What Is Sunscreen Made Of: Chemicals, Minerals, and SPF

Sunscreen is built from a surprisingly short list of functional parts: one or more UV-filtering active ingredients, a base that spreads them evenly across skin, and stabilizers that keep everything working after the bottle is opened. The active ingredients fall into two camps, often called “chemical” and “mineral,” and each blocks ultraviolet radiation through a different physical process. But the story gets more interesting once you look at how SPF numbers translate to real-world protection, why the base ingredients matter more than most people realize, and what happens to these compounds after they leave your skin.

Chemical UV Filters

Chemical sunscreen filters are organic molecules, “organic” in the chemistry sense meaning they contain carbon. They work by absorbing UV photons and converting that energy into small amounts of heat. Each filter molecule is tuned to absorb light in a particular wavelength range, so most chemical sunscreens combine two or more filters to cover the full UV spectrum.

Avobenzone is the most widely used UVA filter in the United States and much of the world. It absorbs well in the UVA range (roughly 320–400 nm), but it has a well-known stability problem: exposure to sunlight triggers a chemical rearrangement that gradually destroys its ability to absorb UV, a process driven by what chemists call keto-enol tautomerism. Formulators address this by pairing avobenzone with photostabilizers, encapsulation techniques, antioxidants, or quencher molecules that absorb the energy before avobenzone can degrade.¹

Octocrylene is one of those stabilizing partners. It absorbs primarily in the UVB range and also acts as a photostabilizer for avobenzone when the two are combined in a single formula. Research into lipid nanocarrier encapsulation of avobenzone and octocrylene together has shown that encapsulation can further boost the photostability of avobenzone while allowing slower release of both filters from the cream.²

Octinoxate (also labeled as octyl methoxycinnamate or ethylhexyl methoxycinnamate) is another common UVB absorber found in many drugstore sunscreens globally. It exists in two structural forms; the more stable form is the one manufacturers rely on for UV absorption, though light exposure can shift a portion of the molecules to the less active form over time.³ Other chemical filters you might see on labels include homosalate, octisalate, and oxybenzone, each covering slightly different slices of the UV spectrum.

Mineral UV Filters

Mineral sunscreens use just two ingredients: zinc oxide and titanium dioxide. These are finely ground inorganic particles suspended in a cream or lotion base. For decades, the standard explanation was that mineral sunscreens “physically block” or “reflect” UV rays like tiny mirrors sitting on the skin. That turns out to be mostly wrong.

A study that directly measured how much UV radiation zinc oxide and titanium dioxide actually reflect found that reflection accounted for only about 4–5% of their UV protection, which is barely enough to register as SPF 2. The vast majority of the UV-blocking work happens through absorption, the same basic mechanism as chemical filters. The difference is that the absorption occurs through a property of the mineral’s crystal structure rather than through a single organic molecule rearranging its electrons.⁴

The traditional drawback of mineral filters is cosmetic: larger particles scatter visible light and leave a white or chalky film on skin. To fix this, manufacturers have increasingly moved to nanoparticle versions (smaller than 100 nm). The nanoparticles are transparent on the skin, but the shift in particle size can change the balance between UVA and UVB protection. There are also ongoing discussions about whether nanoparticles behave differently in terms of skin penetration and cellular effects compared to larger particles. In laboratory cell studies, both titanium dioxide and zinc oxide nanoparticles have shown signs of photo-induced cytotoxicity and genotoxicity, and zinc oxide nanoparticles have occasionally been observed in viable skin layers during long-term exposure experiments.⁵ Whether those lab findings translate to any real risk at the concentrations and exposure durations typical of sunscreen use remains debated; regulatory agencies in the U.S., EU, and Australia still classify nanoparticle mineral sunscreens as safe for topical use.

What SPF Actually Measures

SPF stands for Sun Protection Factor, and it is measured under very specific laboratory conditions. A standardized amount of sunscreen, 2 milligrams per square centimeter, is applied to a patch of skin, and researchers measure how much additional UV exposure it takes to produce redness compared to unprotected skin. SPF 30 means it takes about 30 times as much UV to cause a sunburn. In percentage terms, SPF 15 filters roughly 93% of UVB rays, SPF 30 about 97%, and SPF 50 about 98%. The returns diminish rapidly above SPF 30, and no sunscreen blocks 100%.

