How dangerous is a BMI of under 15?

Low bmi. Make sure the calculation is correct first, but that is a dangerously low bmi and needs to be addressed by a physician.
Low BMI. This is considered critically low. It is associated with many problems including potassium problems and cardiac problems as well as bone problems. The bigger danger is that during the recovery phase there is refeeding syndrome which occurs as the body goes from a catabolic to an anabolic state, causing low phosphorus low calcium and low magnesium which can lead to delirium, coma and death.