So what is a dangerous level of A1C, and when should you actually worry? When high blood sugar is a medical emergencyA high A1C level in and of itself isn’t typically an urgent medical emergency. While you can’t feel your A1C shift over time, you can feel a sudden, dangerous swing in blood sugar. Symptoms of very high blood sugar (acute hyperglycemia)—severe fatigue, dizziness or shortness of breath—warrant an urgent call to your doctor. If your number is climbing toward a dangerous level, talk to your doctor about a treatment plan that fits your age, hormones and overall health.