VOC senstivity varies
I should also point out that individual sensitivity to VOCs varies, perhaps by an order of magnitude in terms of concentration for healthy people, and even more if you're already sick or presensitized.
As for presensitization, here's an anecdote: When I was a kid, I always had to take anti-nausea drugs before flying, otherwise I'd throw up and be miserable all the time (motion sickness!). Before I started taking those drugs, I'd sometimes make it through the flight, feeling quite ill but not quite having to use the barf bag. Then we'd land, go through all the motions, get out of the airport, and get on a city bus that was leaking just a teeny weeny bit of exhaust into the cabin. Bam, throw up right then and there, before the thing would even get in motion!