They always tell you that CO is odorless and invisible and deadly, and they surely speak the truth. So back in the day - before carbon-monoxide detectors were widely used - why didn't more people die of it?
I'm thinking it was because you don't usually encounter CO alone. Usually it's a product - and not the sole product - of combustion. So even if you can't smell CO, you can smell the things that come with it - i.e., you can smell other ingredients of exhaust or smoke.
In this way, smoke
is healthful - no joke -
as a warning sign
for what's more malign.
There was an old-fashioned low-tech CO detector:
Canaries were once regularly used in coal mining as an early warning system. Toxic gases such as carbon monoxide and methane in the mine would kill the bird before affecting the miners. Because canaries tend to sing much of the time, they provided both a visual and audible cue in this respect. The use of so called miner's canaries in British mines was phased out as recently as 1987.Of course this job was very
scary for the canary.