Genetic makeup
Guy is probably correct, but don't think that means there isn't a problem...
3% of the population are pre-disposed to be symptomatic when exposed to long-chain organophospates. Of those, a percentage will be very ill, some will die, and others will have symptoms but be able to function more or less normally.
The problem is related to sheep-dip sundrome and I believe Gulf War syndrome. In the US the Pentagon has accepted GWS exists, and say that only 3% of those exposed to NAPS are symptomatic, which is in line with expectations based on genetic pre-disposition. Meanwhile, the MOD say it can't exist because 97% of those exposed are asymptomatic.
Then the British Air Lines' Puppet Association's General Secretary comes out and says it's nothing to do with contamination and we're all hyperventilating - an idea floated by Dr Mike Bagshaw - who just hapens to work for Airbus and be the retired head of British Airways Health Service.
However, in 2005 BALPA's GS said - and it's in video and can be reached via AOPIS.org - "There is a workplace problem resulting in chronic and acute illness amongst flight crew ... the workplace [is] the cabin environment. This, we conclude, is reulting in significant flight safety issues in addition to unacceptable flight crew personnel health implications"
Go figure!