Years ago I got carbon monoxide poisoning at work. The doctor in the emergency room told me that my threshold for CO poisoning in the future would be lower and to be careful around CO producing equipment. Now this happened in the mid 70's and only recently have I started having problems again. When we warm up the car in the pits I have to move upwind and as far away as possible from the exhaust fumes as I can. And no longer will I accompany the car to the starting line unless there is a good breeze blowing because the fumes trap themselves in the starting line area. So what the doctor told me 35 years ago is happening now and I have to be careful of where I'm at in the pits and near the track.
This also explains why a few years ago when I went to a local dirt track I felt like crap at the end of the evening. I just thought I was coming down with the 24 hour bug, but looking back I think it was the CO affecting me.