Preserving Clay Masks: The Science of Microbiological Control and the Role of Glycols
Any water-containing clay mask needs a full preservative system, because the mineral surface binds preservative molecules by ion exchange. Target pH 5.0–5.5: at pH 7 the active undissociated fraction falls to about 0.16 % for benzoic acid and 0.57 % for sorbic acid. Prefer phenoxyethanol, capped at 1.0 % under UK rules, and challenge-test every formula.
- 70–120 mEq/100 g — cation exchange capacity of smectites such as montmorillonite, against 15–20 mEq/100 g for kaolinite and talc: the measured reason a bentonite mask binds charged molecules far harder than a kaolin one.
- 1.0 % — maximum phenoxyethanol in a ready-for-use product under the UK Cosmetics Regulation (retained Regulation (EC) No 1223/2009, Annex V entry 29).
- 0.5 % (as acid) leave-on / 2.5 % rinse-off for benzoic acid and sodium benzoate, and 0.6 % (as acid) for sorbic acid and its salts — note "as acid", not as the salt you weigh out.
- 0.14 % (as acid) for the sum of butylparaben and propylparaben together; methyl- and ethylparaben sit under a separate 0.4 % single-ester / 0.8 % mixture limit.
- 0.5 % (w/w) — the level at which phenoxyethanol has been reported to pass a cosmetic challenge test in a published benchmark, i.e. below the 1.0 % regulatory cap and below the 1.0 % often dosed in clay.
At the "Walker Formulation Academy" school, we are convinced that a deep understanding of ingredient chemistry is the foundation of professional formulation development, and the preservation of clay masks is one of the most telling examples of how a mineral matrix can completely change the standard logic of working with preservatives.
Clay masks are among the most popular skincare products on the market, but they are also among the most technically challenging products in terms of preservation. The mineral matrix that makes clay so effective as a cosmetic ingredient—its high surface area, ionic charge, and swelling capacity—creates an unfavorable environment for standard preservative systems. Understanding why this happens and how to bypass these limitations at the formulation level is essential knowledge for any serious formulator. It also means accepting an uncomfortable editorial conclusion up front, which this revision states plainly: for clay systems there is no published number that tells you how much preservative is lost to the mineral. The loss is real and mechanistically documented; its size in your formula is established by a challenge test, not by a table.
1. The Microbiological Risk Profile
Not all clay masks present the same level of difficulty in terms of preservation. The risk profile depends almost entirely on water activity—the amount of free water available for microbial growth.
Anhydrous and Dry Systems
Powder masks (dry clay mixed with botanical powders, minerals, or encapsulated actives) and "clay-in-oil" systems are largely self-preserving because bacteria and molds cannot multiply without sufficient free water. The primary chemical risk in such systems is not microbial contamination, but the oxidative rancidity of the lipid components present. Here, an antioxidant system is appropriate rather than a preservative system. The ranges below are the customary starting doses used in this school's teaching formulas, expressed as supplied (w/w) on a clay-in-oil base with an oil phase of 60–80 %; they are not thresholds established by any experiment we can cite, and none of them is a shelf-life guarantee:
- Tocopherol (Vitamin E): 0.1–0.5 % as supplied — interrupts free-radical chain reactions. Note that trade grades differ widely in tocopherol content, so "0.5 % of the product" and "0.5 % active matter" can be several-fold apart; read the specification of the grade you bought.
- Rosemary Extract (ROE): 0.02–0.1 % as supplied — a synergistic antioxidant that adds "botanical" value; again a trade product whose active content is set by its own specification.
- BHT or BHA: 0.02–0.05 % as supplied — more traditional options for industrial formulas.
Rule of thumb: if your formula does not contain an aqueous phase, and the clay is in oil or exists as a dry powder, focus on antioxidants rather than antimicrobial agents. The rule is about water activity, not about the clay.
Systems Containing Water
As soon as water is introduced into the system—whether it is the continuous phase of an emulsion, a gel base, or even a hydrosol added for fragrance and sensory properties—full-spectrum preservation becomes mandatory. This is exactly where the mineral chemistry of clay creates specific challenges.
2. Why Clay is Difficult to Preserve
Clay minerals, especially smectites (bentonite, hectorite) and kaolins, interact with preservative molecules through several mechanisms. To choose the right strategy, it is necessary to understand all three.
