It happens every year. You reach to the back of the bathroom cabinet, the beach bag, or the holiday wash kit, and there it is: a half-used bottle of sunscreen from last summer. It smells fine. It looks fine. So the question is: is it still fine to use?
The short answer is: it depends. The longer answer is worth knowing before you trust last year's protection with this year's skin.
Sunscreen Has an Expiry Date, and an Opening Date
Every sunscreen sold in the UK must display one of two date indicators:
- An expiry date (in the format MM/YYYY): the date until which the product is guaranteed to be effective, assuming it has never been opened.
- A PAO symbol (Period After Opening): the small jar icon with a number followed by "M" (for months). A "12M" symbol, for example, means the product should be used within 12 months of first opening.
Both matter. A sunscreen that has not yet passed its expiry date can still be compromised if it was opened over a year ago, and vice versa. Always check both.
Why Does Opened Sunscreen Degrade?
Sunscreen is a chemically active product. Once opened, it is exposed to air, light, temperature changes, and potential contamination, all of which can break down the UV filters inside.
Chemical UV filters (such as avobenzone or octocrylene) are particularly susceptible to degradation. Over time and with repeated exposure to light and heat, these molecules lose their ability to absorb UV radiation effectively. The SPF number on the bottle refers to the product in its original, stable state, not to a formula that has been sitting open in the sun for twelve months.
Physical UV filters (zinc oxide and titanium dioxide) are generally more stable and less prone to degradation, but the overall formula, emulsifiers, preservatives, texture agents, can still deteriorate, affecting both efficacy and skin safety.
Five Signs Your Sunscreen Is Past Its Best
Even if you cannot remember when you opened it, your sunscreen may tell you itself. Look out for:
- A change in texture: separation, graininess, or a watery consistency suggests the emulsion has broken down.
- A change in colour: yellowing or discolouration is a sign of oxidation.
- An unusual or rancid smell: if it no longer smells as it did when new, the formula has likely degraded.
- Difficulty blending: if it sits on the skin rather than absorbing smoothly, the base has changed.
- A change in appearance on skin: pilling, streaking, or uneven coverage can indicate instability.
If any of these apply, do not use it. No sunscreen is better than compromised sunscreen, because compromised sunscreen can give you a false sense of security.
What About Storage? Does It Make a Difference?
Absolutely. How a sunscreen is stored between uses has a significant impact on how long it remains effective.
Conditions that accelerate degradation:
- Leaving the bottle in direct sunlight (on a beach towel, on a windowsill, in a hot car)
- Exposure to high temperatures, the inside of a car in summer can reach well above 50°C
- Repeatedly opening and closing the product without securing the lid
Best practice for preserving your sunscreen:
- Store in a cool, dry place away from direct light
- Keep the lid tightly closed between applications
- Avoid decanting into smaller, non-airtight containers
- Do not mix old and new product in the same bottle
Sunscreen stored well in a cool, dark cupboard throughout the off-season has a significantly better chance of remaining stable than one that spent July on a sun lounger with the cap off.
The Honest Calculation
Let's be realistic. If you opened a sunscreen in June, used it through August, then stored it properly until the following June, that is roughly ten months of elapsed time since opening. For a product with a 12M PAO, you are within the window, provided the product shows no signs of degradation and was stored correctly.
However, if the same bottle spent part of the summer in a hot car, was left open on the beach, or shows any of the warning signs above, it is not worth the risk.
SPF protection is not something to approximate.
How Much Sunscreen Should You Actually Be Going Through?
If you are applying sunscreen correctly, you should rarely have a near-full bottle left over at the end of the summer. Most adults require approximately 35ml of sunscreen to cover the body in a single application, and reapplication every two hours is recommended during sun exposure.
A standard 200ml bottle, used properly across a two-week holiday with daily sun exposure, will not last long. If yours still has most of its contents after a full summer, it is a sign that either you were not applying enough, or it did not see much use, in which case, it is likely still well within its PAO window.
The Bottom Line
A half-used sunscreen is not automatically a write-off, but it does need to be assessed carefully. Check the expiry date and PAO symbol, consider how it was stored, and look for any physical signs of degradation. When in doubt, replace it.
At Odyskin, our formulas are designed for stability and efficacy, but no sunscreen, however well made, is an exception to the rules of chemistry. Treat your SPF with the same care you treat your skin, and it will do its job properly.
When it comes to sun protection, last year's habits should stay in last year. Your skin deserves a fresh start.
