The average blood sugar level for a healthy adult without diabetes is between 70 and 100 milligrams per deciliter after fasting for eight hours. Within two hours of eating, the level should rise to no more than 140 milligrams per deciliter. Blood sugar levels shouldn't exceed these ranges significantly.
Blood sugar levels above 180 milligrams per deciliter are dangerously high and indicate the subject is hyperglycemic or pre-diabetic. Levels below 50 milligrams per deciliter are dangerously low, in which case the subject is hypoglycemic. Even after fasting, levels should never fall below 60 milligrams per deciliter since the liver in healthy adults converts stored fat and proteins to sugar to maintain the proper levels for the individual. When blood sugar levels are consistently high, glucose acts as a poison to certain organs, such as the pancreas which becomes permanently damaged. Other problems from high blood sugar include cardiovascular disease and blindness.