Some more information:
https://en.wikipedia.org/wiki/Inert_gas_asphyxiation
I recall reading about some inspectors that went into the hold of a ship containing scrap steel. The rusting steel had removed all the Oxygen from the air. The inspectors never knew what hit them.
Back in the 1990s, four men died in my town when a leaking tank of R-22 Freon filled the sump in which they were working.
Actually, two died first. Then a third died when he went down to see what was wrong, and the forth was a supervisor who went down to rescue the other three.
I don’t think any of them knew what had hit them.