hung by a hair 危在旦夕；千钧一发；命悬一线；岌岌可危

【例句】
I saved the baby whose life hung by a hair.
我救了命悬一线的婴儿。

The people's safty hung by a hair when the flood came.
洪水来临，人民的生命安全处在千钧一发的状态。