The surface is acidic, and that is functional
Measured at the surface, intact skin is mildly acidic. The value varies by site, by age, by time of day and by what has recently been done to it, and we are not going to quote a figure as though it were a constant, because it is not one.
What is worth knowing is why the acidity exists and what depends on it. It arises from several sources: products of the breakdown of filaggrin inside corneocytes, free fatty acids in the lipid matrix, components of sweat, and the metabolic activity of resident surface organisms.
And several things depend on it. The enzymes that process secreted lipid precursors into the ordered sheets that make up the barrier are acidic pH optimal. The enzymes that degrade the protein structures holding corneocytes to one another, allowing controlled shedding, work faster at higher pH. Some antimicrobial peptides at the surface are pH sensitive, and the composition of the resident microbial population is influenced by surface acidity.
What raises it
Traditional soap is alkaline by chemistry. It is made by treating fats with an alkali, and what is left is a salt of a fatty acid that raises the pH of anything it is used on. This is not a design flaw; it is what soap is.
Most modern liquid washes are formulated with synthetic surfactants and can be made at a pH close to that of skin. Many are. Many are not, and the label rarely tells you. The words gentle, natural and dermatologically tested carry no information about pH.
Hard water raises effective alkalinity at the surface and interacts with soap to leave residues. Sea water is mildly alkaline. Chlorinated pool water is maintained within a range that is generally near neutral to mildly alkaline, and the water chemistry of pools is properly the subject of a different publication, so we will leave it at that.
How long it stays raised
After a single wash with an alkaline product, surface pH rises and then falls back towards its baseline over a period generally described in hours. The rate at which it recovers depends on the site, the age of the person and the state of the barrier, and it is slower in skin that is already compromised.
Here is the part that matters for anyone in cold water sport or outdoor work. If pH takes some hours to recover and you wash your hands eight times, shower after a session, then shower again after work, the surface may never spend much of the day at its functional value. The insult is not any single wash. It is the interval between them being shorter than the recovery.
This is also why occupational hand skin problems are so common in jobs that require frequent washing, and why the Health and Safety Executive treats work related dermatitis as a preventable occupational health issue rather than a personal misfortune.
| Property of the wash | What it changes | Whether the label usually tells you |
|---|---|---|
| Alkalinity | Slows lipid organisation, speeds up shedding | Almost never |
| Surfactant strength | Removes intercellular lipid along with dirt | No |
| Water temperature | Warm lipids are more mobile and are removed more easily | Not applicable, it is your decision |
| Frequency of use | Whether recovery ever completes between events | Not applicable, it is your decision |
| Fragrance content | Contact sensitisation risk, not barrier function | Sometimes, and see our article on fragrance free |
Ordering framework written by this publication from general skin physiology. It is not a measurement, it is not taken from any study, and no number in it is a reading.
What follows practically
The practical conclusions are unglamorous.
Wash fewer things. Most of the body does not need detergent most days. Rinsing removes salt and sand perfectly well. The parts that need washing are a short list and you know what they are.
Wash the parts that need it, not the parts that do not. A wash used on the whole body because it is convenient is a lipid stripping event applied to shins that were fine.
Prefer a wash whose pH is close to that of skin. Where a product states it, you have information. Where it does not, a syndet bar or a liquid described as soap free is more likely to be near skin pH than a traditional soap bar, which by chemistry is not.
Reduce the temperature. This is a separate mechanism, but it is free and it is applied at the same moment.
Where the pH story is overstated
Skin pH is a genuine and useful concept and it is also a marketing hook. Two overstatements are worth naming.
The first is the idea that any product can restore or rebalance your skin's pH in a lasting way. Surface pH is maintained by the skin's own processes and returns to its own value once the insult stops. What a wash can do is not raise it in the first place.
The second is the idea that a low pH product is automatically good for you. Products can be too acidic for comfort, particularly on a compromised barrier, and there are ingredients whose activity depends on being formulated at a low pH which are not appropriate on skin that is already cracked. Lower is not a scale where more is better.
Hard water, briefly
Water hardness varies enormously across the United Kingdom. Hard water requires more product to lather, leaves more residue when used with true soap, and has been discussed for years in connection with skin symptoms. This is a real area of research and the picture is not simple, so we will describe only the mechanism that is not in dispute: hard water plus soap produces insoluble residues on the skin, and those residues are removed by more washing, which is itself the insult.
If you live in a hard water area and your hands are bad, the cheapest experiment is to use less soap rather than more water. Softening equipment is a plumbing decision and this publication has nothing to sell you.