How low quality is too low for drinking water?

[Vik]

I feel like a real Soviet city planner asking this one, but I'm just wondering since my water quality is at 94%, which seems pretty good, but all my buildings are saying the drinking water is too poor quality. Mind you that there's no one working or living in my city yet, but should this be a cause of concern for me?

[Sir Snuffles]

i think it's anything below 95% always build a water treatment plant, it only takes a few people to work in it and it avoids that headache of everyone getting dysentery and dieing

[Silent_Shadow]

Below 97% is considered low quality. Higher is better for health.
Keep in mind that you can buy a cheap water truck to import water and bring it to residential buildings and most workplaces. If you put it in a technical office, it can even fill up water substations.

[High King Lemur]

94% compared to 97% causes around 10% drop in citizen health.

[ling.speed]

94% is workable at start but is likely to deteriorate over time, so keep an eye in case you decide to stick with it.