I've been just fine at 35m (or at least, I've thought so), and I've been noticeably narked around 30m, so bad that I chose to ascend.
It all depends. But the nark is insidious. Don't trust yourself around or deeper than 30m.
With all due respect that is not entirely correct. Obviosly as a new / newish diver you are almost certainly going to be effected by narcosis at 30m and deeper iniitialy when you venture there. Although not suggesting you do so, the more times you go there the more you will become aware of its insidious effect - or not as the case may be - and then choose whether you want to keep doing that - on air - or switch to a better breathing mixture for those deeper dives.
But before the days of mixed gas there were a lot of folks diving
much deeper on air and getting things done. Before developing the "lust for rust" myself I did my many many hundreds of dives on the Great Barrier Reef and almost all the outer islands in the Coral Sea (it helped to have a friend who owned a liveaboard that did the GBR anf those outer islands) and those consisted of DEEP vertical walls. So I will be first to admit that as a relitively new diver with less than a 100 dives when I first went out there I felt the first noticable effects of narcoss at abot 35m, and noticed a distinct change in 'taste' in my mouth at around 40m; what we reffered to as "the hospital taste". Slowly over the many trips, and given you were out there for ten days to two weeks with up to four dives a day, the opportunuty arose on a consecitive day bases to slowly work my way way deeper, but giving the dramtically shrinking NDL's 'down there', never spent very long 'down there'. But long enogh to notice the changes and awareness of them and my suroundings and complete the tasks at hand (as I was already taking u/w photos). Years later when my interests had shifted to the wrecks off Gudalcanal (Solomon Islands), and before helium was available there, we were doing 60-65m dives with 30 odd minutes of bottom time and taking photos or video to boot then also. And even when helium became available (we had to freight it in ourselves at first) we still used air, or somtimes a topped up a left over mixed gas mix if we had the luxury of that. Of course once CCR's came along then goodbye to deep air diving, save for the odd liveaborad trips in Asia when again helium was not onboard and it was back to OC doing 60 odd meter wreck dives on air.
Would I choose to do so if I had a choice, certainly not, it would be mixed gas every time, but I would have
never foregone a 60m dive because I did not have gas.
So I think its only natural that people feel the effects of narcosis at tne depths you mention, especially having not been down there all that oftan. For me it was the regularity of doing that that 'conditioned' me - for want of a better word - to the effects of narcossis. But as someone else mentioned above about a technical instructor friend getting narced 'shallow', I am not ashamed to say the worst narcosis incedent I
ever had was being underweighted in a drysuit at only 30m, on a cold, well very cold 2c on the wreck,
very dark air dive in the Baltic Sea.