Thirteen-Week Inhalation Toxicity of 2- and 4-Chloronitrobenzene in F344/N Rats and B6C3F1 Mice

Toxicity studies were performed by exposing F344/N rats andB6C3F₁ mice to 2- and 4-chloronitrobenzene (CNB) by whole bodyinhalation 6 hr/day, 5 days/week, for 13 weeks. Animals wereevaluated for clinical chemistry (rats), hematology (rats),histopathology, and body/organ weights. Exposure concentrationswere 0, 1.1, 2.3, 4.5, 9, and 18 ppm for 2-CNB and 0, 1.5, 3,6, 12, and 24 ppm for 4-CNB. All rats in the 2-CNB study surviveduntil the end of the study. Two male mice in the 18-ppm groupin the 2-CNB study, however, died during Week 12; no deathsattributable to 4-CNB exposure occurred in rats or mice. Inboth studies, the mean body weight gains of exposed animalswere similar to those of the respective controls. In rats, inhalationexposure to 2- or 4-CNB resulted in methemoglobinemia leadingto a regenerative anemia and a variety of tissue changes secondaryto the oxidative erythrocyte injury. In the 2-CNB study, methemoglobinemiaresulted in a normocytic, normochiomic, responsive anemia, whereaswith 4-CNB, the methemoglobinemia was more severe and resultedin a niacrocytic, hyperchromic, responsive anemia. Alterationsof erythrocyte morphology were observed in both studies; changesincluded Heinz bodies, poikilocytes, and polychromasia. In rats,both isomers caused increases in serum activities of alanineaminotransferase and sorbitol dehydrogenase and increased bileacid concentrations. Microscopic liver changes included hemosiderindeposition in Kupffer cells (rats and mice exposed to 4-CNB),hepatocytomegaly (mice), and cytoplasmic basophilia (rats).Hepatocellular necrosis and chronic inflammation observed inmice were rather specific to the 2-CNB isomer, as only slightevidence of focal necrosis in the liver was observed in miceexposed to 4-CNB. Splenic lesions included hemosiderin accumulation,capsular fibrosis, and increased hematopoietic cell proliferation.Increased bone marrow hemosiderin and hematopoietic cell proliferationand kidney tubule hemosiderin deposition were also observed.Other findings, attributed to chemical exposure but not to thehematotoxicity, were described. Lesions included hyaline dropletnephropathy and degeneration of the testis in male rats exposedto 4-CNB, inflammation of the harderian gland in rats exposedto 4-CNB, hyperplasia of the nasal cavity epithelium in ratsexposed to 2-CNB, and hyperplasia of the forestomach epitheliumin mice exposed to 4-CNB; these lesions have not been describedpreviously in studies with these chemicals. Based on the exposureconcentrations evaluated, A no-observed-adverse-effect level(NOAEL) for histopathological injury in mice was 4.5 ppm for2-chloronitrobenzene and 6 ppm for 4-chloronitrobenzene; a NOAELwas not determined for rats.