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TOXICOLOGICAL REVIEW of N-HEXANE (CAS No. 110-54-3)

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TOXICOLOGICAL REVIEW of N-HEXANE (CAS No. 110-54-3) EPA/635/R-03/012 www.epa.gov/iris TOXICOLOGICAL REVIEW OF n-HEXANE (CAS No. 110-54-3) In Support of Summary Information on the Integrated Risk Information System (IRIS) November 2005 U.S. Environmental Protection Agency Washington, DC DISCLAIMER This document has been reviewed in accordance with United States Environmental Protection Agency policy and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. ii CONTENTS—TOXICOLOGICAL REVIEW OF n-HEXANE (CAS No. 110-54-3) LIST OF TABLES AND FIGURES ...............................................v FOREWORD ............................................................... viii AUTHORS, CONTRIBUTORS, AND REVIEWERS ................................ ix 1. INTRODUCTION ..........................................................1 2. CHEMICAL AND PHYSICAL INFORMATION RELEVANT TO ASSESSMENTS .....3 3. TOXICOKINETICS RELEVANT TO ASSESSMENTS ............................4 3.1. ABSORPTION .....................................................4 3.2. DISTRIBUTION ....................................................5 3.3. METABOLISM .....................................................8 3.4. ELIMINATION ....................................................17 3.5. PHYSIOLOGICALLY BASED TOXICOKINETIC MODELS ...............21 4. HAZARD IDENTIFICATION................................................25 4.1. STUDIES IN HUMANS—EPIDEMIOLOGY AND CASE REPORTS ........25 4.1.1. Oral Exposure ..............................................25 4.1.2. Inhalation Exposure .........................................25 4.1.2.1. Subchronic Exposure .................................25 4.1.2.2. Chronic Exposure ...................................25 4.2. SUBCHRONIC AND CHRONIC STUDIES AND CANCER BIOASSAYS IN ANIMALS—ORAL AND INHALATION .............................47 4.2.1. Oral Exposure ..............................................47 4.2.1.1. Subchronic Studies ...................................47 4.2.1.2. Chronic Studies .....................................49 4.2.2. Inhalation Exposure .........................................49 4.2.2.1. Subchronic Studies ...................................49 4.2.2.2. Chronic Studies .....................................58 4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES—ORAL AND INHALATION ...................................................58 4.3.1. Oral Studies ................................................58 4.3.2. Inhalation Studies ...........................................59 4.4. OTHER STUDIES ..................................................63 4.4.1. Acute Toxicity Data .........................................63 4.4.2. Studies with Mixtures Containing n-Hexane ......................67 4.4.2.1. Oral Exposure ......................................68 4.4.2.2. Inhalation Exposure ..................................68 4.4.2.2.1. Subchronic studies ...........................68 iii 4.4.2.2.2. Chronic exposure ............................69 4.4.2.2.3. Reproduction/developmental studies .............70 4.4.3. Potentiation and Antagonism Studies ............................73 4.4.4. Mode of Action Studies ......................................82 4.4.5. Genotoxicity Studies .........................................90 4.5. SYNTHESIS AND EVALUATION OF MAJOR NONCANCER EFFECTS AND MODE OF ACTION—ORAL AND INHALATION .....................93 4.5.1. Oral Exposure ..............................................93 4.5.2. Inhalation .................................................94 4.5.3. Mode of Action Information ..................................104 4.6. WEIGHT-OF-EVIDENCE EVALUATION AND CANCER CHARACTERIZATION ..........................................105 4.6.1. Summary of Overall Weight-of-Evidence .......................105 4.6.2. Synthesis of Human, Animal, and Other Supporting Evidence .......106 4.7. SUSCEPTIBLE POPULATIONS AND LIFE STAGES ...................108 4.7.1. Possible Childhood Susceptibility .............................108 4.7.2. Possible Gender Differences ..................................108 5. DOSE RESPONSE ASSESSMENT...........................................110 5.1. ORAL REFERENCE DOSE .........................................110 5.2. INHALATION REFERENCE CONCENTRATION ......................111 5.2.1. Choice of Principal Study and Critical Effect — with Rationale and Justification ..............................................111 5.2.2. Methods of Analysis ........................................116 5.2.2.1. Adjustment to a Human Equivalent Exposure Concentration . 119 5.2.3. RfC Derivation—Including Application of Uncertainty Factors (UFs) . 