Download PDF
February 12, 2013

EPA Issues Chlorpyrifos Assessment:  A Model for Addressing Spray Drift?

The ACTA Group

The U.S. Environmental Protection Agency (EPA) released on February 6, 2013, its preliminary volatilization assessment for the registration review of chlorpyrifos. 76 Fed. Reg. 8522. In a letter to the Natural Resources Defense Council (NRDC) and to the Pesticide Action Network North America (PANNA) dated January 25, 2013, updating these groups on EPA’s response to their September 12, 2007, joint petition regarding chlorpyrifos, EPA stated that “[t]his assessment represents a significant advancement in the evaluation of pesticide risks, as it will be the first probabilistic assessment of the risks posed by the post-application volatilization of a semi-volatile pesticide.” EPA further stated that it is “critical to involve the public in the development of this assessment before it is finalized,” given “the groundbreaking nature of the new assessment and its potential for use in guiding additional risk mitigation.” The implications of the draft assessment and possible regulatory pathways forward could be significant, particularly with regard to EPA’s potential pathway forward to addressing spray drift.

Comments on the preliminary volatilization assessment are due March 8, 2013. EPA states that after reviewing comments received during the public comment period, it will issue a revised volatilization assessment, explain any changes to the preliminary volatilization assessment, respond to comments, and evaluate the need for risk mitigation for chlorpyrifos. The preliminary assessment is available online, and the docket is available online.


The assessment uses the PERFUM model to assess potential bystander exposure to chlorpyrifos and chlorpyrifos byproduct emissions. The PERFUM model was used to assess bystander exposure to soil fumigants. Bystander exposure assessment is a relatively new component of pesticide risk assessment. Bystander exposure assessments played a large role in the Reregistration Eligibility Decision documents for the soil fumigants, but had not previously been widely employed on a large scale.

EPA has created a list of questions for commenters. The questions below are of importance for any semi-volatile chemical that EPA may re-assess in this manner. They include the following:

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)?


The chlorpyrifos assessment document and the broader chlorpyrifos review are especially driven by the non-governmental organization (NGO) petition with particular concerns about chlorpyrifos. The approach can be expected to be viewed as an effective way to prod EPA to tighten restrictions on chlorpyrifos and thus NGOs may well seek to argue that other pesticides, especially other organophosphates, should face similar restrictions for the same reasons. More broadly, however, it appears it might become the approach EPA uses to address issues and concerns about pesticide drift.

EPA claims that this is a groundbreaking assessment and we would agree. The implications of this assessment are far-reaching, as the approach taken may become EPA’s preferred way of tackling the spray drift issues that have been the subject of controversy over the years, which could result in the imposition of new data requirements, buffer zone restrictions, and other risk management requirements for those pesticide products that EPA believes may pose drift issues.

The drift issue has been controversial for decades and EPA has seemed stymied by the inability to define more precisely the issues and implement regulatory responses to control problems with the off-site movement of pesticides. Even when label instructions are followed, EPA believes that for many pesticides, there remains some potential for off-site movement. Such drift may or may not be of consequence (“drift happens”). Many labels include prescriptive mandates such as “Do not allow pesticide drift.” EPA has had difficulty in defining how to craft more appropriate label language to prevent and control unacceptable or damaging levels of drift beyond the simple exhortation to avoid or explicit language to prohibit off-site movement. The subject has been the focus of a Pesticide Program Dialogue Committee (PPDC) workgroup since 2006, with a report of the workgroup issued in 2007 (available online). In 2009, the Office of Pesticide Programs (OPP) issued a draft Pesticide Registration Notice, PR Notice 2009-X Draft: Pesticide Drift Labeling,” which has not been made final (available in the docket at EPA-HQ-OPP-2009-0628-0002). These events of the past decade follow earlier attempts to define and refine an appropriate response to the potential for off-site movement of a pesticide that can occur even when prescribed label directions are followed. Notwithstanding these efforts, for many observers, EPA is no closer to issuing a definitive drift policy than it has been in the past.

The current chlorpyrifos assessment may perhaps be a way for EPA to “resolve,” in a way, the drift issue with a new approach. Instead of overarching policy statements defining “acceptable” drift or what enforcement policies should be used to avoid drift (the focus of much of the past discussions and efforts to address the issue), the approach outlined in the chlorpyrifos document might provide a template of how EPA could assess bystander risk, minimize it to levels below any risk concern, and allow the risk assessment for any individual pesticide to define what level of off-site exposures are acceptable. Label instructions would derive from the analysis, including requirements for buffer zones, application methods, or other requirements to ensure acceptable risks from any calculated off-site potential for off-site movement. At the same time, calculations which underlie any “bystander risk assessment” are likely to include the typical EPA approach of using “conservative” modeling assumptions, which may be disputed by registrants in any particular case. In this way, EPA may be seeking to move any concerns about “drift” as a generic issue to the particular characteristics of an individual pesticide and its specific uses.

Given the broad potential implications, pesticide registrants may wish to consider commenting on the draft assessment.