Can’t be very good happening in the bloodstream either I’d imagine …
I had never really thought this through before, but if someone tries this with a modern car that doesn’t make as much CO, will the compulsion to seek fresh air cause him to end the attempt before the little bit of CO does much damage?
Late-model cars can idle just fine with rather limited O2 intake. At the top of Pikes Peak, for example (14,000+ feet above sea level), the air density about 60% that of sea level. The electronically controlled throttle will open further as necessary to assure enough oxygen is admitted to burn the amount of fuel required to maintain idle RPM.
CO2 will build up to problematic levels before the O2 level drops far enough to compromise combustion. Ultimately the CO2 will compel you to seek fresh air (see my post just a few above this one).
Cold-start emissions can be high, but the cat warms up pretty quickly. The light-off temp for a 3-way cat is about 250C, and idle exhaust temperature is generally around 350C, so tailpipe CO emissions should go low (and stay there) once the engine is warmed up, even if idling for extended periods.
TL,DR: for a late-model car, CO2 buildup in the garage is likely to compel you to leave before the CO kills you.
The latter I understand; ozone is one of several components of smog created by pollutants interacting with the ultraviolet in sunlight.