from an 'Emergency Room' blog:

'Interesting statistics on carbon monoxide concentrations at Wikipedia. The average baseline level of carbon monoxide in homes is 0.5 to 5 ppm (parts per million). Average level near a gas stove is 5 to 15 ppm. Average level of carbon monoxide in a chimney with a wood fire going is 5000 ppm. Average level in car exhaust is 7000 ppm. The level of carbon monoxide in undiluted cigarette smoke is …

30,000 PPM!

Calling all those smokers out there a bunch of "chimneys" is an understatement.'
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Anybody who has to smoke in a car should set the fan on exhaust, that's for sure. But it's better not to smoke, if only because the CO can affect thinking ability and reaction time.