There is an important catch: SPF is only a measure of UVB protection. UVB is the primary cause of sunburn, but UVA penetrates deeper into the skin and contributes to photoaging and skin cancer risk. A sunscreen labeled “broad spectrum” must also provide meaningful UVA protection, though the degree of UVA coverage varies widely between products. If you see only an SPF number with no broad-spectrum claim, the product may do very little against UVA.

The Application Gap

SPF values are tested at 2 mg/cm², but almost nobody applies that much in real life. A study measuring how U.S. consumers actually apply sunscreen at home found that participants believed they were putting on the recommended thickness but actually applied less than half: about 0.89 mg/cm² on the face and 0.85 mg/cm² on the body, versus the 2.0 mg/cm² standard.⁶

This matters enormously for actual protection. When researchers tested SPF at real-world application densities (around 1.33 mg/cm²), they found that SPF drops along a curve, not linearly. To get protection equivalent to SPF 15 tested at the standard thickness, you would need a product labeled SPF 24 at that thinner application. For the equivalent of SPF 30, you would need roughly SPF 48 on the label. And for SPF 50-level protection at real-world thickness, the label would need to read about SPF 79.⁷ This is one of the strongest practical arguments for choosing a higher SPF product: not because SPF 50 is dramatically better than SPF 30 at the tested dose, but because your actual application will likely deliver less protection than the label promises.

The Inactive Ingredients That Make It All Work

UV filters alone would be useless sitting in a jar. The rest of a sunscreen formula, sometimes making up 70–80% of the product by weight, is a carefully engineered delivery system. These “inactive” ingredients are functionally essential.

Emollients and oils form the base of most sunscreens and determine whether the product feels like a lotion, cream, gel, or spray. They dissolve or suspend the UV filters and help the product spread into a thin, even film. Research on emulsion-based sunscreen films has shown that the way the base evaporates on skin matters for UV protection: if the oil phase is non-volatile, the UV filter stays dissolved and the SPF remains roughly constant even as the water or solvent phase evaporates.⁸ Film formers, another class of inactive ingredients, help the sunscreen adhere to skin rather than wiping off or running with sweat.

Antioxidants like vitamin E acetate and stabilized vitamin C (sodium ascorbyl phosphate) are increasingly added to sunscreen formulas. They serve a function beyond marketing: once absorbed into the outer skin layers, these compounds convert into their active vitamin forms and scavenge reactive oxygen species generated by whatever UV radiation gets past the sunscreen’s filters. This adds a layer of biological defense underneath the physical sunscreen film.⁹ Some newer formulations are exploring synergistic antioxidant combinations using ingredients like ferulic acid and ergothioneine alongside vitamins C and E for added skin-protective effects.¹⁰

Do Chemical Filters Get Into Your Blood?

Yes. A randomized clinical trial published in JAMA tested four different sunscreen formulations (two sprays, a lotion, and a cream) applied under maximal-use conditions. All four products sent avobenzone into the bloodstream at concentrations above the FDA’s threshold of 0.5 ng/mL within the first day, and for most formulations this persisted through day 7. Spray formulations produced the highest peak plasma concentrations, with geometric means reaching about 4 ng/mL, while the cream produced a lower peak around 1.8 ng/mL. Concentrations increased from day 1 to day 4, suggesting the compounds accumulate with repeated application.¹¹

The FDA’s 0.5 ng/mL threshold is not a safety limit. It is a regulatory trigger: when any topically applied drug exceeds that plasma level, the FDA requires additional safety testing. Exceeding the threshold does not mean the ingredient is harmful; it means the science on long-term systemic effects is still being gathered. The FDA, the American Academy of Dermatology, and other bodies have consistently maintained that people should continue using sunscreen while this research is underway, because the skin cancer risk from unprotected UV exposure is well established.

