

How potential for spray drift and for volatilization are identified and then managed are likely to be key elements of ongoing and future risk assessments underlying forthcoming EPA registration and reregistration, with significant potential impact on these decisions. Registrants should monitor closely the policies, EPA decisions implementing them, and their potential impact on their products, particularly given the public interest in these issues.

The EPA documents issued in the past eight or so months are significant, particularly given the years of controversy and difficulty in past attempts to propose a clear and “simple” definition of “drift.” The perception by some advocacy groups is that EPA is not adequately addressing alleged harms posed by drift, and resulting appeals for court intervention will undoubtedly complicate the matrix of considerations influencing EPA’s policy. These reasons alone make monitoring the development of these policies critical for registrants.

The EPA guidance documents and recent decisions are discussed below. In addition, as a service to clients and friends, Bergeson & Campbell, P.C. (B&C®) will present a webinar on October 1, 2014, from 1:00 p.m. – 2:30 p.m. (EDT) to review EPA’s current guidance and recent decisions on spray drift and volatilization issues and discuss what registrants may have to do to navigate the registration review process. Panelists will include EPA and industry representatives. If you would like to participate, please contact Chad Howlin by September 26, 2014.


EPA issued key guidance documents on spray drift and volatilization earlier this year. On January 29, 2014, EPA issued for comment draft guidance documents on spray drift, and on March 25, 2014, EPA issued for comment draft guidance documents on volatilization. Both sets of guidance documents raise novel and controversial issues and have been the subject of much discussion and review. EPA has not, to date, taken further formal action with regard to the guidance documents or the comments it received on them.

Assessments of both spray drift and volatilization were conducted for chlorpyrifos prior to release of any formal guidance documents. The initial spray drift assessment of chlorpyrifos released in July 2012 was in part a response to a petition by the Natural Resources Defense Council (NRDC) and the Pesticide Action Network North America (PANNA) seeking to cancel all chlorpyrifos registrations. This was followed by an initial volatilization assessment that EPA issued in January 2013. These assessments were key documents in the formulation of the guidance documents.

Although the initial interim registration review decisions, which EPA began issuing in June (see 79 Fed. Reg. 32284 (June 4, 2014)) and July 2014 (see 79 Fed. Reg. 42786 (July 23, 2014)) do not appear to address spray drift and volatilization issues in any materially different way than past reregistration decisions, at least one more recent decision, issued on July 30, 2014 (see 79 Fed. Reg. 44169 (July 30, 2014)), does and is worth examining, as are the chlorpyrifos decisions, including the June 2014 EPA memorandum addressing volatilization (see EPA, Chlorpyrifos, available online).

The July 30, 2014, interim decision, the June 2014 memorandum, and forthcoming decisions involving similar issues may well herald further evolution of EPA thinking on spray drift and volatilization. The July 30, 2014, interim decision includes an assessment of residential bystander exposure due to potential spray drift during application. In addition, the decision explicitly notes that dicrotophos is listed for further analysis due to possible volatilization (see Memorandum re: Dicrotophos: Human Health Risk Assessment for Registration Review of Dicrotophos (July 10, 2014) at 38, EPA-HQ-OPP-2008-0440-0025). These follow closely the assessments conducted on chlorpyrifos (the spray drift assessment was completed in 2012 while the volatilization issue was addressed in July 2014).

The EPA guidance documents are discussed briefly below, followed by a discussion of how EPA has developed and implemented these in recent decisions.

EPA Guidance Documents and Implementation

Spray Drift

EPA’s January 29, 2014, guidance entitled “Residential Exposure Assessment Standard Operating Procedures (SOPs), Addenda 1: Consideration of Spray Drift (Addenda)” provides procedures for assessing potential exposure and risk to human health associated with spray drift. It is based on the current SOP for assessing exposure and risk from use of pesticides on turf. The Addenda defines when quantitative assessments are needed and provides input factors to be used in the assessment. This methodology was followed for the chlorpyrifos assessment, as well as the dicrotophos assessment.

