If it's scientifically established that a small percentage of humans is biologically vulnerable to certain chemicals, would it not be possible to screen potential employees before giving them jobs in the air? In the same way that air-forces screen for colour-blindness?
Or would that mean that they'd have to do the same thing with passengers?
Overthewing:
Yes and Yes.
Medical information
If they can identify a 500 year old skeleton found under a Leicester council car park as Richard 111, former King of England.....beyond reasonable doubt. ie 99%.
Only 51% Balance of Probability needed for mere mortals and little kiddies....