The above writeups are excellent, but I would like to make a few important additions to both.
1. Most chemical agents are not gases, they are more likely to be vapor. This is an important physical characteristic and is key in predicting downwind hazards and persistancy.
2. Biological warfare has been with us for a very long time. Some historians have speculated that the black plague was caused by biological warfare during a city seige in which corpses, infected with the plague, were dropped in the city's water supply and catapulted over the city walls. The survivors were scattered across the country when the city fell, and the plague spread.
3. Chemical weapons were used much earlier than World War I. Poisons were used in ancient times for assasinations, and "Greek Fire" was used to great effect by the Byzantines on both land and water in a manner similar to napalm. Wooden boats were very succeptible to this method of warfare.
4.Despite the danger, offensive biological weapons research continued well into the sixties in the United States and into the Ninties in the former Soviet Union. Biological agents were stockpiled in large numbers in both countries, though the US offensive stocks have long since been destroyed. 60 Russians were killed when an accidental discharge from an Anthrax manufacture plant spread weaponized anthrax over the village of Sverdlovsk in 1979.
5.Cost Per KM2 Producing Mass Casualties
6."Blood" agents do not, in fact, affect the blood's ability to carry oxygen. Instead, they prohibit the mitochondria's ability to process the oxygen in the cell once the blood gets it there. The hemoglobin transport works just fine, it's the cells that stop working.
7. Only 800 were killed at Ypres when the Germans used Chlorine gas on the French. Of the 15,000 troops there, 5,000 were casualties, but only 800 of those died as a result of their exposure. In fact, for those countries that had good training and equipment, such as the United States, fatality rates were as low at %2 of those soldiers exposed to agent. Even in Russia, where training and equipment were very poor, the fatality rates only ran as high as %12.
8. Though every NATO unit had some method of chemical detection equipment, there were no confirmed cases of the deployment of chemical weapons by the Iraqis duruing the gulf war. Though some soldiers were possibly exposed to trace amounts, this was due to the unintentional destruction of an Iraqi ammunition cache containing chemical munitions.
9. For both CS and Pepper spray, the amounts of agent required to be inhaled or ingested are so great that their role in the deaths that are attributed to them are highly suspect. Basically, you'd have to stand right over a burning CS candle for 6 or 7 hours, inhaling deeply, to inhale a fatal dose.
Most of this data was shamelessly stolen from the good folks at the US Army Research Institute for Chemical Defense, Chemical Casualty Care Division's Field and Medical Management of Chemical and Biological Casualties course. If you're a first responder or medical professional who could benefit from a week-long course of instruction in responding to this sort of attack, visit their webpage to see about attending a course. The training is open to civilians. http://ccc.apgea.army.mil/