Pesticides; tolerances in food, animal feeds, and raw agricultural commodities: Imazethapyr,

[Federal Register: March 14, 2001 (Volume 66, Number 50)]

[Rules and Regulations]

[Page 14846-14852]

From the Federal Register Online via GPO Access [wais.access.gpo.gov]

[DOCID:fr14mr01-6]

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 180

[OPP-301108; FRL-6774-9]

RIN 2070-AB78

Imazethapyr; Time-Limited Pesticide Tolerance

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

SUMMARY: This regulation establishes time-limited tolerances for the combined residues of imazethapyr, as its ammonium salt, and its metabolite in or on rice, grain; rice, straw; rice hulls, and rice, bran. BASF requested this tolerance under the Federal Food, Drug, and Cosmetic Act (FFDC), as amended by the Food Quality Protection Act (FQPA) of 1996. These tolerances will expire on January 1, 2003.

DATES: This regulation is effective March 14, 2001. Objections and requests for hearings, identified by docket control number OPP-301108, must be received by EPA on or before May 14, 2001.

ADDRESSES: Written objections and hearing requests may be submitted by mail, in person, or by courier. Please follow the detailed instructions for each method as provided in Unit VI.. of the SUPPLEMENTARY INFORMATION. To ensure proper receipt by EPA, your objections and hearing requests must identify docket control number OPP-301108 in the subject line on the first page of your response.

FOR FURTHER INFORMATION CONTACT: By mail: Daniel J. Rosenblatt, Registration Division (7505C), Office of Pesticide Programs, Environmental Protection Agency, 1200 Pennsylvania Ave., NW., Washington, DC 20460; telephone number: (703) 305-5697; e-mail address: rosenblatt.dan@epa.gov.

SUPPLEMENTARY INFORMATION:

1. General Information

   1. Does this Action Apply to Me?

      You may be affected by this action if you are an agricultural producer, food manufacturer, or pesticide manufacturer. Potentially affected categories and entities may include, but are not limited to:

      Examples of Categories

      NAICS Codes

      Potentially Affected Entities

      Industry

      111............... Crop production 112............... Animal production 311............... Food manufacturing 32532............. Pesticide manufacturing

      This listing is not intended to be exhaustive, but rather provides a guide for readers regarding entities likely to be affected by this action. Other types of entities not listed in the table could also be affected. The North American Industrial Classification System (NAICS) codes have been provided to assist you and others in determining whether or not this action might apply to certain entities. If you have questions regarding the applicability of this action to a particular entity, consult the person listed under FOR FURTHER INFORMATION CONTACT.

   2. How Can I Get Additional Information, Including Copies of this Document and Other Related Documents?

      1. Electronically.You may obtain electronic copies of this document, and certain other related documents that might be available electronically, from the EPA Internet Home Page at http://www.epa.gov/. To access this document, on the Home Page select ``Laws and Regulations,'' ``Regulations and Proposed Rules,'' and then look up the entry for this document under the ``Federal Register--Environmental Documents.'' You can also go directly to the Federal Register listings at http://www.epa.gov/fedrgstr/. A frequently updated electronic version of 40 CFR part 180 is available at http://www.access.gpo.gov/ nara/cfr/cfrhtml_00/Title_40/40cfr180_00.html, a beta site currently under development.

      2. In person. The Agency has established an official record for this action under docket control number OPP-301108. The official record consists of the documents specifically referenced in this action, and other information related to this action, including any information claimed as Confidential Business Information (CBI). This official record includes the documents that are physically located in the docket, as well as the documents that are referenced in those documents. The public version of the official record does not include any information claimed as CBI. The public version of the official record, which includes printed, paper versions of any electronic comments submitted during an applicable comment period is available for inspection in the Public Information and Records Integrity Branch (PIRIB), Rm. 119, Crystal Mall #2, 1921 Jefferson

      [[Page 14847]]

      Davis Hwy., Arlington, VA, from 8:30 a.m. to 4 p.m., Monday through Friday, excluding legal holidays. The PIRIB telephone number is (703) 305-5805.

