OFFICE OF

 PESTICIDES PROGRAMS

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

Chemical: Chlorpyrifos-methyl

PC Code: 059102

 DP Barcode: 370120

DATE: November 23, 2009

MEMORANDUM

SUBJECT: Registration Review – Problem Formulation for Chlorpyrifos-methyl

FROM: Donna Judkins, Biologist

Mark Corbin, Senior Environmental Scientist

THROUGH: Dana Spatz, Branch Chief

Environmental Risk Branch 3

 Environmental Fate and Effects Division

 Office of Pesticide Programs

TO: Veronique LaCapra, Chemical Review Manager

Reregistration Branch 2

Pesticide Re-Evaluation Division

Office of Pesticide Programs

Attached is the preliminary problem formulation, a part of the registration review, for the 

insecticide Chlorpyrifos-methyl.
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REGISTRATION REVIEW

PROBLEM FORMULATION FOR:

CHLORPYRIFOS-METHYL

Chlorpyrifos-methyl (CAS 5598-13-0)

PREPARED BY: 

Donna Judkins, Ph.D., Biologist

Mark Corbin, Senior Environmental Scientist 

APPROVED BY: 

Dana Spatz, Branch Chief

Environmental Risk Branch 3

Environmental Fate and Effects Division

Office of Pesticide Programs
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STRESSOR SOURCE AND DISTRIBUTION

Chlorpyrifos-methyl is an insecticide used to target and kill a variety of insects including beetles, 

weevils, moths, and grain borers. Chlorpyrifos-methyl has two current registrations (EPA Reg # 

264-992 and 62719-42) for treatment of grains in storage bins and warehouses. Chlorpyrifosmethyl is applied indoors with one application per year by sprayer or automatic sprayer at a rate 

of 0.0253 lbs ai/1,000 square feet. Based on the most recent usage data provided by BEAD 

chlorpyrifos-methyl has been applied predominantly on stored grains with approximately

110,000 lbs applied annually. 

Table 1. Chemical Registration Data Summary

Common name Chlorpyrifos-methyl 

Chemical name Chlorpyrifos-methyl [C7H7Cl3NO3PS]

CAS number 5598-13-0

Pesticide type Insecticide

PC Code 059120

EPA Registration 

Numbers

264-992 (Bayer Crop Science STORCIDE II (21.6% active)) & 62719-42 

(Dow AgroSciences Reldan* F Insecticidal Chemical (97% active)) 

INTEGRATION OF AVAILABLE INFORMATION

The documents available in the Office of Pesticide Programs Regulatory Docket and which serve 

as the basis for this problem formulation include the following:

 1980 Ecological Risk Assessment

 December 2000 Tolerance Reassessment

 October 2000 Pesticide Fact Sheet

 December 2007 Product Cancellation Order

ECOLOGICAL EFFECTS

No ecotoxicological data have been required by EPA in past assessments of chlorpyrifos-methyl 

since all uses were indoor. However, according to product labels, all end-use products are toxic 

to extremely toxic to fish, birds and other wildlife; therefore, it is not allowed to be used on or 

near water or on seed to be fed to birds or wildlife. Data from earlier risk assessments indicate 

that chlorpyrifos-methyl is practically nontoxic to moderately toxic to avian wildlife based on 

dosing studies (LD50, 100->2000 mg/Kg); practically nontoxic to slightly toxic to avian wildlife 

based on dietary studies (LC50, 1835->5000 mg/L); moderately toxic to very highly toxic to fish 

based on acute data (LC50, 0.014-4.58 mg/L); and slightly toxic to very highly toxic to aquatic 

invertebrates based on acute tests (LC50, 0.00017-45.8 mg/L); and very highly toxic to honeybees 

based on acute studies (LD50, 0.088-0.383 μg/bee). 
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EXPOSURE CHARACTERISTICS

No acceptable environmental fate data have been submitted to EPA for chlorpyrifos-methyl. The 

environmental fate and metabolism of chlorpyrifos-methyl are expected to be similar to the 

environmental fate and metabolism for chlorpyrifos. Previously, data summaries were prepared 

by EFED that present overviews of the environmental fate as captured in open literature. These 

data suggest that chlorpyrifos-methyl is relatively non-persistent under biotic (aerobic soil 

metabolism and field dissipation studies) and abiotic conditions (hydrolysis) based on both 

laboratory and field data with half lives generally less than 30 days. An exception to this is that 

hydrolysis half lives are longer (i.e., up to 76 days) at colder temperatures (i.e., 15 degrees 

Celsius) and is more stable under acidic conditions. Finally, chlorpyrifos-methyl is relatively 

immobile with Koc’s between 3,000 and 4,500. 

Chlorpyrifos-methyl does not appear in any of the surface or groundwater monitoring data sets 

reviewed, however, this could be due to the fact that most monitoring programs are not analyzing 

for it. 

Table 2. Physical and Chemical Properties of Chlorpyrifos-methyl

Empirical formula C7H7Cl3NO3PS

Molecular mass (g/mol) 322.6

Vapor pressure 4.22 x 10-5 mm Hg at 25 oC

Solubility in water 4 mg/L

Kow 4.3

pKa NA

Hydrolysis half-life 3 days to 76 days

ANALYSIS PLAN 

Chlorpyrifos-methyl is used as an indoor treatment for small grains in storage bins and 

warehouses. The product is applied by manual and automatic sprayer in self-contained units 

applied in discrete locations with a spatial extent limited to building size areas with negligible 

potential for offsite transport. The application method does not allow for drift to terrestrial areas 

or to water bodies. The units are not intended for moist areas and do not contain amounts of 

active ingredient sufficient for runoff to be a concern. Direct or indirect exposure to humans or 

ecological receptors is not expected. If used as directed on the product label, then the likelihood 

of exposure to non-target wildlife is minimal. Therefore, ecological exposure and effects are not 

expected from this use. In addition, the use pattern is unlikely to result in exposure in aquatic 

resources or groundwater and thus drinking water is unlikely to be impacted by this use.

According to 40 CFR Part 158, avian oral toxicity, freshwater fish acute toxicity and freshwater 

invertebrate acute toxicity data are conditionally required for the indoor use pattern, depending 

on whether or not there is a potential for environmental exposure. Since the use of the currently

registered products containing chlorpyrifos-methyl is not expected to result in environmental 

exposure, no additional ecological effects data are required.
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The Agency anticipates that no quantitative ecological risk assessment or drinking water 

assessment will be needed. Because these uses are indoors, it is unlikely that listed species will 

be impacted and thus a quantitative endangered species assessment will not be conducted.