Thanks for the caution.
The funny-looking top on the can is sort of a gas mask and seals around the mouth if you don't have facial hair. You can breathe in through your mouth and out through your nose.
Balancing the risks, as you did, I think I'd take my chances on exhaling "used" air perhaps containing some extra oxygen and being able to get out of Dodge.
The trigger valve on it releases gas only when held down. Hey, I'm not recommending any use for it besides what's in their literature, though, just blue-skying.