Sunblock works by absorbing ultraviolet radiation before it can damage your skin cells. Both mineral and chemical sunscreens rely primarily on absorption, though they use different molecular processes to get the job done. The end result is the same: UV photons are intercepted at the skin’s surface and converted into small amounts of heat that dissipate harmlessly.
What UV Radiation Actually Does to Your Skin
The sun emits two types of ultraviolet radiation that reach your skin: UVB (290 to 320 nanometers) and UVA (320 to 400 nanometers). They behave differently once they hit you. UVB penetrates only about 10 to 20 micrometers deep, roughly the outermost layer of your epidermis. That’s enough to damage the DNA in surface skin cells, causing sunburn and driving the mutations behind most skin cancers.
UVA goes further. At 341 nanometers, UVA penetrates up to 135 micrometers into skin, reaching well into the deeper layers of the epidermis and toward the dermis. That deeper reach is why UVA is responsible for premature aging, wrinkles, and loss of skin elasticity. It also contributes to skin cancer risk, though less directly than UVB. Effective sunblock needs to handle both.
Chemical Filters: Absorbing UV and Releasing Heat
Chemical (also called organic) sunscreen filters are carbon-based molecules designed to absorb UV photons. When a UV photon hits one of these molecules, it excites electrons to a higher energy state. The molecule then releases that energy as a tiny amount of heat as the electrons drop back down. This cycle repeats with each incoming photon, essentially converting damaging radiation into warmth you’ll never notice on your skin.
Different chemical filters target different parts of the UV spectrum. Some are tuned to absorb UVB, others UVA. Most sunscreen formulas blend several filters together to cover the full range. One widely used UVA filter, avobenzone, has a well-known weakness: it exists in two molecular forms and flips between them when exposed to light. The form that absorbs UVA well is stable, but the other form breaks down quickly, generating free radicals and losing its protective ability. To solve this, manufacturers pair avobenzone with stabilizing compounds that prevent the breakdown, keeping the sunscreen effective longer.
This photostability issue is why reapplication matters beyond just sweat and water washing your sunscreen away. Even sitting in direct sun, chemical filters gradually lose potency as their molecules degrade.
Mineral Filters: Not Mirrors, but Absorbers
Zinc oxide and titanium dioxide are the two mineral sunscreen ingredients available. For decades, they were described as tiny mirrors that physically reflect UV rays off your skin. That turns out to be mostly wrong. Research measuring the actual reflectance of these minerals found they reflect only 4 to 5 percent of UV radiation, the equivalent of less than SPF 2. That’s negligible protection.
What they actually do is absorb UV photons through a process called semiconductor band gap absorption. The mineral particles act like semiconductors: when a UV photon carries enough energy (which UV wavelengths do), the particle absorbs it. The interesting twist is that at longer wavelengths, in the visible light range, these minerals become reflectors, bouncing back up to 60 percent of light. That’s why old-school mineral sunscreens left a white cast. Newer formulations use nanoparticles small enough to reduce visible light reflection while still absorbing UV effectively.
What SPF Numbers Actually Mean
SPF measures protection against UVB only, and the scale is not linear. SPF 15 blocks 93 percent of UVB rays. SPF 30 blocks 97 percent. SPF 50 blocks 98 percent, and SPF 100 stops 99 percent. The jump from SPF 15 to 30 cuts the amount of UVB reaching your skin roughly in half (from 7 percent getting through to 3 percent). But going from SPF 50 to 100 only cuts it from 2 percent to 1 percent.
For a sunscreen to be labeled “broad spectrum,” it must pass an FDA test showing its protection extends to a critical wavelength of at least 370 nanometers, well into the UVA range. Only products with both broad spectrum protection and an SPF of 15 or higher can make that claim on the label. If your sunscreen isn’t labeled broad spectrum, it may protect against sunburn but leave you exposed to the deeper-penetrating UVA damage that ages skin.
Water Resistance and Reapplication
No sunscreen is waterproof. The FDA allows only two water resistance claims: 40 minutes and 80 minutes. These numbers come from standardized testing where SPF is measured after subjects spend that amount of time in water. After the labeled time, protection drops significantly. Even without swimming, sweating, toweling off, and simply rubbing your face throughout the day remove sunscreen. The standard recommendation of reapplying every two hours accounts for all of these factors plus the natural photodegradation of chemical filters.
Absorption Into the Body
A 2020 study published in JAMA tested six common chemical sunscreen ingredients and found that all of them entered the bloodstream after normal application. Plasma concentrations exceeded 0.5 nanograms per milliliter, the FDA’s threshold for requiring additional safety studies, after just a single application on day one. This doesn’t mean these ingredients are harmful. It means the FDA flagged them for more research because their systemic absorption was higher than the level at which safety can simply be assumed without further data. Zinc oxide and titanium dioxide were not flagged in the same way, which is one reason mineral sunscreens are often recommended for young children and people who prefer to minimize chemical exposure.
Environmental Effects on Coral Reefs
Some chemical UV filters cause measurable damage to marine ecosystems. Oxybenzone, one of the most common chemical filters, has been shown to cause four specific types of harm to developing coral: increased bleaching susceptibility, DNA damage, abnormal skeleton growth through hormone disruption, and gross deformities in coral larvae. Even a closely related compound, benzophenone-2, can kill juvenile corals at very low concentrations. Hawaii, Key West, and several other coastal regions have banned sunscreens containing oxybenzone and octinoxate for this reason.
If you’re swimming near reefs, mineral sunscreens or formulas labeled “reef safe” (typically free of oxybenzone and octinoxate) reduce your impact. “Reef safe” isn’t an FDA-regulated term, though, so checking the active ingredients list is more reliable than trusting front-of-bottle marketing.