Tear Gas

TL;DR

something.

While there are many so-called tear gases 2-chlorobenzalmalononitrile (aka CS gas) is the most commonly used.

Banned for use in warfare but generally not for civilians.

Solid at room temperature so typically one technique or another is used to create an aerosol from the powder.

Neutralisation

Rinsing/flushing irritated areas with clean water is considered the safest way to neutralise the effects.

While rinsing/flushing with a 4% sodium-bicarbonate solution is likely more effective, it does carry some risk unless you can be absolutely certain the gas being used is actually CS and the chemicals used to create and disperse the aerosol don't contain anything the sodium-bicarbonate will react with to make things worse.

Try not to directly touch or rub effected areas.

Avoiding Exposure

Move away from any clouds, canisters etc.

Seek higher ground, most tear gases are heavy and have a higher concentration nearer the ground.

Further Reading

I found this article very informative: https://phr.org/our-work/resources/preparing-for-protecting-against-and-treating-tear-gas-and-other-chemical-irritant-exposure-a-protesters-guide/.