Good question! Baking soda and other high pH cleansers like traditional soaps (castile, cold processed, hot processed, etc.) are frequently recommended on all kinds of natural/DIY skin care blogs, and on Pinterest you’re never more than five pins away from a baking soda scrub that promises to solve all of your skin problems—and these frequent recommendations are the entire reason I ever started using baking soda on my face—so I realize this is a bit of an odd stance to take in this “space”.
However, I’ve been doing lots of research, and it turns out that using basic (or high pH) substances (like soap and baking soda) on your skin (and especially your face) is really, really not good for your skin in the does-damage-over-time kind of way. Eek!
Our skin has a protective acid mantle that forms in the days and weeks after we are born (source, source). This acid mantle has a pH around 4.5–5.5 (it fluctuates with age, gender, and skin tone [source]), and that’s where our skin does best. Our acid mantle is a huge part of what keeps our skin effortlessly happy: “In the last decade it has been demonstrated that skin pH largely influences barrier homeostasis, stratum corneum integrity and cohesion, and antimicrobial defense mechanisms” (source). A healthy acid mantle means your skin naturally protects itself from gross stuff like bacteria, fungi, pollution, and drying out. When you repeatedly assault that acid mantle with products that have a drastically different pH, you start to destroy that natural protective layer and alter the pH of your skin (source).
When our acid mantle is unhappy, so is our skin. Your defenses are down and your skin is left open to inflammation, infection, dehydration, acne, general sensitivity, and a bunch of stuff that’s definitely on your oh dear heavens no to-don’t list for your skin.
Now, your skin will mostly correct its pH within 2–6 hours, so the odd exposure to a high pH substance isn’t the end of the world. However, “Small and sustained pH increases, like those caused by daily use of soap-based cleansers multiple times a day, adversely influence the barrier repair mechanism (source).” This fascinating study looked at two groups of subjects: one group washed with alkaline soap over four weeks, and the other washed with acidic syndet bars for four weeks. Both groups noticed a significant bump in skin pH directly after washing, but this was mostly resolved within two hours. However, over time, those using the acidic syndet maintained a stable or slightly declined pH, while those who used soap noticed an increase. “There are long-lasting effects with as few as two washing procedures of 1 minute each a day… according to a randomized open crossover trial, skin surface pH increases on the regular use of a conventional soap and decreases again after the change to an acidic cleanser (of pH 5.5) and vice versa… Hence there is amply evidence that there is both a short-term and a long-term effect on skin surface pH if a cleanser is used whose pH deviates from the pH of the skin surface to which it is applied. In keeping with this hypothesis, so-called natural cleansers [like cold process soap] are by no means neutral in a biologic sense (source)”.
These results were duplicated using two syndet bars to see if perhaps it was the soap, and not the pH that was the issue. One syndet bar had a pH of 8.5, while the other had a pH of 5.5. The participants using the pH 8.5 bars had a significant increase in propionibacteria population. “The results suggested that even minor differences—in the order of a single pH unit—in skin surface pH markedly influence the resident flora, in particular propionibacteria (source).” They went on to study soap vs. acidic syndet in acne-prone individuals over 4 weeks: those using soap experienced an increase in acne and irritation, while those using the syndet bar experienced a decrease in acne and significantly less irritation. Another study demonstrated “that high-pH
solutions even in the absence of surfactants increase stratum corneum swelling and lipid rigidity (source),” so we do know that it’s the pH, and not the soap that causes issues.
Because infants are born without an acid mantle and gradually acquire one over the first month or so of life, they’re great little case studies on life without an acid mantle. “Elevated pH is known to increase activity of serine proteases, kallikrein 5 and 7, which are involved in desquamation and degradation of corneodesmosomes” (source)—basically, when your skin pH is higher than it should be, you start to see your skin dry out as its barrier function degrades. “Additionally, key enzymes involved in the synthesis of the permeability barrier, β-glucocerebrosidase and acidic sphingomyelinase, which require an acidic pH are not fully activated in the newborn period resulting in decreased skin hydration (source)”. So, babies are prone to dry, easily irritated skin because their acid mantle hasn’t developed yet.
Skin pH: From Basic Science to Basic Skin Care discusses how several enzymes that are essential to the barrier performance (and barrier regeneration) of our skin are pH dependent. “Two key lipid-processing enzymes, β-glucocerebrosidase and acidic sphingomyelinase have pH optima of 5.6 and 4.5, respectively. Both are involved in the synthesis of ceramides, critical components of the permeability barrier. Activity of β-glucocererbrosidase is 10 times lower in situ at pH 7.4 than at pH 5.5.” If the pH of your skin is higher than it should be, the ability of your skin to protect itself drops significantly. “Studies have shown that elevations of pH in normal skin creates a disturbed barrier, linked to increased activity of serine proteases and reduced activities of ceramide-generating enzymes (source).”
Messing with the pH of your skin also throws off its microbiome—that is, it makes the good bacteria unhappy, and rolls out the welcome mat for the types of bacteria and fungus that causes problems. For instance, sweat contains Dermicidin, an antimicrobial peptide. In sweat it has been shown to have a bacteriocidal effect over 90% at a pH of 5.5, but that effect dropped to 60% in a pH of 6.5 (source).” That’s an efficacy drop of over 30% with a pH change of just 1! Having a higher than optimal skin pH means the bacteria in your sweat can more easily thrive, which means more body odor. Neat, eh?
