And if you happen to have confined spaces, particularly a cellar or
similar, there is a real asphyxiation risk from a leak. ...

I don't think so - breathing just a little would be so distressing that
you'd immediately get-the-hell-out. It's the rise in CO2 concentration
in our lungs that makes it so hard to hold your breath for very long.
There's still plenty of oxygen in your lungs but the rising CO2 says:
BREATH! BREATH!! You start experiencing that in your cellar and you'll
be out of there long before asphyxiation is a risk.

Bob

In the UK I believe there's about one fatality a year, often people cleaning
fermenting vessels in breweries. That said, I did some measurements and
"calcs" for my local small brewery (Uley) which is in Victorian premises and
managed to convince the Health & Safety inspector that the natural
ventilation meant there was no risk in the cellar and that they therefore
didn't need multiple linked detectors.