2.1 Adsorption of Preservatives
This is the most significant and, at the same time, the most underestimated problem. Clay minerals carry a negative layer charge and a high surface area, and the charge is the part that can be quantified from the literature. A 2023 review of clay minerals in pharmaceutical and biomedical use puts hard numbers on the difference between clay families: kaolinite and talc have minimal layer charge and low cation exchange capacities of 15–20 mEq/100 g, while smectites such as montmorillonite carry tetrahedral and octahedral substitutions and high ion exchange capacities of 70–120 mEq/100 g. That is a four- to eight-fold difference in the very property that drives ionic binding — which is why "clay" is not one preservation problem but at least two. Preservative molecules bind to these surfaces due to:
- Ion exchange — cationic preservatives (e.g., quaternary ammonium compounds) bind strongly and are poorly recovered. On a smectite with 70–120 mEq/100 g of exchange capacity, a cationic preservative is competing against the clay's entire reason for existing. Do not build a preservation strategy on a cation here.
- Hydrogen bonding — at the edge sites of the clay and in the interlayer water.
- Hydrophobic partitioning — some molecules diffuse into the interlayer spaces of the clay.
The consequence of this is that the free concentration of the preservative — that is, the portion that is actually available to inhibit the growth of microorganisms — is lower than the total concentration added to the formula. Formulators who use standard dosages without accounting for adsorption may end up with a cosmetically acceptable product that fails the preservative efficacy test.
Correction to an earlier version of this article: we previously printed a figure of "20–50 % lower free concentration in formulas with 15–25 % clay". We have removed it. We could not find a published measurement supporting that range, and a number without a source does not become acceptable by being called typical. What can be said is directional and testable: free preservative is lower than nominal, the gap grows with clay content and with exchange capacity, and the only way to size it in your system is a challenge test on your own formula.
2.2 pH shift
Most smectite clays give an alkaline reaction in suspension and pull the pH of a formula upward. Measure your own dispersion rather than assuming a range — the value depends on the deposit, the grade and the water. Why it matters is not a matter of opinion, because the activity of the common food-style preservatives is governed by the Henderson–Hasselbalch equation and their acid dissociation constants. Taking the standard values (benzoic acid pKa ≈ 4.20, sorbic acid pKa ≈ 4.76), the undissociated — that is, active — fraction works out as follows. This is our own arithmetic from published pKa values, not a measurement in a clay system:
- Sodium benzoate: ≈86 % undissociated at pH 3.4, ≈14 % at pH 5.0, ≈0.16 % at pH 7.0.
- Potassium sorbate: ≈37 % undissociated at pH 5.0, ≈5.4 % at pH 6.0, ≈0.57 % at pH 7.0.
- Parabens are esters rather than weak acids in this sense; their practical ceiling is hydrolysis, which is base-catalysed, so an alkaline clay dispersion works against them over storage.
- Phenoxyethanol is non-ionic and is not switched off by pH in the way the weak acids are — one of the reasons it is the default here.
Read as a general study rather than a cosmetic one, the food-microbiology literature points the same way: in a 2024 study of 21 short-chain carboxylic acids against Salmonella enterica, inhibition was consistently higher in an acidic environment (pH 4.5) than at near-neutral pH. That work is about a food pathogen in a food matrix and does not transfer to a clay mask — we cite it as directional support for the pH mechanism, not as evidence about your product.
The technologist must measure and adjust the pH after adding the clay, not before. The standard choice for acidification is citric acid, weighed as a defined percentage of your batch and recorded together with the measured final pH. A target final pH of 5.0–5.5 keeps the weak-acid systems meaningfully active — see the fractions above — while remaining skin-compatible.
Note: never measure the pH of a clay dispersion with indicator strips. A colloidal suspension gives false readings. Use a calibrated pH meter with an electrode, record the temperature of the measurement, and rinse the electrode thoroughly between measurements.
2.3 Physical interactions with the formula matrix
In addition to adsorption, the gel network formed by swelling clays can trap and immobilize preservative molecules, further reducing their diffusion rate in the product. A preservative that cannot freely diffuse through the formula is unable to reach and inhibit microbial cells at the packaging walls, under the lid, or on the surface of the product after opening. This is a mechanism, and we present it as one; we have no published diffusion coefficients for a cosmetic clay gel to offer alongside it.
