Coat and skin
Dog shampoo pH: why a human product does not suit
A dog's skin sits close to neutral, human skin is acidic. What that one difference actually changes when you are choosing a shampoo bottle off the shelf.
pH, in one minute
pH measures how acidic something is, on a scale from 0 to 14. Below 7 is acidic. At 7 it is neutral. Above, it is alkaline. Skin, like any damp surface, has its own pH, and it is not the same in every species.
In adult humans, the skin surface is distinctly acidic, around 5. We talk about the acid mantle, and it is part of the skin's defence: many unwanted bacteria settle badly in an acidic environment, while the resident flora does well there.
In dogs, the skin surface sits close to neutral. The values recorded vary with breed, body area, age and measuring method, but they stay clearly higher than in humans. It is an established finding in veterinary dermatology, and it is the starting point for the whole shampoo question.
The hydrolipidic film, explained simply
On the surface of the skin there is a thin mixture of water and fats, produced by the skin itself. The fats come from sebum and from the lipids made by the cells of the horny layer. The water comes from perspiration and from natural evaporation. That mixture forms a continuous layer, the hydrolipidic film, on which a normal microbial flora lives.
This film does three things. It holds water inside the skin and limits drying out. It provides a physical and chemical barrier against irritants and passing micro-organisms. And it keeps the surface pH in its usual zone.
Any shampoo removes part of that film. That is in fact its job. The whole question is how much it removes, and how long the skin takes to rebuild the rest. A suitable product takes off what is needed and lets the skin recover within a few hours. A product that strips too hard takes off more than necessary and leaves behind dry skin that feels tight and itches.
Why human shampoo is not suitable
Two reasons add up, and pH is only the first.
pH first. A human shampoo is adjusted for acidic skin. Applied to skin close to neutral, it pushes the surface towards a state that is not its own. A dog's skin comes back to its own pH by itself, but in the meantime the ground has changed, and repeated use leaves no time for recovery.
Surfactants next. These are the cleaning agents. The ones in human shampoos are dosed for a scalp that produces a lot of sebum and that tolerates frequent washing. On a dog, they degrease beyond what is useful. Added to that are perfumes and cosmetic agents designed for human hair, which bring nothing to a dog's coat.
Finally, a dog's horny layer is structurally thinner than a human's. The margin for error is therefore narrower.
| What you read on the bottle | What it means | What it does not say |
|---|---|---|
| Neutral pH | The product is adjusted to a dog's skin | Nothing about how mild the cleaning agents are |
| Soap-free | No classic alkaline soap | Nothing about the presence of harsh surfactants |
| Natural, organic | Where the ingredients come from | Nothing about how well the skin tolerates it |
| Hypoallergenic | The commonest cosmetic allergens are absent | No guarantee for any individual dog |
What counts as much as pH
Rinsing. Product residue left in the coat irritates and itches, even with a faultless formula. You rinse until the water runs completely clear.
Temperature. Warm water is enough. Hot water speeds up drying out and increases the sensation of itching.
Frequency. The best shampoo in the world, used too often, damages the hydrolipidic film just as surely as a poor product.
Essential oils added to a shampoo or to the rinse water are not a harmless step, and they are particularly dangerous for cats, including through simple contact or through being diffused in the room. Home recipes with vinegar, lemon or bicarbonate of soda shift the skin's pH one way or the other with no control at all.
One last marker. If your dog's skin stays dry, scaly or sore despite a suitable product and careful rinsing, it is no longer a question of which bottle. Skin that does not return to its normal state indicates something a label will not settle, and that is shown to a vet.
Frequently asked questions
What pH should a dog shampoo have?
A product intended for dogs is formulated close to neutral, so a pH near 7, whereas human shampoos sit nearer 5. That value is not always stated on the bottle, and its absence is not a bad sign in itself: a product formulated for dogs by a serious manufacturer is already calibrated. In any case pH is only one criterion among several.
Is baby shampoo suitable for a dog?
No, despite its reputation for mildness. Baby shampoo is still a human product, formulated for a more acidic skin pH and for a scalp, and its mildness is mostly about being kind to eyes. Occasional use as a stopgap is not a disaster, but it should not become a habit.
What if I have already washed my dog with human shampoo once?
A one-off wash on healthy skin generally passes with no visible consequence. Rinse thoroughly with clean, warm water, then watch the skin over the following days. If redness, dandruff or scratching appear and settle in, show your dog to a vet rather than trying another product on top.
This article explains and points the way. It is not a diagnosis and is not a substitute for a veterinary consultation. It is published as a preliminary version and will be signed once a veterinary surgeon has approved it.
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Human shampoo on dogs: what the pH really changes
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