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Overview of the Preliminary Cumulative Organophosphorus Risk Assessment

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 Although the information provided here was accurate and current when first created, it is now outdated.

PESTICIDES AND USES INCLUDED IN THE ASSESSMENT

EPA'S APPROACH TO CUMULATIVE RISK ASSESSMENT

THE FOOD ASSESSMENT

THE RESIDENTIAL ASSESSMENT

THE WATER ASSESSMENT

THE PRELIMINARY CUMULATIVE ASSESSMENT

RISK CHARACTERIZATION

FUTURE WORK

REGIONAL ASSESSMENTS

December 3, 2001: EPA has completed the preliminary cumulative risk assessment for the organophosphorus pesticides. The assessment is a preliminary view of the results of a new way of analyzing data about potential exposure to multiple pesticides with the same mechanism of toxicity. The Agency's methods result in measurements of the probability of exposure to more than one organophosphorus pesticide. While we are evaluating the results of these analyses, it is too soon to draw conclusions about risks or consider risk management possibilities.

This preliminary assessment introduces several advances in risk assessment methods, particularly in the hazard, drinking water, and residential components, as well as in methods of combining pathways to produce a total risk profile. EPA in cooperation with the U.S. Department of Agriculture (USDA) has developed a set of followup analyses to assist in interpretation of results and to prepare a revised organophosphorus pesticide cumulative risk assessment. In addition, new information from the public comment period and any further risk mitigation measures taken after November 2001 will be incorporated.

The risk mitigation measures that have already been taken on individual members of this group of pesticides have led to significant reduction in potential risk, and EPA is continuing to address risks as they are identified for individual organophosphorus pesticides. EPA has confidence in the continued safety of our food supply and emphasizes the importance of eating a varied diet rich in fruits and vegetables. Drinking water appears to not be a significant contributor to risk from the use of organophosphorus pesticides. Although most indoor uses of organophosphorus pesticides have been eliminated through earlier risk reduction actions, some remaining uses may be of concern.

PESTICIDES AND USES INCLUDED IN THE ASSESSMENT

When EPA began the cumulative assessment process there were 39 cholinesterase-inhibiting organophosphorus pesticides registered by EPA. The cumulative assessment takes into account regulatory actions that EPA has taken on several pesticides, such as eliminating uses. Eight pesticides were excluded from the assessment because they are being phased out based on specific legal agreements or have negligible, if any, exposures according to their individual risk assessments. The pesticides considered in this analysis were selected based on their detection in USDA's Pesticide Data Program (PDP), which collects monitoring data on pesticide residues in commonly eaten children foods such as fruits, vegetables, and milk, as well as their potential for human exposure through residential or non-occupational uses and/or drinking water.

The table below shows the pesticides considered in the preliminary cumulative assessment of potential organophosphorus exposure. The letters following the names indicate in which assessment(s) they appear, food (F), water (W), residential (R).

The following pesticides were excluded from the assessment:

Pesticides Considered in the Preliminary Cumulative Risk Assessment
  • acephate-F,W,R
  • azinphos-methyl-F,W
  • bensulide-W,R
  • chlorethoxyfos-W
  • chlorpyrifos-F,W
  • chlorpyrifos-methyl-F
  • disulfoton-F,W,R
  • diazinon-F,W
  • dichlorvos (DDVP)-F,W,R
  • dicrotophos-W
  • dimethoate-F,W
  • ethoprop-F,W
  • fenamiphos-F,W,R
  • fenthion-R
  • malathion-F,W,R
  • methamidophos-F,W
  • methidathion-F,W
  • methyl parathion-F,W
  • mevinphos-F
  • naled-W,R
  • oxydemeton-methyl (ODM)-F,W
  • phorate-F,W
  • phosolone-F
  • phosmet-F,W
  • phostebupirim-W
  • pirimiphos methyl-F
  • profenofos-W
  • terbufos-F,W
  • tetrachlorvinphos-R
  • tribufos-F,W
  • trichlorfon-R

EPA'S APPROACH TO CUMULATIVE RISK ASSESSMENT

The Food Quality Protection Act (FQPA) has challenged EPA and its stakeholders to consider the combined toxic effects of chemicals that have a common mechanism of toxicity. This statutory challenge has required development of new methods and tools for conducting cumulative risk assessment. EPA has relied on reviews by the FIFRA Scientific Advisory Panel as well as the work of its own scientists and many others within and outside the federal government to ensure that the Agency is using appropriate methods and sound science.

EPA's approach to cumulative risk assessment relies on a careful review of the data on toxicity of individual pesticides and information on potential for exposure. EPA's preliminary cumulative risk assessment is data-rich, and EPA has high confidence in the data. EPA has considered all available data in this preliminary assessment. For example, EPA worked with USDA to increase the data on children in the Continuing Survey of Food Intake by Individuals. Now, there are about 16,000 days of diet information for children age under 1 to 11, which provides sufficient variety to ensure that analysis of potential exposure is based on realistic values, not outliers. Each component of the risk assessment uses the best available data.