3. Choosing a preservative for clay formulas
Given these limitations, when choosing preservatives, priority should be given to systems that are resistant to adsorption and stable across pH levels. Two conventions before the list. Every dosage below is the trade product as supplied (w/w), not active matter. And every regulatory number is taken from the UK Cosmetics Regulation — the retained Regulation (EC) No 1223/2009, whose Annex V lists the preservatives allowed in cosmetic products and their maximum concentrations in the ready-for-use preparation. The reference system for this section is an aqueous clay mask: 8–15 % kaolin or bentonite, water phase 70–85 %, glycol 3–8 %, adjusted to pH 5.0–5.5.
Phenoxyethanol + Ethylhexylglycerin (e.g., Euxyl PE 9010)
- Dosage: 0.8–1.0 % of the trade product (as supplied). Ashland's technical documentation for euxyl PE 9010 is not reachable from our network, so we cite the product by name and do not hyperlink it; take the recommended range from the supplier's current data sheet, not from us.
- Regulatory ceiling: phenoxyethanol is capped at 1.0 % in the finished product (Annex V, entry 29). Ethylhexylglycerin is not listed in Annex V as a preservative at all — it is dosed as a multifunctional ingredient. The blend's name implies a 90:10 ratio, which would make 1.0 % of the product ≈0.9 % phenoxyethanol; we could not open a composition document confirming that split, so check the specification before relying on the arithmetic.
- pH range: broad; phenoxyethanol is non-ionic and does not lose activity to pH the way the weak acids do.
- This is the best general choice for clay. A 2024 study on a plant-extract preservative notes in passing that the industry benchmark, phenoxyethanol, has been reported to pass the cosmetic challenge test at 0.5 % (w/w) in a conventional formula. Read that as the useful comparison it is: if 0.5 % suffices without clay and you are dosing 1.0 % with clay, you have already doubled the input — and you still do not know whether that covers the adsorption loss until you test.
Sodium Benzoate + Potassium Sorbate
- Dosage: commonly 0.5 % each as the salt. Watch the units: Annex V sets 0.5 % as acid for leave-on and 2.5 % as acid for rinse-off products for benzoic acid and sodium benzoate, and 0.6 % as acid for sorbic acid and its salts. Because you weigh the salt and the law counts the acid, the two numbers are not the same and the conversion belongs in your calculation sheet.
- pH range: below 5.5 only, for the reasons calculated above.
- Effective only with strict pH control. Requires firm acidification of the clay dispersion. Risk: pH drift upward during storage, which silently switches the system off.
Benzyl Alcohol + Dehydroacetic Acid (e.g., Ecocert-approved blends)
- Dosage: according to the supplier's specification for the specific blend.
- Regulatory ceiling: benzyl alcohol 1.0 % (Annex V entry 34); dehydroacetic acid and sodium dehydroacetate 0.6 % as acid, and not to be used in aerosol dispensers (entry 13).
- Often chosen for natural/organic certification claims. Whether a given certifier accepts it is decided by that certifier's current standard, not by Annex V. Less forgiving than phenoxyethanol-based systems in our teaching batches; always verify with a challenge test.
Parabens (Methylparaben + Propylparaben)
- Dosage: 0.1–0.4 % as supplied, within the limits below.
- Regulatory ceiling: methyl- and ethylparaben (Annex V entry 12) — 0.4 % as acid for a single ester, 0.8 % as acid for mixtures of esters. Butyl- and propylparaben (entry 12a) — 0.14 % as acid for the sum of the individual concentrations, not 0.14 % each. An earlier version of this article described that limit as applying to propylparaben alone; the correction matters if you dose both.
- Historically reliable, with consumer-perception issues, and with a real risk of hydrolysis in alkaline conditions — which is precisely what an unadjusted clay dispersion provides.
In almost all cases, phenoxyethanol-based systems are the most practical choice for aqueous clay formulas. The mechanistic argument is that the non-ionic, relatively non-polar nature of phenoxyethanol makes it less susceptible to ionic adsorption on the clay surface than charged alternatives. We are labelling that as a mechanistic argument rather than a measurement: we could not open a published study comparing recovery of phenoxyethanol and of a cationic preservative from the same clay dispersion.
