Chicago is at 580 ft. but this weekend I visited my sister in Boulder and we went mountain biking (my first time! ) up at Silverthorne, which I think is about 9000. I felt great on the ride (working the heart harder, of course) but way bad after I was done. Some altitude weirdness - but not the usual kind you hear about with headaches and nausea, just a strange anxious feeling! You mountain dwellers could come down here and kick our butts at any time with all that extra hemoglobin in your blood.