1. School of Mathematics and Computational Science Xiangtan University, Xiangtan, Hunan, China
2. Department of Toxicology and Sanitary Chemistry, School of Public Health, Tianjin Medical University, Tianjin, China
3. Department of Mathematics, School of Public Health, Tianjin Medical University, Tianjin, China
4. Tianjin Key Laboratory of Environment, Nutrition and Public Health, Tianjin, China