Sunscreen works by placing a layer of active ingredients on your skin that absorb ultraviolet radiation before it can damage your cells. Some ingredients also scatter a small amount of UV light away from the skin. The result is the same either way: less UV energy reaches the living layers of your skin, reducing sunburn, premature aging, and cancer risk.
What UV Radiation Does to Your Skin
Sunlight contains two types of ultraviolet radiation that reach your skin: UVA (320 to 400 nm wavelength) and UVB (280 to 320 nm). They cause different kinds of damage at different depths.
UVB is higher in energy and hits the outermost layers of skin, the epidermis and superficial dermis. It’s the primary cause of sunburn, and it directly damages DNA in skin cells in ways that can lead to melanoma and other skin cancers over time. UVB also triggers delayed tanning, the kind that shows up hours after sun exposure and is usually preceded by redness.
UVA penetrates much deeper, reaching through the epidermis into the dermis and even subcutaneous tissue. It causes immediate pigment darkening and breaks down collagen by ramping up enzymes that degrade the structural fibers in your skin. This is the main driver of photoaging: wrinkles, loss of elasticity, and dark spots. UVA also contributes to cancer risk, though less directly than UVB.
Chemical vs. Mineral Filters
Sunscreen ingredients fall into two categories, but they work more similarly than most people think. Chemical (organic) filters absorb UV radiation and convert it into a small amount of heat that dissipates from your skin. Mineral (inorganic) filters, zinc oxide and titanium dioxide, also primarily absorb UV radiation. They do scatter and reflect a small fraction of light, but absorption is their main mechanism too. The old idea that mineral sunscreens work like tiny mirrors bouncing light away is mostly a myth.
The practical difference is more about texture, skin tolerance, and which wavelengths each ingredient targets. Many chemical filters are designed to absorb specific slices of the UV spectrum, so sunscreen formulas often combine several to cover both UVA and UVB. Zinc oxide is unusual in that it absorbs across a very wide range of UV wavelengths on its own, which is one reason it shows up in so many mineral and hybrid formulas.
What SPF Actually Measures
SPF stands for Sun Protection Factor, and it’s a ratio. Testers apply sunscreen to a patch of skin at a standardized thickness, then expose it to UV light alongside an unprotected patch. The SPF number equals the dose of UV needed to produce visible redness on protected skin divided by the dose needed on bare skin. So SPF 30 means the protected skin required 30 times the UV energy to burn.
This is not the same as “30 times longer in the sun.” Real-world UV intensity changes throughout the day, you sweat and rub sunscreen off, and most people apply far less than the amount used in testing. The standard application density in lab testing is 2 milligrams per square centimeter of skin. For your face alone, that’s roughly a nickel-sized dollop. Studies consistently find that people apply about half that amount, which means you’re getting significantly less protection than the label suggests.
SPF also only measures protection against UVB-induced redness. It tells you nothing about UVA protection. To ensure you’re getting both, look for “broad spectrum” on the label. A sunscreen earns that designation by passing a test showing its protection extends to a critical wavelength of at least 370 nm, well into the UVA range. Only broad spectrum sunscreens with SPF 15 or higher can claim protection against skin cancer and aging.
Why Some Formulas Break Down in Sunlight
Here’s an underappreciated problem: the same UV radiation sunscreen is supposed to block can degrade the ingredients doing the blocking. When a UV filter molecule absorbs energy and reaches an excited state, it normally returns to its baseline and is ready to absorb again. But sometimes the molecule breaks down into a new compound that no longer absorbs UV. This is called photodegradation, and it’s the reason reapplication matters even if you haven’t been swimming.
Avobenzone, one of the most common UVA filters, is particularly prone to this. It’s effective when fresh but loses potency under sustained UV exposure. Formulators solve this by pairing avobenzone with stabilizing ingredients like octocrylene, which absorbs some of the excess energy and prevents avobenzone from degrading as quickly. If you see both ingredients on a label, that’s why. Mineral filters like zinc oxide and titanium dioxide are inherently photostable, meaning they don’t break down under UV exposure.
Water Resistance and Reapplication
No sunscreen is waterproof. The FDA allows only two water resistance claims: 40 minutes and 80 minutes. To earn the 40-minute label, the product must retain its labeled SPF after two cycles of 20 minutes in water followed by 15 minutes of air drying. The 80-minute claim requires four of those cycles. After that tested window, you need to reapply.
Even without swimming, the general recommendation is to reapply every two hours. Sweating, toweling off, and the natural photodegradation of ingredients all reduce protection over time. If you’re in and out of water, reapply immediately after drying off regardless of what the label says.
Safety and Absorption Into the Body
Chemical sunscreen filters do absorb through the skin into the bloodstream. Modeling studies have found that oxybenzone produces the highest plasma concentrations of any common filter after application. Several other ingredients, including avobenzone, octocrylene, and octinoxate, reach measurable levels in the blood as well, though limiting use to facial application keeps most of them at or below the FDA’s safety threshold of 0.5 nanograms per milliliter.
The FDA currently classifies only zinc oxide and titanium dioxide as generally recognized as safe and effective without additional data. For 12 other chemical filters, including avobenzone, homosalate, oxybenzone, and octocrylene, the agency has requested more safety data before making a final determination. Two older ingredients, PABA and trolamine salicylate, are considered not safe for sunscreen use. None of this means chemical sunscreens are dangerous. It means the regulatory process is still catching up, and mineral options have the clearest safety profile right now.
Impact on Coral Reefs and Marine Life
Several chemical UV filters wash off in the ocean and harm aquatic ecosystems. NOAA identifies oxybenzone, octinoxate, and octocrylene among the chemicals that can damage corals and other marine life. A 2022 National Academy of Sciences review confirmed that chemical UV filters can harm corals and called for more comprehensive ecological risk assessments.
There’s no regulated definition of “reef safe,” so that label on a bottle doesn’t guarantee much. Mineral sunscreens are generally considered a better option for ocean environments because they have fewer documented effects on aquatic organisms, though nano-sized particles of titanium dioxide and zinc oxide are still flagged as potential concerns. If you’re swimming in coral reef areas, wearing UV-protective clothing and seeking shade during peak hours (10 a.m. to 2 p.m.) reduces the amount of sunscreen entering the water in the first place.