Mommy always told me to never do that.

Hmmm, if we ignore Mommy's advice, think of the possibilities with one of those big plastic dry cleaning bags!

Ugh, sorry, I haven't had my morning coffee yet...

Seriously, it's a good idea in certain situations, if it comes to that. Better than getting a big whiff of tear gas or anthrax spores or whatever, until I can hightail it out of immediate danger. Think about many of those smoke or "escape" hoods on the market--many are essentially a plastic bag you put over your head, except these have a way to breathe through them (I'm being safe, Mom, really!)