• Laura Chaparro, science journalist

    The ‘acid touch’ of the largest human organ

    24 Nov

    Research has revealed that human skin is more acidic than was previously thought. To keep skin healthy and hydrated, use of less alkaline soaps and cosmetics is recommended.


    The skin, the largest organ of the human body, measures two square metres in size. However, when we think of the most important organs, we always think of the heart or brain, overlooking this fine sheath that completely encloses us. The skin is our main barrier against external agents, preventing viruses, harmful bacteria and mites from entering our bodies. The skin also plays a key role in controlling body temperature and eliminating toxins. One skin layer is partly composed of slightly acidic lipids that make it hard for germs, microorganisms and other parasites to get through.

    The importance of pH
    The acidity of a substance is measured on a pH scale. Pure water has a neutral pH of 7 and European tap water has a pH of 8. Substances with a pH below 7 are acidic and those with a pH above 7 are alkaline. Until fairly recently average skin pH was believed to be 5.5, with most scientists considering it to range between 5 and 6, depending on body zone and sex; skin pH is generally lower in men than in women.

    However, a 2006 International Journal of Cosmetic Science study by Lambers and coworkers, demonstrated average skin pH to be closer to 4.7. Skin is therefore more acidic than was previously thought.

    These conclusions were based on pH measurements for 330 people from Spain, the Philippines, the Netherlands and Germany. As tap water (pH 8) and soaps were known to increase pH, none of the individuals in the study could shower or use cosmetics during the experiment.


    Is it better to have slightly acidic skin?
    The slightly acidic pH of the skin is due to the lipid defence layer known as the acid mantle. Acid pH levels support the correct development of the skin flora, composed of the microorganisms with which we live in harmony. Many billions of bacteria from a thousand different species, along with a wide range of fungi and yeasts, live on the skin of each human being. This microflora plays a role in stimulating the immune system and is responsible for body odours. The Lambers study pointed to a symbiotic relationship between the skin and the flora: the skin provides the microorganisms with lipids, minerals and proteins, whereas the microorganisms bolster the human body’s first line of defence - the acid mantle - by blocking invasion by harmful bacteria. The study also demonstrated a link between higher (less acid) pH values and diseases such as atopic dermatitis, irritant contact dermatitis, acne and ichthyosis.


    Ensuring a healthy skin
    How can we help our flora fight off external agents? The first answer is obvious: we should maintain our natural skin acidity and so safeguard the flora’s protective capacity. Since tap water itself is alkaline, even after washing without soap the skin can take up to six hours to regain normal acidity (below pH 5).
    Skins with a pH between 4 and 5 are in better condition than those with a pH above 5. Consequently, experts recommend the use of gels, soaps and cosmetics with a pH as close as possible to 5, but never above 6.5, as values higher than this can have a negative impact on the lipid mantle. Manufacturers have therefore been developing formulations with increasingly lower pH levels.

    The importance of hydration
    One simple procedure can also enhance care of our skin: hydration with a suitable product. Various components in moisturizing products are essential for proper maintenance of the epidermis, especially the outer layer containing the lipids that maintain the protective flora (the stratum corneum). Moisturizers usually contain combinations of humectants (to absorb and retain water), occlusive agents (to create a water-retaining fat layer) and emollients and epidermal lipids (to add fat to the inner layers). Moisturizers therefore fulfil two functions: they retain sufficient water to ensure constant skin-cell reproduction and replacement and they provide the fat necessary for flora to proliferate in our protective shield.

    International Journal of Cosmetic Science, 2006, Vol. 28, Issue 5, 359-370