apias:

There's really a very simple experiment that can be performed to test this; you don't even have to drive anywhere and no expensive instruments are required. You can perform this test with either a petrol or diesel engine, the results will be the same.

First, with your car in the driveway, facing upwind, get in the car, start the engine, close the windows and turn on the ventilation system to draw air in from the outside. Do this for 30 minutes and note how you feel breathing this air being "breathed into" the car.

Now, shut off the ventilation system, turn off the engine, connect a hose to an exhaust pipe and run the hose into the car through one of the windows, partially opened, tape some plastic over the window opening to seal it and make sure any other windows are closed. Get in the car, start the engine, make sure the ventilation system is off. Do this for 30 minutes and note how you feel breathing this air coming from the exhaust.

In the unlikely event you are still alive after this second trial, compare your impressions of the air quality, and let us know which you feel is "cleaner".

You go first. 

While exhaust is obviously poisonous it is still possible that other elements are cleaned up and eliminated during the combustion process. Btw Clarkson made the same point when testing the 997 Turbo for Top Gear.