Cardiovascular problems, abnormally high blood pressure/very high resting pulse, extremely low blood pressure/very low resting pulse, aneurysms 

epilepsy and seizures

if you are taking strong medications

severe psychiatric symptoms, especially psychosis or paranoia, bipolar disorder 

severe osteoporosis, recent surgery, glaucoma 

a current pregnancy 

asthma (people with asthma should bring their own inhaler and consult their physician or respiratory trainer before participating)