The Science

How Alert. knows when
your SPF is gone.

A photochromic molecule inside every patch changes structure when UV light hits it — turning the patch purple. Your sunscreen blocks that UV. When it stops blocking, the patch tells you.

☁️ No UV reaching patch
Closed statePatch stays clear
UV light
☀️ UV hitting patch
Open statePatch turns purple
🧴
SPF active
Sunscreen absorbs UV before it reaches the patch — molecule stays closed, patch stays clear
⚠️
SPF worn off
UV passes through to the patch — molecule opens, patch turns purple
SPF 50+
Patch stays fully clear — UV almost entirely blocked
SPF 30
Slight tint possible — more UV getting through the SPF layer
No sunscreen
Patch turns fully purple — UV reaching your skin unblocked
Why we recommend SPF 50+ — SPF 30 blocks ~97% of UV, SPF 50 blocks ~98%. That extra 1% means more UV reaches the patch under SPF 30, which can cause a faint tint even when you're still protected. With SPF 50 the patch starts fully clear and the colour change to purple is unmissable. Alert. works with SPF 30 — it's just more accurate and dramatic with SPF 50.
Why It Matters

Sunscreen breaks down.
Most people don't know when.

⚠️
SPF degrades invisibly
UV filters in sunscreen break down through exposure to sunlight, heat, water, and sweat. This happens completely invisibly — you can't see, smell, or feel when it stops working.
⏱️
Reapplication timing is critical
Most sunscreens need reapplying every 90–120 minutes in direct sunlight. Studies show the majority of people either don't reapply at all, or reapply far too late — after UV damage has already begun.
🔬
UV damage is cumulative
UV radiation causes DNA damage in skin cells that accumulates over time. Even a single unprotected exposure contributes to long-term skin ageing and significantly increases skin cancer risk.
The Technology

UV-reactive photochromic ink

Alert. patches use a photochromic compound — a molecule that changes its structure, and therefore its colour, when exposed to specific wavelengths of UV light. This is the same class of technology used in photochromic lenses (glasses that darken in sunlight).

The compound is embedded in a thin, skin-safe adhesive layer. When sunscreen is active on the skin beneath the patch, it absorbs the UV before it reaches the photochromic compound — keeping the patch clear.

When your sunscreen degrades and UV passes through unchecked, the compound activates — turning the patch a vivid purple that's impossible to miss.
UV reaching the patch
No SPF
100% UV → Patch turns purple
SPF fading
~60% UV → Changing colour
SPF active
Minimal UV → Clear patch
SPF active
Clear
→
SPF fading
Changing
→
SPF gone
Reapply now
How It Resets

The photochromic
reset mechanism

When you reapply sunscreen, the fresh SPF layer once again blocks UV from reaching the patch. Without UV activation, the photochromic compound reverts to its original colourless state — the patch clears again within minutes.

This reset is completely reversible and can happen dozens of times on a single patch. The compound doesn't "run out" from normal use — it responds purely to the presence or absence of UV reaching the patch surface.

One patch lasts a full day of sun exposure. The chemistry is self-contained, requiring no battery, charging, or external activation.
Reset cycle
1
Apply SPF → patch clears
2
UV exposure → SPF breaks down
3
Patch turns purple
4
Reapply SPF → patch resets
5
Repeat all day
90min
Average SPF reapplication window in direct sun
80%
Of people don't reapply sunscreen correctly
3x
Higher burn risk when SPF isn't reapplied on time
24
Patches per pack — enough for a full week's holiday
Safety & Testing

Gentle by design.
Safe for all skin types.

🌿
Gentle adhesive
The adhesive layer is designed for gentle, comfortable wear and leaves no residue when removed. If you experience any irritation, remove the patch and discontinue use.
🚫
No harmful chemicals
The photochromic compound used in Alert. patches contains no toxic or harmful substances. It operates entirely as a colour-change indicator — nothing transfers to skin.
👶
Safe for children
Alert. patches are safe for use on children's skin. As with all adhesive products, adult supervision is recommended for children under 3.

Ready to protect your skin
the smart way?

Get 24 patches delivered to your door — from €9.99.

Shop Alert. Patches