The environmental risk of the anionic surfactant, linear alkylbenzene sulfonate (LAS), in Japanese surface waters is presented using a probabilistic exposure and effects assessment. A chronic toxicity Species Sensitivity Distribution (SSD) (20 species) is used to define the 5th percentile hazardous concentration and compared to experimental stream mesocosm findings following toxicity normalization to various LAS carbon chain lengths (CL) ranging from C10 to C14. CL-dependent ecotoxicity data are combined with environmental monitoring in Japan where CL distributions of LAS are also quantified. Over 9,000 surface water measurements with CL specific LAS concentrations were compiled. Because LAS displays a common polar narcotic mode of action across all CL, a Toxic Unit (TU) concentration-addition approach can be followed whereby TU exceeding 1 correspond to environmental risk of cumulative Predicted Exposure Concentrations (PEC)/Predicted No Effects Concentration (PNEC) also exceeding 1. SSD, mesocosm, and monitoring data confirm that an extremely small number of water samples exceed a TU of 1 (5 of 4748 for SSD PNEC; 0 sites for mesocosm PNEC). Total LAS measurements from > 25,000 sites were compared to CL normalized PNECs demonstrating > 99.99% probability that the PEC would be less than the PNEC indicating negligible risk from LAS in Japan surface waters.