The answer is 5.0. Hair and skin naturally sit on the slightly acidic side of the pH scale, well below neutral (7).
That number is the benchmark for judging products — pH-balanced shampoos and conditioners are formulated to sit near 5 so they don't disturb hair's natural acidity. Plain water, by comparison, is close to neutral and noticeably more alkaline than hair and skin, which is part of why hair can swell when it's oversaturated.
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