Tim Blair has been getting some lip for this quote.
Is it possible for a car to clean the air? Well, you sure wouldn't want to have to breath through a carburetor. Some pollutants are combustible, such as vapors escaping from gasoline tanks. Run them through a car engine and they may well wind up as little more than CO2 and water. If CO2 is to be called a pollutant, then Bambi is a polluter too. (Like I needed another reason to shoot deer - they're living proof that Satan copulates with rats.)
The point is that in principle an automobile's exhaust could be cleaner than its intake air. Whether that ever happens is something else entirely.
This reminds me of my nuclear power days, when I met fellow engineers from all over the country. One of them told me of a plant on the lower Mississippi River which had a special variance for the radioactivity levels permitted in its discharge water. Why? Because the water it was drawing in was already out of spec.