I was wondering if there comes a point where HIIT becomes dangerous. Are there any warning signs if it is?

Is HIIT Safe? - It depends how clever you are and how well you listen to your body. The only long term negative effects would come from over training and bad form.

Short term we are talking about heart attacks, and passing out. both of which can be avoided with basic listening skills and sensible diet.

You wouldnt advise a 90year old smoker to get up 1st thing and do an intense 20min HIIT session without eatting breakfast. You just gotta be carefull and find your limits slowly by building up the intensity.

Well I'm 24 so that doesn't apply to me.lol I felt a little light headed when I got my pulse at 191 so I guess I don't need to get that high.

I cannot even get above 150 with HIIT. I try, but it doesnt work. I take my pulse a few seconds after I am done with HIIT with my stop watch. Mine is usually around 140-150. I dont know how people can get 160 and above. It's really hard. I tried that, it doesnt work. is 140-150 a good thing or bad thing? sometimes even after 1 minute or so my HR goes between 120-130. I recover easily, so maybe right after I am done I should do my HR instead of a few seconds. BTW I take my pulse for 15 seconds, and then I add a 1 in front of it to determine my HR

Are you sprinting all out???? When I sprint all out it only takes me about 20 seconds and my pulse is flying. Especially after a couple times...

It applys to everyone. Thats my point. - take it slow and push yourself a little more each time. If you hadnt taken such a leap you wouldnt have been light headed at 191, you will be able to get there, just not yet.

when I'm at school only - I do like 20 seconds walking, 10 seconds 25% jog, 10 seconds 50% jog, 10 seconds 75% jog, 10 seconds sprint all out..and repeat for about a mile and then I do stairs (jog the straight a-ways, sprint up the stairs) for 10 minutes and then with my soccer ball on the grass 45-50 second jog and then 10-15 second sprint with the soccer ball for 10 minutes.

Training for soccer - sprints do not last that long and its a mixture of jogging, walking, and sprinting

My HIIT session speeds are like this

walking 2 min - 6km/h
jogging 1 min - 10 km/h
sprinting 1 min - 16km/h

After sprinting, I check my heartrate and it is always above 160, hitting 170 occasionally. I am 24. Should I decrease sprint speeds?

Why would you decrease sprint speeds? It's HIGH INTENSITY INTERVAL TRAINING.

I am worried about my HR going past 170. I CAN do that, but is it safe? What is the upper limit HR for a 24 year old male?

That formula is simply a human construct to give order to chaos.

The truth: 220-age is the theoretical upper limit, but the reality is that it's individual.

So if you go above your theoretical upper limit it is dangerous than?

In theory, but the truth is that you'll likely collapse from exhaustion before you do any damage.

Your problem is that you're trying to sprint for a minute: physiologically, that's impossible. The sprint period should be no more than 15, *possibly* 20 seconds in general (and can be as low as 10 seconds if you're truly pushing yourself) if you're attempting true HIIT, with another 30-60 seconds in recovery, depending on your conditioning. 10-15 cycles are all you should be able to accomplish before you're spent if you're doing it "correctly."

Yeah I can't imagine sprinting as hard as I could for 1 minute, and then doing jogging, walking, and doing it all over again.

Decrease your interval time.