Apologies if I've missed it, but I don't think anyone contributing to this thread has considered the "other" altitude-related medical condition - not hypoxia, but decompression sickness.
Irrespective of the mixture or pressure of breathing air, if you spend enough time above 18,000' cabin altitude then symptoms of DCS (otherwise known as "the bends") can start to appear, although the risk at that altitude is confined mainly to those who have been diving recently. The figure most commonly quoted for the population at large is 25,000'. This is why the RAF uses that as its maximum cabin altitude, and therefore operating ceiling for unpressurised aircraft like the Tucano. Aircraft which operate for extended periods at cabin altitudes of greater than 25,000' employ partial- or full-body pressure suits to provide counterbalancing force for two purposes: 1) on the chest to allow prolonged pressure breathing and 2) over as much of the body as possible to prevent the development of DCS. As cabin altitude increases further, only a full-body suit such as that worn by U-2 pilots or astronauts is capable of preventing the latter. The FAA has produced a useful factsheet on DCS
here which sets out causes, symptoms, effects and treatments.
Prolonged flight above 25,000' cabin altitude would risk DCS in either crew or passengers. This would be particularly unwise on flights departing diving destinations as the prescribed waiting times between diving and flying only take into account the pressure conditions expected on a normal flight. But there is no practical way of telling which of your passengers are vulnerable to DCS until they start complaining with symptoms. Unlike mild hypoxia, from which passengers should make a full recovery without medical treatment after descent, DCS may require protracted and expensive therapy and may result in permanent injury - not something your average airline is going to want to foot the bill for!