The water in the thermos can be drunk overnight, but the tea can not be drunk overnight.
There is no carcinogen in the boiled water overnight. If there is no material basis in the overnight water, the carcinogen will not be born out of thin air. The most feared carcinogen nitrite must be produced on the basis of nitrates, but ordinary mineral water or pure water is only minerals and trace elements, or nothing, in this case carcinogenic The substance will not be born out of thin air.
As long as it can ensure that the water source itself meets the safety standards, how to burn the water will not produce carcinogens. However, the tea in the night will produce amino acids and other substances. Over time, the microorganisms will easily multiply and should not be consumed.