in a way that cannot continue over a period of time

Prices have been set unsustainably high for years. 多年来价格一直处于不可忍受的高位。

in a way that causes damage to the environment and is therefore not able to continue for a long time

Water is being used unsustainably and is running out. 水在以一种不可持续的方式使用，而且正在耗尽。