Hong-Giang Hoang, Chitsan Lin*, Chow-Feng Chiang, Xuan-Thanh Bui, Wisanukorn Lukkhasorn, Thi-Phuong-Thuy Bui, Huu-Tuan Tran, Thi-Dieu-Hien Vo, Van-Giang Le and Long D. Nghiem (2021)/The individual and synergistic indexes for assessments of heavy metal contamination in global rivers and risk: A review