'CS gas' is a misnomer. CS stands for ortho-chlorobenzylmalononitrile, which is a white solid powder. It is usually dispersed into the air with an agent such as methylene chloride to form something often referred to as 'tear gas'.
It is an incapacitant commonly used by military and law enforcement agencies worldwide. It has a distinctive peppery smell, which means it is sometimes incorrectly referred to as 'pepper spray' (true pepper spray being literally a suspension of cayenne pepper derivative in an aerosol propellant).
CS is also a popular training aid for armed forces. Training to use NBC (nuclear, biological, and chemical warfare), protective equipment can be made more realistic if the trainee knows that making a mistake will lead to a dose of CS.
CS gas causes a severe burning sensation in the eyes, streaming tears, coughing, excessive production of mucus from the nose, and often panic. The blood pressure rises, and the breathing and heart rate slow. It is also a skin irritant and causes a burning sensation on any skin exposed to it. Nausea and vomiting may also be caused.
CS can be delivered from handheld spray canister, by thrown grenade, or in bulk by larger spray devices. It is not recommended for use indoors due to the panic and disorientation it tends to cause.
A small but significant minority of people is immune to the effects of CS. They may experience some of the symptoms in a mild form, or may only be aware of its presence by the distinctive smell. The reasons for this immunity are unclear.
One of the main advantages of CS is that the effects wear off in only a few minutes if the affected person is exposed to fresh air. There are no known long-term effects of exposure, and there is no known case of a death due to CS exposure anywhere in the world.
What to Do
If you are exposed to CS in any environment, immediately hold your breath, and do not panic. Almost completely close your eyes, and expect to be blinking a lot. If you are a long way from fresh air, cover your face with a clean cloth - your outer sleeve is probably covered with the stuff so pull your sleeve up or inside out if necessary.
Remember, CS is not a gas. It's a powder, so it's relatively easy to stop the stuff getting up your nose. Don't breathe through your mouth if you can avoid it - it will give you a bigger dose quicker and will cause coughing and possibly vomiting.
Your first priority should be to get to fresh air. If you are indoors, get out as quickly as possible. If you are outdoors, try to see where the CS is coming from and get out of the way. As soon as you get into fresh air, keep moving. Your clothes will be coated with the stuff, but the majority will blow away in the wind, so hold your arms up and turn around. When you get the chance, wash the clothes thoroughly separately from any others. Above all, remember that the effects will wear off very quickly when you reach fresh air, so do not panic.