Before beginning the cumulative assessment process, EPA generally assesses risks associated with individual pesticides. For each individual pesticide, EPA performs an aggregate risk assessment considering exposures to the pesticide from food, drinking water, and residential. At this stage, if risks exceed EPA's level of concern, the Agency takes steps to reduce risks associated with the chemical to acceptable levels. EPA's revised guidance on aggregate risk assessment ("General Principles for Performing Aggregate Exposure and Risk Assessment") is available on the web at www.epa.gov/pesticides/trac/science.

Identify Pesticides with a Common Mechanism

Under FQPA, EPA is required to consider pesticides that have a common mechanism of toxicity when making decisions about whether pesticide uses meet the current safety standard. Once a common mechanisn has been identified, these pesticides are called a "common mechanism group." Such a group consists of chemicals for which scientifically reliable data demonstrate that the same toxic effect occurs in or at the same organ or tissue by essentially the same sequence of major biochemical events. EPA guidance on the process for identifying whether chemicals have a common mechanism ("Guidance for Identifying Pesticide Chemicals and Other Substances that Have a Common Mechanism of Toxicity") was issued on February 5, 1999 (64 FR 5795) or on the web at www.epa.gov/pesticides/trac/science.

Overview of the Process

Perform a Cumulative Assessment

After establishing the common mechanism group, EPA performs the cumulative risk assessment in four steps:

1. Hazard Assessment and Characterization-For the cumulative assessment, hazard assessment focuses on the potential for health effects from exposure to multiple pesticides that share the common mechanism of toxicity, including consideration of conditions that will allow the effects to cumulate and whether specific subgroups might have increased sensitivity to the common toxic effect.

2. Dose-Response Assessment and Characterization-This step determines the relative toxic strength of each pesticide included in the assessment and establishes a dose that is used to estimate the potential joint risk of this group.

3. Exposure Assessment and Characterization-This step assesses who is exposed, how they are exposed, and how much of the pesticide people are exposed to through food, drinking water, and various non-agricultural uses, such as use in and around the home. For the cumulative assessment, EPA will assess the potential for humans to be exposed to multiple members of the common mechanism group and whether there are regional or subpopulation concerns.

4. Risk Characterization-This step identifies the major pesticide contributors or sources of the cumulative risk, and any sub-populations that are at increased risk. This step also evaluates the confidence in results as well as the uncertainties and assumptions.

The first two steps include use of a weight-of-the-evidence approach to determine the harmful effect that occurs through a common mechanism of toxicity and to establish a common measure of toxic potency. A weight-of-the-evidence approach involves reviewing all pertinent data and information, assessing the strengths and weaknesses of the data, and reaching conclusions based on the overall picture provided by the data, rather than based on any one specific study. The Relative Potency Factor method is one way of comparing toxic potency. In this method, once the absolute potency of each member of the common mechanism group is determined, one pesticide is selected as the "index" chemical (generally the one for which the Agency has the best information on effects at a variety of doses). The Agency then compares the other chemicals to it to determine the relative potency of each (e.g., one might be half as toxic as the index chemical, while another might be twice as toxic).

Steps 3 and 4 include estimating exposure and risks for the food, drinking water, and residential/non-occupational pathways and combining the exposures to yield an estimate of the joint risk.

EPA published draft guidance on conducting cumulative risk assessments in June 2000. The revised guidance will be available at www.epa.gov/pesticides/trac/science when it is published. In the process of developing the methods for cumulative risk assessment, EPA consulted outside experts, obtained scientific peer review, and sought and considered public comment on many occasions.

Selection of the effect of concern (endpoint)

EPA selected inhibition of acetylcholinesterase in brain as the effect of concern or endpoint for this preliminary cumulative risk assessment. Inhibition of brain cholinesterase is known to be a harmful effect of organophosphorus pesticide exposure in both humans and laboratory animals. Organophosphorus pesticides affect the nervous system by reducing the ability of cholinesterase to control acetylcholine. Acetylcholine helps transfer nerve impulses from a nerve cell to a muscle cell or another nerve cell. If acetylcholine is not properly controlled by cholinesterase, the nerve impulses or neurons remain active longer than they should, overstimulating the nerves and muscles. Cholinesterase is active (and can be inhibited) in brain tissue and peripheral nerve tissue. Using the brain tissue to determine response is more direct and does not introduce any error or uncertainty due to extrapolation between a surrogate tissue (red blood cells or plasma) and the target tissue (brain or peripheral nerve tissue). The data from measurements of cholinesterase inhibition in brain tissue have less variability than measurements of red blood cell or plasma, which allows EPA to be more confident about their accuracy. (The statistical term for this is "confidence limits," which are shown as bars on either side of a data point on a graph.)

