That was the first thing my mom told us when as kids went to protests against Pinochet. CS reacts with water, even if you are not feeling the effects water can make the chemical react again.
The CS gas effects are just temporal, no longer than 5 minutes... try to survive them.
I've been documenting social movements in chile since 10 years ago, i've been exposed to CS directly more than a hundred times. I know you will panic, but never try to use clean water on an affected area.
salud
ps. I don'know if sodium metabisulphate works, but cops use a chemical when they got hit by CS, if this is the thing they use ill give you a pizza when you come around here.
when the gas comes, remain calm. When you feel yourself panicking from the gas, breath through the onion. For some reason, the smell of the onion (in Turkey I've also seen people using lemons) seems to re-center you and mitigate some of the instant psychological effects of the gas (i.e. suddenly you're in a poison cloud).
Never been sprayed directly, but I was cleaning out a dorm room for a summer-time job and someone had left a bottle that went off by accident. And somebody in a club I was in decided to mace the room one time... ugh...
To combat the effects carry a bottle of solution made up from water and sodium metabisulphate (sold as Campden tablets used in home brewing) as this combination neutralises the effects of CS. If you cannot get this use clean water to rinse the eyes and skin affected.