How sunscreen works

Sunscreen works by forming a layer of UV filters over the skin that absorbs or scatters ultraviolet light before it reaches living cells. Mineral filters — zinc oxide and titanium dioxide — sit on the surface. Chemical filters absorb into the upper skin layers. Both convert most of the UV they capture into a small amount of heat, and both protect only where they are actually present, which is why how much you apply matters enormously.

Understanding the mechanism makes every practical rule about sunscreen obvious rather than arbitrary.

What UV does to skin

Ultraviolet light carries enough energy to damage DNA in skin cells directly, and to generate reactive oxygen species that damage it indirectly. Most of that damage is repaired; some is not. Accumulated unrepaired damage is what drives both photoaging and skin cancer, and ultraviolet radiation is classified as a known human carcinogen.

That last row is the reason the broad-spectrum claim exists as a separate regulated test. A high SPF with poor UVA coverage stops you burning while leaving the deeper damage largely unaddressed.

How the filters actually work

Mineral filters

Zinc oxide and titanium dioxide form a layer of fine particles on the skin surface. UV hitting that layer is partly scattered away and partly absorbed and re-emitted as heat. Because the particles sit on top of the stratum corneum rather than penetrating it, protection begins the moment the layer is even.

The common description — mineral reflects, chemical absorbs — is a simplification that is not quite right. Mineral filters absorb a substantial share of the UV they capture. The real distinction is location, not mechanism. Mineral versus chemical.

Chemical filters

Organic compounds such as avobenzone, octocrylene and homosalate absorb into the upper skin layers, where their molecular structure absorbs UV photons and releases the energy as heat. They are conventionally given 15 to 30 minutes before exposure to settle into the skin.

They are also consumed by doing their job: absorbing UV progressively degrades the molecules, which is part of why reapplication matters. Avobenzone is notably unstable and is almost always paired with a stabilizer.

Why the amount matters so much

SPF is measured at 2 milligrams of product per square centimeter of skin, spread evenly. A filter only protects the skin it is actually covering, so a thinner layer leaves more gaps and lets more UV through — and the relationship is not linear. Applying roughly half the tested dose leaves you with substantially less than half the labeled SPF.

Real-world application is consistently between a quarter and a half of the tested amount. That single fact explains most of the gap between the number on a bottle and the protection people actually get. How much sunscreen to apply.

Why you have to reapply

1. Chemical filters degrade as they absorb UV.
2. Everything rubs off — on towels, clothing, sand and hands.
3. Sweat and water move it, which is what the 40 and 80-minute water-resistance claims measure.
4. Toweling removes it mechanically, regardless of what the water-resistance number said.

Hence the standard rule: every two hours, and immediately after swimming, sweating or toweling off. Water-resistant sunscreen explained.

What sunscreen does not do

• Block everything. SPF 50 filters about 98% of UVB. Nothing reaches 100%, which is why the word sunblock was banned.
• Cover visible light, which contributes to melasma and hyperpigmentation. Iron oxides in tinted formulas do. Iron oxides and visible light.
• License longer exposure. Using sunscreen to stay out longer rather than to be safer during the same time is the behavior it was never meant to enable.
• Protect where you did not apply it. Ears, the back of the neck and the tops of the feet are missed constantly.