Non-linear dose-response relation between urinary levels of nicotine and its metabolites and cognitive impairment among an elderly population in China oleh Tian Li, Wei Liu, Ya-jun Yue, Shao-you Lu, Lu-lin Nie, Xi-fei Yang, Qing-qing Zhu, Bo Zhu, Lu Wang, Fei-qi Zhu, Li Zhou, Jia-fei Zhang, Er-wei Gao, Kai-wu He, Li Liu, Fang Ye, Jian-jun Liu, Jing Yuan, Lin Wang