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I posted this last night with the wrong link. Here's the right one.

I think @dulcima mentioned this happened at one or another protest. At the time it was reported as a sonic weapon, but those don't make people's face burn.

Anyhow, long story short, some auminium wire mesh or aluminium foil can be used as a shield. If you bring a poster to the protest put aluminium in the middle. If it's made of corriboard it should also help against auditory weapons and water cannons.