Yeah I didn't know better and was lulled into a false sense of security. For a couple of years, I always started my car first and opened the garage door and always shut the car off after the door came down. Why? Because my car's built-in garage door opener doesn't work with the car off. This hasn't been an issue for a long time and after a while I figured "a few minutes is okay". Also, I didn't grow up in a place with garages (coastal part of Florida) Gas heating is really not a thing there either (again because Florida). So CO poisoning wasn't a thing I learned to worry about. So the combination of ignorance and "it was okay if I ran the car inside for 15 seconds so it must be okay if I ran it a little longer".
Dumb for sure but thankfully I did know enough about gas water heaters and gas stoves to have CO detectors installed throughout my house.
A 2L engine uses 1L of air every cycle. At 1000 rpm, that's 500L (or .5 m^3) of air every minute.
A single car garage is 4m x 2m x 3m or 24 m^3. Running the car for 5 minutes just exchanged ~10% of the air for exhaust.
I'm sure my math is wrong somewhere and someone will come by to mention it's the CO concentration, not the % of air consumed (which is true), but it's interesting math none the less.
Dumb for sure but thankfully I did know enough about gas water heaters and gas stoves to have CO detectors installed throughout my house.