Carbon monoxide is a product of incomplete combustion. Too much carbon fuel for the available amount of oxygen. Modern vehicles avoid this condition to improve mileage and decrease emissions. They typically do a great job, and there are stories of unsuccessful suicide attempts by folks who try to off themselves by sitting in a running vehicle in a closed garage. All they get is a headache.
But if something goes wrong with a vehicle or if the vehicle consumes almost all of the available oxygen, you may have real problems. And I suspect that more CO is produced when vehicles first start up.
There is another problem with CO, though. Exposure to fairly small amounts can eventually accumulate to something significant because CO bonds much better (something like 20 times better) to hemoglobin in the blood than does oxygen. It comes out of the blood very slowly, unless you have access to a hyperbaric chamber.
So it pays to be very careful, even though the hazard from BBQs and kerosene heaters and malfunctioning furnaces and other sources is probably greater. Cars, at least, have oxygen sensors and will complain if there's too much incomplete combustion.
As for the earlier topic, I suspect that a good job of diverting the exhaust pipe out of the side of a 4Runner would improve the CO problem in comparison to the existing rear-exit approach.
