Ancient Learning Curve

In AD 256 during a siege at the Syrian city of Dura-Europos a number of Roman soldiers in a tunnel under the walls died a terrible death. The theory is that the opposing Persian soldiers killed them by toxic fumes. The story is:

http://www.livescience.com/13113-ancient-chemical-warfare-romans-persians.html

This sets out the information and what is known fully. It seems the ancients knew about the toxic properties of certain petro-chemical substances in confined spaces and subject to chemical reactions.

As yet, it seems either we have forgotten or are just beginning the process of relearning.