When is bradycardia harmful to the body?

It depends: Some people can tolerate a heart rate of 40 without problems especially athletes, however any heart rate less than 60 accompanied by symptoms of dizzyness, syncope or fatigue is considered harmful and may necessitate a pacemaker.

Bradycardia: Bradycardia is harmful when low cardiac output is the result and various organs are underperfused.

Not usually: Only if accompanied by symptoms like enervation, dizziness, shortness of breath. If naturally occurring (no heart rhythm disturbance or medication adverse effect) and no symptoms, probably would simply observe. Otherwise, underlying conditions need to be sought and treated accordingly. Some may require a pacemaker if symptomatic and not responsive to conservative therapy.

Depends: It depends on your overall cardiac output. If your heart rate is low but your heart can pump a large volume per beat (stroke volume) then no. The best way to evaluate this is on how you feel. If you are dizzy, tired, lightheaded, short of breath, or having chest pain then it is likely too low and you should see your doctor.