120 5.2.4 Previous RfC ..............................................122 5.3. CANCER ASSESSMENT ...........................................123 6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD AND DOSE RESPONSE ..........................................................125 6.1. HUMAN HAZARD POTENTIAL ....................................125 6.2. DOSE RESPONSE ................................................126 6.2.1. Noncancer ................................................126 6.2.2. Cancer ...................................................127 7. REFERENCES ...........................................................128 APPENDIX A: SUMMARY OF EXTERNAL PANEL PEER REVIEW AND PUBLIC COMMENTS AND DISPOSITION ............................................ A-1 APPENDIX B: BENCHMARK DOSE (BMD) ANALYSIS ..........................B-1 iv LIST OF TABLES AND FIGURES Table 3-1. Concentration of n-hexane in blood and tissues of pregnant F344 rats immediately after a 6-hour exposure to 1000 ppm n-hexane .................................6 Table 3-2. Tissue distribution of radioactivity in male F344 rats 72 hours after a 6-hour inhalation exposure to various concentrations of [1,2-14C ......................... 6 Table 3-3. Apparent steady state concentration of n-hexane concentrations in male F344 rats after 6 hours inhalation exposure to [1,2-14C]-n-hexane ..........................7 Table 3-4. Metabolism of n-hexane following a 6-hour exposure of pregnant F344 rats on gestation day 20 ........................................................11 Table 3-5. Apparent kinetic parameters for n-hexane hydroxylation in rat liver and lung microsomes ...........................................................13 Table 3-6. Metabolites excreted in urine during a 72-hour period following inhalation exposure to n-hexane in male F344 rats .............................................20 Table 3-7. n-Hexane metabolite levels in urine of Wistar rats coexposed to n-hexane and toluene ...............................................................21 Table 4-1. Persistent and transient neurological symptoms following occupational exposure to n-hexane in a tungsten carbide alloy factory ..................................27 Table 4-2. Results of neurological tests in control subjects and those occupationally exposed to n-hexane in a tungsten carbide alloy factory ..................................28 Table 4-3. Nerve stimulation in control subjects and those occupationally exposed to n-hexane in a tungsten carbide alloy factory ..........................................29 Table 4-4. Motor neurographic findings in patients with n-hexane polyneuropathy ......... 37 Table 4-5. Nerve conduction study findings in printers with n-hexane-induced polyneuropathy ........................................................38 Table 4-6. FM-100 hue test error scores of n-hexane-exposed and nonexposed groups ...... 40 Table 4-7. Nerve conduction changes in male Wistar rats exposed 12 hours/day for 24 weeks to 200 or 500 ppm n-hexane ................................................50 Table 4-8. Experimental protocol for Phases I and II of a 6-month inhalation study of n-hexane and a mixture containing hydrocarbon isomers plus n-hexane in male Sprague-Dawley rats ..................................................................53 v Table 4-9. Relative organ weights of male Sprague-Dawley rats exposed to n-hexane 22 hours/day, 7 days/week for 6 months .......................................54 Table 4-10. Incidence of nasal turbinate and neuropathological lesions in B6C3F1 mice exposed to n-hexane for 13 weeks .................................................56 Table 4-11. Skeletal variations in live fetuses of pregnant Sprague-Dawley rats exposed to n- hexane via inhalation ....................................................60 Table 4-12. Total red blood cells and nucleated cells in bronchial lavage from n-hexane- challenged New Zealand white rabbits ......................................63 Table 4-13. Enzyme activities in lung homogenates of rabbits exposed to n-hexane ........ 64 Table 4-14. Concentration of biochemicals and enzyme activities in bronchial lavage fluid from male Sprague-Dawley rats exposed to n-hexane ...............................64 Table 4-15. Changes in sciatic and sural nerve action potentials induced by n-hexane and 2,5­ hexanedione ...........................................................67 Table 4-16. Incidence of liver and pituitary tumors in male and female B6C3F1 mice exposed to commercial hexane for 2 years ............................................70 Table 4-17. Time-to-onset for the appearance of axonal swelling in explanted cultures of fetal mouse spinal cord incubated with mixtures of n-hexane and methyl ethyl ketone ..... 76 Table 4-18. Effect of 2,5-hexanedione, acetone, ethanol, and mixtures of 2,5-hexanedione with acetone or ethanol in drinking water on average MCV ..........................79
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