We briefly looked at how a higher skin pH can cause propionibacteria to grow, so let’s look at that a bit more. There are three different types of propionibacteria that camp out on our skin: propionibacteria acnes (acne causing!), propionibacteria avidum, and propionibacteria granulosum. P. acnes (which causes acne!) is the most prominent. “While P.acnes grows very well at pH values such as 6.0 and 6.5, this is not the case at a pH of 5.5… a pH of 6.0 clearly promotes propionibacteria growth, while the opposite applies to pH values of 5.5 and 5.0.” (Source)
“Repeated washings with soap led to increased [propionibacteria] bacterial counts, after changing to the acidic syndet, counts decreased again. Moreover, there was a correlation between the skin pH and density of both bacterial species at the forehead. These observations indicate that (i) repeated washings with either soap or acidic syndet produce long-term changes in skin pH; (ii) different bacterial species forming the resident flora can be influenced differently in the long run by the type of skin cleanser.” (Source) Given the increase in propionibacteria bacterial counts in the group using the soap bar, we can conclude that their skin pH must’ve elevated to at least 6.0, while those using the sydet bar maintained a pH of 5.5 or lower. “In contrast to alkaline soap, [synthetic detergents with a pH of 5.5 or below] do not interfere with the cutaneous microflora, whose composition is linked to the skin surface pH.” (Source)
Now, let’s talk about the acid mantle in relation to pH: “The key to the bilayer formation and water-retaining capcity of the epidermal lipids is the pH of the system. Only if the pH is adjusted to that of normal skin (pH 5.5) are bilayers of these lips formed which are essential for the prevention of skin dryness and roughness… on the basis of these data, it may be suggested that individuals with sensitive skin such as atopics should preferably not use alkaline soaps.” (Source) Basically, the ability of our acid mantle to exist and do things like keep your skin smooth and hydrated relies on the pH of the skin being around 5.5 or lower. So, even if you’re not fussed about acne, there’s still plenty of reason to care! Studies have shown that people suffering from eczema, itchy skin, dry skin, ichthyosis vulgaris, Candidal intertrigo, irritant contact dermatitis, and athlete’s foot all have higher skin pH in the affected areas (source). Of course having a higher skin pH doesn’t mean one will get any of these conditions, just like walking about topless doesn’t necessarily mean you’ll end up with sunburned nipples—but in the event of exposure, one is much more susceptible! “Indeed, a large proportion of the general population—those with a polar constitution of the skin surface that is either seborrheic or sebostatic skin—might profit from the regular use of an acidic cleanser, and there is no reason to believe that it might be disadvantageous in the rest.” (Source)
You can learn even more from F.C. over at his wonderful Simple Skincare Science blog. His post on the importance of the pH of our skin care products is incredibly comprehensive and informative, and worth every second it takes to read.
Some soap makers say that using acidic cleansers is a fad and unnecessary because the pH of your skin will revert to pre-wash levels within about 6 hours (source). Setting aside the fact that these assertions come from people who make their living selling soap, which makes them a touch biased, the evidence shows that this isn’t completely true as pH changes do accumulate over time with frequent washings (source). The key here seems to be the frequency. This study (which was sponsored by Kao Corporation, a company that used to be known as the Kao Soap Company and now owns brands like Biore… so a study sponsored by a company that sells both soap and syndet products found that both soap and syndet are appropriate cleansers…) looked at people who routinely used soap on their body vs. those who used syndets, and found little difference in the pH of their forearms. The studies showing long term pH change looked at washing twice a day, which one typically does not do to their arms. The arm study was not particularly comprehensive, either; they looked at 43 individuals who self-reported that they’d either used exclusively soap or exclusively acidic sydets over the past five years (and self-reporting is notoriously unreliable). The pH of their skin was then measured over the course of one day; once before washing, and then at 0h, 1h, 3h, and 6h after washing with their preferred soap or syndet. The study states that “By 3 h following washing, there were not any significant differences between any of the user groups at any measured time.”, but if one looks at the included chart those differences were typically at least 1 pH, and we’ve already looked at how a difference that small can mean the difference between the proliferation of p. acnes and a substantial decrease in the performance of naturally occurring antibacterials, so I don’t agree with their definition of “significant”.
This excerpt from this informative post explains why while your skin will strive to correct its pH, that doesn’t mean you should ignore the pH of your facial cleansers. “It literally takes a split second for an alkaline product to degrade the skin barrier enough for an irritant or damaging detergent to penetrate. Some people can handle this better then others, but long term daily use on the skin can contribute to long term issues on all skin types. As skin ages, or the barrier function degrades, it has more difficultly dealing with this type of stress. Even after 20 min or so, when the skin re-adjusts to its more normal pH (4.5 to 5.5) – it is already damaged, irritated and stressed. The damage recovery involves longer term healing. 14 to 17 days for acid mantle repair.” (Source)
Long story short—alkaline facial cleansers are out on Humblebee & Me. I won’t take down the pre-existing recipes, but I won’t be publishing any new ones ’cause I don’t want to encourage any kind of acid mantle disrupting silliness! Instead I’ll be focusing on gentle, pH balanced cleansers. I’ve been doing this personally for a couple months and I can’t believe how much happier my face is!
From what I can figure, this research doesn’t mean there are no uses for baking soda or soap in your DIY routine. In applications where baking soda is reacted with an acid (like bath bombs), it’s not an issue, nor is it an issue when a small amount is added to a huge amount of water, like in an occasional bath soak. I’m also happy to continue using soap on my body given the infrequency of use (though I am giving more acidic hand washes a go as I wash my hands more than anything). I may change my mind on this with more research, though!
Posted in: Safety