Smell
N1 Vibes point is spot on. Also it is important to realise that not everyone is affected by breathing this contaminated air.
It is pretty clear now that some are very susceptible others not. So caution is required when listening to those who say "I flew this type for X number of years without problem" they were probably saved by their genes.
Also none of this is necessary. There's a French company making oil without OPs/TCPs, it's used by the UK military among others.......funny old thing that.