Relative potencies and dose addition

One of the many challenges in conducting a cumulative risk assessment is determining how to place the pesticides on a common footing so the exposures can be combined. EPA resolved this issue by developing "relative potency factors." This is the ratio of the toxic strength of any pesticide to the strength of an "index chemical." EPA uses relative potency factors to convert exposures of all chemicals in the group into exposure equivalents of the index chemical.

Because of its high quality dose response data for all routes of exposure, EPA selected methamidophos as the Index Chemical for standardizing the toxic potencies and calculating relative potency factors for each OP. The high quality response data for methamidophos permits reliable estimates of the point in the dose-response curve for each route of exposure at which a change in response can be reliably said to be due to dosing with the chemical. EPA uses these point of departure values with exposure information to estimate potential joint risk of these chemicals to humans.)

Relative Potency Factors for the Oral, Dermal, and Inhalation Routes of Exposure

Chemical Oral Dermal Inhalation
RPFs based on brain cholinesterase activity measured from female rats.
Acephate 0.13 0.0025 0.208
Azinphos-methyl 0.092    
Bensulide 0.003 0.0015  
Chlorpyrifos 0.10    
Chlorpyrifos-methyl 0.012    
Diazinon 0.024    
Dichlorvos 0.037   0.677
Dicrotophos 1.95    
Dimethoate 0.33    
Disulfoton 1.23 0.47 6.596
Ethoprop 0.049    
Fenamiphos 0.039 1.5 0.315
Fenthion 0.35 0.015  
Fosthiazate 0.16    
Malathion 0.0003 0.015 0.003
Methamidophos 1.00 1.00 1.000
Methidathion 0.37    
Methyl-parathion 0.058    
Mevinphos 1.36    
Naled 0.083 0.075 0.820
ODM 0.90    
Phorate 0.39    
Phosalone 0.024    
Phosmet 0.020    
Pirimiphos-methyl 0.029    
Terbufos 0.84    
Tetrachlorvinphos 0.0008 0.00075  
Tribufos 0.045    
Trichlorfon 0.014 0.0075 0.087

Note: EPA included fosthiazate, a new chemical that might be considered for future registration (methyl bromide alternative), in the hazard assessment, but it is not considered in the preliminary cumulative risk assessment. Two pesticides included in the preliminary risk assessment, phostebupirim and profenofos, are not quantified in the hazard assessment, but their contribution to drinking water risk is very low, based on use of artificially high relative potency factors.

Monitoring data show that people generally have had some level of OP exposure, making it unlikely that any individual would encounter exposure to OP pesticides without having a previous exposure from other sources. Therefore, EPA does not consider the use of single-day exposures to be reflective of the actual human exposure situation. Furthermore, the effects of OP exposure can persist for several days to weeks depending on the magnitude of exposure, making the exposed individual potentially more vulnerable to subsequent exposures during that period. Data from studies on rats showed that for most pesticides, the cholinesterase level did not change appreciably after about 21-30 days of exposure, making the determination of relative potencies more reliable. Thus, EPA chose to include data from exposure of rats of 21 days or longer.

This assessment should not be categorized in the traditional terms of acute, chronic, or subchronic. In the past, there had to be clear lines between different time periods due to the nature of the computer programs used to make exposure calculations. Now, with better software available, timeframes can be more flexible, to reflect more real-world exposure situations.

Relative potency factors for the inhibition of female brain cholinesterase activity of OPs

(error bars are 95% confidence limits)

chart of relative potency factors for female brain cholinesterase inhibitor data

THE FOOD ASSESSMENT

EPA's assessment of exposure to organophosphorus pesticides from food is highly refined and is based solely on monitoring data. It provides a view of the potential exposure of various population groups to this group of pesticides.

Pesticides and Crops Included and Excluded

EPA conducted the food assessment nationally because food is generally distributed nationally. The assessment includes only chemical/crop combinations currently registered in the U.S. or with import tolerances; it does not include residues that violate tolerance requirements or uses that have been canceled or phased out based on the single-chemical assessments. The OPs that are not included in the assessment meet one of these criteria:

The foods included in the cumulative dietary exposure assessment come from the USDA Continuing Survey of Food Intake by Individuals (CSFII) for the years 1994-1996 with supplementary data on children obtained in 1998. (Table III.C.1 in the preliminary risk assessment appendix lists all of the foods in CSFII 1994-1998 in decreasing order of their relative per capita consumption by children 3-5 years old.) Based on available sources of information, EPA considered foods accounting for about 96% of the diets of young children.

The food forms not included in the current assessment account for almost 4% of the per capita consumption distributed among many food forms. Table I.C.1 summarizes the above discussion on inclusion of foods in the assessment and the information detailed in Table III.C.1.

