NEVER RUN AN ENGINE IN A GARAGE WITH THE DOOR CLOSED
From the 1934 Wills Cigarettes 'Safety First' album.
"Never run an engine in a garage with the door closed. The practice of running the engine for a short time before starting out has everything to recommend it. The engine, however, must not be run in a closed garage as in our picture; in fact, it is never really safe to run it except in the open, as the exhaust fumes are highly poisonous. It is not the constituents of the gas that one can see or smell that are dangerous, but the invisible carbon monoxide, which is both tasteless and odourless. There is no known method of working this poison rapidly out of the system and repeated small doses can cause death."