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For @sciam I took a look at less-lethal weapons. As is probably unsurprising, weapons designed for narrow use in edge cases are easily & commonly misused against crowds. People suffer as a result while police forces shrug & say "would you prefer batons?" https://www.scientificamerican.com/article/what-less-lethal-weapons-actually-do/
One of the more fascinating pieces of teargas history I learned reporting this is that CS (the standard tear gas in use today) was simultaneously fielded in Vietnam and in the US, with each instance referencing the other as a justification. https://www.scientificamerican.com/article/what-less-lethal-weapons-actually-do/
For the US forces in Vietnam, selling tear gas as a "riot control agent" helped the counter-insurgent mission justify itself as using effective but less-than-deadly tools to flush people out of cover. For police stateside, the Vietnam use case branded CS as effective and proven.
A broader theme, woven into the piece & drawing heavily from the work of @stschrader1, is that much of what we know as policing today has its orgins & evolution as the practices of occupying colonial forces. "Baton rounds" start in British Hong Kong, "rubber bullets" in Ireland.
Another part which didn't quite fit in the final version is that while tear gas is prohibited from battlefield use in war, the Geneva Conventions *explicitly* have a carve out for "riot control agents," which marks tear gas as what it most clearly is: a weapon of colonial warfare
The reason tear gas is prohibited from battlefield use between uniformed militaries (nations, practically speaking) isn't so much about the gas itself, but about the inability to distinguish between the less-lethal tear gas and explicitly lethal other chemical weapons in war.
It's the sort of thing where, if a nation had a stockpile of, say, mustard gas, and heard that their forces were hit by an unknown gas, they might respond with what they think is in-kind and escalate in an especially grim way. Laws of war are weird, but that's what that is.
One other note: using tear gas to disperse people is, as these things go, a use that mitigates the harms from the gas, as people leave a space and the aerosol dissipates. Using tear gas to pin people in place for arrests, say in conjunction with kettling, exacerbates these harms.
