This ominous color can be a sign of a serious condition, paraphenylenediamine toxicity, which is caused by ingredients in some hair dyes. Consequences include rhabdomyolysis or a breakdown of muscle tissue that causes toxins in the blood, leading to kidney damage or even failure. People with this condition often require dialysis. In some cases, the cause could be alkaptonuria, an inherited condition that can cause the urine to turn black when exposed to air.

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