PAO Symbol
— What the shelf life after opening really says
The little open jar icon on packaging is full of science: how preservative testing works, why storage and application methods significantly influence shelf life, and when the printed number can be misleading.
The small symbol on the back of a cosmetic bottle—an open jar with a number—is one of the most overlooked yet significant pieces of information in skincare. The PAO symbol (Period After Opening) indicates how long a product should remain microbiologically stable and cosmetically effective after it is first opened. While it sounds simple, it is much more complex in practice.
The PAO indication is backed by standardized stability and preservation tests that manufacturers are required to conduct in accordance with EU Cosmetic Regulation 1223/2009. Nevertheless, in literature, the symbol is increasingly discussed as a simplified approximation: storage conditions, application behavior, and formulation type can cause the actual shelf life to deviate significantly from the printed timeframe. It is worth taking a closer look at the science behind it.
Preservation efficacy, oxidation drift, and water activity: The physics behind the jar symbol
The PAO symbol is the result of a defined testing procedure, the so-called Challenge Test (also: Preservative Efficacy Test, PET), which is carried out according to international standards such as ISO 11930. In this test, a finished cosmetic formulation is deliberately inoculated with standardized microorganisms—including Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans, and Aspergillus brasiliensis—and the reduction kinetics are measured over several weeks. The Field Notes article on Challenge Testing and Preservative Efficacy in multi-use formulas explains how profound this correlation is for product safety.
Preservatives react over time with formulation components, UV light, or introduced germs. This depletion effect—described in literature as "preservative depletion"—reduces antimicrobial capacity even when the product appears unchanged on the outside. The PAO number marks the point up to which the manufacturer has verified this capacity is sufficient.
Every time the container is opened, oxygen, light, and potentially moisture are introduced. Oxidation-sensitive molecules—including certain polyphenols, vitamins, and unsaturated fatty acids—can be inactivated or converted into skin-irritating degradation products. The PAO takes this drift into account, but can significantly underestimate it under unfavorable storage conditions.
The free water content of a formulation (aw-value) is the decisive factor for potential microbial growth. Anhydrous oils, anhydrous formulas, and highly concentrated glycerin systems have intrinsically low aw-values and require fewer or no classic preservatives—which explains why pure facial oils are often provided with longer or even missing PAO indications.
Four scenarios where the PAO indication fails in practice
The PAO symbol is a regulatorily valid minimum statement, not a guarantee under everyday conditions. Anyone who takes storage temperature, application hygiene, and packaging type into account can judge actual product safety better than by relying solely on a printed number. This is especially true for oxidation-sensitive active ingredients—the PAO describes microbiological, not necessarily photochemical or oxidative, stability.
Routine design, packaging choice, and rhythm: What the PAO means for your skincare
- Note the opening date on the packaging with a waterproof pen
- Prefer pump dispensers and airless systems—lower real-use contamination
- Store products away from light and cool (15–20 °C), not in a steam room
- Fingers used directly in jars without washing hands in between
- Storing products next to the shower or window (heat, UV, humidity)
- Continuing to use products after the PAO has expired because they "still smell good"
The Porcelain Skin Serum accompanies the day skincare routine as a Day Rhythm with a delivery form geared towards pump application, which reduces oxygen intake during daily use. It contains pullulan, two forms of hyaluronic acid, Kigelia extract with bioactive flavonoids, amino acid-based active ingredients, functional silk polypeptides, and licorice root extract—all of which are active ingredients whose formulation stability is taken into account within the Bioactive Infusion Complex™. For the night, the Blue Crystal Drops facial oil completes the Night Rhythm: as a low-water system with bioactive phytosterols, vitamin C, bisabolol, and essential oils from blue lotus and blue tansy, it is subject to different stability dynamics than aqueous emulsions. Both products are part of the temporally structured skincare approach that NATURFACTOR® pursues under the principle of skin chronobiology—The Science of Rhythm. The Perfect Duo shows how day and night products can be sensibly combined.
For specific skin concerns—such as persistent irritation—you should seek a specialist medical opinion.
Frequently asked questions
Does the PAO indication apply from the date of purchase or from the first opening?
Exclusively from the first opening. Before opening, the best-before date (BBD) printed on the packaging applies, or—for products with a shelf life of more than 30 months—there is no mandatory indication. The PAO symbol only comes into effect from the moment the packaging is opened for the first time. It is therefore recommended to note the date with a pen directly on the bottle.
Why do some oils have no PAO symbol?
Anhydrous formulations with a proven low aw-value are inherently microbiologically stable and do not require classic preservatives. In this case, EU Cosmetic Regulation 1223/2009 does not mandate a PAO, provided that stability is confirmed by a safety assessment. However, this does not exclude the possibility that oxidation processes (rancidity) can impair sensory and functional quality.
Can I still use a product after the PAO has expired?
Formally, the PAO is the usage period guaranteed by the manufacturer. Once it has expired, it can no longer be ensured that the preservative system is sufficiently active or that the active ingredients are present in their declared concentration. Smell, color, and texture are not reliable indicators of microbiological safety—a changed appearance is often a sign of far-advanced degradation.
Does the PAO differ between a serum and a facial oil?
Usually yes, as the formulation types have fundamentally different microbiological profiles. Aqueous serums provide a favorable growth environment for microorganisms and require active preservation—accordingly, they follow classic PAO guidelines (often 12M). Pure facial oils without water content can, due to their nature, do without a PAO symbol, but exhibit their own oxidation dynamics that are significantly influenced by storage conditions.
- Cosmetics Europe (2019). Guidelines on Stability Testing of Cosmetic Products. Cosmetics Europe Technical Guidelines, Rev. 2019.
- Brannan, D. K. (1997). Cosmetic microbiology: A practical handbook. CRC Press, Boca Raton.
- Orth, D. S. & Kabara, J. J. (1998). In-use testing: defining and measuring the efficacy of cosmetic preservative systems. Cosmetics & Toiletries, 113(4), 57–66.
- ISO 11930:2019. Cosmetics – Microbiology – Evaluation of the antimicrobial protection of a cosmetic product. International Organization for Standardization, Geneva.
- Steinberg, D. C. (2006). Preservatives for Cosmetics (2nd ed.). Allured Publishing, Carol Stream, IL.
This article is for informational purposes only and does not constitute medical advice. For specific skin concerns, we recommend seeing a board-certified dermatologist.