The exposure and risk assessment focuses on pesticide movement from an applied field to a neighboring lawn where children might play. Dermal and hand-to-mouth exposure and risk are estimated. The standard SOPs for dermal and hand-to-mouth exposure to children playing on lawns include modifications to include a factor for the percentage of applied pesticide that drifts onto the neighboring lawn. These factors or “drift fraction values” are estimated using AgDrift® for different application methods (i.e., groundboom, airblast, and aerial) and for differing distances from the field to the lawn. Three tables provide the drift fraction values for the three different application methods.

The resulting potential risk estimates from dermal and hand-to-mouth exposure incorporating the drift fraction values in the SOP equations are used to determine mitigation options (e.g., buffer zones, reduced application rates). These “bystander risk” estimates may lead to new issues for registrants, as critics of pesticide use will likely cite conservative EPA estimates suggesting possible harm even if EPA registration decisions about appropriate label requirements allow for continued use.

In the documents it released on July 30, 2014, in connection with its interim decision on dicrotophos, EPA followed the spray methodology exactly as it was presented in the draft. The default assumptions were used for the assessment of groundboom and aerial application methods (i.e., only a Tier I assessment was conducted for aerial application). The methodology used in the assessment does not resolve any of the issues raised by the regulated community during the guidance document comment period. Registrants have commented that the current default assumptions are overly conservative and calculations will imply a need for unnecessarily large buffer zones. In addition, some believe that the AgDrift® model itself is inappropriate for use, and that drift reduction strategies currently used are not included in the default assessments.

The original spray drift assessment of chlorpyrifos resulted in untenable buffer zones that many in industry believed not necessary or supported by science. EPA and the registrants agreed on mitigation decisions, including lowering application rates resulting in buffer zones that were acceptable from an agricultural use perspective.


In 2009, EPA conducted a Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Scientific Advisory Panel (SAP) review of issues regarding the assessment of volatilization for conventional pesticides. This was followed in 2013 by EPA’s assessment of the volatility of chlorpyrifos (see EPA-HQ-OPP-2008-0850). The purpose of the assessment was to determine if buffer zones were needed to protect bystanders from the potential volatilization of applied chlorpyrifos. The chlorpyrifos assessment used the Probabilistic Exposure and Risk model for FUMigants (PERFUM) and field volatility studies (flux rate data) to estimate emission rate profiles. These studies are not typically conducted for conventional pesticides. This assessment proved to be very labor- and data-intensive. Thus, EPA reviewed other possible models to assess volatilization.

EPA believes that the results of the 2009 SAP and the 2013 chlorpyrifos assessment identified the need for a tool that could be used to screen conventional pesticides to determine if a more complete assessment of possible volatilization was needed.

On March 1, 2014, EPA published for public review and comment a Volatilization Screening Tool that “provides a consistent and health protective framework to assess the potential inhalation bystander risks resulting from volatilization of conventional pesticides.” Five documents were made available for public review and comment:

  • Human Health Bystander Screening Level Analysis: Volatilization of Conventional Pesticides (Background Document);
  • Appendix A: Volatilization Screening Tool Guidance Document;
  • Appendix B: Inhalation Equivalent Concentration Calculations for the Registration Review Chemical Analysis;
  • Appendix C: Data Entry Sheets for the Registration Review Chemical Volatilization Screening Analysis; and
  • Appendix D: Registration Review Chemical Volatilization Screening Analysis Results.

The Background Document provides an overview of the history of the screening tool development, as well as a statement of guiding principles. The screening tool inputs are described, followed by a discussion of the results of EPA’s use of the tool to assess chemicals under registration review. Finally, interpretation issues regarding the screening results are presented.

Appendix A provides a step-by-step guide on how to use the screening tool. It is presented as an Excel spreadsheet and can be downloaded from the EPA docket. The tool is based on the EPA air model, AERSCREEN, where air concentrations are directly related to the emission rate from the field at issue. AERSCREEN estimates off-field air concentrations based on application rate, different release rates (e.g., hourly flux rates), meteorological conditions, and crop conditions (e.g., bare ground vs. orchards). The tool then estimates the distance from the edge of the field where air concentrations are below the level of concern for that specific chemical. Estimation of the level of concern is based on the toxicology of the chemical.

