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Relative Potency Factors

Information provided for informational purposes only

Note: This information is provided for reference purposes only. Although the information provided here was accurate and current when first created, it is now outdated.

EPA's Points of Departure and Revised Relative Potency Factors for the Cumulative Risk Assessment of the Organophosphate Pesticides

The Environmental Protection Agency (EPA) has revised the relative potency factors (RPFs) included in the Preliminary Cumulative Risk Assessment for the Organophosphate Pesticides (OPs) after refining RPF calculations and analyzing new toxicology data for several OPs. EPA discussed the refined statistical procedures at the February 5-7, 2002 meeting of the FIFRA Scientific Advisory Panel (SAP), and the SAP supported the proposed refinements. The revised RPFs are included in a table below, along with the Points of Departure (PODs) for methamidophos, the index chemical for the OPs.

The revised RPFs incorporate a more accurate statistical model that better reflects the toxicology data used in the RPF calculations. Relative potency factors allow levels of exposure to different OPs to be expressed in terms that are considered toxicologically equivalent. The Agency will use these factors to determine the combined risk presented to human health from all the OPs identified in the risk assessment. Therefore, the accuracy of the potency factors are critical to the success of the assessment.

In addition, EPA has calculated RPFs for chlorethoxyphos, phostebupirim, and profenofos, to replace default RPFs utilized in the Preliminary Cumulative Risk Assessment. EPA has also received additional toxicology data for disulfoton, fenamiphos, phosalone, tetrachlorvinphos, and tribufos. New data for omethoate, the oxon metabolite of dimethoate, have been utilized to calculate an RPF not included in the Preliminary Risk Assessment.

The Points of Departure were not impacted by the revisions. A point of departure represents the dose level of a chemical below which there is no demonstrable difference between the responses of test animals exposed to the substance and animals which have not been exposed. The PODs listed in the table below are the same as those listed in figure I.B-10 on page 54 of the Preliminary Cumulative Risk Assessment's "Hazard / Relative Potency Factor" section (http://www.epa.gov/pesticides/cumulative/pra-op/i_b-f.pdf).


For additional information, please contact:

Dr. Anna B. Lowit at lowit.anna@epa.gov

The table below replaces table I.B-7 on page 43 of the Preliminary Cumulative Risk Assessment's "Hazard / Relative Potency Factor" section, available on-line at: http://www.epa.gov/pesticides/cumulative/pra-op/i_b-f.pdf

Relative Potency Factors for
Female Brain Cholinesterase Activity
Oral Dermal Inhalation
Acephate 0.08 0.0025 0.208
Azinphos-methyl 0.10
Bensulide 0.003 0.0015
Chlorethoxyfos 0.13
Chlorpyrifos 0.06
Chlorpyrifos-methyl 0.005
Diazinon 0.01
Dichlorvos 0.03 0.677
Dicrotophos 1.91
Dimethoate 0.32
Disulfoton 1.26 0.47 6.596
Ethoprop 0.06
Fenamiphos 0.04 1.5 0.315
Fenthion 0.33 0.015
Fosthiazate 0.07
Malathion 0.0003 0.015 0.003
Methamidophos 1.00 1.00 1.00
Methidathion 0.32
Methyl-parathion 0.12
Mevinphos 0.76
Naled 0.08 0.075 0.82
Omethoate 0.93
Oxydemeton-methyl 0.86
Phorate 0.39
Phosalone 0.01
Phosmet 0.02
Phostebupirim 0.22
Pirimiphos-methyl 0.04
Profenofos 0.004
Terbufos 0.85
Tetrachlorvinphos 0.001 0.00075
Tribufos 0.02
Trichlorfon 0.003 0.0075 0.087


PODs listed in the following table are the same as those presented in the original Preliminary Cumulative Risk Assessment in figure I.B-10 on page 54 of the "Hazard / Relative Potency Factor" section


Points of Departure for Methamidophos, the index chemical
Oral 0.08 mg/kg/day
Dermal 2.12 mg/kg/day
Inhalation 0.39 mg/kg/day


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