Effective Ancient Chemical Warfare?

I was looking at the Ancient History of Chemical Warfare and considering why it was not more effective at the time. We know on OTL that it was of some effectiveness to be known and used again and again but not a ‘main weapon’. My first thoughts were that the LD50 (or deadliness) of the substances being used was so low that they were only useful if they were employed in a few niche applications: confined space like countermining or surprise or harassing. I thought that the advances in chemistry of the 1700s would be necessary for the truly “useful” agents. But looking at the toxic smokes and the resources available to the users at the time I began to consider the lack of protective equipment. If the agent is very lethal or lachrymatory or blistering then it is cumbersome thing. What is your motivation besides the niches listed above to unleash or even develop an effective agent?

What if: you developed a protective mask and other equipment?

You’d be safe if the wind changed and be able to fight in it and exploit the effects of the contaminated environment. Technologically a charcoal mask, transparent visor of glass or perhaps of sheer cloth smeared with grease, adequate clothing to cover and seal exposed skin for protection do not require new developments. There are a few hints in Ancient Rome and Greece of respirators being used in mining.

How could history of been nudged if the Chinese with their history of using Arsenical Smokes had confronted the Mongols as they became ill with fresh protected troops?

My central question is: is it feasible / plausible? How would you make it feasible (if you time traveled there for instance)
What technology/scientific advancement would it have spawned?
I imagine a new focus on poisons and antidotes/protection, meteorology, humidity? Perhaps learning to protect food & water and to use chemical warfare strategically in sieges?

Would it have made a difference in China or say Rome?
 
I think even in the 1700s no European state had the industrial capacity to produce and develop chemical weapons in a non-confined setting (so excepting siege/mining operations).

Sure, various nations did experiment with poison gases even in antiquity, but the sealing/gas production & concentration technology to use it in field battles just didn't exist, or at least weren't advanced enough to justify the opportunity cost of simply making more conventional armaments.

In sieges, again you have same problems with production/technology, but at least you have a useful niche for such weapons in counter-mining operations, and if I recall they were used until they were banned in the 1675 Strasbourg Agreement.

If I were to make poison gas feasible... I'd probably first engineer the creation of a large, centralized, despotic empire that had the resources and the time to invest in these unconventional weapons (so like a Persia), then have that country develop economically in such a way that mining becomes an important industry (where they can come into contact with a lot of poisonous gases).

The advances this form of warfare would spawn... probably meteorology is the most important one; a focus on weather prediction would be immensely helpful for generals to see whether they could actually use their poison gases. Use of poison gas would also lead to a revolution in defense as sealants will need to be found in order to gas-proof structures. Alternatively, another response could be to build underground structures where one could retreat to in the advent of above-ground gas attack, which would help develop mining technology.
 
Greek fire was essentially an incendiary device. You did have noxious gas use with burning sulfur among others. Except by accident you won't get even small amounts of true "war gasses" produced, the theory of chemistry isn't there. Furthermore even if you gave them the formulas to make mustard or phosgene the capability to do so in any significant quantity is not there even if you assume the workers are expendable. Storing and delivering the chemical weapons in a reasonable way is also not happening.

Chemical weapons in quantity and usable before at least 18th century not happening. Unfortunately protective gear is a serious problem - and until you have vulcanized rubber you can't make effective masks...
 
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