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Who has never thrown away a cosmetic product that was still half full because they were unsure whether it had expired? And who has never wondered whether an expired product could irritate the skin on their face? Today we analyze cosmetic expiration and PAO, because knowing these details helps save money and protect your skin.
We will look at exactly what PAO means, how it is established through laboratory tests, the actual risks of using an expired cosmetic, and how to store products so you can use them up without waste.
What is PAO in cosmetics?
PAO is an acronym for the English term Period After Opening and indicates the period during which a cosmetic remains safe and effective after the packaging has been opened.
By law, you will find this symbol on packaging, particularly on products whose shelf life exceeds 30 months from production. How can you recognize it? It is an open jar accompanied by a number such as 6M, 12M, or 24M: it indicates how many months you have to use the product once opened.
The reason is simple: as soon as it is opened, a cosmetic comes into contact with oxygen, moisture, bacteria, and the fingers of the person using it. All of this encourages microbial growth and the oxidation of active ingredients — vitamin C and retinol are the most sensitive examples. The texture may separate, the fragrance may change, and effectiveness may decrease.
The legal basis: Article 19
The requirement comes from Article 19 of Regulation (EC) No 1223/2009, the legislation governing cosmetics throughout the European Union. It establishes that the label must show the minimum durability date, meaning the date until which the product, stored under appropriate conditions, continues to fulfill its original function and remains safe.
Then comes the exception that explains what you see on the packaging: when the minimum durability exceeds 30 months, that indication is not mandatory, and the period after opening is given instead. That is why you almost always find the open jar on a face cream and hardly ever the hourglass: it means that, unopened, the product lasts more than two and a half years.
| Symbol | What it indicates | When it appears |
|---|---|---|
| Hourglass + date | Minimum durability date when unopened | If the shelf life is less than 30 months |
| Open jar + 12M | Months of safe use after first opening | If the shelf life unopened exceeds 30 months |
| Neither | Single-use product or product not subject to alteration | Single-dose products, sealed aerosols, and some anhydrous products |
How companies establish the PAO
It is neither a randomly chosen number nor a cautious estimate: it is the result of documented tests that are compiled in the product information file, which must be available to the regulatory authorities. There are three, and they measure different things.
The challenge test
This is the central test, governed by ISO 11930. The finished product is deliberately contaminated with standardized microbial strains — including Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans, and Aspergillus brasiliensis — and, at precise intervals of up to 28 days, the rate at which the preservative system eliminates them is measured. If the reduction does not reach the required thresholds, the formula is unsuitable and must be revised: this is not a formality; reformulation is genuinely sometimes necessary.
Stability tests
At the same time, the product is kept for months at different temperatures — typically 4 °C, room temperature, and 40 °C — and checks are carried out to ensure that the emulsion does not separate, the pH remains within range, color and odor do not change, and the active ingredients do not degrade beyond a certain threshold. The 40 °C test is used to simulate the aging that occurs over many months in just a few weeks.
Container-content compatibility
It is verified that the formula and packaging do not interfere with each other: that the container does not release substances into the product and that the product does not degrade the container. It may seem like a technical detail, but it is actually one of the reasons why the same formula can have different PAOs in two different types of packaging.
The practical consequence is that the PAO is a property of the finished product in its container, not of the formula in the abstract. A wide-mouth jar, a tube, and an airless system expose the same cream to very different conditions: with the first, air and fingers enter with every use, while with the last, the formula never comes into contact with air. This is also why the choice of packaging, which is often perceived as an aesthetic decision, is a technical decision for formulators.
If you want to see the entire journey that brings a cosmetic product to market, we described it in the article on how a cosmetic product is made.
How to interpret the symbol
A product with a 24M PAO offers two years of use after opening, provided it is stored correctly. Note an important nuance: the PAO guarantees that the product will remain safe and effective within that period, not that it will be identical to how it was on the first day. A slight decline in performance toward the end is normal and differs from deterioration.
One practical tip that is worth more than any theory: write the opening date on the bottom of the package with a permanent marker. It’s the simplest way to avoid having to guess six months later.
PAO and expiration date: two different things
The two should not be confused. The expiration date applies even when the package is unopened; the PAO starts from the moment of opening. If a product displays both, the expiration date is the non-negotiable limit. For most cosmetics, however, the PAO is what sets the rules.
European regulations require clear labels. If a product has a shelf life of less than 30 months from production, it must display a precise expiration date; beyond that threshold, the PAO is sufficient. Within those limits, the manufacturer guarantees safety and efficacy, provided the specified storage conditions are followed.
How to store a cosmetic product correctly
Proper storage allows the product to remain fully effective until the end of its PAO. The main enemies are heat, direct light, and humidity.
Cosmetics should be stored in cool, dry, dark places: a bedroom drawer or a well-closed toiletry bag works perfectly. Avoid keeping them near heat sources such as radiators or in humid environments—and here’s the counterintuitive point: the bathroom is the worst place, even though that’s where almost all of us store our skincare products. Shower steam creates a warm, humid microclimate that is ideal for microbial growth.
The refrigerator isn’t a good idea either, unless the manufacturer explicitly recommends it: temperature fluctuations can destabilize emulsions. The inside of a car in summer should be avoided absolutely, as temperatures can easily exceed 60 °C: no stability test covers those conditions.
One final tip—obvious but effective: don’t open a product if you don’t intend to use it. Opening it just to smell its fragrance or try its texture starts the degradation processes without providing any benefit.
How to avoid contamination
- Don’t put your fingers in jars: use a clean spatula instead.
- Wash brushes and spatulas regularly.
- Always close the packaging tightly after use.
- Whenever possible, choose airless packaging or dispenser bottles, which limit contact with air.
Signs of a deteriorated product
In addition to the dates, it’s worth trusting your nose and eyes. The warning signs are:
- the color changes or darkens;
- the cream separates into an oily phase and a watery phase;
- the smell changes, or the notes perceived at first disappear;
- the texture becomes lumpy or excessively runny;
- a bothersome sensation occurs on contact.
The risks of using a cosmetic product beyond its PAO
When the formula's stability changes, the preservative system may no longer be able to counteract microorganisms. The most common consequences are skin irritation, the appearance of small blemishes, and intolerance reactions, particularly for people with sensitive or reactive skin or an already weakened skin barrier.
There is also a second, less discussed risk: an oxidized product no longer works. A vitamin C serum that has turned orange is not necessarily irritating, but it also no longer does what you bought it for.
If a persistent skin reaction appears after using a product, stop applying it and consult a dermatologist or your pharmacist. Temporary irritation is one thing; a reaction that does not go away should be evaluated by a professional.
In summary
The PAO indicates the number of months during which a product remains safe and effective after opening; the expiration date applies regardless. Following both protects your skin and, at the same time, prevents you from throwing away products that are still good or using products that are no longer effective.
The most useful rule is still the simplest: buy what you can realistically finish. It is also the most practical way to reduce waste, as we explain in the article on eco-friendly packaging.
Sources for this article
01 Regulation (EC) No 1223/2009 of the European Parliament and of the Council on cosmetic products, Article 19 (labelling, minimum durability date and period after opening).
02 ISO 11930 standard, evaluation of the antimicrobial protection of a cosmetic product.
03 Commission Regulation (EU) No 655/2013, common criteria for the justification of claims relating to cosmetic products.
On the website: how a cosmetic product is made · vitamin C · the skin barrier.
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