Ingredients for Sunscreen Formulation: A Chemist's Guide to UV Filters and Beyond

Sunscreen formulation relies on UV filters (organic absorbers like avobenzone, octocrylene, or mineral filters like zinc oxide and titanium dioxide), plus emulsifiers, film-formers, antioxidants and rheology modifiers that keep filters stable, evenly spread and photostable on skin. In the UK/EU, only filters listed in Annex VI of Regulation (EC) 1223/2009 may be used, each with a fixed maximum concentration.

• Titanium dioxide and zinc oxide are permitted up to 25% in Annex VI of the EU Cosmetics Regulation
• Avobenzone (Butyl Methoxydibenzoylmethane) is capped at 5% under EU/UK rules, a lower ceiling than in some other markets
• The US OTC monograph, for comparison, lists 16 historically permitted actives with limits such as octinoxate at 7.5% and oxybenzone at 6%
• SPF testing and broad-spectrum claims depend on both the filter blend and the vehicle, not the active alone, per peer-reviewed photostability studies
• A well-designed sunscreen base typically carries 8–20% total UV filter load, blending two to four actives to cover UVB and UVA1/UVA2 ranges

Here's the part that surprises most people starting out: the UV filter is rarely the reason a sunscreen formula fails. It's usually the base — the emulsifier that can't hold 15% oil-soluble actives, the film former that streaks, or the missing antioxidant that lets avobenzone degrade within an hour of sun exposure. Formulating SPF is less about picking a filter and more about building a system that keeps that filter working.

UV Filters: The Organic and Mineral Toolkit

Every sunscreen starts with its actives, and in the UK and EU these are governed strictly by Annex VI of Regulation (EC) 1223/2009 (retained in UK law) — a closed list. If a UV filter isn't named there, with its maximum authorised concentration, it cannot be used in a marketed product, regardless of how it performs in other regions' monographs. This is worth repeating because formulators often reference US FDA lists out of habit: the US OTC monograph permits actives like aminobenzoic acid up to 15% and homosalate up to 15%, but UK/EU permissions and ceilings for the same chemical can differ, so each filter's status has to be checked against Annex VI directly, not assumed by analogy.

Organic (Chemical) Filters

These absorb UV photons and re-emit the energy as heat. Commonly used organic filters in UK/EU formulation include:

• Ethylhexyl Methoxycinnamate (Octinoxate) — a strong UVB absorber, typically used at 3–7%
• Butyl Methoxydibenzoylmethane (Avobenzone) — the main UVA1 filter, capped at 5% in Annex VI, but notoriously photounstable unless paired with a stabiliser
• Octocrylene — a UVB filter that also helps stabilise avobenzone photochemically
• Ethylhexyl Salicylate (Octisalate) — a weaker UVB filter, often used as a solvent for other crystalline UV actives
• Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine (Tinosorb S) — broad-spectrum, oil-soluble, and prized for photostability

Mineral (Inorganic) Filters

Zinc oxide and titanium dioxide work by a combination of absorption, scattering and reflection. Both are permitted up to 25% in the EU/UK Annex VI listing, and both come with a separate regulatory question when used in nano form — nano-titanium dioxide and nano-zinc oxide have their own entries and restrictions (including a ban on nano-TiO2 in sprayable formats that could be inhaled). This is one detail that trips up newer formulators buying "micronised" mineral filters without checking whether the grade counts as nano under the regulation's definition.

Beyond the Actives: What Actually Makes SPF Work

An SPF value on a jar is a property of the finished film on skin, not of the raw filter. This is why two creams with identical filter percentages can test at SPF 30 and SPF 50 — the difference lives in the vehicle.

Emulsifiers and the Oil Phase

Organic filters are oils or oil-soluble solids, and many need to be dissolved at elevated temperature before they crystallise into the wrong polymorph and lose efficacy. This makes emulsifier choice critical: a sunscreen's oil phase often runs 15–25% by weight once you add filters, emollients and waxes, which is a heavier load than a typical face cream discussed in our Face Cream Recipe breakdown. Emulsifiers with a high HLB and strong oil-holding capacity — think PEG-100 Stearate/Glyceryl Stearate combinations or polymeric emulsifiers — are standard choices because they resist the destabilising effect of high internal phase volume.

Film-Formers and Water Resistance

Water resistance claims depend on film-forming polymers such as VP/Eicosene Copolymer or acrylate copolymers, which create a flexible, water-repelling network that keeps filters distributed evenly rather than pooling into micro-droplets — pooling is one direct cause of lower measured SPF, since it leaves UV-unprotected "gaps" on skin.

