[spockz]

You hit it even easier when driving in a car with the internal circulation turned on to keep nasty fumes out.

[uxhacker]

In a car with Recirculation Mode on Levels routinely spike between 1,500 and 4,000 ppm within 20 to 30 minutes.

I wonder how many driving accidents can be saved by having a co2 monitor in the car.

[eurleif]

I had to drive a rental car for a bit recently, after getting rear-ended, and I was shocked to discover that it defaulted to recirculation mode every time it was turned on, regardless of whether you'd turned it off the previous time. I felt light-headed in it a few times before I realized I needed to manually turn recirculation off every time. Horrible behavior, and I don't doubt that it's responsible for many accidents.