There's a few diagrams where they show ideal placement of the thermostat/electrical switches to minimize the danger of explosion in case of a gas leak into a confined space like the inside of an icebox.
I think this is what they're talking about - normally the charge in these systems is so small that a leak would not be dangerous...but if it leaked into a confined space with an ignition source you could be in trouble!
However, based on the information that article contains about the size of the charge needed in a small R290 system and the LEL (lower explosive limit) the danger of explosion seems nonexistent in the kind of setups we run
*disclaimer, if you've ever seen a butane lighter thrown into a camp fire, you know that even a tiny amount of a flammable substance under the right circumstances can be VERY dangerous...be careful friends
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