The Science
Watch the shade,
Watch the shade,
not the colour.
A UV-reactive ink inside every patch deepens in colour as more UV reaches it. Suncream on top of the patch slows that down. As it wears away, the shade deepens — and that's your prompt to reapply.
☁️ Little UV reaching patch
Resting statePalest lilac
UV light
☀️ More UV hitting patch
Reacted stateDeep purple
SPF 50+
Patch sits at its palest lilac — least UV getting through to the ink
SPF 30
Starts a touch deeper — a little more UV reaching the ink from the outset
No suncream
Patch goes deep purple — most UV reaching the ink
The patch never goes completely colourless. Even under a generous layer of SPF 50 it settles to a pale lilac, not clear. That is normal and it is how the patch is meant to look. What you're watching for is the shade deepening through the day — not a change from invisible to purple.
Why we suggest SPF 50. Published figures put SPF 30 at roughly 97% of UVB filtered and SPF 50 at roughly 98%. A little less UV reaching the ink means the patch starts paler, so the deepening is easier to spot. Alert. works with SPF 30 — the contrast is just clearer with SPF 50.
Alert. is a reapplication reminder. It does not block UV, does not measure SPF, and does not replace suncream. It also sits pale in shade, indoors or under clothing, where little UV reaches it — so a pale patch on its own is not proof that you are covered.
Alert. is a reapplication reminder. It does not block UV, does not measure SPF, and does not replace suncream. It also sits pale in shade, indoors or under clothing, where little UV reaches it — so a pale patch on its own is not proof that you are covered.
Why It Matters
Sunscreen wears off.
Most people don't notice when.
It wears off invisibly
Sunscreen is worn away by sunlight, heat, water, sweat and towelling. None of that is visible — you can't see, smell or feel how much is left on your skin.
Reapplying is the hard part
Public health guidance is to reapply roughly every two hours, and after swimming or towel drying. The advice is simple. Remembering it, hours into a beach day, is the bit people struggle with.
UV exposure adds up
UV exposure accumulates across a lifetime and contributes to skin ageing and skin cancer risk. That's why staying on top of reapplication matters, not just applying once in the morning.
The Technology
UV-reactive ink
Alert. patches use a UV-reactive ink — a compound that changes colour in response to UV light. It's the same broad class of technology as photochromic lenses, the glasses that darken in sunlight.
The ink sits in a thin, skin-safe adhesive layer. You wear the patch on bare skin and put your suncream on top of it. The suncream over the patch slows how much UV reaches the ink, so the patch sits at its palest. As that layer wears down, more UV gets through and the patch deepens toward purple.
The deeper the purple, the more UV is reaching the patch. That's your prompt to reapply.
Worth knowing: the patch never goes completely colourless. Even under a generous layer of SPF 50, it settles to a pale lilac rather than fully clear. That's normal and it's how the patch should behave — what you're watching for is the shade deepening, not a change from invisible to purple.
Alert. is a reapplication reminder. It doesn't block UV, doesn't measure SPF, and doesn't replace suncream. It also sits pale in shade, indoors or under clothing, where little UV reaches it — so a pale patch on its own isn't proof you're covered.
UV reaching the patch
No suncream
Wearing off
Freshly applied
Freshly applied
Palest lilac
Palest lilac
→
Wearing off
Deepening
Deepening
→
Deep purple
Reapply now
Reapply now
Illustrative only — shades vary with UV strength, how much suncream you apply and where you wear the patch.
How It Resets
It fades back
when you top up
Reapply your suncream over the patch and less UV reaches the ink again. Without that UV, the compound fades back toward its palest shade over the following minutes.
The change is reversible and can happen several times across a day, so you don't need a fresh patch every time you top up. Use a new patch the next day.
One patch, one person, one day. No battery, no charging, no app.
The cycle
1
Patch on bare skin
2
Suncream over the top
3
Patch settles to its palest
4
Out in the sun — shade deepens
5
Deep purple? Reapply over the patch
6
Fades back — repeat all day
2 hrs
How often health guidance says to reapply suncream in the sun
80%
Of UV can pass through light cloud, according to the World Health Organization
1
Patch per person, per day
24
Patches per pack — 24 person-days
Safety & Skin
Gentle by design.
Gentle adhesive
The adhesive is designed for comfortable everyday wear and leaves no residue. Apply to clean, dry, unbroken skin. If you have sensitive skin, test one patch on a small area first. Remove and stop using if any irritation occurs.
Nothing goes into your skin
Alert. is not a transdermal or topical product and delivers no active ingredient. The UV-reactive ink is held within the patch layer, and the only part designed to contact your skin is the adhesive.
Using them with children
Patches should be applied by an adult, patch-tested on a small area first, and supervised throughout. Keep them away from small children's hands and mouths. The patch is a prompt for the adult supervising, not something a child should rely on.
Not a substitute for suncream
Alert. does not block UV and does not protect your skin. It's a visual reminder to reapply. Always follow the directions on your suncream, and seek shade and cover up when UV is strong.
Never wonder whether
it's time to reapply.
24 patches delivered to your door — €9.99. Buy 2 or more and save 20%.
Shop Alert. Patches