Be VERY carefull...Carbon Monoxide(CO) is(if I am not mistaken) heaver than air, and so will not go out the cracked windows(at least not in any car I have ever seen <img src="/images/graemlins/wink.gif" alt="" /> )
CO is a slow killer, and bonds with the oxygen carriers in your blood preferentially to oxygen, and it does not UNBOND!
That means that you can be getting a little bit all the time and not know it, then get a short exposure to a higher concentration and have it kill you without warning.