The Proportion of the Diet of Children (3-5 years old)
Covered in the Cumulative Food Assessment

Source of Residue Estimate Percent of Per Capita Consumption
PDP 85.7
Translation of PDP 1.3
FDA Monitoring and TDS 6.3
Assumed Negligible 3.1
Not Included in Current Assessment 3.6

Children have some special considerations

There are two issues unique to children that should be noted. First, EPA has not incorporated any organophosphorus pesticide exposure from breast milk in the risk assessment. EPA reviewed literature to determine whether there is potential for transfer of organophosphorus pesticides to human breast milk and found no evidence that this is a concern. Organophosphorus pesticides also are not found in cow's milk when cattle are fed a diet containing organophosphorus pesticides.

Second, EPA assumed that all baby food is prepared at home. This may overestimate the potential exposure for young children consuming commercial baby food, since some manufacturers prohibit residues of organophosphorus pesticides on fruits and vegetables that will be used in its products.

Results

THE RESIDENTIAL ASSESSMENT

EPA has for the first time conducted a probabilistic assessment of residential exposure to pesticides. EPA has used the proprietary calendar-based model (Calendex) to address the temporal aspects of the residential use of pesticides in 12 distinct geographic regions throughout the United States. These regions, based on major crop growing areas and their influence on surface and ground water, also present an opportunity to consider the unique climate patterns, pest patterns, and potential socioeconomic patterns that influence residential pesticide use and expected exposure.

Pesticides and Uses Included and Excluded

The residential assessment includes the potential for exposure to pesticides from uses in and around the home, parks, and schools, as well as from exposures from golf courses where pesticides have been used. It also includes the wide-area application of pesticides by government officials for public health reasons (e.g., mosquito control). Residential exposure occurs via inhalation or dermal exposure in most cases. Non-dietary oral exposure also is possible, due to hand-to-mouth behavior by children.

Before the reassessment process began, 17 OPs had registered uses in residential and public areas. EPA has excluded seven of these from the residential assessment since residential uses have been eliminated: chlorpyrifos, diazinon, dimethoate, ethoprop, fenitrothion, phosmet, and propetamphos. Six of the OPs have completed individual risk mitigation; the cumulative assessment reflects the most up-to-date residential use picture for these chemicals: acephate, bensulide, disulfoton, naled, and trichlorfon. Five of the 11 remaining OPs are still in the assessment process, and any future risk mitigation actions will be incorporated into the revised cumulative assessment: dichlorvos (DDVP), fenamiphos, malathion, fenthion, and tetrachlorvinphos.

Two OP pesticides are currently registered for use on pets, tetrachlorvinphos (shampoo/dip) and DDVP (flea collars). EPA did not have sufficient data on exposure for these uses to include them in a calendar based probabilistic assessment. The screening-level assessments for these uses indicate risks of concern. EPA is identifying exposure data that could help refine the assessments. For example, EPA needs data on the fate of the pesticide on the fur and skin of the animal and data on transfer of the pesticide during the contact with humans. The exposure assessment for these products also needs to take into account the frequency of pesticide use on pets and the transfer of residues from the pet to residential surfaces.

Other OP uses were not included because they resulted in low exposures or because their single chemical REDs showed low risk. These low exposure uses include ant baits, paint additives and post application residential exposure from sod farm application of pesticides. Ant baits are contained inside enclosed packages. The treatment of individual fire ant mounds has very low applicator exposure and reentry or significant play on fire ant mounds is unlikely.

Use of Data in the Assessment

EPA used several reliable data sources to define how pesticides are used, dissipation of pesticide residues, how people may come into contact with pesticides (e.g., via dermal or inhalation exposure), and the length of time people might be exposed based on certain activities (e.g., playing on a treated lawn). The primary sources of data for the residential/non-occupational component are:

EPA employs data on pesticide use to determine, for example, how many households use a pesticide, when pesticide treatments are made and how often, and how long exposure continues. Use data are also important in identifying geographical regions where the pesticide will be applied. Pesticide residue dissipation data address the fate of the pesticides once applied to an environment (e.g., lawns). Exposure contact data are exposure-specific measures that relate human exposure to pesticide residues-humans are exposed to the residues by contacting the product directly or by contacting the residues left after the pesticide applications are made. Human exposure factors include breathing rates, body weight, and body surface area.

EPA has incorporated data in the cumulative risk assessment that have not been used previously, such as data on time children and adults spend performing certain activities. The results of these studies allowed EPA to develop uniform distributions for children and adults for use in the risk assessment scenarios. EPA also obtained data to evaluate non-dietary ingestion through hand-to-mouth behavior in young children, although it remains difficult to quantify the contribution of this route to total exposure.

Results

Residential exposures are the major source of risk at the higher percentiles of population exposure.

Inhalation exposure from residential exposures is a significant contributor to risk for all population groups, largely from no-pest strips and crack and crevice treatments using DDVP. (Note, however, that this assessment does not include recent label changes that may alter estimated risks from the pest strip use.)

Hand-to-mouth exposure to children is a major source of risk to children in conjunction with lawn scenarios, but there is significant uncertainty associated with these exposure estimates.

