My HR constantly stays above 85%.
Oh ha ha!! I just recalculated it again. My 85% is 153. Good lord, that's so low! That is not "working out". If I kept my HR that low I would not feel like I was achieving anything.
So I've spent years exercising at 175 (running) or 160ish (lower impact activities) and not had any issues.
Remember the 220-age thing is hugely generalised. Your own personal actual 85% may not be aligned with the 85% you get from that.
Can you sustain a harmfully high HR? No. You can't. Your own body will rebel and you'll stop. Try "sprinting" (not just running) for 1 minute. Can't do it, can you? Nobody can. Same principle. If you attempt to work out "too hard" your body will simply stop you, you just can't do it.
Do watch for symptoms of overtraining though! (Google that term).
Deb, in New Zealand