The short answer

Cosmetics expiration dates are determined by running a "stability test" which evaluates how well a product maintains its efficacy and safety while undergoing various environmental conditions, such as changes in temperature and light, over a set period of time.

The long answer

Why do cosmetics need expiration dates?

First off, why do cosmetics – such as makeup, hair dye, skin cream, and deodorant – need expiration dates in the first place? It mostly comes down to the issue of microorganisms like bacteria and fungi (yeast and mold) becoming present in cosmetics.

Here are some fun reasons why cosmetics start to break down and become unsafe to use:

• Dipping your fingers into a product and using applicators (like mascara wands) adds microorganisms each time you use them

• Preservatives break down over time, allowing microorganisms to grow

• Exposure to moisture (like in a steamy bathroom) also make it easier for microorganisms to thrive

• Products can dry out, causing them to harden and crack

• Temperature changes and exposure to air and sunlight can cause changes in texture and color.

All of these issues (and more) can lead to your favorite cosmetic to break down over time and become a treasure trove of bacteria, yeast, and/or mold. So it makes sense that government regulations would dictate including an expiration date on the label, like we do with food.

Government regulation (?)

Here's where it gets interesting. If you're in the United States, you might be surprised to find out that, according to the Food and Drug Administration:

There are no U.S. laws or regulations that require cosmetics to have specific shelf lives or have expiration dates on their labels. However, manufacturers are responsible for making sure their products are safe.

However, the European Union does require all cosmetics products to undergo a standardized "stability test" to determine both the estimated shelf life and the Period After Opening.

What’s the difference between Shelf Life and Period After Opening?

In the EU, all cosmetics are tested for shelf life and PAO. What's the difference?

• Shelf life is how long a product can be used – whether or not a consumer opens the container.

• Period After Opening (PAO) is how long a product can be used after a consumer opens the container.

If a cosmetic’s shelf life is determined to last longer than 30 months, a manufacturer does not need to include it on the container. But they must include the PAO period. Conversely, if a cosmetic’s shelf life is determined to be under 30 months, a manufacturer needs to include the shelf life period on the container and a PAO is not necessary.

What is a cosmetic stability test?

A stability test is an evaluation of how a cosmetic retains its efficacy and safety in a variety of environmental conditions for a set period of time. The goal is to recreate what happens to a product while it gets shipped, arrives on store shelves, and sits in consumer's bathrooms. The product undergoes changes in light and temperature and is evaluated at set intervals to see if its physical, chemical, and performance has changed (if at all). Characteristics such as color, odor, pH value, viscosity, and microbial content are monitored.

If a product is found to have a shelf life lasting beyond 30 months, the PAO is calculated (rounded to the nearest 3-month interval) according to various parameters, such as water content, frequency of use, whether it is rinse-off vs leave-on type of product, and preservative content.

Sources

FDA. (2022, February 25). Shelf Life and Expiration Dating of Cosmetics. U.S. Food and Drug Administration. https://www.fda.gov/cosmetics/cosmetics-labeling/shelf-life-and-expiration-dating-cosmetics

Lebreux, F. (2023, June 19). Cosmetic Expiration Date. Biorius. https://biorius.com/biorius-group/expiration-dates-a-comprehensive-compliance-guide/

Romanowski, P. (2018, July 2). Expiration Date of a Cosmetic Formula. Chemists Corner. https://chemistscorner.com/expiration-date-of-a-cosmetic-formula/

Teles, L. (2021, September 20). Cosmetic Products Testing in the European Union. Critical Catalyst. https://criticalcatalyst.com/cosmetic-products-testing-in-the-european-union