THE WATER ASSESSMENT

The regional water exposure assessments are representative of exposures from typical OP usage conditions at one of the more vulnerable surface watersheds in the region. Each regional assessment focuses on areas where combined OP exposure is likely to be among the highest within the region as a result of total OP usage and vulnerability of the drinking water sources. In this manner, EPA is confident that if the regional cumulative risk assessment finds that exposure in water is not a significant contributor to the overall OP exposure in that area, it will not be a significant contributor in other areas in the region. EPA has used monitoring data when available to corroborate the results of the assessment.

Pesticides and Uses Included in the Assessment

The cumulative water exposure assessment includes 24 pesticides plus the potentially toxic transformation products of 20 of the pesticides. The water assessment focuses on crops grown and pesticides used in the area that is the focus of the regional water assessment. These are listed in the table at the end of this document.

Use of Data in the Assessment

The primary sources of data for the water component of the assessment:

Evidence from the available monitoring studies confirms that OP pesticides occur in drinking water sources. The frequency of detections is generally low, except for chlorpyrifos, diazinon, and, in some instances, malathion. The magnitude generally ranges from sub-parts per billion to a few parts per billion. Targeted monitoring studies have higher frequencies of detection of OPs.

OP pesticides can occur together in the same water source at the same time. Chlorpyrifos, diazinon, and malathion are the three OP pesticides most often detected. However, other OP pesticides may also occur with one or more of these three in local areas. The USGS National Ambient Water Quality Assessment detected multiple OP pesticides in the same water samples at the same time in almost all of its study units.

In general, surface water sources are more likely to be vulnerable to OP contamination than are ground water sources. OP pesticides are found in streams draining through predominantly urban/residential as well as agricultural watersheds. Chlorpyrifos, diazinon, and malathion are frequently detected in urban streams. While the residential uses of chlorpyrifos and diazinon are being canceled, residential uses for malathion remain, as do agricultural uses for all three.

Scenarios in the Assessment

Selecting a location within the region for drinking water assessment

Each region in the assessment is represented by a geographic area with the highest apparent potential for cumulative exposure to OPs in drinking water. Each selected geographic area has a relatively high usage of multiple OP pesticides (in relation to other parts of the region) coinciding with surface and/or ground water sources of drinking water that are vulnerable to potential contamination by these OPs. Because OP usage varies within the region, the initial evaluation focused on the areas of highest pesticide use. These areas are dependent upon the crops grown, which OP(s) are used on these crops, how much OP pesticides are applied, and when they are used. Since the purpose of the assessment is to identify the effects from multiple OPs occurring in water in the same area, the area(s) selected for the assessment do not necessarily represent the highest exposure of a single chemical, but rather the highest multiple OP exposure within the region.

First, EPA identified the high OP usage areas and high agricultural intensities within each region; these are shown on a national scale in Figure I.E-4. Next, in each high usage area within the region, EPA determined the types and locations of drinking water sources. The final step in choosing a location is to assess the vulnerability of drinking water sources within the high usage area within the region. EPA compared locations of surface drinking water intakes on runoff vulnerability maps (Figure I.E-5) with the OP use areas to determine whether potentially vulnerable surface water sources of drinking water coincided with high use areas. For ground water, EPA compared OP use areas with a pesticide leaching vulnerability map (Figure I.E-6).

Cumulative Water Assessment Uses Improved Methods

The assessment of exposure to organophosphorus pesticides from drinking water provides a distribution of daily residue values over multiple years rather than the single point estimates that were used in the individual chemical assessments.

The PRZM-EXAMS/IR model

The co-occurrence of OP residues in water is primarily estimated from modeling. After considering several predictive tools, the Agency chose its paired PRZM and EXAMS models for the Index Reservoir to estimate a daily concentration of drinking water concentrations that could be used for multiple chemicals over several years of predictions across the country. EPA ran the model for each chemical-crop combination reported to be used in the counties selected for assessment.

Monitoring data are not available consistently enough to be the sole basis for the assessment. However, monitoring data are used to corroborate the modeling results. The estimated residues for each region represent typical pesticide uses and reflect seasonal variations as well as regional variations in cropping and OP use. This analysis represents a major step forward because it is based on a probabilistic modeling approach that considers the full range of data and not a single high-end estimate. EPA is confident that these estimates represent reasonable approximations of pesticide concentrations in water.

Results

The results of the preliminary OP cumulative risk assessment indicate that drinking water is not a major contributor to the total cumulative risk. Contribution to cumulative risk from drinking water is generally at least 10-100 times lower than the contribution from food at percentiles of exposure above the 95th for all population subgroups evaluated.

