Given the carcinogenicity of nitrosamines as nitrogenous disinfection byproducts, this study investigated the impacts of different pre-oxidation technologies on nitrosamine formation through different full-scale treatment approaches from a health risk assessment perspective. The cancer risk posed by six nitrosamines and their formation potentials (FPs) in a surface water source and three subsequent plants with comparable treatment approaches in southern Taiwan were estimated to investigate the total toxicity variations through different treatments. In the results, sufficient environmental exposure suppressed the risks by nitrosamine-FPs in the source, with appreciable risks (the median ranging from 6.4 x 10(-5) to 1.0 x 10(-4)) present in the water near the intake. In three treatment plants, the risks decreased through the treatment processes (1%434% reduction). Pre-chlorination suppressed the risk by nitrosamine formation during post-chlorination (dropped by 58%-86%), whereas preozonation elevated the risk in the next oxidation steps. Removal of organic matter before ozonation reduces the risk increase during oxidation. Total organic carbon (TOC) (r = -0.89) or dissolved organic carbon (DOC) variations (r = -0.86) were negatively associated with the cancer risk variations in three DWTPs. The discrepancy between the risks estimated by using different toxicity data sources occurred. Effectively minimizing the formation of nitrosamines with proper source water quality, treatment approaches, and correct parameters to monitor are critical for minimizing the toxicity posed by various nitrosamines in drinking water. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.