Microcystins are a group of biologically active hepatotoxic peptides produced by bloom-form- ing cyanobacteria in water. The harmful cyanobacte- rial blooms in natural water have become an increas- ing environmental problem all over the world and the exposure hazards are threating due to difficulties in controlling nutrients specially in tropical coun- tries. Total of 72 water samples were collected from 18 locations representing three water types (river, irrigation and wastewater) from Sohag and Assiut Governorates. All water samples were analyzed for chemical oxygen demand, total nitrogen, total phos- phorus and dissolved oxygen. Intracellular and extra- cellular microcystins (MC-LR, MC-RR and MC-YR) were quantified. Microcystins were estimated in all river water samples and 40% of the samples con- tained concentration higher than that established by
World Health Organization and the highest detected concentration was 41.45 μg/L. The concentration of extracellular microcystins was higher than the intra- cellular in 40%, 20% and 62.5% of the river water, irrigation water and wastewater sample’s locations, respectively. Additionally, the mean concentration of MC-LR in river water samples was 4.04±7.12 ug/L and it was the most common type as it was detected in all samples followed by MC-YR and MC-RR detected in 40% and 20% of the samples, respectively. Hazard quotient of microcystins through intake of river water in Sohag Governorate was higher than 1 indicat- ing high potential risk for children. Furthermore, the health risk degree due to ingestion of Nile river drink- ing water in Sohag Governorate is high and medium risk degree for children and adult, respectively. Therefore, extra consideration should be paid because microcystins not only cause acute toxic effect but also have the ability to initiate cancer. Conclusively, there are ready steps to protect the human health include educating people about the dangers of microcystins and about water hygiene and proper waste disposal methods, with the need for regular observation of the physical appearance of the water.