You've also got to contend with less life threatening onboard gases such as the good old fart;
Farts on planes among strange studies - Yahoo! New Zealand - Totaltravel
I recall a certain colleague I used to fly with who would purposely eat cabbage, beans, broccoli or a Big Mac the day before he flew with F/O's he didn't like, and he did this for obvious reasons!
But back on tropic, although studies into onboard toxicity isn't fully inclusive, there certainly is a lot of evidence pointing towards specific causes. And it would appear, as has already been suggested, that some people are more susceptible to these toxins than others, which is not entirely unusual in life.