OT but sort of DIY

[Dave Liquorice]

filled

I see plenty of refrigerated trucks about but they don't have that notice, even ones that are mobile freezers rather than just chilled. Those levels of cold won't make you keel over in 30 seconds, low levels of oxygen will.

It's rather a while ago and there additional words like "caution" and "something something atmosphere".

Sorry, not giving the full picture, something I hate when others do it. .

[Andrew Gabriel]

Nowadays they're mostly real refrigeration units. They used to use a lump of evaporating dry ice (CO2) or tank of slowly boiling liquid nitrogen, either of which will slowly displace the air in the container.