I exercise on a treadmill 3-4 times a week, 2-4 miles of walking per day. today, i used a heart rate monitor for the first time. walking uphill between 2-2.5 miles an hour, 15 degrees elevation, brought my hr to 208. is this dangerous?

A pulse of "208" may just indicate exercise intolerance. Suggest seeing a cardiologist for stress testing to rule out other causes, and providing a template for gradation of exercise parameters.

A rough calculation of maximum HR is 220 minus age. In your case 153. It is best you check with a cardiologist and follow her advice re degree of exercise. Good luck