4. The role of glycols
Glycols occupy a special and often underestimated place in the preservation of clay masks. They are usually added as humectants or conditioning agents, but their contribution to the preservation system is real from a mechanistic point of view. This is also the section where the original version of this article carried the most unsupported arithmetic, so it is the section that has changed most.
4.1 Activity as co-preservatives
Some glycols demonstrate direct antimicrobial activity, and the honest statement of it is that the concentration depends entirely on the glycol, the organism and the rest of the formula. The one measurement we can hand you comes from a 2024 study developing a sugarcane-straw preservative, where nine cosmetic solvents were screened: the ingredient built on 1,2-hexanediol (20 % dry extract dispersed in 25 % 1,2-hexanediol in water) gave a minimum inhibitory concentration between 3 % and 5 % against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and Candida albicans, and 5 % (w/v) of that ingredient met USP 51 criteria. Note carefully what that is and is not: it is a specific extract-plus-glycol ingredient in a specific test, not a licence to quote an MIC for caprylyl glycol in your mask.
- Propylene Glycol (PG): used at 3–8 % as a humectant in clay systems. The often-repeated thresholds ("inhibits bacteria above 5–8 %", "self-preserving at 15–20 %") have been removed from this article: we could not source them, and at 15–20 % the sensory penalty in a clay mask makes the question academic anyway.
- Butylene Glycol: a better skin-feel humectant at comparable levels; we have no MIC data to offer for it and are not inventing any.
- Caprylyl Glycol and Pentylene Glycol: widely sold as multifunctional co-preservatives at 0.2–1.0 % as supplied. Treat the supplier's use level as the starting point, and treat the claim "broad-spectrum at 0.2–0.5 %" as a marketing statement until your own challenge test says otherwise — in the screen above, caprylyl glycol was not the solvent that produced the best antimicrobial performance.
The defensible summary: glycols at 3–8 % contribute to the overall antimicrobial burden of the formula and let you build a margin against adsorption losses. How much margin is not a number anyone has published for clay.
4.2 Competitive adsorption — a partial solution
This is the mechanism most directly related to the problem of adsorption onto clay. Glycol molecules are small, polar, and capable of forming hydrogen bonds — the very properties that let them compete with preservative molecules for binding sites on the clay surface.
The reasoning is that glycol molecules partially occupy hydrogen-bond donor and acceptor sites at the clay's edge regions and hydration shell, reducing the number of sites available to bind preservative molecules, and therefore raising the free preservative fraction. Mechanistically this is coherent, and it is consistent with the layer-charge and edge-site chemistry described in the clay review cited above.
What has been withdrawn: an earlier version of this article stated that "published data on bentonite/phenoxyethanol systems show that the addition of glycol can reduce adsorption losses by approximately 20–40 %". We searched for that publication and could not find it. Since the sentence attributed a specific number to a specific system and a specific literature, and we cannot produce the source, the number is gone rather than hedged. The mechanism stays; the quantity was never ours to give.
An important nuance, offered as reasoning rather than measurement: glycerin is often assumed to act the same way. Its molecular geometry and its strong tendency to stay in the bulk water make that assumption unsafe, and we have no comparative adsorption data for glycerin versus propylene glycol on cosmetic clay. Treat them as non-interchangeable for this purpose until your own testing says otherwise.
PEG-containing polyols carry a different theoretical risk: bridging of clay platelets, which could change the effective surface available for adsorption rather than reducing it. Use them with caution and always verify via a challenge test.
4.3 The Question of Addition Order
Among experienced technologists, there is a practice that, while not formally confirmed in peer-reviewed literature, is nonetheless widely discussed: pre-dispersing the preservative in the glycol fraction before introducing the clay.
The logic stems from the kinetics of competitive adsorption: if the clay surface is already partially saturated with glycol molecules at the moment the preservative is introduced, the equilibrium position should shift towards a higher proportion of free preservative compared to a situation where the preservative is introduced into an already formed clay dispersion with available active sites.
From a chemical standpoint, this logic is sound. However, there is no published data where this influence is isolated as a separate variable. Formulators using this technique should verify it in their own system using a challenge test rather than simply assuming the effect exists.
Practical note: the technique based on the order of addition is low-risk and chemically reasonable. In educational materials, it is correct to present it as "mechanistically justified but not empirically verified as an isolated variable"—and use it as an opportunity to discuss the methodology of preservative challenge testing.
