It's something I've noticed a long while ago, back when we got the Zaun and Piltover lore update at the beginning of the season...But, I think there are narrative clues suggesting that Piltover and Zaun's artificial hextech & chemtech industries have some very horrific and downright morally inhumane implications going for them. And I mean _ Extremely dark_ implications.
INC. text mountain ahead:
First of all, Camille's bio states that her family couldn't rely traditional hextech crystals to propagate their business forever, due to the fact that they were a limited natural resource from an endangered species that they were quickly driven to extinction thanks to their efforts. So they "utilized shadow investments in chemtech and runic alchemy to produce and market the less powerful, but easier to procure, synthetic hex-crystals" to maintain their power in the tech industry, as directly stated from her bio page on the Universe site. The same artificial hextech crystals that are responsible for the pollution that's slowly infecting & killing off so many of Zaun's inhabitants.
I mean really, "shadow investments in runic alchemy." If we know anything from reading Ryze's lore, then there's some seriously powerful dark magic evolved. Definitely not the kind of dark magic that mortals should be dabbling in, for sure. After all, the ancient Rune Wars nearly destroyed Runeterra and Ryze's home-city was practically nuked out of existence because of it.
It get's even more suspicious with the recent lore bits that Riot added into their in-game loading screen durations this season. Here's one that always stood out for me in particular:
"The Incognium Runeterra in Piltover was designed to locate anyone in Runeterra, yet its creator mysteriously died before revealing the formula that powers it, leaving it inoperable."
Apparently, there are people very, very high on both Piltover and Zaun's social ladder who_ do not_ want this secret formula getting out and will do anything to keep said secret from being exposed to the public. Including outright murder. So that alone indicates that whatever they're doing is downright illegal and morally wrong on a massive scale.
And when we look back at traditional hextech crystals - we already know that those contain the souls of the Brackern, Skarner's race. In fact, this is already already common knowledge to Runeterra's population at large; Camille's family house did set up their public mining operations in Shurima after all. It's just that nobody ever knew that the Brackern were fully sentient beings in a state hibernation - assuming they were mindless beasts instead. So the Brackern aren't the "big secret" here, as everyone else found it excusable to drive them into extinction thanks to the misunderstandings mentioned above.
BUT... if you took took human souls and forcibly bound them into artificial hextech crystals.. well, that's bound to create some serious moral strife, isn't it?
Even moreso if the primary source of faux-hextech manufacturing is Zaun's impoverished working class, which Piltover and Zaun's elite already treat as a disposable resource. It's genius when you think about it, really. Repress the bulk of their working class from climbing up the social ladder so that they're forced to live in and die young from the Zaun grey. Younger life spans = more deaths. More deaths = more souls to amass and harvest as faux-hextech crystals. More crystals = more global expansion and profit for Piltover and the Zaunite elites. It's an endless cycle that works in their favor, and gives more purpose for Piltover & the Chem Barons to retain that massive working class population under their complete control. After all, those worker-drones even have a use after death! It's Runeterra's own Soylent Green scenario.
But what would happen if the secret was finally exposed? Well there would be a massive Zaunite social uprising overnight, no doubt. Innocent & unknowing Piltovan civilians would be massacred in the streets by the thousands. And there would no longer a steady supply of faux-hextech crystals. It's enough to completely destroy both Piltover and Zaun's economies altogether; taking Runeterra's global economy with them since both cities are the mercantile powerhouses of their world.
Also, I would think that human souls don't exactly function like Brackern souls. It probably isn't natural for them to be incased in crystals, let alone crystals that were artificially created from malevolent & dark runic magic - hence the Zaun grey. Maybe it's a stretch - but I think you're right in that the Zaun grey could very well be a supernatural "curse" of sorts. My crackpot theory is that the Zaun Grey is actually mass of both human and Brackern souls that have taken on a gaseous form after breaking free from their expelled & used crystals, and are attempting to "possess" victims in an effort to regain a new living form.
Over time, those "gaseous" souls eventually take on a collective, liquefied form.
And that's where Zac and the sentient pool of ooze come in.
Also some things about Ionia and Warwick, but this post is getting to long so I'm just gonna end it for now and maybe finish the rest of it tomorrow. Call it a stretch if you want, but those are the theories and conclusions I've come to. Would be neat if they turned out to be true but I'm not holding my breath, lol.