2. Background and Statutory Findings

   In the Federal Register of September 27, 2000 (65 FR 58074) (FRL- 6744-6), EPA issued a notice pursuant to section 408 of the FFDCA, 21 U.S.C. 346a as amended by the FQPA of 1996 (Public Law 104-170) announcing the filing of a pesticide petition (PP OF6186) for tolerance by American Cyanamid. This company has now merged with BASF Corporation. This notice included a summary of the petition prepared by American Cyanamid, the initial registrant. There were no comments received in response to the notice of filing.

   The petition requested that 40 CFR 180.447 be amended by establishing a tolerance for the combined residues of the herbicide imazethapyr, 2-[4,5-dihydro-4-methyl-4-(1- methylethyl)-5-oxo-1H- imidazol-2-yl]-5-ethyl-3-pyridine-carboxylic acid) as its free acid or its ammonium salt (calculated as the acid), and its metabolite 2-[4,5- dihydro-4-methyl-4-(1- methylethyl-5-oxo-1H-imidazol-2-yl]-5-(1- hydroxyethyl)3-pyridinecarboxylic acid both free and conjugated in or on the raw agricultural commodity (RAC) rice. In this time-limited tolerance rule, EPA is promulgating tolerances for rice grain at 0.3 part per million (ppm), rice straw at 0.2 ppm, rice hulls at 1.5 ppm and rice bran at 2.5 ppm. These tolerances will expire on January 1, 2003. Establishing a time-limited tolerance will permit the EPA to evaluate confirmatory data that has not yet been fully evaluated. .

   Section 408(b)(2)(A)(i) of the FFDCA allows EPA to establish a tolerance (the legal limit for a pesticide chemical residue in or on a food) only if EPA determines that the tolerance is ``safe.'' Section 408(b)(2)(A)(ii) defines ``safe'' to mean that ``there is a reasonable certainty that no harm will result from aggregate exposure to the pesticide chemical residue, including all anticipated dietary exposures and all other exposures for which there is reliable information.'' This includes exposure through drinking water and in residential settings, but does not include occupational exposure. Section 408(b)(2)(C) requires EPA to give special consideration to exposure of infants and children to the pesticide chemical residue in establishing a tolerance and to ``ensure that there is a reasonable certainty that no harm will result to infants and children from aggregate exposure to the pesticide chemical residue....''

   EPA performs a number of analyses to determine the risks from aggregate exposure to pesticide residues. For further discussion of the regulatory requirements of section 408 and a complete description of the risk assessment process, see the final rule on Bifenthrin Pesticide Tolerances (62 FR 62961, November 26, 1997) (FRL-5754-7).

3. Aggregate Risk Assessment and Determination of Safety

   Consistent with section 408(b)(2)(D), EPA has reviewed the available scientific data and other relevant information in support of this action. EPA has sufficient data to assess the hazards of imazethapyr and to make a determination on aggregate exposure, consistent with section 408(b)(2), for a tolerance for the combined residues of imazethapyr (2-[4,5-dihydro-4- methyl-4-(1-methylethyl)-5- oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridine carboxylic acid and 2- [4,5- dihydro-4-methyl-4-(1-methyethyl-5-oxo-1H-imidazol-2-yl]-5-(1- hydroxyethyl)-3-pyridine carboxylic acid (free or conjugated) on rice grain, rice straw, rice hulls, and rice bran. EPA's assessment of exposures and risks associated with establishing the tolerance follows.

   1. Toxicological Profile

      EPA has evaluated the available toxicity data and considered its validity, completeness, and reliability as well as the relationship of the results of the studies to human risk. EPA has also considered available information concerning the variability of the sensitivities of major identifiable subgroups of consumers, including infants and children. The nature of the toxic effects caused by the combined residues of imazethapyr (2-[4,5-dihydro-4-methyl-4-(1- methylethyl)-5- oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridine carboxylic acid and 2-[4,5- dihydro- 4-methyl-4-(1-methyethyl-5-oxo-1H-imidazol-2-yl]-5-(1- hydroxyethyl)-3-pyridine carboxylic acid (free or conjugated) are discussed below. The toxicological data considered in support of the proposed tolerances include:

      1. Several acute toxicology studies placing technical-grade imazethapyr in Toxicity Category III and Toxicity Category IV.

