EPA/635/R-08/004F www.epa.gov/iris TOXICOLOGICAL REVIEW OF NITROBENZENE (CAS No. 98-95-3) In Support of Summary Information on the Integrated Risk Information System (IRIS) January 2009 U.S. Environmental Protection Agency Washington, DC i DISCLAIMER This document has been reviewed in accordance with U.S. 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 NITROBENZENE (CAS No. 98-95-3) LIST OF TABLES......................................................................................................................... vi LIST OF FIGURES ........................................................................................................................ x LIST OF ACRONYMS ................................................................................................................. xi FOREWORD xiii AUTHORS, CONTRIBUTORS, AND REVIEWERS ............................................................... xiv 1. INTRODUCTION ..................................................................................................................... 1 2. CHEMICAL AND PHYSICAL INFORMATION ................................................................... 3 3. TOXICOKINETICS .................................................................................................................. 5 3.1. ABSORPTION .................................................................................................................. 5 3.1.1. Gastrointestinal Tract Absorption Studies.............................................................. 5 3.1.2. Pulmonary Absorption Studies............................................................................... 6 3.1.3. Dermal Absorption Studies .................................................................................... 7 3.2. DISTRIBUTION ............................................................................................................... 8 3.3. METABOLISM............................................................................................................... 10 3.3.1. Microbial Reduction of Nitrobenzene (the Three-Step, Two-Electrons-per- Step Transfer Process).......................................................................................... 13 3.3.2. Hepatic and Erythrocytic Reduction of Nitrobenzene (the Six-Step, One- Electron-per-Step Transfer Process)..................................................................... 17 3.3.3. Microsomal Oxidation of Nitrobenzene............................................................... 22 3.4. ELIMINATION............................................................................................................... 23 3.5. PHYSIOLOGICALLY BASED TOXICOKINETIC MODELS .................................... 25 4. HAZARD IDENTIFICATION................................................................................................ 26 4.1. STUDIES IN HUMANS—EPIDEMIOLOGY AND CASE REPORTS........................ 26 4.1.1. Oral Exposure....................................................................................................... 26 4.1.2. Inhalation Exposure.............................................................................................. 28 4.1.3. Dermal Exposure .................................................................................................. 29 4.2. SUBCHRONIC AND CHRONIC STUDIES AND CANCER BIOASSAYS IN ANIMALS—ORAL AND INHALATION .................................................................... 31 4.2.1. Oral Exposure....................................................................................................... 31 4.2.1.1. Subchronic Studies ................................................................................. 31 4.2.1.2. Chronic Studies....................................................................................... 43 4.2.2. Inhalation Exposure.............................................................................................. 43 4.2.2.1. Subchronic Studies ................................................................................. 43 4.2.2.2. Chronic Studies....................................................................................... 46 4.2.3. Dermal Exposure .................................................................................................. 57 4.2.3.1. Subchronic Studies ................................................................................. 57 4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES—ORAL AND INHALATION... 61 4.3.1. Oral Exposure....................................................................................................... 61 4.3.2. Inhalation Exposure.............................................................................................. 68 4.4. OTHER STUDIES .......................................................................................................... 73 4.4.1. Acute and Short-Term Toxicity Data ................................................................... 73 iii 4.4.2. Structure-Activity Relationships .......................................................................... 77 4.4.3. Immunotoxicity Studies........................................................................................ 81 4.4.4. Neurotoxicity Studies ........................................................................................... 84 4.4.5. Genotoxicity Studies ............................................................................................ 86 4.4.6. Other Studies in Support of Mode of Action........................................................ 92 4.5. SYNTHESIS OF MAJOR NONCANCER EFFECTS ................................................... 92 4.5.1. Oral Exposure....................................................................................................... 93 4.5.2. Inhalation Exposure.............................................................................................. 97 4.5.3. Mode-of-Action Information.............................................................................. 101 4.6. EVALUATION OF CARCINOGENICITY ................................................................. 103 4.6.1. Summary of Overall Weight of Evidence .......................................................... 103 4.6.2. Synthesis of Human, Animal, and Other Supporting Evidence ......................... 103 4.6.3. Mode-of-Action Information.............................................................................. 105 4.7. SUSCEPTIBLE POPULATIONS AND LIFE STAGES.............................................. 107 4.7.1. Possible Childhood Susceptibility...................................................................... 107 4.7.2. Possible Gender Differences .............................................................................. 107 4.7.3. Other ................................................................................................................... 108 5. DOSE-RESPONSE ASSESSMENTS................................................................................... 109 5.1. ORAL REFERENCE DOSE (RfD) .............................................................................. 109 5.1.1. Choice of Principal Study and Critical Effect—with Rationale and Justification......................................................................................................... 109 5.1.2. Method of Analysis—Including Models ............................................................ 112 5.1.3. RfD Derivation—Including Application of Uncertainty Factors (UFs)............. 115 5.1.4. Previous RfD Assessment .................................................................................. 117 5.2. INHALATION REFERENCE CONCENTRATION (RfC) ......................................... 117 5.2.1. Choice of Principal Study and Critical Effect—with Rationale and Justification......................................................................................................... 117 5.2.2. Methods of Analysis—Including Models .......................................................... 121 5.2.3. Evaluation of Human Equivalent Concentrations .............................................. 123 5.2.4. RfC Derivation—Including Application of Uncertainty Factors (UFs)............. 126 5.2.5. Previous RfC Assessment................................................................................... 127 5.3. CANCER ASSESSMENT ............................................................................................ 127 5.3.1. Choice of Principal Study and Target Organ—with Rationale and Justification......................................................................................................... 127 5.3.2. Benchmark Concentration Modeling ................................................................. 130 5.3.3. Inhalation Dose Adjustments, Inhalation Unit Risk, and Extrapolation Methods .............................................................................................................. 130 5.3.4. Uncertainties in Cancer Risk Values.................................................................. 133 6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD AND DOSE RESPONSE...................................................................................................... 134 6.1. HUMAN HAZARD POTENTIAL ............................................................................... 134 6.1.1. Exposure Pathways............................................................................................