CS gas (its military designation; its real name is orthochlorobenzalmalononitrile, which is a mouthful) is a fine particulate, dispersed in a fluid vehicle and propelled to its target in any number of ways. The effect of the chemical agent is mainly respiratory; you feel as if you're choking, your nose and mouth start running and you may even vomit. Also your eyes will slam shut, because CS is also an irritant on moist flesh, so if you're sweating you will feel it.
I speak from experience on this. Trust me.
The US Army does not use CS (sometimes misnamed tear gas) or any other chemical riot control agent in Iraq due to severe restrictions on its use in international treaties and rules of engagement.
But in 2005, it seems that a group of American soldiers at a Green Zone checkpoint in Baghdad were exposed to CS.
Two words: Blackwater Worldwide.
In an article in the New York Times, it was revealed today that Blackwater apparently dropped canisters of CS from a hovering helicopter as well as tossed canisters from a ground vehicle. Nothing was going wrong at the time, so it is possible that the mercs were just trying to either claer the intersection or screw with the Iraqis below. The US troops were just in the wrong place at the wrong time, it would appear.
Unfortunately, Blackwater operates under its own rules - that is to say, no rules at all - and it's protected by the ukase decreed by our second Viceroy of Iraq, L. Paul Bremer. This shit happened two years ago, and we're only now finding out about it.
It's time to see the end of this company and its involvement in Iraq before their actions get our soldiers killed and not just suffering runny noses and watery eyes.