The Earliest Use of Chemical Warfare

New evidence from the Roman city of Dura-Europos. The city was under siege in the later years of the empire by the Sassanians. The archeologists there found twenty Romans dead with their weapons still clasped in their hands in a small tunnel or mine. What they think happened was that the Romans established a counter mine underneath the city, to counter the Sassainian mine that was being dug under the walls to collapse them. The Sassainians then released a gas made up of sulfur crystals and bitumen. This would and did immediately render the Romans unconcious and kill them within  minutes.

http://www.world-science.net/othernews/090116_chemical.htm