I agree with Jason ... someone is either running an additive or "non-traditional" fuel. It may be legal, or not. Based on your physical reaction, I'd guess there's a good chance it ought to be "not".

We've smelled stuff like this from our corner stations a few times. Unfortunately, we've never been able to narrow down which car it was. But we did let Race Control know about it and which group of cars we smelled it from. I don't know if anything ever came of those reports.

Luckily, out on the corners, we're not likely to get ill effects even from fairly nasty fumes, since we're in the open air and the cars circulating keeps the air stirred up. If I smelled it from inside my car, race or no race, I'd either zoom around the suspect cars if I could or pull off if I couldn't. Some additives cause really nasty stuff in the exhaust, and I don't know which are which so I don't want to risk it.