The inhalation Point of Departure (POD) is the “chemical-specific human-health point of departure selected for the inhalation route of exposure in a chemical’s most recent human health exposure or risk assessment.” EPA describes three options and the uncertainty factors (UF) for this value:

  • If a Human Equivalent Concentration (HEC) based on a route-specific inhalation toxicity study was calculated in a recent risk assessment using EPA’s inhalation reference concentration (RfC) methodology, this value would be used. The UF for interspecies may be reduced to 3X per the RfC methodology, while the intraspecies 10X UF is retained. Any additional UFs (e.g., database uncertainty, Food Quality Protection Act (FQPA)) are also retained;
  • If a route-specific inhalation toxicity study was used in a recent risk assessment without calculation of an HEC, a no observed adverse effect level (NOAEL) from the inhalation study would be used. Inter- and intra-species UFs are retained at 10X each, in addition to any additional UFs used in the risk assessment; and
  • If a route-specific inhalation toxicity study is not available in a recent risk assessment, EPA would convert a NOAEL from an oral toxicity study to an inhalation equivalent concentration (IEC). Appendix B provides the IECs for all chemicals currently assessed. Inter- and intra-species UFs are retained at 10X each, and any additional UFs used in the risk assessment are retained. In addition, an FQPA UF of 10X is applied to address database uncertainty due to the lack of an inhalation study.

For both chlorpyrifos and dicrotophos, use of the Volatilization Screening Tool without refinement would result in buffer zones of over 4,500 meters. The chlorpyrifos registrants conducted two inhalation toxicity studies and on the basis of those studies a determination was made by EPA in July 2014 that there was no human health risk from volatilization of chlorpyrifos, despite the results of the model that is now the Volatilization Screening Tool.

Volatilization was not addressed quantitatively in the dicrotophos assessment, rather, EPA states:

Volatilization of pesticides may be a source of post-application inhalation exposure to individuals nearby pesticide applications. The agency sought expert advice and input on issues related to volatilization of pesticides from its Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel (SAP) in December 2009, and received the SAP’s final report on March 2, 2010. . . The agency has evaluated the SAP report and has developed a Volatilization Screening Tool and a subsequent Volatilization Screening Analysis. . . During Registration Review, the agency will utilize this analysis to determine if data (i.e., flux studies, route-specific inhalation toxicological studies) or further analysis is required for dicrotophos.

(See EPA-HQ-OPP-2008-0440-0025 at 38.)


EPA appears now prepared to use the guidance documents it issued earlier this year to help it make registration review decisions. Some in the registrant community are concerned with this, believing the models add another layer of conservatism to EPA’s evaluation of pesticide safety and do not produce realistic risk estimates. Registrants will need to respond to estimates of risk and in so doing try to create a pathway for successfully addressing model outcomes and regulatory decisions based on those outcomes. For example, it is not clear at this point in time what type of study or other information EPA would consider to rebut model outcomes.

Registration-specific evaluations of possible bystander risk, based on evolving draft EPA guidance documents on specific related issues, might also be viewed as a case-by-case application of de facto drift and volatilization policies. As more pesticides move through registration review, some of which can be expected to present bystander risk concerns, EPA will effectively develop its policies by virtue of the evolution of its case-by-case decisions. Close review of these decisions will provide guidance to registrants regarding what they may have to do to navigate the registration review process successfully.

As noted, B&C is hosting a webinar for clients and friends on October 1, 2014, from 1:00 p.m. – 2:30 p.m. (EDT) to discuss EPA’s recent decisions involving spray drift and volatilization, and what companies must do to navigate the registration review process in light of those decisions. Panelists will include EPA and industry representatives. If you would like to participate, please contact Chad Howlin by September 26, 2014.