Even if a car has "Air Conditioning" it is taking the air from the same source (e.g. the outside) but closer to ground level, and changing it only once every few minutes. This air will include all the impurities that are in that air.
The Cyclist, however, is in the Open Air, which is constantly refreshed and moving. The only potential problem is Particulates are easily removed. Indeed the "Nasal Hair is specifically designed for the task and this can be improved with a simple mask.
I find it difficult to believe that the Motor Industry would do research on the "Build-up" of CO2 or CO in a Car as it is unlikely to be critical and negative results could prove detrimental to Passenger perception of Car Safety.
Modern cars have a filter designed to clean the air before it enters the car, i have seen the result of this when my garage changed the filter during a service, the muck was unbelievable. You can, for short periods, stop any outside air from entering the car as well.