Sunscreen is made of UV-filtering active ingredients suspended in a base of moisturizers, emulsifiers, and stabilizers. The active ingredients fall into two categories: mineral filters (zinc oxide and titanium dioxide) that physically block UV rays, and organic filters (like avobenzone and oxybenzone) that absorb UV energy and release it as heat. Most sunscreens on the market use one type or a combination of both, along with inactive ingredients that hold the formula together and make it feel good on skin.
Mineral Filters: Zinc Oxide and Titanium Dioxide
Mineral sunscreens rely on two metal oxides: zinc oxide and titanium dioxide. These are tiny insoluble particles that form a thin, opaque film on the skin’s surface. That film scatters, reflects, and absorbs incoming UV radiation before it reaches living skin cells. Zinc oxide protects across both UVA and UVB wavelengths, while titanium dioxide is strongest against UVB and short-wave UVA.
Both ingredients can be used at concentrations up to 25% by weight. The U.S. FDA classifies zinc oxide and titanium dioxide as “generally recognized as safe and effective,” making them the only two sunscreen active ingredients that have fully cleared the FDA’s safety review. The European Commission and Mercosur regulations set the same 25% cap.
The traditional drawback of mineral sunscreens is the visible white cast they leave on skin. To address this, many modern formulas use nanoparticle-sized versions of these minerals, which spread more transparently. A comprehensive review by Australia’s Therapeutic Goods Administration found that nanoparticle zinc oxide and titanium dioxide do not penetrate past the outermost dead-cell layer of skin, even on compromised skin. They stay on the surface and in the stratum corneum, making systemic absorption extremely unlikely.
Organic (Chemical) Filters
Organic UV filters work differently. Instead of sitting on top of the skin and deflecting light, these carbon-based molecules absorb UV photons and convert that energy into small amounts of heat. Each chemical filter targets a specific slice of the UV spectrum, so sunscreen manufacturers typically combine several to cover both UVA and UVB.
The most common organic filters you’ll see on ingredient labels include:
- Avobenzone, the most widely used UVA filter
- Oxybenzone, which covers both UVA and UVB but has drawn safety and environmental scrutiny
- Octinoxate (octyl methoxycinnamate), a UVB absorber
- Octocrylene, a UVB filter that also helps stabilize other ingredients
- Homosalate and octisalate, both UVB absorbers
The FDA has not yet made a final safety determination on any of these 12 organic active ingredients. That doesn’t mean they’ve been found unsafe. It means the FDA has requested additional data from manufacturers to complete its review. In the meantime, these filters remain legally available in over-the-counter sunscreens.
How Filters Are Stabilized
One challenge with organic filters is that UV exposure can break them down, which is ironic for a product designed to sit in the sun. Avobenzone is particularly prone to this. It degrades when hit by UV light, losing its protective ability over time.
To counteract this, formulators add photostabilizers. Octocrylene is the most effective stabilizer for avobenzone, though it’s expensive and difficult to work with in formulations. Antioxidants like vitamin E, vitamin C, and ubiquinone (CoQ10) also help. Research on avobenzone degradation found that ubiquinone was the most effective antioxidant stabilizer, with the added benefit of boosting SPF. These antioxidants pull triple duty: they stabilize the UV filter, increase sun protection, and help prevent the kind of free-radical damage that accelerates skin aging.
The Inactive Ingredients
Active UV filters typically make up 5% to 25% of a sunscreen formula. The rest is a carefully engineered base that determines whether the product feels like a lightweight lotion, a thick cream, or a dry-touch spray. These inactive ingredients don’t block UV, but without them the sunscreen wouldn’t spread, stay on your skin, or remain shelf-stable.
Common categories include emulsifiers (which blend oil and water phases together), thickeners (like carbomers or xanthan gum), moisturizers (glycerin, dimethicone, or plant oils), preservatives to prevent bacterial growth, and film-forming agents that help the product resist sweat and water. Silicones are especially common in modern formulations because they create a smooth, non-greasy feel while helping distribute UV filters evenly across the skin.
What SPF Numbers Actually Measure
SPF stands for sun protection factor, and it measures only UVB protection. SPF 30 blocks about 97% of UVB rays, SPF 50 blocks 98%, and SPF 100 blocks 99%. The jump from 30 to 50 sounds meaningful, but in practice you’re gaining just one additional percentage point of protection. The bigger factor is how much you apply and how often you reapply, since real-world protection always falls short of what’s measured in the lab.
For UVA protection, look for “broad spectrum” on the label. This designation means the sunscreen has been tested to confirm it provides UVA coverage proportional to its SPF rating. A sunscreen that only filters UVB will prevent sunburn but won’t protect against UVA rays, which penetrate deeper into the skin and drive premature aging and long-term damage.
Absorption Into the Body
One question that drives a lot of sunscreen concern is whether these chemicals get into your bloodstream. For mineral filters, the answer is essentially no. The weight of evidence shows nanoparticle zinc oxide and titanium dioxide stay in the outer dead-cell layer of skin and don’t reach viable cells or the general circulation. One study did detect trace amounts of zinc in blood and urine after sunscreen use, but the levels were negligible compared to the zinc already present in the body from a normal diet.
Organic filters are a different story. Modeling studies have found that oxybenzone produces the highest predicted plasma concentrations of any sunscreen ingredient after facial application. Other filters like avobenzone, ensulizole, octocrylene, and octinoxate showed median plasma levels at or below 0.5 nanograms per milliliter under typical facial use. The FDA flagged these absorption findings as the reason it needs more data before classifying organic filters as fully safe. Detectable absorption doesn’t automatically equal harm, but it’s the gap the FDA wants manufacturers to close with further testing.
Environmental Concerns
Several common sunscreen chemicals have documented effects on marine ecosystems. NOAA identifies oxybenzone, octinoxate, and octocrylene among the ingredients that can accumulate in coral tissue, induce bleaching, damage DNA, deform young coral, and even kill it. Nano-sized titanium dioxide and zinc oxide are also on NOAA’s list of potentially harmful ingredients for marine life, even though they’re considered safe for human skin.
Hawaii, the U.S. Virgin Islands, Key West, and Palau have all passed laws restricting sunscreens containing oxybenzone and octinoxate. Products marketed as “reef safe” typically avoid these ingredients, though there’s no regulated standard behind that label. If you’re swimming in coral-rich waters, a non-nano mineral sunscreen is generally the lowest-impact option.