The natural pH of healthy hair and skin is approximately:

The natural pH of healthy hair and skin is approximately 4.5 to 5.5, which is slightly acidic. This slight acidity, sometimes called the skin's 'acid mantle,' helps protect against bacteria and keeps the hair cuticle compact and smooth. Products designed to be 'pH balanced' aim to stay close to this range. Strongly alkaline products (like some relaxers) raise the pH and swell/open the cuticle, which is why an acidic neutralizing or balancing step is often needed afterward to restore the natural pH.