      2. A 1-year feeding study with dogs fed diets containing 1, 1,000, 5,000, or 10,000 ppm with a systemic no-observed adverse effect level (NOAEL) of 1,000 25 milligrams/kilogram/day (mg/kg/day) based on decreased packed cell volume, hemoglobin, and erythrocytes in the blood of female dogs at the 5,000 ppm 125 mg/kg/day dose level.

      3. A 78-week carcinogenicity study in mice fed diets containing 0, 1,000, 5,000, or 10,000 ppm (equivalent to 0, 150, 750, or 1,500 mg/kg/ day) with a systemic NOAEL of 5,000 ppm based on decreased body weight gain in both sexes at the 10,000 ppm dose level. No carcinogenic effects were observed under the conditions of the study.

      4. A 2-year chronic feeding/carcinogenicity study in rats fed diets containing 0, 1,000, 5,000, 10,000 ppm (equivalent to 0, 50, 250, or 500 mg/kg/day) with no treatment-related systemic or carcinogenic effects observed under the conditions of the study.

      5. A multi-generation reproduction study in rats fed diets containing 0, 1,000, 5,000, or 10,000 ppm (equivalent to 0, 50, 250, or 500 mg/kg/day) with no treatment-related systemic or reproductive effects observed under the conditions of the study.

      6. Developmental toxicity studies in rats and rabbits with no developmental toxicity observed under the conditions of the studies at dose levels up to and including the highest dose tested (HDT) (1,125 mg/kg/day in rats and 1,000 mg/kg/day in rabbits. In the rat prenatal developmental study, the maternal (systemic) NOAEL was 375 mg/kg/day based on clinical signs of toxicity seen in dams at the LOAEL of 1,125 mg/kg/day. The developmental (fetal) NOAEL was 1,125 mg/kg/day HDT. The developmental LOAEL was not established in this study. In a prenatal developmental study in rabbits, the maternal (systemic) NOAEL was 300 mg/kg/day based on maternal deaths observed at the LOAEL of 1,000 mg/ kg/day HDT. The developmental (fetal) NOAEL was 1,000 mg/kg/day HDT. In this study, the developmental LOAEL was not established. In a 2- generation rat reproduction study, the parental and offspring toxicity NOAEL was 500 mg/kg/day. The parental and offspring toxicity LOAEL was not established.

      7. Mutagenicity studies include gene mutation assays in bacteria cells (negative) and Chinese hamster ovary cells (no-dose response); structural chromosomal aberrations assays in vivo in rat bone marrow cells (negative) and in vitro in Chinese hamster ovary cells (positive without activation at levels toxic to cells and negative with activation); and other genotoxic effects (did not induce unscheduled DNA synthesis in rat hepatocytes cultured in vitro).

   2. Toxicological Endpoints

      The dose at which the NOAEL from the toxicology study identified as

      [[Page 14848]]

      appropriate for use in risk assessment is used to estimate the toxicological level of concern (LOC). However, the lowest dose at which adverse effects of concern are identified (LOAEL) is sometimes used for risk assessment if no NOAEL was achieved in the toxicology study selected. An uncertainty factor (UF) is applied to reflect uncertainties inherent in the extrapolation from laboratory animal data to humans and in the variations in sensitivity among members of the human population as well as other unknowns. An UF of 100 is routinely used, 10X to account for interspecies differences and 10X for intra species differences.

      For dietary risk assessment (other than cancer) the Agency uses the UF to calculate an acute or chronic reference dose (acute RfD or chronic RfD) where the RfD is equal to the NOAEL divided by the appropriate UF (RfD = NOAEL/UF). Where an additional safety factor is retained due to concerns unique to the FQPA, this additional factor is applied to the RfD by dividing the RfD by such additional factor. The acute or chronic Population Adjusted Dose (aPAD or cPAD) is a modification of the RfD to accommodate this type of FQPA Safety Factor.

      For non-dietary risk assessments (other than cancer) the UF is used to determine the LOC. For example, when 100 is the appropriate UF (10X to account for interspecies differences and 10X for intraspecies differences) the LOC is 100. To estimate risk, a ratio of the NOAEL to exposures (margin of exposure (MOE) = NOAEL/exposure) is calculated and compared to the LOC.

