Carbon monoxide is deadly. It is also invisible, odourless and tasteless so it is impossible to know if you are breathing in CO (typically from a faulty exhaust or cabin heater system) until it is too late – unless you have a CO detector.
All powered aircraft cockpits should be fitted with at least one CO detector.
The simplest are the chemical reaction type, like this, which have a coloured spot which goes black in the presence of carbon monoxide.
These need to be periodically replaced (see the detector itself for the relevant time period, usually six months from opening the pack).