I think it's a multi criteria formula.
Passenger specific: existing medical conditions, jetlag (from previous flights), illness induced during travel commencing prior to taking the flight, cabin choice (Y/J/F) etc.
Flight specific: duration, altitude (related to how much a cabin is pressurised) route taken, delay at departure, inflight service etc
Plane/airline specific: cabin settings (temperature, oxygen levels, seat arrangements)
Other factors?!
This would make an interesting research topic for a journal article, given there is a wealth of historical data on the matter.
I know what you're thinking, Nerdie Nerd Nerd! but hey, that's what you get when you're half way through a PhD