      The linear default risk methodology (Q*) is the primary method currently used by the Agency to quantify carcinogenic risk. The Q* approach assumes that any amount of exposure will lead to some degree of cancer risk. A Q* is calculated and used to estimate risk which represents a probability of occurrence of additional cancer cases (e.g., risk is expressed as 1 x 10-\6\ or one in a million). Under certain specific circumstances, MOE calculations will be used for the carcinogenic risk assessment. In this non-linear approach, a ``point of departure'' is identified below which carcinogenic effects are not expected. The point of departure is typically a NOAEL based on an endpoint related to cancer effects though it may be a different value derived from the dose response curve. To estimate risk, a ratio of the point of departure to exposure (MOEcancer= point of departure/exposures) is calculated. A summary of the toxicological endpoints for imazethapyr used for human risk assessment is shown in the following Table 1:

      Table 1.--Summary of Toxicological Dose and Endpoints for imazethapyr for Use in Human Risk Assessment

      FQPA SF* and Level of Exposure Scenario

      Dose Used in Risk

      Concern for Risk Study and Toxicological Assessment, UF

      Assessment

      Effects

      Acute dietary females 13-50 years of None

      None

      A dose and endpoint age and general population including

      attributable to a infants and children

      single exposure were not identified from the reviewed and acceptable oral toxicity studies, including both maternal and developmental toxicity in the developmental toxicity studies.

      Chronic dietary all populations

      NOAEL = 25 mg/kg/day FQPA SF = 10

      1-year feeding study-- UF = 100............... cPAD = chronic RfD/FQPA dog Chronic RfD = .25 mg/kg/ SF =.

      LOAEL = 125 mg/kg/day day.

      0.025 mg/kg/day........ based on decreased packed cell volume, hemoglobin and erythrocytes seen in females.

      Short-term dermal (1 to 7 days), and None

      None

      EPA concluded that no intermediate term dermal (1 week to

      hazard is identified several months)

      to support quantifying (Residential)........................

      risk for these exposure scenarios.

      Short-term Inhalation (1 to 7 days) Inhalation (or oral) LOC for MOE = 1,000 Rabbit Developmental (Residential)........................ study

      (Residential).......... Study NOAEL= 300 mg/kg/day

      LOAEL = 1,000 mg/kg/day (inhalation absorption

      based on maternal rate = 100%).

      deaths seen at the HDT.

      Intermediate-Term Inhalation (1 week Inhalation (or oral) LOC for MOE = 1,000 Rabbit Developmental to several months)

      study

      (Residential)

      Study (Residential)........................ NOAEL = 300 mg/kg/day

      LOAEL = 1,000 mg/kg/day (inhalation absorption

      based on maternal rate = 100%).

      deaths seen at the HDT.

      Cancer (oral, dermal, inhalation) EPA determined that a

      Rat and mouse cancer risk assessment

      carcinogenicity is not necessary.

      studies were negative for carcinogenicity.

      * The reference to the FQPA Safety Factor refers to any additional safety factor retained due to concerns unique to the FQPA.

   3. Exposure Assessment

      1. Dietary exposure from food and feed uses. Tolerances have been established (40 CFR 180.447) for the residues of the herbicide imazethapyr, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H- imidazol-2-yl]-5-ethyl-3-pyridine carboxylic acid as its ammonium salt, and its metabolite, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H- imidazol-2-yl]-5-(1-hydroxyethyl)-3-pyridine carboxylic acid, both free and conjugated, in or on a variety of raw agricultural commodities including legume vegetables, soybeans, alfalfa, peanuts, corn grain, endive, and lettuce. Risk assessments were conducted by EPA to assess dietary exposures from combined residues of imazethapyr in food as follows: i. Acute exposure. Acute dietary risk assessments are performed for a food-use pesticide if a toxicological study has

      [[Page 14849]]

      indicated the possibility of an effect of concern occurring as a result of a one day or single exposure. EPA did not perform a quantified acute dietary risk assessment. In acceptable toxicity studies, no appropriate endpoint was identified for this exposure duration. ii. Chronic exposure. In conducting this chronic dietary risk assessment the Dietary Exposure Evaluation Model (DEEM\TM\) analysis evaluated the individual food consumption as reported by respondents in the USDA 1989- 1992- nationwide Continuing Surveys of Food Intake by Individuals (CSFII) and accumulated exposure to the chemical for each commodity. The following assumptions were made for the chronic exposure assessments: The exposure assessment that supports this time-limited tolerance is conservative. Tolerance level residues, 100 percent crop treated, and default processing factors were assumed for all registered and proposed uses. Percent crop treated estimates and anticipated residue assumptions were not used in this risk assessment.

