The two symbols and what they mean
UK cosmetic labelling rules give manufacturers two ways of communicating product life, and which one appears depends on how long the product lasts unopened.
If the product has a shelf life of thirty months or less, it must carry a date of minimum durability, printed as a best before date or an hourglass symbol with a date. If the shelf life exceeds thirty months, a date is not required, but a period after opening symbol must appear instead. That is the small open jar icon with a number and the letter M, meaning the number of months the product remains suitable once opened.
The period after opening clock starts when you first open the container, not when you buy it. A bottle opened in June last year and closed in September has been running its clock the whole time, sitting in a cupboard. Twelve months from first opening is the common figure for a sunscreen, and once it passes there is no way to verify from the outside whether the emulsion and the filters are still doing their jobs.
What actually degrades
Three separate things can go wrong in an ageing sunscreen, and only one of them is visible.
The first is the emulsion. Sunscreen is a suspension of oil and water phases held together by emulsifiers, with filters distributed through one or both. When an emulsion breaks, the phases separate. A watery layer at the top, an oily film, or a texture that has gone grainy all indicate that the filters are no longer evenly distributed. Even if every molecule of filter is intact, an uneven film protects unevenly.
The second is the filters themselves. Some organic filters degrade with prolonged heat and light exposure. This is a slow process in a sealed opaque bottle and a much faster one in a clear bottle on a windowsill or in a car parked in the sun. Zinc oxide and titanium dioxide do not degrade in this way, which is a genuine practical advantage of the formats reviewed in our mineral sunscreen review.
The third is the preservative system. Preservatives are consumed over time and by repeated contamination from fingers, sand and water. A product past its period after opening may support microbial growth, which is a hygiene problem rather than a protection one, but it is a reason to discard rather than persist.
Heat is worse than time
The date on the pack assumes reasonable storage. It does not describe a bottle that spent July in a car door pocket, a beach bag in direct sun, or a poolside table for six hours a day for a fortnight.
Temperatures inside a parked car in summer sun climb far above ambient and stay there. That is the single most destructive environment a sunscreen encounters, and it is also the most common one, because the car is exactly where the bottle gets left. A summer in those conditions can undo a formulation well inside its printed life.
Practical storage rules are simple. Keep the bottle in a bag rather than on the sand. Keep it in shade rather than sun. Do not leave it in a car. Close the cap properly, because an open cap admits contamination and lets volatile components evaporate, which concentrates and thickens what remains.
The signs a bottle should go
- Separation. A watery or oily layer that does not reincorporate after vigorous shaking means the emulsion has broken.
- Changed smell. A rancid, sharp or plasticky note where there was none suggests oxidation of the oil phase.
- Grittiness. A gritty or grainy texture in a formerly smooth product indicates the mineral or filter phase has aggregated.
- Colour change. Yellowing or browning in a white formula is a common sign of degradation.
- Different feel on the skin. If it drags, beads up or refuses to spread the way it used to, the base has changed.
None of these tests is definitive individually, and their absence does not prove a product is fine. They are reasons to discard rather than reasons for confidence.
The economics of keeping an old bottle
There is a straightforward argument against caution here, which is that using a possibly degraded sunscreen is better than using none. It is a fair point and it applies in a genuine emergency, on a day when nothing else is available.
It is a poor basis for planning a holiday. The cost of a new bottle is small against the cost of a burn, and the deeper problem with an old bottle is that it invites under application. People ration what is left rather than using the six to seven teaspoons a body needs, which our application guide sets out. A generous new bottle protects far better than a careful old one.
If you are using the correct quantity, expiry becomes largely academic. A household applying properly will finish a bottle long before its period after opening runs out. Finding leftovers next spring is itself a signal that last year's application was too thin. For general UK sun safety advice, the NHS guidance remains the plainest reference available.