I constantly measure 15 ppm of CO inside my car after a long commute (~1 hour of continuous driving, but the overall trip can take from 1.5 to 2 hours depending on my destinations before heading home). 15 ppm is the typical reading. Sometimes it's a little less (12), one time it reached 20, but 15 seems to be the typical measurement. I always leave the windows closed and set the AC to circulate the air inside (no intake from outside). My question is: is this normal for a long commute on busy roads, or should the value be lower and I may have a leak/problem with isolation?
This NIH (National Institutes of Health) Study monitoring CO inside the cabin of a vehicle during commutes over an extended period has some pretty interesting data.
From the study
the mean CO concentration was 9.8 ppm, with a standard deviation of 5.8 ppm
Here is another study Here is another study that reflects older data ('80's and '90's) sampled from several metro areas that you may be familiar with.
I discovered some additional studies that would show cars idling on warmup with the vent sent to 'fresh air' in the winter where the CO levels went as high as 8,000 ppm!
So it seems that are within normal operation limits or that your exposure to CO does not seem out of the normal variance. It would be interesting to measure the CO with fresh air on driving in the country far from normal metropolitan contaminants.