Is it dangerous to have a low heart rate? how dangerous?

Not per se: A low heart rate is not in itself dangerous. Many well conditioned athletes have low resting heart rates with excellent cardiac health. A low heart rate that associated with cardiac disease may be an entirely different story. Resting heart rates less than 30 or low rates associated with symptoms always need investigation.

Usually not if no sx: One sign of good cardiovascular fitness is a low resting heart rate. A low hr that does not increase with exercise might present with dizziness with exercise and could represent sinus node dysfunction (a problem with the normal pacemaker) or a problem with electrical conduction in the heart. If you have dizzy spells, an evaluation with an ECG might be worthwhile.