The increase of acute toxicity of plasma-treated ultrapure water during the 30–120 s treatment was also significant ( P < 0.05) ( Fig. 8 ). Because ultrapure water was free from contaminants, the acute toxicity increases observed for plasma-treated raw water and filtra- tion effluent were not caused by the plasma induced degradation of pollutants in water. Rather, the increased toxicity of treated water was mainly due to the chemical reaction of water to the effects of plasma. Furthermore, these effects were long-lasting; ultrapure water still exhibited acute toxicity for 2 h following plasma treat- ment because some long-life active materials still existed. Although the life of free-radical active materials was relative short, the generated liquid phase of oxidizing substances such as ozone and hydrogen peroxide were long-life active materials that could cause acute toxicity. In some cases, the active particles causing acute toxicity inevitably contribute to the microbes’ sterilization in water. Moreover, the presence of acute toxin as found in the present research may contribute to water disinfection in the plasma dis- charge method. However, with regard to the safety evaluation of water quality, the increase of acute toxicity resulting from plasma treatment would restrict the extension of this technology to appli- cations that provide treated water for sensitive recipients; such applications include provision of drinking water, medical water and the aquaculture water. Similarly, the plasma treatment of wastewater may require careful consideration due to the possible adverse influence of the discharge on biota in receiving waters.