For an adult, dangerous glucose levels can be either high readings that are over 240 mg/dL or very low reading that are under 70 mg/dL, states WebMD. The normal blood glucose range is between 70 and 125 milligrams per deciliter in most cases, explains MedlinePlus.
Some factors that can cause a rise in glucose levels are an infection or different medical conditions that include tumors or pancreatic cancer, reports MedlinePlus. When diabetes is not under control, diabetics also can experience a rise in blood glucose levels. This condition is called hyperglycemia. If the blood sugar level is 300 mg/dL or higher on two consecutive readings, then a person should see or call his doctor, according to WebMD.
Hypoglycemia is the term used when blood glucose levels are too low. Although diabetics can experience hypoglycemia, other conditions may cause it, including adrenal or pituitary gland problems, reports Mayo Clinic.