5. Chelators as Preservative Boosters
Disodium EDTA at 0.05–0.1 % as supplied is an important addition to any water-based formula containing clay. The textbook mechanism differs entirely from that of glycols and primary preservatives: it chelates divalent metal ions (Mg²⁺, Ca²⁺, Fe²⁺) that stabilise the outer membrane of Gram-negative bacteria, making those organisms more sensitive to preservatives.
There is now a measurement to put next to the textbook. A 2026 single-cell study combining time-lapse microscopy with a microdevice found that EDTA enhanced the bactericidal effect of 2-phenoxyethanol and markedly reduced the dividing subpopulation that survives preservative exposure — most cells divided once or not at all, against up to five divisions over 23.4 hours under phenoxyethanol alone. Two details are worth keeping rather than flattening: the organism was Staphylococcus ureilyticus, a Gram-positive contaminant, so the booster effect observed there cannot be explained by the Gram-negative outer-membrane mechanism above; and population-level kill curves looked similar with and without EDTA, meaning the benefit was invisible to the conventional assay. For a clay mask that is a useful reminder that a challenge test result of "pass" is a population measurement.
Importantly, EDTA is not adsorbed onto clay in the way organic preservatives are — its chelating action is realised in the aqueous phase. We present that as the standard mechanistic reason it is expected to contribute additively in a clay matrix, not as something quantified for clay.
In natural-format formulas where EDTA is not used, sodium phytate (0.1–0.5 % as supplied) or gluconic acid can perform a partial chelating function. Both are acidic and will move your pH, so re-measure after adding them.
6. Recommended development strategy
The following protocol combines the principles discussed above into a practical approach to preserving aqueous clay masks. It is a development protocol, not a safety guarantee: none of these steps replaces the challenge test in step 6 or the safety assessment required of the responsible person under the UK Cosmetics Regulation.
-
Target pH 5.0–5.5
Keeps the weak-acid preservatives meaningfully undissociated (≈14 % benzoic, ≈37 % sorbic at pH 5.0 by the calculation above). Add citric acid after the clay has been fully dispersed. Re-check the pH after 24 hours — clay systems can drift upward on standing. -
Include glycol at 3–8 % (as supplied)
Propylene glycol or butylene glycol, in the reference system named in section 3. Humectancy plus a contribution to the antimicrobial burden plus, mechanistically, competition for adsorption sites. Do not budget a specific percentage of "recovered" preservative against it. -
Primary preservative at 0.8–1.0 % of the trade product
Phenoxyethanol + ethylhexylglycerin, dosed at the upper end of the supplier's range to leave headroom for adsorption, while staying inside the 1.0 % phenoxyethanol ceiling of Annex V. -
Add disodium EDTA at 0.05–0.1 % (as supplied)
Chelator and potentiator, acting in the aqueous phase. -
Consider the order of addition
Pre-disperse the preservative in the glycol fraction before adding the clay. Mechanistically justified; not verified as an isolated variable. Verify it in your own system. -
Conduct a challenge test for every formula
A preservative efficacy test to ISO 11930 or USP <51> is the only step here that produces evidence rather than expectation. For clay it is not optional, because the matrix makes prediction unreliable and, as section 5 shows, population assays can hide surviving subpopulations. -
Select appropriate packaging
An airless pump or tube minimises contamination after opening. Wide-mouth jars are the worst option, and the difference is largest for products a user applies with fingers.
7. A note on natural claims
Formulators working within organic or natural certification schemes (COSMOS, NaTrue, ECOCERT) face restrictions beyond Annex V, and those restrictions come from the certifier's own permitted-preservative list. An earlier version of this article stated that "phenoxyethanol is permitted by COSMOS up to 1 %". That conflated two separate things: 1.0 % is the regulatory cap in Annex V, and whether a private natural standard admits phenoxyethanol at all is decided by the current version of that standard — which for several schemes it does not. Check the standard you are certifying to, in its current revision, rather than any secondary summary including this one. Where phenoxyethanol is excluded:
- Benzyl alcohol (up to 1.0 %, Annex V entry 34) + dehydroacetic acid (up to 0.6 % as acid, entry 13) — the standard fallback; check your certifying body's current requirements as well as the regulatory ceilings
- Caprylyl glycol (0.5–1 % as supplied) as a co-preservative broadens the protection — as a contributor, not as the primary system
- The challenge test becomes even more critical — these systems have a narrower window of effectiveness and less tolerance for the adsorption losses described above
Keep the clay percentage as low as the product concept allows, and prefer the lower-exchange-capacity clay where the sensory profile permits. Each additional percentage of clay adds surface for adsorption, and on the cation exchange figures in section 2.1, swapping bentonite for kaolin changes that binding capacity by a factor of four to eight. If 8 % kaolin gives you the sensory profile you want, there is no formulation benefit to 15 %.
