Water Activity Testing in Cosmetics: The Microbial Safety Gap Hiding in Your EU Product Information File

Most cosmetic brands approaching their Product Information File (PIF) treat microbial safety as a preservative question. They run a challenge test to EN/ISO 11930 or European Pharmacopoeia 5.1.3, generate pass criteria, and move on. The problem is that this approach answers only half the question Regulation (EC) No 1223/2009 is actually asking.

The other half is about water activity — and most PIFs we review don’t mention it at all.

This isn’t a minor documentation oversight. It’s a gap that assessors are increasingly flagging, particularly for products claiming “natural preservation,” reduced-preservative formulas, or waterless formats where the claim itself creates regulatory scrutiny. The SCCS Notes of Guidance for the Testing of Cosmetic Ingredients and their Safety Evaluation (11th revision, SCCS/1602/18) explicitly addresses microbiological quality criteria, and water activity is part of that framework whether brands acknowledge it or not.

What Water Activity Actually Measures — and Why It Differs from Water Content

Water content is a compositional figure. It tells you how many grams of water are present per hundred grams of formula. Water activity (Aw) is a thermodynamic parameter. It tells you how much of that water is actually available to microorganisms.

The distinction matters enormously. A cream with 60% water by weight might have an Aw of 0.96. A slightly reformulated version with the same water content — but with added glycerol or sorbitol — might register 0.91. Both formulas are 60% water. Only one of them is hospitable to Pseudomonas aeruginosa.

Water activity is measured on a scale from 0 to 1.0, where 1.0 represents pure water and 0 represents a completely desiccated substrate. Microorganisms have well-characterised growth thresholds:

• Most gram-negative bacteria (including Pseudomonas spp., E. coli) require Aw above 0.97 to proliferate.
• Gram-positive bacteria such as Staphylococcus aureus can grow at Aw as low as 0.86, with halophilic strains tolerating even lower.
• Yeasts are generally inhibited below Aw 0.87.
• Xerophilic moulds — the most tolerant category — can grow at Aw values approaching 0.70.

For cosmetic safety assessors, these thresholds are not abstract microbiology. They’re the basis on which a reduced preservative concentration, or the complete absence of a conventional preservative, can be scientifically justified.

What EU Regulation 1223/2009 Actually Requires in the PIF

Article 10 of Regulation (EC) No 1223/2009 mandates that the Responsible Person maintain a Product Information File before placing any cosmetic product on the EU market. Annex I of the Regulation specifies the PIF’s contents, and Part A, section (f) requires microbiological data where relevant to the type of product and its safety.

That phrase — “where relevant” — carries more weight than it might appear. Under the SCCS Notes of Guidance, microbiological quality is considered relevant for all aqueous-phase cosmetic products and, critically, for products where the absence of adequate preservation is justified by physico-chemical parameters rather than by the conventional challenge testing route.

In practice, this means two things for brands:

1. If you rely on a challenge test result, the test protocol, the test standard applied (EN ISO 11930:2019, Ph.Eur. 5.1.3, or CTFA guidelines), and the criteria used must all be documented and traceable in the PIF.

2. If you claim that physico-chemical conditions — including low Aw — make adequate preservation self-evident, you need the measurement data to prove it. A statement that your product is “anhydrous” is not sufficient. An Aw reading of 0.63 ± 0.02, obtained by a calibrated benchtop analyser and recorded as part of the in-process QC batch record, is.

The DGCCRF (Direction générale de la concurrence, de la consommation et de la répression des fraudes) — France’s market surveillance authority — has shown increasing attention to exactly this kind of documentation gap during PIF audits. A PIF that contains a challenge test for a formula subsequently reformulated to remove a solvent, without updated Aw data, is a liability.

Challenge Testing Versus Aw-Based Justification: Two Different Answers to Two Different Questions

A preservative challenge test (sometimes called a PET — Preservation Efficacy Test) introduces known challenge organisms at defined inoculum levels and measures how quickly the product eliminates or suppresses them. It’s a functional test: it tells you whether your current formulation protects itself under controlled conditions.

Water activity testing is a predictive parameter. It tells you whether the conditions that enable microbial growth are present in the first place.

A challenge test tells you whether your preservative system works under worst-case microbial pressure. Water activity tells you whether that pressure can even develop in the first place — they answer different questions.

This is why, for certain product categories, Aw testing can replace challenge testing entirely. The SCCS and ISO 22716:2007 GMP framework both recognise physico-chemical hurdles as a legitimate alternative to biological testing, provided the data is robust, reproducible, and measured with calibrated equipment (typically chilled mirror dew-point instruments or capacitance-based sensors, each of which carries its own calibration requirements).

