Just saw an AP photo of muslim women in Indonesia preparing to protest with toothpaste daubed under their eyes – like ballplayers smear black goo under their eyes to protect against glare. The caption said the toothpaste was to protect against the anticipated use of tear gas. How can that help?
Well the reason is that it acts as sort of a shield. For example, put some vaseline on the back of your hand, and run some water over it. What happens? The water rolls right off. Same principle basically. I forget exactly which kinds now, but this only works on CERTAIN KINDS OF GAS. Mace, pepper spray, CS, all have different properties that require different precautions. Putting vaseline (and toothpaste i imagine) on your face will protect against some kinds of gasses, but it will actually cause others to bind to you more tightly, making the effects FAR worse! Again, different precautions for different gasses…
Hope that’s the answer you were looking for… 
-Dani
Dani - nice try, but you’re suggesting that the toothpaste acts as a protective covering, if that were the case, they should smear the toothpaste all over their faces, not just daub under each eye.
Actually, if toothpaste were at all effective, then a large enough daub under the eye would be quite enough. Basically, the incoming chemicals would hit the toothpaste and bind, leaving the eye alone. Sure, you’d still get some in the eye, but instead of having your bone structure act as a funnel, with the gas bouncing its way into your ocular cavity, the toothpaste absorbes a significant portion of it. Better than nothing.
From looking around the web, it appears that even the activists aren’t certain that it’s effective. For instance:
Seems like the kind of situation where 90% of the information is just passed around by word-of-mouth and there are no real experts. So anyone with some cred and a good-sounding rationale can influence other people to do this stuff, even though it may not work. To tell whether it actually works, you’d have to be tear-gassed twice, and have used the toothpaste only the second time. I’m sure that’s a huge population of people.