      2. Dietary exposure from drinking water. The Agency lacks sufficient monitoring exposure data to complete a comprehensive dietary exposure analysis and risk assessment for imazethapyr in drinking water. Because the Agency does not have comprehensive monitoring data, drinking water concentration estimates are made by reliance on simulation or modeling taking into account data on the physical characteristics of imazethapyr.

      The Agency uses the Generic Estimated Environmental Concentration (GENEEC) or the Pesticide Root Zone/Exposure Analysis Modeling System (PRZM/EXAMS) to estimate pesticide concentrations in surface water and SCI-GROW, which predicts pesticide concentrations in groundwater. In general, EPA will use GENEEC (a tier 1 model) before using PRZM/EXAMS (a tier 2 model) for a screening-level assessment for surface water. The GENEEC model is a subset of the PRZM/EXAMS model that uses a specific high-end runoff scenario for pesticides. GENEEC incorporates a farm pond scenario, while PRZM/EXAMS incorporate an index reservoir environment in place of the previous pond scenario. The PRZM/EXAMS model includes a percent crop area factor as an adjustment to account for the maximum percent crop coverage within a watershed or drainage basin.

      None of these models include consideration of the impact processing (mixing, dilution, or treatment) of raw water for distribution as drinking water would likely have on the removal of pesticides from the source water. The primary use of these models by the Agency at this stage is to provide a coarse screen for sorting out pesticides for which it is highly unlikely that drinking water concentrations would ever exceed human health levels of concern.

      Since the models used are considered to be screening tools in the risk assessment process, the Agency does not use estimated environmental concentrations (EECs) from these models to quantify drinking water exposure and risk as a %RfD or %PAD. Instead drinking water levels of comparison (DWLOCs) are calculated and used as a point of comparison against the model estimates of a pesticide's concentration in water. DWLOCs are theoretical upper limits on a pesticide's concentration in drinking water in light of total aggregate exposure to a pesticide in food, and from residential uses. Since DWLOCs address total aggregate exposure to imazethapyr they are further discussed in the aggregate risk sections below.

      Based on the GENEEC and SCI-GROW models the estimated environmental concentrations (EECs) of imazethapyr for acute exposures are estimated to be 6.34 parts per billion (ppb) for surface water and 2.2 ppb for ground water. The EECs for chronic exposures are estimated to be 6.13 ppb for surface water and 2.2 ppb for ground water.

      3. From non-dietary exposure. The term ``residential exposure'' is used in this document to refer to non-occupational, non-dietary exposure (e.g., for lawn and garden pest control, indoor pest control, termiticides, and flea and tick control on pets).

      Imazethapyr is not registered for use on any sites that would result in residential exposure.

      4. Cumulative exposure to substances with a common mechanism of toxicity. Section 408(b)(2)(D)(v) requires that, when considering whether to establish, modify, or revoke a tolerance, the Agency consider ``available information'' concerning the cumulative effects of a particular pesticide's residues and ``other substances that have a common mechanism of toxicity.''

      EPA does not have, at this time, available data to determine whether imazethapyr has a common mechanism of toxicity with other substances or how to include this pesticide in a cumulative risk assessment. Unlike other pesticides for which EPA has followed a cumulative risk approach based on a common mechanism of toxicity, imazethapyr does not appear to produce a toxic metabolite produced by other substances. For the purposes of this tolerance action, therefore, EPA has not assumed that imazethapyr has a common mechanism of toxicity with other substances. For information regarding EPA's efforts to determine which chemicals have a common mechanism of toxicity and to evaluate the cumulative effects of such chemicals, see the final rule for Bifenthrin Pesticide Tolerances (62 FR 62961, November 26, 1997).

   4. Safety Factor for Infants and Children

      1. In general. FFDCA section 408 provides that EPA shall apply an additional ten-fold margin of safety for infants and children in the case of threshold effects to account for prenatal and postnatal toxicity and the completeness of the data base on toxicity and exposure unless EPA determines that a different margin of safety will be safe for infants and children. Margins of safety are incorporated into EPA risk assessments either directly through use of a margin of exposure (MOE) analysis or through using uncertainty (safety) factors in calculating a dose level that poses no appreciable risk to humans.

