Sunscreen works by placing a thin layer of active ingredients on your skin that either absorb or scatter ultraviolet radiation before it can damage your cells. The specific mechanism depends on whether the sunscreen uses mineral filters, chemical filters, or a combination of both, but the end result is the same: UV energy gets neutralized before it reaches the living layers of your skin.
The Two Types of UV You Need to Block
Sunlight contains two types of ultraviolet radiation that reach your skin. UVB rays (280 to 315 nanometers) are shorter in wavelength and more energetic. They burn the outer layers of your skin and are the primary driver of sunburn and skin cancer. UVA rays (315 to 400 nanometers) are longer, less intense per photon, but far more abundant, making up roughly 95% of the UV radiation that hits the Earth’s surface. UVA penetrates deeper into the skin, breaking down collagen and elastin, accelerating aging, and contributing to skin cancer risk in ways scientists previously underestimated.
No single sunscreen ingredient handles both types of UV equally well. That’s why most formulas combine multiple active ingredients to cover the full UV spectrum.
How Mineral Sunscreens Work
Mineral sunscreens use two inorganic compounds: zinc oxide and titanium dioxide. These sit on the surface of your skin as tiny particles that interact with UV light in three ways at once. They reflect some UV radiation, scatter it in different directions, and absorb a portion of it directly. The old idea that mineral sunscreen acts purely as a physical “mirror” bouncing light away is incomplete. Absorption plays a significant role too.
The two minerals complement each other. Titanium dioxide is primarily a UVB absorber, meaning it’s strongest against the rays that cause sunburn. Zinc oxide is more efficient in the UVA range, handling the deeper-penetrating rays linked to aging and long-term skin damage. A mineral sunscreen containing both ingredients covers a wider swath of the UV spectrum than either one alone. One tradeoff: because these particles also scatter visible light, mineral sunscreens tend to leave a white or chalky appearance on the skin, especially in formulas with larger particle sizes.
How Chemical Sunscreens Work
Chemical (organic) sunscreens take a different approach. Their active ingredients are carbon-based molecules with electron-rich structures that act like tiny energy sponges. When a UV photon hits one of these molecules, it excites the electrons into a higher energy state. The molecule then releases that energy as a tiny, imperceptible amount of heat rather than letting it penetrate into your skin cells. In a photostable filter, the molecule snaps back to its original state and is ready to absorb the next photon, repeating the cycle over and over.
Different chemical filters target different parts of the UV spectrum. Some are tuned to absorb primarily UVB, others primarily UVA. Formulas labeled “broad spectrum” combine multiple filters so nothing slips through. To earn that label in the United States, a sunscreen must pass a standardized test showing its protection extends to a critical wavelength of at least 370 nanometers, which falls well into the UVA range.
Why Some Filters Break Down in Sunlight
Not every UV filter handles the absorption cycle gracefully. Avobenzone, one of the most widely used UVA filters, is notoriously unstable. After absorbing UV energy, it can undergo structural changes that cause the molecule to fragment rather than return to its original state. When this happens, the filter stops working and can even produce reactive byproducts on your skin.
Sunscreen manufacturers counter this with several strategies. One common approach is pairing avobenzone with stabilizing ingredients like octocrylene, which absorbs some of the excess energy and prevents avobenzone from breaking apart. Other stabilizers work as antioxidants, neutralizing the free radicals generated during photodegradation. Newer broad-spectrum filters developed outside the U.S. (sold under names like Tinosorb S and Mexoryl SX) are inherently more photostable and provide strong UVA protection without needing the same stabilization tricks. Mexoryl SX-containing formulas have been shown to provide superior UVA protection compared to formulas with the same SPF but without that filter.
What SPF Actually Measures
SPF, or sun protection factor, measures how well a sunscreen blocks UVB radiation specifically. The numbers follow a curve of diminishing returns:
- SPF 15 blocks 93% of UVB rays
- SPF 30 blocks 97% of UVB rays
- SPF 50 blocks 98% of UVB rays
The jump from SPF 30 to SPF 50 adds just one percentage point of protection. That doesn’t mean higher SPFs are pointless, though. In real-world use, people almost always apply less sunscreen than the amount used in lab testing. SPF is measured with a standardized application of 2 milligrams per square centimeter of skin. Most people apply roughly half that, which means the protection you actually get is lower than what’s on the label. A higher SPF gives you a larger margin of error.
How Much You Actually Need to Apply
The reason SPF underperforms in practice comes down to application thickness. To match lab conditions, dermatologists recommend the “teaspoon rule” for full-body coverage:
- Face, head, and neck: 1 teaspoon
- Each arm: 1 teaspoon
- Front torso: 1 teaspoon
- Back torso: 1 teaspoon
- Each leg: 2 teaspoons
That totals about 9 teaspoons, or 45 milliliters, for your entire body. For context, that’s roughly a shot glass and a half. Most people use far less, which is why reapplication every two hours matters as much as the initial coat.
Which Ingredients Are Considered Safest
The FDA currently recognizes 16 sunscreen active ingredients under its existing regulatory framework. However, in its proposed updated rule, only two, zinc oxide and titanium dioxide, have enough publicly available safety data to be proposed as generally recognized as safe and effective (GRASE). Two older ingredients, PABA and trolamine salicylate, are proposed as not safe for sunscreen use. The remaining 12 chemical filters, including avobenzone, oxybenzone, homosalate, and octocrylene, fall into a middle category: the FDA says it needs more data before making a final determination on their safety.
This doesn’t mean those 12 ingredients are dangerous. It means the FDA wants additional studies, particularly on how much of these compounds absorb through the skin and what effects that absorption might have over time. All sunscreens currently on U.S. shelves remain legal to sell and are still considered effective at blocking UV radiation. The protection they provide against skin cancer and sun damage is well established regardless of which filter type they use.