True, especially amongst the elderly or weak.

I've also seen the self-induced form (hyperventilation) due to anxiety in the altitude chamber (not passed out, but displaying hypoxia symptoms).

That's the mass-hysteria angle. One person does due to their low tolerance. Another---a nurse---gets lightheaded from exhertion on said patient. Some other armrest-gripper sees this and thinks "We aren't getting any air!!" Panic, etc.

Other than high cabin altitude, the only other thing I know of that can affect people physiologically would be engine oil present in the bleed air, which goes to the cabin. Such oil can originate from a pinhole leak at the first bearing, in front of the compressor.

Jet turbine oil contains Tricresyl phosphate as a stabilizing additive for high and low temerature extremes.

Tricresyl phosphate - Wikipedia, the free encyclopedia

It is also a nerve agent and extremely toxic.