Conclusion
Preserving clay masks is not just a matter of adding a standard preservative blend to a formula. The mineral matrix actively counteracts common preservation strategies through adsorption, pH influence, and the physical entrapment of active molecules.
Glycols solve one part of this problem—competitive adsorption—while simultaneously contributing as co-preservatives and improving sensory properties. They are not a standalone solution, and after this revision they are not a quantified one either: the mechanism is sound, the size of the effect in a clay mask is not published, and the honest answer to "how much preservative do I add to compensate?" is that no universal figure can be established from the available documents. That is a real result, not a gap in the article.
A complete solution requires attention to pH control, preservative selection and dosage, chelation, packaging, and empirical validation through a challenge test. Formulators who understand why each of these interventions is necessary, rather than just what exactly needs to be added, are much better prepared to troubleshoot failures and adapt a formula when ingredient availability or certification requirements change.
If you want to learn how to develop complex formulations with a deep understanding of the chemistry of each ingredient, join the courses at the Walker Formulation Academy school. We analyse exactly these kinds of non-trivial tasks: from preservative selection to finished product validation.
Learn more about the Walker Formulation Academy school courses
Why does a bentonite mask need more preservative than a kaolin mask at the same water content?
Because the binding capacity differs by roughly four to eight times: smectites such as montmorillonite carry cation exchange capacities of 70–120 mEq/100 g against 15–20 mEq/100 g for kaolinite. More exchange sites means more preservative held on the mineral and less free in the water. The size of the difference in your formula is established by challenge-testing both versions, not by scaling the dose to the ratio.
Can I rely on a challenge test I ran on a similar formula without clay?
No. The clay is the variable that changes the answer, through adsorption, pH shift and reduced diffusion, and it is the one part of the system a prior test did not contain. Re-test whenever the clay type or percentage changes, and note that a population-level pass can still hide a surviving subpopulation, as the 2026 single-cell work on phenoxyethanol showed.
Do the Annex V limits apply to the salt I weigh or to the acid?
To the acid. Benzoic acid and sodium benzoate are limited to 0.5 % as acid in leave-on and 2.5 % as acid in rinse-off products; sorbic acid and its salts to 0.6 % as acid; butylparaben and propylparaben to 0.14 % as acid for the sum of both. You weigh sodium benzoate or potassium sorbate, so the conversion from salt to acid belongs in your calculation sheet before you declare compliance.
Sources
- Regulation (EC) No 1223/2009 on cosmetic products, Annex V — List of preservatives allowed in cosmetic products (UK retained legislation, legislation.gov.uk)
- Nunes C. et al., “Natural and Synthetic Clay Minerals in the Pharmaceutical and Biomedical Fields”, Pharmaceutics 15(5):1368, 2023 (review)
- “EDTA suppresses bacterial perseverance to 2-phenoxyethanol”, Microbiology Spectrum, 2026 (single-cell experimental study)
- “New Natural and Sustainable Cosmetic Preservative Based on Sugarcane Straw Extract”, Molecules 29(16):3928, 2024 (experimental study; source of the 1,2-hexanediol MIC and the phenoxyethanol 0.5 % benchmark)
- Cosmetic Ingredient Review — Phenoxyethanol ingredient record (expert-panel safety assessment index)
- “Environmental pH and compound structure affect the activity of short-chain carboxylic acids against planktonic and biofilm Salmonella enterica”, Microbiology Spectrum, 2024 — food-matrix study, cited only as directional support for the pH mechanism
- Ashland, technical documentation for euxyl® PE 9010 (phenoxyethanol + ethylhexylglycerin) — supplier data sheet, cited by document name; ashland.com is not reachable from our network, so no hyperlink is given
- ISO 11930 (Cosmetics — Microbiology — Evaluation of the antimicrobial protection of a cosmetic product) and USP <51> Antimicrobial Effectiveness Testing — standards cited by designation
