Chlorine Used as a Weapon in Iraq: 2006 - 2007

From Securing the City (2009):

Chlorine gas ... was the first of what have since become known as weapons of mass destruction. The Germans used it with devastating effect at the World War I battle of Ypres in 1915. When released from a container by opening the nozzle or setting off an explosive charge, it billows out as a heavy greenish fog that smells, not surprisingly, like chlorine bleach. When it mixes with the tears in your eyes, or the blood and soft tissue in your lungs and throat, it turns to searing hydrochloric acid. Outside, it settles into low-lying terrain—trenches during World War I. And in a closed space—a building or a subway—the effect is both gruesome and lethal. ...
[It showed up in] Iraq in late 2006, as the insurgents started adding canisters of the gas to their suicide bombs. By early 2007, these crude devices began attracting some attention in Washington. One exploded at a military base in Ramadi in January. It killed sixteen people, but the U.S. claimed the casualties came from the blast, not the gas. Then in February the insurgents set off a truck with two big chlorine gas tanks in it at the town of Taji north of Baghdad, killing six people and sickening scores more. A day later, they blew up canisters with a car bomb along the Baghdad airport road, killing two and sickening twenty-five. All of these attacks were in relatively open spaces. If they’d taken place in, say, the tunnels of New York, their impact would have been much worse.