Skin Reactions and Contact Sensitivity

A small percentage of people develop skin reactions to sunscreen ingredients. The most commonly reported culprits among chemical filters include PABA and PABA esters (rarely used today), cinnamates, benzophenones like oxybenzone, and dibenzoylmethane derivatives. These can trigger allergic contact dermatitis or, when the skin is simultaneously exposed to sunlight, photocontact dermatitis.¹²

If you consistently get rashes or itching from sunscreen, switching from a chemical to a mineral formula (zinc oxide or titanium dioxide) is the usual first step, since mineral filters cause contact allergies far less often. The fragrances and preservatives in any sunscreen can also be the real trigger, so a fragrance-free mineral product tends to be the least reactive option.

What Sunscreen Does to Coral Reefs and Marine Life

Organic UV filters wash off swimmers and enter coastal waters in concentrated amounts at popular beaches. Research has demonstrated that sunscreen chemicals can cause rapid and complete bleaching of hard corals, even at very low concentrations. The mechanism involves the organic UV filters activating dormant viral infections in the algae that live symbiotically inside coral tissue, triggering the algae to die and the coral to bleach.¹³ Oxybenzone and octinoxate have drawn the most attention, and several jurisdictions, including Hawaii and Palau, have banned or restricted sunscreens containing them.

Mineral filters are often marketed as “reef safe,” but that label is not regulated and not entirely honest. Zinc oxide nanoparticles in marine environments can release zinc ions, interact directly with algae cells, and generate reactive oxygen species, all of which are toxic to marine microalgae.¹⁴ The environmental picture is not as simple as “mineral equals safe, chemical equals harmful.”

Tinted Sunscreens and Visible Light

Standard UV filters, whether chemical or mineral, do almost nothing against visible light in the 400–700 nm range. For most people this is not a concern, but visible light, particularly high-energy blue-violet wavelengths, can worsen hyperpigmentation in darker skin tones. This is where iron oxides come in. Iron oxides are the pigments used in tinted sunscreens and foundations, and they absorb visible light in a way that neither zinc oxide nor chemical filters do. A literature review found that iron oxide-containing sunscreens improve visible light-induced pigmentation outcomes.¹⁵ If you are treating melasma or post-inflammatory hyperpigmentation, a tinted sunscreen with iron oxides offers a layer of protection that a non-tinted product simply cannot.

Stability and Shelf Life

Sunscreen does degrade over time, but under normal conditions the process is slow. A study that exposed an emulsion sunscreen to typical summer temperatures (25°C and 29°C) and an extreme condition (40°C) over 15 days found that SPF values remained stable throughout. A slight dip of about 5% was measured at 25°C after the first 24 hours, and a similar drop around 4% appeared at 29°C by day 15, but these differences were too small to compromise protection in practice. Even at 40°C, protection held.¹⁶ The FDA requires sunscreens to carry an expiration date or to remain stable for at least three years. A bottle left in a hot car for an afternoon is not ruined, but a bottle that has been sitting in your beach bag since two summers ago may have drifted below its labeled SPF.

Oral Supplements as a Sunscreen Adjunct

A growing category of products claims to boost your skin’s UV defenses from the inside. The most studied ingredient is Polypodium leucotomos extract (PLE), derived from a tropical fern. A clinical evaluation found that short-term oral supplementation with PLE was associated with measurable increases in the minimal erythemal dose, meaning participants’ skin tolerated more UV before turning red, along with reduced redness after exposure.¹⁷ The effect is real but modest and nowhere close to replacing topical sunscreen. Think of it as a small additional buffer rather than an alternative, most useful for people who are already diligent about topical protection and want an extra margin.

Ingredients Borrowed from the Ocean

Some of the most promising next-generation sunscreen ingredients are not synthesized in a lab but harvested from marine organisms. Mycosporine-like amino acids (MAAs) are small molecules produced by algae and cyanobacteria as their own built-in sunscreen. They absorb UV radiation across a broad range, are remarkably photostable compared to avobenzone, and double as antioxidants.¹⁸ These organisms evolved under constant UV stress, so their photoprotective chemistry has been pressure-tested by hundreds of millions of years of natural selection.

Researchers are actively investigating MAAs as substitutes for current commercially available chemical UV filters, with the appeal being their natural origin, biodegradability, and multifunctionality.¹⁹ The challenge is producing them at scale and at a cost that competes with synthetic filters, but several companies are working on fermentation-based production methods. If MAAs do reach commercial sunscreen formulations, they could address both the skin-safety and environmental concerns that hang over current chemical filters.