Drinking overnight boiled water will not cause cancer, because the water will be boiled to kill most of the bacteria in the water, although the boiled water is easy to be contaminated by microorganisms after being placed overnight, but the nature of the boiled water itself will not be changed, so overnight boiled water is not a carcinogen.
Carcinogenic substances in the nitrite must contain nitrate on the basis of certain conditions will lead to its production, ordinary drinking mineral water or boiled water in the presence of minerals and trace elements, will not be out of nowhere to produce nitrite and become carcinogenic.
Overnight water is not carcinogenic, but still need to cover the water cup to prevent microbial contamination of water quality. Long-standing boiled water is also not recommended to drink, if abnormal symptoms occur after drinking, should seek medical attention without delay.