Your skin’s pH: why slightly acidic matters

The surface of your skin is slightly sour. Not in a way you’d ever taste, but on the pH scale it sits on the acidic side, and that turns out to matter for much more than keeping germs away. Here’s what skin pH is, what it does, and what nudges it in the wrong direction.

A quick refresher on pH

pH is a scale from 0 to 14 that measures how acidic or alkaline something is. Seven is neutral, like pure water. Lower numbers are acidic (lemon juice, vinegar), and higher numbers are alkaline (baking soda, bar soap). Each step is a tenfold change, so pH 4 is ten times more acidic than pH 5.

Inside your body, things are kept close to neutral. Blood, for example, sits around 7.4. Your skin’s surface is different. Healthy skin runs acidic, usually somewhere between pH 4 and 6, which creates a steep gradient of two or three pH units between the surface and the living layers just underneath.

Almost a century ago, German researchers named this the “acid mantle”. The name stuck, even though we now know the acidity comes from several sources working together rather than one coat.

Where does the acidity come from?

It isn’t one thing. Researchers have identified several contributors:

• Filaggrin breakdown. Filaggrin is a protein in your outer skin cells. As it breaks down, it releases amino acids and urocanic acid, which help acidify the surface.
• Fatty acids. The free fatty acids in the lipid “mortar” between skin cells add acidity.
• A proton pump. A molecule called NHE1 (a sodium-hydrogen exchanger) pushes acid into the spaces of the outer layer.
• Sweat. It contains lactic acid, which adds to the mix.

A large study of 330 people found an average forearm pH of about 5.1. When participants skipped showering and products for 24 hours, it dropped to about 4.9, and the researchers estimated that untouched skin sits at around 4.7, a little lower than the 5-to-6 range often quoted.

pH also varies across your body. Armpits, the groin, under the breasts and between the toes run less acidic, which helps explain why those spots are prone to odour and yeast infections, according to one review. Newborns start out close to neutral, and their skin becomes more acidic over the first weeks of life. Skin in older adults tends to drift back up a little.

Why acidic matters: the enzymes

The biggest reason pH matters is that it switches enzymes on and off. Enzymes are proteins that do chemical work, and each one has a pH where it works best.

Two enzymes that make ceramides, key lipids in your barrier, work best in acidic conditions. One of them is about ten times less active at pH 7.4 than at pH 5.5. So an acidic surface helps your skin build and organize the lipid layers that keep water in.

Other enzymes do the opposite job. A group called serine proteases snips the tiny rivets that hold dead skin cells together so they can shed. They prefer a more neutral pH. When the surface becomes less acidic:

• the lipid-building enzymes slow down
• the shedding enzymes speed up
• the barrier gets leakier and the outer layer holds together less well

In lab and animal studies, a barrier damaged by stripping recovered faster under an acidic solution than a neutral one. After experimental barrier damage, pH goes up and takes many hours to return to normal.

Your skin’s bacteria like it acidic too

Your skin is home to a community of microbes, the skin microbiome. Most of the usual residents grow best at acidic pH. Staphylococcus aureus, a bacterium that often causes trouble in eczema, prefers a neutral pH and is markedly held back around pH 5.

In one lab comparison, a natural antimicrobial substance from sweat killed more than 90% of Staphylococcus aureus at pH 5.5, but only 60% at pH 6.5. The study of 330 people also found that acidic skin keeps friendly resident bacteria attached, while an alkaline pH encourages them to drift off.

What pushes pH up?

Plenty of ordinary things raise skin pH, usually for a few hours.

• True soap. Soap-based cleansers are typically alkaline, around pH 10, while syndets (cleansers made with synthetic detergents) are usually neutral or acidic. One study found washing hands with soap raised palm pH by an average of three units, and it was still elevated 90 minutes later. Reviews link more alkaline cleansers to more irritation.
• Plain tap water. Tap water is often slightly alkaline. Health Canada’s operational guideline for drinking water is a pH of 7.0 to 10.5. The large pH study found tap water alone raised skin pH for up to six hours before it drifted back.
• Hard water. Hard water carries lots of calcium and magnesium. In a 2018 study of 80 people, washing with a cleansing agent in hard water left more residue on the skin, which increased water loss and irritation, especially in people with eczema and a filaggrin gene variant. Softened water reduced the effect.
• Occlusion and moisture. Diapers, tight dressings and skin folds trap urine, sweat or moisture, which can make those areas more alkaline.

For healthy skin, these bumps are normal and temporary. The concern is small, repeated rises day after day, which reviews suggest can wear down the barrier’s repair system.

The eczema connection

Skin pH is consistently higher in atopic dermatitis (the most common form of eczema). In one study of 100 children with eczema, pH was higher both in the patches and in skin that looked normal, compared with healthy children. Higher pH has also been measured in the itchiest and driest areas.

There are several reasons. Eczema skin often has less filaggrin, so fewer natural acids. It may produce less sweat. And inflammation disturbs the lipid system. The higher pH then feeds back: it speeds up the shedding enzymes, slows ceramide production and favours Staphylococcus aureus. Researchers now think abnormal pH is a meaningful part of the problem, not just a side effect.

Reviews describe acidifying the skin as helping in lab and animal studies, but the research on acidic products in people is still limited. If eczema is flaring, see your doctor or pharmacist about treatment, rather than relying on pH alone.

What you can do

You don’t need a pH meter. A few habits keep the acid mantle happy:

• Choose a gentle, soap-free cleanser, especially for your face, hands and any dry or eczema-prone skin. A fragrance-free cleanser is a good starting point.
• Keep showers short and lukewarm, so tap water isn’t sitting on your skin longer than it needs to.
• Rinse well, and moisturize soon after washing.
• Don’t over-wash, and save the cleanser for the areas that really need it.

Your skin is built to be slightly acidic, and that acidity quietly runs the enzymes that make your barrier, keeps shedding in check and favours friendly bacteria. Soap, hard water and long showers push it up for a few hours; eczema-prone skin tends to stay higher. Gentle cleansing, short showers and regular moisturizing let your skin get back to its natural, slightly sour best.