Background The benchmark dose (BMD) method calculates the dose associated with a specific change in response based on a specific dose-response relationship. Compared with the traditional no observed adverse effect level (NOAEL) method, the BMD method has many advantages, and the 95% lower confidence limit of benchmark dose lower limit (BMDL) is recommended to replace NOAEL in deriving biological exposure limits. No authority has yet published any health-based guideline for rare earth elements. Objective To evaluate genotoxicity threshold induced by acute exposure to neodymium nitrate in mice using BMD modeling through micronucleus test and comet assay. Methods SPF grade mice (n=90) were randomly divided into nine groups, including seven neodymium nitrate exposure groups, one control group (distilled water), and one positive control group (200 mg·kg⁻¹ ethyl methanesulfonate), 10 mice in each group, half male and half female. The seven dose groups were fed by gavage with different concentrations of neodymium nitrate solution (male: 14, 27, 39, 55, 77, 109, and 219 mg·kg⁻¹; female: 24, 49, 69, 97, 138, 195, and 389 mg·kg⁻¹) twice at an interval of 21 h. Three hours after the last exposure, the animals were neutralized by cervical dislocation. The bone marrow of mice femur was taken to calculate the micronucleus rate of bone marrow cells, and the liver and stomach were taken for comet test. Results The best fitting models for the increase of polychromatophil micronucleus rate in bone marrow of female and male mice induced by neodymium nitrate were the exponential 4 model and the hill model, respectively. The BMD and the BMDL of female mice were calculated to be 31.37 mg·kg⁻¹ and 21.90 mg·kg⁻¹, and those of male mice were calculated to be 58.62 mg·kg⁻¹ and 54.31 mg·kg⁻¹, respectively. The best fitting models for DNA damage induced by neodymium nitrate in female and male mouse hepatocytes were the exponential 5 model and the exponential 4 model, respectively, and the calculated BMD and BMDL were 27.15 mg·kg⁻¹ and 11.99 mg·kg⁻¹ for female mice, and 16.28 mg·kg⁻¹ and 10.47 mg·kg⁻¹ for male mice, respectively. The hill model was the best fitting model for DNA damage of gastric adenocytes in both female and male mice, and the calculated BMD and BMDL were 36.73 mg·kg⁻¹ and 19.92 mg·kg⁻¹ for female mice, and 24.74 mg·kg⁻¹ and 14.08 mg·kg⁻¹ for male mice, respectively. Conclusion Taken the micronucleus rate of bone marrow cells, DNA damage of liver cells and gastric gland cells as the end points of genotoxicity, the BMDL of neodymium nitrate is 10.47 mg·kg⁻¹, which can be used as the threshold of genotoxic effects induced by acute exposure to neodymium nitrate in mice.

Background The respiratory toxicity of inhaled polyhexamethylene guanidine (PHMG) has been extensively studied since the humidifier disinfectant incident. However, the impacts of inhalation of PHMG on the immune system are not comprehensively studied yet. Objective To explore the effects of inhalation of PHMG disinfectant aerosol on major immune organs and immune cells in mice. Methods Thirty male C57BL/6J mice (6-8 weeks old) were randomly divided into three groups: control, low-dose (0.1 mg·m⁻³ PHMG), and high-dose (1.0 mg·m⁻³ PHMG), with ten mice in each group. The mice were administered by oral-nasal inhalation of PHMG aerosol for 4 h per day, 5 d per week for 4 weeks consecutively. After designed treatment, venous blood was collected from the inner canthus of the eyes of mice and peripheral hematological indicators were measured with a blood analyzer. Then the mice were sacrificed by cervical dislocation and the lung, thymus, spleen, and femur were isolated. Lung, thymus, and spleen were weighed and organ coefficients were calculated, and single cell suspensions of thymus, spleen, and bone marrow were prepared to analyze lymphocytes phenotypes and proportions by flow cytometry. Results The body weight of mice in the high-dose group was lower than that of mice in the control group (P<0.01) from the 7th day of inhalation, and decreased by 15.74% compared with that of mice in the control group at the end of inhalation (P<0.01). The lung coefficients of both the low-dose and high-dose groups were higher than that of the control group (P<0.01), the thymus coefficient of mice in the high-dose group was lower than that of the control group (P<0.05), but the spleen coefficient did not change significantly (P>0.05). Leukocyte count [(1.49±0.22)×10⁹·L⁻¹], lymphocyte count [(0.96±0.36)×10⁹·L⁻¹] and its proportion [(63.13±14.96)%] in the peripheral blood of mice in the high-dose group were lower than those in the control group [(2.69±0.25)×10⁹·L⁻¹, (2.33±0.28)×10⁹·L⁻¹, and (86.23±3.40)%, respectively] (P<0.01), whereas red blood cell count [(12.32±0.46)×10¹²·L⁻¹], hemoglobin count [(175.25±4.65) g·L⁻¹], and hematocrit [(53.55±0.70)%] in the peripheral blood of mice in the high-dose group were higher than those in the control group [(11.11±0.37)×10¹²·L⁻¹, (160.67±4.04) g·L⁻¹, and (45.10±9.75)%, respectively] (P<0.05). Compared with the control group, the proportion of CD4+ CD8+ double-positive T cells decreased (P<0.05), the proportions of CD4+ T cells and CD8+ T cells increased (P<0.05), and the amounts of CD8+, CD4+ CD8+, CD4+, and CD4- CD8- cells decreased (P<0.05) in the thymus of mice of the high-dose group, the proportion of CD4+ T cells in the spleen of the high-dose group increased (P<0.05), the proportions and amounts of T cells, CD4+ T cells, and CD8+ T cells in the bone marrow of the high-dose group increased (P<0.05). Conclusion Inhalation of PHMG may cause thymic atrophy, disrupt T-lymphocyte development, and lead to an imbalance in the number of immune cells in the bone marrow, peripheral blood, and spleen, suggesting that inhalation of PHMG induces immune dysfunction.