THE PRELIMINARY CUMULATIVE ASSESSMENT

EPA focuses its risk assessment on exposure and resulting risk to the population, not to risk to an individual. This distinction is an important one with regard to defining how the components of the assessment will be combined. Available data on food and water consumption are adequate for describing the behavior of the population over time, but they do not adequately reflect the day-to-day variability in behavior of an individual. For this reason, EPA decided to develop a series of daily exposure distributions and show them as a distribution across time.

The distribution of daily exposures and resulting Margins of Exposure (MOEs) are developed such that the exposures from OPs in foods, drinking water and from residential uses are all calculated simultaneously for each hypothetical individual in the subpopulation. (MOEs describe how far away the exposure is from the point at which the chemical begins to have effects. Risk concerns decrease with increasing MOEs.) EPA uses the Calendex software to develop the distributions and MOEs. Calendex permits incorporation of time course information with regard to residential uses of pesticides, but does not permit specific allowance for regional variability. EPA addressed this issue by running separate risk assessments for each of twelve regions of the US. The regions correspond to agronomic cropping areas and reflect climatic and soil conditions that are likely to affect pest pressure and resulting pesticide use. Regional differences in pesticide use are major considerations in appropriately estimating exposure from pesticides in drinking water and residential uses.

To generate a daily distribution of exposure for the subpopulation of interest, a consumption record is selected from the food consumption survey (CSFII) that corresponds to the age group of interest. Calendex uses this consumption record to estimate OP exposure from food by randomly assigning a residue value from PDP for each food included. After multiplying each amount of food consumed by its selected residue value, the total exposure for this individual from food is summed. At the same time, all appropriate residential scenarios that may be encountered for the calendar day 1 (January 1) are reviewed. A probability-based decision is made as to whether or not that scenario will be encountered (e.g., a lawn treatment; probably not in January in most parts of the country). If the scenario is assigned a "yes" answer, then the appropriate values defining the residential exposure selected. The exposures for dermal, oral and inhalation exposures are calculated for all selected residential scenarios. A drinking water value taken from the PRZM/EXAMS output for January 1 is selected and paired with the water consumption reported in the CSFII consumption record. These values are used to calculated exposure from drinking water for that date. All of the exposures are converted to route-specific MOEs to define the total exposure to the hypothetical individual on January 1. The process is repeated for each consumption record for the age group in the CSFII ten times to build a distribution of exposures for January 1. This process is repeated for January 2, January 3 and so forth across the year. The 365 daily exposure distributions are arrayed together in order to provide a profile of possible exposures by each route and in total as MOEs. By comparing the results of different percentiles of exposure, EPA will be able to draw conclusions about significant exposure routes and sources.

RISK CHARACTERIZATION

Risk characterization is the interpretation phase of the assessment process. Appropriate interpretation of results is especially important for an assessment as complex as the OP cumulative risk assessment. EPA has combined many types of data, derived from a variety of sources, to produce detailed estimates of risk from multiple OPs in food, drinking water, or from residential use. The outputs of the assessment should be evaluated in a variety of ways. Risk characterization identifies potential biases in input parameters, the direction of the bias, and the uncertainty surrounding the inputs and the exposure model with regard to their potential impact on the results of the assessment.

The results of the OP cumulative assessment indicate that the contribution to OP cumulative risk from drinking water is generally at least 10 times lower (one order of magnitude) than the contribution from OPs in food at percentiles of exposure above 95th for all population subgroups evaluated. As the percentile of exposure increases, the difference between the food and water contributions increases. Below the 95th percentile of exposure, the water risk comes within one order of magnitude of the food contribution. This pattern is consistent for all regions in the current risk assessment. Those regions with the lowest total MOEs at the upper percentiles in the exposure distribution generally reflect the contribution of the inhalation route of exposure from residential indoor uses of DDVP. The exposures occur from the No Pest Strips and crack and crevice treatments. This observation is consistent for all regions evaluated. The same pattern of risk from each pathway is observed for all regions. At these higher percentiles of population exposure, residential uses are the major source of risk-in particular exposure from hand-to-mouth activity by children and inhalation exposure by all age groups. These patterns occur in all population sub-groups, although estimated risks appear to be higher for children than for adults regardless of the population percentile considered. EPA believes that the results of the current assessment provide a highly refined, health protective estimate of the cumulative risk to the US public from the use of OPs.

FUTURE WORK

The preliminary OP cumulative risk assessment provides a detailed picture of possible exposure to organophosphorus pesticides. Details in the assessment provide the basis to evaluate the effects of the methods and assumptions on the results of the assessment. This process is particularly important for a cumulative OP assessment because it reflects additional data compared to previous, single-chemical assessments. It uses distributions of data in place of point estimates to the extent possible, and introduces new data sources, particularly in the residential portion of the assessment. EPA has used the OP cumulative risk assessment as a vehicle to introduce a number of advances in its risk assessment methodology. These changes are most evident in the hazard, drinking water and residential components, as well as in the methods used to combine the pathways to develop a total risk profile for all of the OPs together. Therefore, EPA plans to conduct additional analyses of the data before reaching final conclusions. At this point in the planning process, EPA in cooperation with USDA has developed a set of followup analyses that will be conducted to assist in interpreting the results of the preliminary analysis, and to prepare an OP cumulative risk assessment for making regulatory decisions. Some examples of planned analyses are:

As noted previously in this document, incorporation of new information submitted during the comment period that will serve to improve the accuracy of the assessment will be a goal of activities to complete the assessment. In addition, further mitigation activities will be incorporated.