The criteria most assessors will accept for a complete exemption from challenge testing are:

• Aw ≤ 0.60 (below the growth threshold of virtually all cosmetically relevant organisms)
• pH ≤ 3.0 or pH ≥ 10.0 in combination with an Aw of ≤ 0.75
• Anhydrous formulas that remain sealed and dry throughout the product’s period after opening

For formulas that fall between these clear thresholds — say, an Aw of 0.82, which inhibits Pseudomonas but not xerophilic moulds — a combined approach is often required: physico-chemical data to explain the partial hurdle effect, plus targeted microbiological testing focused on the organisms that could realistically survive.

The “Waterless” Myth: Why Anhydrous Claims Still Need Microbial Data

Waterless cosmetics are genuinely having a moment. Shampoo bars, balm cleansers, dry serums, stick foundations — the commercial rationale is clear, and the sustainability narrative resonates. But from a regulatory standpoint, “waterless” is a marketing descriptor, not a safety category.

A lip balm primarily composed of plant waxes, butters, and oils with no added water phase typically has an Aw well below 0.60. No measurable challenge testing is needed, and this should be documented as such in the PIF. Fine.

But consider a gel-format product reformulated to “water-free” by swapping the aqueous phase for aloe vera juice at 70% — a move we’ve seen several French brands make in response to consumer demand. Aloe vera juice is roughly 99% water by composition. The Aw of the finished product may well be 0.95 or higher. Calling it “water-free” because no tap water was added doesn’t change the thermodynamic reality.

This is where Aw measurement becomes not just useful but essential. The number is objective. It tells the assessor, and any market surveillance inspector, exactly what the formula’s microbiological risk environment looks like — independent of the ingredient list or the product name.

Integrating Aw Measurement Into Your EU GMP Workflow Under ISO 22716

ISO 22716:2007, the GMP standard specifically cited under Regulation 1223/2009, covers quality control requirements for cosmetic manufacturing. Section 8.3 on in-process controls is the natural home for Aw measurement in your quality system.

A practical integration approach typically looks like this:

During formulation development: Measure Aw of the finished formula as part of the preliminary safety data package. Confirm stability of the reading across the expected shelf life temperature range (accelerated stability at 40°C/75% RH will drive Aw changes in some emulsions — document whether it does).

During batch manufacture: Include Aw as a release criterion for any product where the safety justification relies on it. This is a five-minute measurement on a benchtop instrument. The result goes into the batch record and becomes part of the traceability chain.

At renewal or reformulation: If any ingredient is changed — particularly humectant concentrations, surfactant ratios, or the water phase composition — re-measure Aw before updating the PIF. A PIF that references Aw data from a different formula version is not compliant.

The equipment investment is modest relative to the regulatory exposure it covers. A calibrated dew-point analyser suitable for cosmetic emulsions and semi-solid products runs between €2,500 and €6,000. Recalibration with certified salt standards is typically annual. The cost of a non-compliant PIF, a rejected CPNP notification, or a DGCCRF enforcement action is considerably higher.

What This Means If You’re Preparing or Updating a CPSR

The Cosmetic Product Safety Report requires Part A to include microbiological specifications and, where applicable, preservation efficacy data. If your safety assessor is thorough — and under EU law, they must be qualified and take professional responsibility for their conclusions — they will ask about the basis of your microbiological safety claim.

If that basis is a challenge test alone and your formula has since been modified, you’ll need to re-test. If the basis is physico-chemical parameters, you’ll need the Aw data, the measurement method, the calibration records, and — ideally — a statement confirming the parameter is controlled as part of your ISO 22716-compliant GMP process.

Our team regularly works with European brands at exactly this stage — reviewing existing PIFs, identifying gaps in the microbiological safety rationale, and coordinating with partner laboratories to generate the missing data quickly without delaying the CPNP notification timeline.

The brands that handle this best are the ones that build Aw measurement into their formulation workflow from day one rather than retrofitting it when an assessor asks a question they weren’t prepared for.

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Written by Nour Abochama, Quality & Regulatory Advisor, Care Europe | VP Operations, Qalitex. Learn more about our team

Talk to our team about EU market entry and microbiological safety documentation. Contact us

Related from our network

• ISO 17025-accredited cosmetic ingredient testing in the US — Qalitex Laboratories provides challenge testing, Aw measurement, and microbial identification under a UKAS/A2LA-equivalent accreditation framework for brands exporting to the US market.
• Health Canada GMP compliance for cosmetic and NHP products entering Canada — Androxa supports European brands navigating Health Canada’s microbiological and GMP expectations for cosmetics and natural health products.