A high potassium level of 7.0 millimoles or more is considered dangerous and a cause for immediate medical intervention, and a low potassium level of 2.5 millimoles or less is life-threatening, according to Mayo Clinic. High potassium is called hyperkalemia, and low potassium is referred to as hypokalemia.
The symptoms of hyperkalemia are an abnormal heart rhythm, slow heart rate and weakness, as stated by WebMD. The symptoms of hypokalemia are fatigue, muscle damage, numbness and tingling sensations, heart palpitations, abnormal heart rhythms, and constipation. Severe hypokalemia can cause the heart beat to slow or stop completely, according to MedlinePlus.