For those "I'm alright jack" types...
It's possible that repeat mild exposure causes sensitization. It's surprisingly easily done. I've managed to become sensitized to "zip kicker" an aromatic amine used to fast cure superglue. It took around 20 years of very infrequent use (less than weekly) without symptoms before I noticed a problem appear. More info on request.
Just when I thought that this might be a bit OT...
It seems there is quite a lot of different chemicals, including amines in bleed air...
http://www.caa.co.uk/docs/33/CAPAP2004_04.pdf
"There are over 40 different chemicals contained in oil breakdown
products and many have no published toxicity data..."
http://dspace.dsto.defence.gov.au/ds...ndle/1947/3349
"It was also claimed that contamination by the anti-oxidant amine additive, phenyl-α-naphthylamine, may also occur and it has been suggested that the compound causes sensitisation and may be carcinogenic."
I'm not a professional pilot but I'd be more worried about the possibility of continuous low level exposure than the infrequent but more obvious/noticable incidents.