Choosing ingredients for custom sunscreen starts with the UV-filter system, but SPF alone is not enough. UVA covers about 320–400 nm, while UVB covers about 290–320 nm under the EU sunscreen definition. A finished formula also needs solvents or dispersants, film formers, emollients, humectants, emulsifiers, rheology modifiers, preservatives, and antioxidants that remain compatible during storage and application. In the EU, recommended UVA protection is at least one-third of the labeled SPF, with a critical wavelength of at least 370 nm. In the U.S., water-resistance claims must be supported for either 40 or 80 minutes. Ingredient selection therefore has to be planned around the finished product, target market, dosage form, and required testing.
The first formulation choice is the UV-filter combination because each filter covers a different section of the solar UV spectrum. Zinc oxide is widely used in mineral formulas because its optical protection extends across UVA and UVB wavelengths, while titanium dioxide is especially useful in UVB and shorter UVA regions. Organic filters can be combined to widen spectral coverage, and the exact choice has to follow the rules of the country where the sunscreen will be sold. In June 2026, for example, the FDA added bemotrizinol to the U.S. OTC sunscreen monograph, the first new sunscreen active ingredient added there in roughly 20 years.
A filter percentage on a raw-material sheet does not tell you how evenly the finished sunscreen will protect skin. Mineral powders have to remain finely dispersed; agglomerated zinc oxide or titanium dioxide can increase whitening, roughness, and uneven application. The dispersion system may use compatible esters, hydrocarbons, polyhydroxystearic acid or other supplier-approved dispersing materials, depending on particle coating and vehicle. For organic filters, solvent capacity matters just as much because an oil-soluble filter that recrystallizes after several weeks of storage is no longer distributed in the same way as it was when the batch was filled.
Sunscreen testing is performed on the finished formulation, not on a theoretical sum of individual filter percentages. FDA SPF testing specifies an application amount of 2 mg/cm², followed by at least 15 minutes before UV exposure during the test procedure.
That testing quantity explains why spreadability deserves attention during ingredient selection. A formula that drags, pills, sets too quickly, or leaves a heavy residue may be applied less evenly in normal use. Lightweight esters can reduce greasy feel, while triglycerides may add richness and improve glide. Volatile or low-residue emollients can make facial sunscreen more comfortable, but replacing one oil with another can change UV-filter solubility, crystallization behavior, viscosity, and film formation, so sensory changes should be followed by stability and performance checks rather than treated as simple ingredient swaps.
The same relationship applies to emulsifiers. A lotion containing a water phase and a large oil-soluble filter load may place much more stress on an emulsion than an ordinary moisturizer. Oil-in-water systems often produce a lighter skin feel, while water-in-oil structures can be useful when a more hydrophobic continuous phase is wanted. Formulators usually choose an emulsifier system according to oil-phase polarity, electrolyte content, processing temperature, and targeted viscosity instead of relying on one standard emulsifier for every SPF level.
| Ingredient group | Main formulation job | Common issue to check |
|---|---|---|
| Zinc oxide / titanium dioxide | UV attenuation | Agglomeration, white cast, settling |
| Organic UV filters | UVA and/or UVB absorption | Solubility, crystallization, photostability |
| Esters and emollients | Spreading and filter solubilization | Greasiness, filter compatibility |
| Film-forming polymers | Film continuity and water resistance | Tack, pilling, flexibility |
| Humectants | Water retention and skin feel | Stickiness at higher use levels |
| Rheology modifiers | Viscosity and suspension | Drag, poor dispensing |
| Preservatives | Microbial control | pH and formula compatibility |
Once the emulsion structure is stable, film-forming ingredients determine how well the product remains distributed after it dries. Acrylate-based polymers and other water-resistant film formers may improve adhesion and reduce wash-off, which is useful for sport, beach, and high-humidity products. The presence of a film former alone cannot support a water-resistance claim. U.S. labeling permits water resistance to be stated as 40 or 80 minutes only after the finished sunscreen passes the applicable test procedure. A 40-minute test uses two 20-minute water-immersion periods separated by drying; an 80-minute claim uses four immersion-drying sequences.
Texture modifiers then need to keep that protective film practical to apply. Carbomers, cellulose derivatives, acrylate polymers, natural gums, waxes, and oil-phase thickeners can adjust viscosity or suspend insoluble material. A stick normally requires more internal structure than a lotion because the product has to remain solid in the package while releasing enough material when rubbed across skin. A portable sunscreen stick may therefore use waxes, structurants, oils, dispersants, and UV filters in proportions very different from a fluid facial emulsion, even when both products carry the same labeled SPF.