Antioxidants and Photostabilisers

Avobenzone degrades under UV exposure unless protected. Octocrylene is the classic photostabiliser pairing, but formulators also add antioxidants like tocopherol or BHT (within IFRA-referenced limits for fragrance-related use) to slow oxidative breakdown of both the filters and the surrounding emollients — a concern we've covered from the oil side in our piece on oxidative stability, induction period, and shelf life.

Rheology Modifiers

Because mineral filters are dense powders, sunscreen emulsions need robust suspension: xanthan gum, cellulose derivatives, or acrylate crosspolymers keep zinc oxide from settling in the bottle and help control the notorious "white cast" that comes from uneven particle distribution rather than the filter itself.

Why the Base Formula Decides the SPF Number

A film that spreads unevenly, or that a wearer rubs off within twenty minutes because it feels heavy, will never deliver its theoretical SPF in real life. This is the gap between the lab-tested number, generated under controlled application rates of 2 mg/cm², and the number consumers experience, which typically involves a much thinner, patchier application. Formulators compensate partly through sensory design — silicones, esters like C12-15 Alkyl Benzoate, and lightweight emollients that encourage full-strength application without the greasy after-feel. Getting this balance wrong is one reason so many mineral sunscreens historically felt chalky; modern coated, surface-treated zinc oxide grades have largely solved this, but the underlying emulsion still needs to carry a heavy powder load without separating, which circles back to the same emulsifier and rheology choices discussed above and echoed in our look at white soaping effect in emulsions — a related failure mode when surfactant and electrolyte balance goes wrong.

Reading and Reverse-Engineering an Existing Sunscreen

If you're studying a commercial SPF product, the INCI list order tells you roughly how the formulator prioritised the oil phase, filter blend, and finishing agents — much like the general label-reading principles in our breakdown of what's on the label vs. what's behind it. For a systematic approach to pulling apart a competitor's SPF formula ingredient by ingredient, the method described in How to Break Down a Competitor's INCI List with AI applies directly, since sunscreen INCI lists tend to be long and layered with functional ingredients beyond the actives themselves.

Common Formulation Mistakes

• Ignoring filter compatibility: some organic filters recrystallise or destabilise each other; blends need photostability testing, not just SPF-additivity assumptions
• Underestimating oil phase load: a 15% filter blend behaves nothing like a 15% emollient blend in terms of viscosity and crystallisation risk
• Skipping pH checks: some UV filters and film-formers are pH-sensitive, and a shift outside their stable range can cause haze or reduced efficacy over shelf life
• Assuming nano-free equals simpler compliance: non-nano mineral grades still fall under the same Annex VI concentration caps, they just avoid the additional nano-specific labelling and restriction requirements

Anyone building toward a formulator's toolkit eventually has to treat SPF as its own specialised discipline — it borrows emulsion science from everyday creams but adds a strict regulatory ceiling on actives and a testing burden (SPF, UVA-PF, water resistance) that few other cosmetic categories carry. If this is a direction you want to take further, our broader path from hobbyist to professional is mapped out in How to Become a Cosmetic Chemist.

FAQ

What are the main ingredients for sunscreen formulation?

The core categories are UV filters (organic absorbers such as avobenzone and octocrylene, or mineral filters like zinc oxide and titanium dioxide), plus emulsifiers to stabilise the heavy oil phase, film-forming polymers for water resistance, antioxidants to prevent filter degradation, and rheology modifiers to suspend mineral powders evenly.

Which UV filters are approved in the UK and EU?

Only UV filters listed in Annex VI of Regulation (EC) 1223/2009, retained in UK law, are permitted, each with its own maximum concentration. This list should always be checked directly, since permissions and caps can differ from those used in other regulatory frameworks such as the US OTC monograph.

Why do sunscreens need photostabilisers like octocrylene?

Avobenzone, a widely used UVA1 filter, breaks down under prolonged UV exposure unless paired with a photostabiliser. Octocrylene is commonly used for this purpose, along with antioxidants that slow degradation of both the filter and surrounding emollients.

Does a higher percentage of UV filter always mean a higher SPF?

Not directly. SPF depends on the filter blend, how evenly it forms a film on skin, and the vehicle's stability — a poorly emulsified formula with a high filter load can test lower than a well-engineered formula with less active ingredient.

Sources

1. European Commission CosIng, Annex VI — List of UV Filters Allowed in Cosmetic Products
2. U.S. FDA — Sunscreen: How to Help Protect Your Skin in the Sun
3. PMC — peer-reviewed literature on UV filter photostability
4. GOV.UK — Cosmetic Products Regulation guidance
5. CTPA — Cosmetic, Toiletry and Perfumery Association, UK regulatory guidance