CPR is an abbreviation for Cardio-Pulmonary-Resuscitation. It is manually pumping a persons heart and breathing for them.
Is it dangerous?
It is recommended that unless the situation is dire you should not make mouth to mouth contact without a barrier. If you carry a wallet or purse you could easily fit a disposable mask in case of emergency.
Is it tiring?
CPR is an extreme workout. It takes tons of energy and physical strength.
When should I stop giving CPR?
There is only two reasons you should stop giving CPR. If the persons regains a heartbeat and can breath or if you feel so tired that if you continue you'll pass out as well. Other than that you should give CPR until further help arrives.