REGIONAL ASSESSMENTS

EPA conducted the drinking water and residential assessments for each of 12 regions. The regions are shown in the following table, along with information about the uses and pesticides that formed the basis for the drinking water and regional assessments in each region.

Regional Basis for the Water and Residential Assessments-Crops/Uses and Pesticides

Region Water Exposure-Crops Selected as Primary OP Contributors Pesticides Represented in the Selected Crops Residential-Scope and Registrations
Region 1-Heartland (for water assessment, Central Illinois)
(includes Illinois, Indiana, most of Ohio, Iowa, Minnesota, Nebraska, South Dakota, parts of Western Kentucky, and most of Missouri)
Corn Terbufos
Chlorethoxyphos
Chlhorpyrifos
Tebupirimphos
  Lawn: DDVP, Malathion, Trichlorfon
Golf course
: Bensulide, Trichlorfon
Gardens: Malathion, Acephate, Disulfoton
Indoor: DDVP
Region 2-Northern Crescent
(for water assessment, South Central Pennsylvania)
(includes Maine, Vermont, New Jersey, New Hampshire, Massachusetts, Connecticut, New York, Rhode Island, most of Pennsylvania and parts of Maryland, part of Ohio)
Apple
Pear
Peach
Corn
Alfalfa
Pumpkin
Cantaloupe
Corn
  Chlorpyrifos
Dimethoate
Azinphos-methyl
Diazinon
Terbufos
Methyl parathion
Phosmet
Methidathion
Tebupirimphos
  Lawn: Malathion, Trichlorfon
Golf course
: Bensulide, Trichlorfon
Indoor
: DDVP
Gardens
: Acephate, Disulfoton, Malathion
Public health
:
Malathion, Naled
Region 3-Northern Great Plains
(for water assessment, Red River Valley-Minnesota, North Dakota)
(includes North Dakota, most of South Dakota, parts of Nebraska, parts of Minnesota, Colorado, and Montana)
Corn
Wheat
Sugar Beet
Potato
  Chlorpyrifos
Dimethoate
Azinphos- methyl
Phorate
Terbufos
  Lawn: DDVP, Trichlorfon
Golf course: Bensulide, Malathion
Gardens: Acephate, Disulfoton, Malathion
Indoor: DDVP
Region 4-Prairie Gateway
(for water assessment, Central Hills, Texas)
(includes Kansas, part of Texas, Oklahoma, New Mexico, Colorado, and Nebraska)
Cotton
Corn
Alfalfa
Wheat
Potato
Sorghum
  Acephate
Azinphos-methyl
Chlorpyrifos
Dicrotophos
Malathion
Methyl parathion
Phorate
Phostebupirim
Terbufos
Tribufos
Lawn: Bensulide, DDVP, Malathion, Trichlorfon
Golf course: Acephate, Bensulide, Fenamiphos, Malathion, Trichlorfon
Public health: Malathion
Gardens: Acephate, Disulfoton, Malathion
Indoor: DDVP
Region 5-Eastern Uplands
(for water assessment, Western North Carolina)
(includes West Virginia, most of Tennessee, part of Virginia, parts of North Carolina, most of Kentucky, Alabama, Ohio, Pennsylvania, Arkansas, Oklahoma, Missouri, and Kentucky)
Apple
Alfalfa
Corn
  Azinphos-methyl
Chlorpyrifos
Dimethoate
Methyl parathion
Phosmet
Terbufos
  Lawn: DDVP, Malathion, Trichlorfon
Golf course: Acephate, Bensulide, Fenamiphos, Malathion, Trichlorfon
Gardens: Acephate, Disulfoton, Malathion
Indoor: DDVP
Region 6-Southern Seaboard
(for water assessment, East Coastal Plain, North Carolina)
(includes most of Virginia, North Carolina, South Carolina, Alabama, and Georgia; part of Arkansas, Mississippi, Louisiana, Texas, Maryland, and Delaware)
Cotton
Tobacco
Corn
Peanuts
  Acephate
Chlorpyrifos
Dimethoate
Disulfoton
Ethoprop
Fenamiphos
Phorate
Terbufos
Tribufos
  Lawn: Malathion, Trichlorfon
Golf course: Acephate, Bensulide, Fenamiphos, Trichlorfon
Public health: Malathion
Gardens: Acephate, Bensulide, Disulfoton, Malathion
Indoor: DDVP
Region 7a-Fruitful Rim, CA
(for water assessment, North Central Valley, California)
(includes most of California and Arizona)
