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?

1 Answer 1


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.

  • I'm glad you posted ... I misread the question as CO2 not CO ... thanks for making this answer right! Dec 13, 2015 at 20:34
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    I'm familiar with that study. The thing is, even though 15 ppm is within the standard deviation range, my mean value appears to be 15 ppm, while the study reports a mean value of 9.8 ppm. Therefore I'm still not convinced that there is no problem.
    – user13662
    Dec 13, 2015 at 20:39
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    @NikitaKiryanov: I think the study was saying that some vehicles had more than 9.8, while others had less than 9.8; a vehicle that had 15ppm would be just beyond the normal standard deviation (which doesn't mean that it's harmul, or that 14.7 would be okay; it merely means the percentage of vehicles with less than that is much greater than the percentage with more).
    – supercat
    Dec 13, 2015 at 23:45
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    @supercat: From the statistical side, 6.8% of all cars have 9.8-10.8ppm. 6.6% have 10.8-11.8ppm. 6.3% have 11.8-12.8ppm. 5.7% have 12.8-13.8ppm. 5.1% have 13.8-14.8ppm. 4.3% have 14.8-15.8ppm. So: A car with 15ppm isn't that rare compared to the average car. However, you are right, 81.5% of all cars have less than 15ppm. But: 34.1% have 9.8-15.6ppm, so again, the car isn't that bad. (Assuming the distribution has a perfect gaussian shape)
    – sweber
    Dec 14, 2015 at 9:22
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    @sweber: Another way of looking at is that even if only 0.000001% had more than 15ppm that wouldn't mean 15ppm is dangerous, but the fact that the number of cars driving around with 15ppm or higher is vastly greater than the number of motorists suffering from CO poisoning strongly implies that that it's not.
    – supercat
    Dec 14, 2015 at 16:36

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