Stick hardness also has to be balanced against product transfer. Too much wax can produce a hard surface that deposits a thin or patchy layer, while too little structure may create sweating, deformation, or breakage under warm storage conditions. Temperature cycling is useful during development because a product may encounter warehouse, vehicle, bathroom, and beach temperatures during its shelf life. For U.S. sunscreens, FDA directions also state that the container should be protected from excessive heat and direct sun, showing that packaging and storage conditions remain part of product performance rather than an afterthought.
Humectants can be added when the sunscreen is intended to replace part of a daily moisturizer. Glycerin, propanediol, butylene glycol, sodium PCA, and hyaluronic-acid materials are common options, but their sensory effects differ. A high humectant load can increase tack or slow the dry-down of an otherwise light sunscreen. Facial formulas usually benefit from selecting humectant levels together with emollients and film formers rather than maximizing hydration ingredients independently, because all three groups affect how evenly makeup, skincare, or a second sunscreen application sits on top.
Antioxidants may support the oil phase or add skincare functions, but they should not be counted as substitutes for regulated UV filters. Tocopherol is often added to oil-containing products, while other antioxidant materials require closer attention to solubility, oxidation, color change, and pH. SPF remains primarily a measure of protection against erythema-producing UV radiation, and FDA consumer guidance notes that broad-spectrum protection is needed when protection against both UVA and UVB is claimed. Products that are not broad spectrum, or have SPF below 15 under U.S. labeling rules, cannot make the same skin-cancer and early-skin-aging risk-reduction statement permitted for Broad Spectrum SPF 15 or higher when used as directed with other sun-protection measures.
Broad-spectrum design also changes the way filter combinations should be judged in European-market development. The European Commission's 2006 recommendation defines UVA as 320–400 nm and UVB as 290–320 nm, and recommends a UVA protection factor of at least one-third of the labeled SPF plus a critical wavelength of at least 370 nm. An SPF 30 product therefore should not be developed as though strong UVB attenuation alone were enough; the recommended UVA protection rises with the SPF category. The same recommendation places labeled SPF 30 in the “high protection” range and SPF 50+ in the “very high protection” category, with SPF 50+ corresponding to a measured SPF of at least 60.
Preservation becomes relevant as soon as the formula contains enough available water to support microbial growth. Phenoxyethanol-based systems, organic acids, multifunctional glycols, chelating agents, and other permitted materials may be considered according to pH and market requirements. A preservative should not be selected from its recommended supplier percentage alone: the completed product may contain surfactants, botanical materials, mineral powders, packaging contact surfaces, or pH conditions that change preservative performance. Microbiological quality and preservative efficacy testing therefore belong in the development program for water-containing sunscreen.
Fragrance and essential oils deserve similar restraint. A daily face product used around the eye area may benefit from a fragrance-free design, while a body sunscreen can tolerate a different sensory brief. “Natural” origin does not automatically make an aromatic material suitable for sun-exposed skin; oxidation products, fragrance allergens, and photoreactive constituents have to be reviewed at the actual use concentration. Removing fragrance can also simplify a formula that already contains 10 or more functional material groups competing for stability, sensory feel, and compatibility.
Packaging should be selected while the formula is still being developed. Pumps need viscosity that allows reliable priming and evacuation, tubes need enough structure to avoid leakage, and sticks need a melt profile that works with the chosen mechanism. Some oils, solvents, or fragrance materials can soften plastics, affect seals, or migrate into packaging components over time. A formula that looks stable in a laboratory glass jar can behave differently after months in its commercial container, so compatibility checks should use the intended package rather than a substitute whenever possible.
The final ingredient list should be tested as a complete product. ISO 24444:2019 provides an internationally recognized in-vivo method for determining SPF and was confirmed again by ISO in 2026; an amendment was published in 2022. U.S. water-resistance and broad-spectrum claims follow FDA requirements, while European sunscreen recommendations use their own UVA/SPF relationship. Regulatory route, filter choice, dosage form, manufacturing process, packaging, and claim language should therefore be fixed early enough that the laboratory is developing the product against the requirements it will actually have to meet.
A custom formula intended for oily facial skin may use lower-residue emollients, carefully controlled powder content, modest humectancy, and a flexible film former. A dry-skin body product may tolerate more triglycerides or richer emollients, while an 80-minute water-resistant sport sunscreen needs much greater attention to film retention after repeated water exposure. Mineral sticks place more emphasis on particle wetting, wax structure, payoff, and white-cast control. Developing those products from one shared base and only changing one or two marketing ingredients may save laboratory time, but it can also produce very different application films and therefore requires renewed stability and claim testing whenever the modification can affect sunscreen performance.