Corn
Alfalfa
Almond
(Walnut)
Pear
Peach
Apricot
Nectarine
Plum
Apples (Pear)
Broccoli
Grape
Asparagus
Melons
Cucumber
Pumpkin
Squash
Tomato
Dry beans
Sugar Beet
Cantaloupe
Acephate
Azinphos-methyl
Chlorpyrifos
Diazinon
Dimethoate
Disulfoton
Fenamiphos
Fonofos
Malathion
Methamidophos
Methidathion
Methyl parathion
Naled
ODM
Phorate
Phosmet
Lawn: Malathion, Trichlorfon
Golf course: Bensulide, Fenamiphos, Trichlorfon
Indoor: DDVP
Gardens: Acephate, Disulfoton, Malathion
Region 7b-Fruitful Rim, CA (for water assessment, South Central Valley, California)
(includes most of California and Arizona)
Cotton
Citrus
Alfalfa
Almond
(Walnut)
Apple
(Pear)
Orange
Melon
Cantaloupe
Peach
Apricot
Nectarine
Plum
Grape
Sugar Beet
Lettuce
Prune
Broccoli
Acephate Azinphos-methyl
Bensulide
Chlorpyrifos
DDVP
Diazinon
Dimethoate
Disulfoton
Fenamiphos
Malathion
Methamidophos
Methidathion
Methyl parathion
Naled
ODM
Phorate
Phosmet
Profenofos
Tribufos
Lawn: Malathion, Trichlorfon
Golf course: Bensulide, Fenamiphos, Trichlorfon
Indoor: DDVP
Gardens: Acephate, Disulfoton, Malathion
Region 8-Basin and Range
(includes Nevada, Utah, most of Oregon, part of California, Washington, Idaho, Wyoming, Colorado, New Mexico, Arizona, and Montana)
Region 3 information used. No specific watershed was representative of the region. Region 3 information used. No specific watershed was representative of the region. Lawn:Malathion, Trichlorfon
Golf course: Bensulide, Trichlorfon
Indoor: DDVP
Gardens: Acephate, Disulfoton, Malathion
Region 9-Mississippi Portal (for water assessment, Southern Louisiana)
(includes part of Arkansas, Louisiana, Mississippi, and Tennessee)
Cotton
Corn
Soybean
  Acephate Chlorpyrifos
Dicrotophos
Dimethoate
Disulfoton
Malathion
Methamidophos
Methyl parathion
Phorate
Profenofos
Terbufos
Tribufos
Phostebupirim
Tebupirimphos
Lawn: Malathion, Trichlorfon
Golf course: Acephate, Trichlorfon, Malathion
Public health:
Malathion, Fenthion
Gardens: Acephate, Disulfoton, Malathion
Indoor: DDVP
Region 10-Fruitful Rim, NW
(for water assessment, Willamette Valley, Oregon)
(includes most of Washington, part of Oregon, part of Idaho)
Apple
Pear
Cherry
Pea
Broccoli
Cauliflower
Cabbage
Nursery (Trees and Shrubs)
Blackberries
Blueberries
Raspberries
Cucumber
Squash
Onion
Sweet Corn
Mint
Hazelnut
Sweet and Tart Cherries
Christmas Trees
Snap Beans
Acephate
Azinphos-methyl
Bensulide
Chlorpyrifos
Diazinon
Dimethoate
Disulfoton
Ethoprop
Malathion
Methidathion
Methyl parathion
Naled
ODM
Phosmet
Lawn: Malathion, Trichlorfon Golf course: none
Indoor: DDVP
Gardens: Acephate, Disulfoton, Malathion
Region 11-Fruitful Rim, TX (for water assessment, Central Hills, Texas)
(includes part of Texas)
Cotton
Corn
Alfalfa
Wheat
Potato
Sorghum
  Acephate
Azinphos-methyl
Chlorpyrifos
Dicrotophos
Malathion
Methyl parathion
Phorate
Phostebupirim
Terbufos
Tribufos
Dimethoate
Golf course: Acephate, Bensulide, Fenamiphos, Malathion, Trichlorfon
Indoor: DDVP
Public health: Malathion
Gardens: Acephate, Disulfoton, Malathion
Lawn: Malathion, Trichlorfon
Region 12-Fruitful Rim, FL
(for water assessment, South Central Florida)
(includes Florida, part of Georgia, and South Carolina)
Tomato
Pepper
Cucumber
Watermelon
Sweet Corn
Lettuce
Citrus
Corn
Sugarcane
Acephate Chlorpyrifos
Diazinon
Ethoprop
Methamidophos
Phorate
  Lawn: Malathion, Trichlorfon
Golf course: Acephate, Bensulide, Fenamiphos
Indoor: DDVP
Public health: Malathion, Naled
Gardens: Acephate, Disulfoton, Malathion

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