If you’ve just pulled a half-used tube of sunscreen out of a drawer and paused to wonder whether it’s still doing anything, you’re not alone. Plenty of people end a summer with a bottle half gone, tuck it away somewhere sensible, and meet it again the following spring.

The printed date on the back is the first thing most of us check, and once the cap’s been twisted off it’s also close to the least useful line on the package. What matters more is when you first broke the seal and what the bottle has been through since. In this article, we’ll cover what happens to ultraviolet filters as they age, why the expiry date and the period-after-opening symbol are two separate numbers, how storage quietly shortens both, and a six-point check you can run in about a minute.

Why That Drawer Tube Deserves a Second Look

Finishing the season with a half-empty tube, dropping it in a drawer and rediscovering it the next spring is an ordinary thing to do. It feels wasteful to bin a product that’s still half full, and the date stamped on the crimp usually sits comfortably in the future, which makes the decision feel easy.

Here’s the catch. That date was assigned on the assumption that the container was never opened. From the moment the cap comes off, the formula starts meeting air, fingertips, bathroom humidity and whatever temperature its hiding place happens to be. Two identical tubes can therefore be in very different condition today: one that sat sealed in a cool cupboard, and one that rode around in a tote bag and a car door pocket for three months.

That’s why the honest answer isn’t a flat yes or no. It’s a short list of things to look at, and it’s a question we field often at Beautystone, a dermatology clinic in the Hapjeong area of Seoul, because after a laser session or a peel the state of the sunscreen already in someone's bag stops being theoretical.

What Happens to UV Filters Over Time

Sunscreen filters aren’t inert. Some of them change shape when they absorb light, and avobenzone is the textbook case. Research on the stability of avobenzone describes it as a leading filter for the UVA range, particularly 340 to 400 nanometers, and explains that the molecule shifts between a keto-enol form and a diketo form. That shuttling is what makes it unstable, and as it happens the filter blocks less ultraviolet light than it did on day one.

The same work adds a second point that’s easy to miss: the photodegradation* products formed along the way are highly reactive radicals, and those can set off inflammation in the skin. So an aged sunscreen isn’t only a weaker sunscreen. It can also be a more irritating one.

*Photodegradation: the change a light-exposed ingredient goes through when its structure breaks down and it loses the job it was designed to do.

None of this is a manufacturing fault. It’s a known property of the ingredient, and formulators build around it, pairing unstable filters with stabilizing partners or wrapping them in capsules so the molecule is shielded. That engineering isn’t unconditional, though. It assumes the product is kept roughly the way the label suggests, and it comes under strain when it isn’t. A 2014 review in Photodermatology, Photoimmunology and Photomedicine pulled together what was known about the photostability and safety of ultraviolet filters, and it’s a useful reminder that stability is something a formula holds onto rather than something it owns for good.

Expiry Date vs. Period After Opening

Stability testing shows why one number can’t settle the question on its own. A stability trial of sunscreen held in plastic containers reported that after three months at a room temperature of 22 degrees Celsius, non-encapsulated octocrylene measured 113.32 percent near the top of the container and 102.26 percent near the bottom. Same bottle, different readings depending on where the sample was drawn. The encapsulated version stayed in a tighter band, between 99 and 107 percent. The same protocol also ran a condition that cycled between 20 and 45 degrees, because temperature swings are treated as a variable worth isolating.

Guidance from the American Academy of Dermatology on replacing makeup and sunscreen supplies the regulatory half of the picture. It notes that the US Food and Drug Administration requires sunscreens to keep their original strength for three years, and it tells you to throw a product out if the color has visibly shifted, the smell is off, or the formula has split into layers. The same guidance singles out direct sunlight and hot, closed spaces, a parked car being the obvious one, as conditions that bring the breakdown forward.

Which means the markings are best read as a set rather than as a single verdict.

*PAO (period after opening): the figure printed beside the open-jar symbol and followed by an M, showing how many months the product is meant to be used once the cap is off.

A tube you opened last June and carried through an entire summer usually belongs in one of the bottom two rows, whatever the front of the package says.

Storage, Heat, and What the Safety Research Says

Storage is the variable most people underestimate, partly because it leaves no visible trace. A bottle that spent July in a car door pocket looks exactly like the one that spent July in a bedroom drawer. The difference shows up in the chemistry rather than on the label, and it’s the reason the printed numbers are a starting point instead of a promise.

The safety angle’s worth stating plainly. Because the byproducts of photodegradation are reactive, an old bottle can irritate the skin as well as protect less than it used to. Redness, stinging or small breakouts after applying a product that has been through a hot summer are worth paying attention to. If that happens, stop using it and speak to a dermatologist rather than pushing through and hoping it settles.

Individual results vary, and so do storage histories, which is why we’ll ask about the second one before recommending anything about the first. When we plan a laser or peeling course, the season and the amount of time someone spends outdoors come into the conversation early, and if the sensible move is simply to open a fresh bottle, that is what gets said.

The Six-Point Check You Can Run Right Now

None of this needs a lab, and you don’t need the box it came in. Six quick checks will tell you most of what you need to know.

• Find the open-jar symbol: the number printed beside it, followed by an M, is the period after opening.

• Try to recall the day you opened it: if you can’t, that blank is itself a piece of information.

• Look at the color: a yellow or grayish cast that wasn’t there before is a reason to stop.

• Smell it: a scent that doesn’t match your memory of the product suggests the formula has moved.

• Squeeze a little onto the back of your hand: if water and cream come out separated, lean toward replacing it.

• Retrace where it lived: months in a car or on a sunny windowsill make the printed numbers far less reliable.

If even one of the six applies, opening a new bottle is the easier call. With sunscreen the awkward failure isn’t applying too little. It’s applying a full, careful layer that isn’t doing the job you believe it’s doing.

The Bottom Line

To pull the threads together:

• The printed expiry date describes a sealed bottle, not the one sitting in your drawer.

• Filters such as avobenzone lose ultraviolet protection as they degrade, and the byproducts can irritate skin.

• The period after opening counts from the day you opened it, and rough storage can shorten either number.

• Color, smell and separation are reasons to discard. Their absence isn’t proof that a product’s still working.

Like anything else in skincare, this comes with trade-offs. Replacing a bottle that still looks fine feels wasteful, and there’s no way to measure remaining protection at home. Ultimately the choice depends on how the product was stored, how long ago you opened it, and how much sun you’re about to meet.

If you’re planning a treatment where sun protection carries real weight, a consultation is the best way to work out what fits your routine. Beautystone is a dermatology clinic in the Hapjeong area of Seoul, and current offers are listed at our promotions page. If last summer's tube is still on your shelf, the one-minute check above is a reasonable place to start today.