As I mentioned in past comments, we did experiments where my wife got a headache at 9 ppm and up. She is more sensitive than most; if I spray Windex on a rag to clean the Ex windows, and she comes out, she covers her face with her shirt.
This is why I liked using the carbon monoxide data logger. It runs every 10 seconds for three days, and then I print out a report of the graph showing the results. For example, I will write on my notepad that on Tuesday at 2PM, there was an exhaust odor and the wife got sick. Then later when I print out the graph from the last three day period, I will look at the Tuesday at 2 PM, and see if there was carbon monoxide recorded and how high the ppm was.
I propose that you reverse engineer the CO level for you by finding out when you get sick, see what level of CO you record inside the cabin. The lowest level of CO that you get sick maybe your threshold level.