      2. Prenatal and postnatal sensitivity. The pre- and postnatal toxicology data base for imazethapyr is complete. There is no evidence of increased susceptibility to infants and children.

      3. Conclusion. There is a complete toxicity data base for imazethapyr and exposure data are complete or are estimated based on data that reasonably accounts for potential exposures. There is no evidence of qualitative or quantitative susceptibility to infants and children. No evidence of increased susceptibility was observed in rat and rabbit fetuses following in utero exposure in the prenatal developmental toxicity study and also in the 2-generation reproduction study in rats. However, EPA has not concluded its review process regarding application of the additional safety factor for infants and children as to imazethapyr and, therefore, for the purpose of this action will retain the statutory default factor of an additional 10x. Once EPA has the opportunity to complete its review of the data on imazethapyr in this area, the 10x safety factor may be reduced or removed.

   5. Aggregate Risks and Determination of Safety

      To estimate total aggregate exposure to a pesticide from food, drinking water, and residential uses, the Agency calculates DWLOCs which are used as a point of comparison against the model

      [[Page 14850]]

      estimates of a pesticide's concentration in water (EECs). DWLOC values are not regulatory standards for drinking water. DWLOCs are theoretical upper limits on a pesticide's concentration in drinking water in light of total aggregate exposure to a pesticide in food and residential uses. In calculating a DWLOC, the Agency determines how much of the acceptable exposure (i.e., the PAD) is available for exposure through drinking water, e.g., allowable chronic water exposure (mg/kg/day) = cPAD - (average food + residential exposure). This allowable exposure through drinking water is used to calculate a DWLOC.

      A DWLOC will vary depending on the toxic endpoint, drinking water consumption, and body weights. Default body weights and consumption values as used by the USEPA Office of Water are used to calculate DWLOCs: 2Liter (L)/70 kilogram (kg) (adult male), 2L/60 kg (adult female), and 1L/10 kg (child). Default body weights and drinking water consumption values vary on an individual basis. This variation will be taken into account in more refined screening-level and quantitative drinking water exposure assessments. Different populations will have different DWLOCs. Generally, a DWLOC is calculated for each type of risk assessment used: Acute, short-term, intermediate-term, chronic, and cancer.

      When EECs for surface water and groundwater are less than the calculated DWLOCs, OPP concludes with reasonable certainty that exposures to the pesticide in drinking water (when considered along with other sources of exposure for which OPP has reliable data) would not result in unacceptable levels of aggregate human health risk at this time. Because OPP considers the aggregate risk resulting from multiple exposure pathways associated with a pesticide's uses, levels of comparison in drinking water may vary as those uses change. If new uses are added in the future, OPP will reassess the potential impacts of residues of the pesticide in drinking water as a part of the aggregate risk assessment process.

      1. Acute risk. EPA did not select a toxicological endpoint for this exposure duration. A dose and endpoint attributable to a single exposure were not identified from oral toxicity studies. Therefore, no quantified acute risk assessment is necessary.

      2. Chronic risk. Using the exposure assumptions described in this unit for chronic exposure, EPA has concluded that exposure to imazethapyr from food will utilize 1% of the cPAD for the U.S. population, 3% of the cPAD for All Infants (1 year) and 3% of the cPAD for Children (1-6 years). There are no residential uses for imazethapyr that result in chronic residential exposure to imazethapyr. There is potential for chronic dietary exposure to imazethapyr in drinking water. After calculating DWLOCs and comparing them to the EECs for surface and ground water, EPA does not expect the aggregate exposure to exceed 100% of the cPAD, as shown in the following Table 2:

      Table 2.-- Aggregate Risk Assessment for Chronic (Non-Cancer) Exposure to imazethapyr

      Surface Water EEC Population Subgroup

      cPAD mg/kg/day

      % cPAD (Food)

      parts per billion Ground Water EEC Chronic DWLOC (ppb) (ppb)

      (ppb)

      U.S. Population

      0.025

      1

      6.13

      2.2

      864 All Infants (01-6329Filed3-13-01; 8:45 am] BILLING CODE 6560-50-S