How Sunscreen Actually Works: The Science Behind UV Protection

September 10, 2026 · 6 min read

Most people put on sunscreen every summer and never once wonder what it’s doing once it’s on their skin. It absorbs UV radiation and turns it into heat. Or it scatters that radiation away. A lot of the time it’s doing both at once. For years, the story was that mineral sunscreen works like a tiny wall, physically blocking rays from getting through. That’s not really accurate. Studies from the past ten years or so point to something different: both mineral and chemical filters mostly work by absorbing UV at a molecular level. Blocking isn’t really the mechanism, even though the packaging sometimes makes it sound that way.

The Two Main Types of Sunscreen Filters

Mineral (Physical) Sunscreens

Two ingredients do basically all the work in mineral sunscreens: zinc oxide and titanium dioxide. Both are inorganic, ground into fine particles, and they sit on top of the skin rather than soaking in. People often assume that’s the whole story, but there’s more happening underneath.

Some of the protection does come from scattering and reflection. Picture the particles knocking a few UV photons off course before they reach skin cells. But that part is smaller than most people think. When researchers measured it with optical equipment, reflection accounted for only around 4 to 5 per cent of UV protection. The bulk of it, somewhere between 90 and 95 per cent, comes from absorption. The particles take in the UV energy and let it go again as a small amount of heat right at the skin’s surface.

Chemical (Organic) Sunscreens

Chemical sunscreens take a different route entirely. They’re built from carbon-based compounds — avobenzone, homosalate, octisalate, octocrylene, octinoxate, oxybenzone among them — and none of them scatter or reflect anything. It’s absorption from start to finish.

A UV photon hits one of these molecules and gets absorbed, pushing the molecule into what chemists call an excited state. It doesn’t stay excited for long. The energy comes back out as heat through a process called internal conversion, and the molecule settles back to normal almost instantly. This happens over and over, across every molecule in the product, the entire time you’re out in the sun.

Understanding UV Radiation: UVA vs. UVB

There are two kinds of UV radiation to worry about, and they don’t act the same way on skin.

UVA sits at 320 to 400 nanometers and travels deep, reaching down into the dermis. This is the ray tied to premature ageing — wrinkles, sun spots, that leathery look people associate with sun damage over decades. It’s also linked to skin cancer, just more quietly than UVB, since it won’t necessarily leave you sunburned.

UVB runs shorter wavelength, 290 to 320 nanometers, and mostly affects the epidermis, the outer layer. This is the one responsible for actual sunburn, and it damages DNA directly enough to raise cancer risk on its own.

Broad-spectrum sunscreen is supposed to cover both. Zinc oxide in particular tends to get singled out here since it holds up across nearly the full UV range by itself.

What SPF Really Means

SPF only measures UVB protection. Nothing about UVA is captured in that number at all. SPF 30 blocks around 97 per cent of UVB rays. SPF 50 gets to roughly 98 per cent. The jump from 30 to 50 sounds bigger than it actually is.

A few things worth knowing:

Higher SPF numbers don’t add much. SPF 100 only reaches about 99 per cent.

Lab testing happens under controlled conditions. It doesn’t account for sweat, water, or the fact that most people apply less than they should.

SPF has nothing to say about UVA protection. Check for a broad-spectrum label, or a PA rating if the sunscreen was made for markets in Asia.

Common Misconceptions Debunked

Mineral sunscreens only reflect UV rays — that’s the claim, but it’s mostly wrong. Reflection is a small piece of the puzzle. Most of the protection comes from absorption through semiconductor band-gap behaviour, which sounds technical but basically means the mineral particles soak up UV energy the same way chemical filters do, just through a different physical process. The “tiny mirror” image is easy to picture and wrong more often than it’s right.

Chemical sunscreens are inherently unsafe — this one shows up online constantly. Avobenzone, octinoxate, and similar ingredients absorb UV and release it as heat without turning into anything harmful under normal use. Some chemical filters do raise real environmental questions, mostly around coral reefs, but as far as human safety goes, they’re FDA-approved and have been studied for decades.

You only need sunscreen on sunny days — also not true. Clouds let through a surprising amount of UV, commonly cited around 80 per cent. Glass doesn’t stop UVA either, so sitting near a window all day still adds up. Daily use makes sense regardless of what the sky looks like.

How to Choose the Right Sunscreen for Your Needs

Sensitive or reactive skin usually does better with mineral sunscreen, non-nano zinc oxide specifically, since it stays on the surface instead of soaking in. Dermatologists tend to recommend it after procedures and for people dealing with rosacea or eczema.

Chemical sunscreens shine for daily wear, especially under makeup. They’re light, absorb fast, and don’t leave a white cast. The catch is that fifteen to twenty-minute wait before they’re fully effective.

For the most complete broad-spectrum coverage, look for that label paired with SPF 30 or higher. Doesn’t matter much whether it’s mineral, chemical, or a hybrid — a lot of dermatologists actually prefer hybrids that mix zinc oxide with chemical filters.

If reef health matters to you, skip oxybenzone and octinoxate. Both are already banned in Hawaii and Key West. Non-nano zinc oxide is generally viewed as the gentler option for marine environments.

Application Tips for Maximum Protection

Most people simply don’t use enough sunscreen. Estimates put it at 25 to 50 per cent of what’s actually needed. A decent rule of thumb: about one ounce, roughly a shot glass, to cover the whole body. Two finger-lengths for the face and neck.

Reapply every two hours. That might sound like overkill until you consider that sun exposure alone breaks the product down, before sweat or water even enter the picture. Even water-resistant formulas only hold up for 40 to 80 minutes in water before losing real effectiveness.

Some spots get missed constantly. Ears. The back of the neck. Tops of feet. Along the hairline. Small areas, easy to burn.

Apply it last in your skincare routine, right before makeup, not somewhere in the middle.

The Bottom Line

Mineral and chemical sunscreens both work when they’re formulated properly and applied the way they’re supposed to be. There isn’t really a universal winner. The best one is whichever you’ll actually put on every day without skipping it — a light chemical formula for daily wear, or a gentler mineral one if your skin reacts easily. What actually matters is broad-spectrum coverage, SPF 30 or higher, using enough product, and reapplying throughout the day.

Sources

Topics:

Chemical Sunscreen Dermatology Mineral Sunscreen Skin care Tips Skincare Sun Protection Sunscreen UV Protection

Reviewed by editorial staff before publication.
Fact-checking and source verification applied.
Updated regularly for accuracy and clarity.
Aligned with newsroom ethics and publishing standards.

About The Author

Oliver Hughes Senior Lifestyle Correspondent

Oliver Hughes is a Senior Lifestyle Correspondent at New York Editor, where he covers lifestyle, health, wellness, travel, fashion, entertainment, food, home living, relationships, and modern culture.

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