Yes a heart rate of 120 beats per minute can be dangerous especially if you haven't been exercising and or the heart rate does not slow down within 5 to 20 minutes of resting.
A heart rate of 100 or more is considered not good and is called Tachycardia and you should see a doctor or go to urgent care or the emergency room to get checked out.
A bad heart rate is when your heart is beating faster than 100 beats per minute.
If your heart is beating less than 100 beats per minute then it's usually okay but if your heart continues beating as fast as 100 beats per minute or more than you may be experiencing a health problem such as Tachycardia which is when your heart beats too fast.
It's normal for your heart to beat rapidly after exercise, when being stressed out or suffering from anxiety or even when you're exerting yourself working.
A heart beat of around 100 beats per minute is usually normal but your heart should not beat more than 100 times per minute.
If you've rested and your heart is beating more than 100 times per minute and doesn't slow down then go to the ER to get checked out.
You could be having heart palpitations or your heart could be telling you that you may be having a heart attack.
Not all heart attacks will cause pain and there's a thing such as as silent heart attack.
So it's best to get checked out just in case.
Some people have heart beats of around 160 beats per minute or less and go to the ER.
Remember that your heart should not continue to beat more than 100 times per minute after you're resting.
Heart Palpitations are rarely serious enough to need to visit the ER.
Most heart palpitations are nothing to worry about however they can seem scary.
if the heart palpitations do not go away within 24 to 48 hours or the heart palpitations continue or get worse then you should go to the ER for the heart palpitations.
I get some heart palpitations myself about twice per month and I did go to the ER for them because I was unsure about them the first time it happened.
But nothing was wrong with me and they said the heart palpitations were nothing to worry about until I had any chest pain, tightness, arm numbness etc.
Heart Palpitations are where you have a fast-beating, fluttering or pounding heart which can be caused by stress, anxiety, nervousness, medication and exercise.
Some medical conditions may cause heart palpitations but the good news is most heart palpitations are not serious or life threatening.
Usually resting and relaxing will stop the heart palpitations.