Cosmetics carry two different lifespans: how long they last sealed, and how long they last once opened. The second is much shorter, and it is governed by contamination rather than by the ingredients degrading.
Water is what makes a product vulnerable
Bacteria and fungi need moisture to grow. A product with a substantial water phase provides that; a dry pressed powder or an anhydrous oil balm does not.
This single distinction explains most of the variation in shelf life. Liquid foundations, creams and mascaras are perishable in a way that eyeshadows and powders are not.
Preservative systems exist precisely to hold that growth back, and their capacity is finite. Each contamination event consumes some of it.
The applicator decides how much contamination arrives
A mascara wand travels from the eye area into a narrow tube and back, wiping bacteria against the neck of the container each time and reintroducing them into the formula.
A pump dispenses without anything re-entering the bottle, so a pumped foundation stays far cleaner than one poured from an open-necked jar.
Jars sit at the vulnerable end because fingers enter the product directly, which is why the same formula is often given a shorter open life in a jar than in a tube.
The open jar symbol is an estimate from testing
Many markets require a symbol showing a period after opening, expressed in months, based on stability and preservative challenge testing done by the manufacturer.
Those tests simulate repeated contamination under controlled conditions, so the figure describes typical handling rather than a hard expiry point.
Labelling requirements differ between jurisdictions and change over time, so the same product can carry different markings in different markets.
Storage conditions shorten the period considerably
Heat accelerates both microbial growth and chemical breakdown, and a bathroom is warm and humid for a large part of every day.
Light degrades pigments and certain active ingredients, which is why some formulas are packaged in opaque or tinted containers rather than clear ones.
A product kept in a bag in a hot car has effectively aged faster than its label assumes, regardless of how recently it was opened.
The signs of a turned product are physical
Separation that will not remix, a change in smell, a change in texture, or a colour shift all indicate the formula has broken down.
Products used near the eyes deserve the most caution because the tissue there is thin and easily irritated, and a persistent reaction is a reason to see a professional rather than to switch brands.
Nothing about a product looking normal guarantees it is fine, which is the reason the open-jar period exists as a guide at all.