Sunscreen-Stick White Cast: Filter Class and Base as the Two Structural Variables

Definition

White cast on a sunscreen stick is a function of two structural variables, neither of which is fully readable from an INCI list. 1) FILTER CLASS. - Inorganic (particulate) filters - zinc oxide and titanium dioxide - scatter visible light because their primary particles are on the order of the wavelength of visible light (~400-700 nm) unless engineered smaller. This particle-size dependence is OBSERVED and quantified in primary-patent literature: US Patent 7,390,355 B2 (Croda International, filed 2004, granted 2008) specifies zinc oxide dispersions with median volume particle diameter 70-130 nm (preferred 99-101 nm) and titanium dioxide dispersions at 24-42 nm to achieve 'change in whiteness ΔL of less than 3', versus conventional dispersions the patent describes as producing 'unacceptable whitening'. US Patent Application 2017/0065505A1 (IBM, published 2017) states the MECHANISM (hypothesis-grade physical reasoning, not a controlled clinical measurement): 'particle scattering coefficients decrease as the particle size of ZnO decreases,' and proposes reducing ZnO below 100 nm (30-80 nm) plus refractive-index-matched carriers to cut scattering. A separate patent search returned OBSERVED formulation data of mean primary particle sizes ~380 nm (range 150-550 nm) for ZnO and ~1100 nm (range 150-10,000 nm) for TiO2 in conventional (non-transparent) dispersions - these figures conflict in magnitude with the Croda/IBM transparent-grade figures above precisely because they describe different, non-transparent product classes; both are kept here rather than averaged. A 2025 peer-reviewed clinical study (Maldonado López et al., PLOS One) measured this in vivo: at 5% ZnO (coated with triethoxycaprylylsilane, stated particle size >100 nm) white cast scored 'reasonable'; at 20-30% ZnO it scored 'unacceptable' across all tested Fitzpatrick II-VI groups, with L*-value (CIEL*a*b* whiteness) rising with % ZnO (r≥0.822, p<0.001, n=13 in vivo panel). This is the strongest OBSERVED, sourced, quantitative link between an inorganic filter variable and measured cast in this record. - Organic (chemical/soluble) filters do not scatter visible light as particulates and are not associated with white cast in any source reviewed; their role in this system is that raising inorganic-filter load to hit a target SPF (or lowering it, by co-formulating with organics) is a formulation lever that indirectly changes cast - this composite point is HYPOTHESIS/inference, not a value taken from a single measured source. 2) BASE / CARRIER. - The wax-ester-silicone base does not itself cause or prevent cast by absorbing/scattering UV, but patent literature describes it as a lever for masking cast optically: search-summarized patent language (not independently re-verified against a single readable patent PDF in this session) describes 'masking agents' such as PPG-10 butanediol and dimethicone copolyols as reducing 'apparent whiteness' by changing how light interacts with the particulate filter at the skin surface. This is HYPOTHESIS-grade (mechanism claimed by patent applicants, not an independent clinical measurement) and is flagged as UNTESTED in the academic literature reviewed here. - A real, publicly published anhydrous stick formula (Koster Keunen 'Natural Sunscreen Stick, SPF 30', a supplier formulation record) shows how base and filter combine in practice: Beeswax 12%, Theobroma Cacao (Cocoa) Seed Butter 9%, Oryza Sativa (Rice) Bran Wax 7.5%, Polyhydroxystearic Acid 6%, Lauryl Laurate (ester) 9%, Cocos Nucifera (Coconut) Oil 26%, Tocopheryl Acetate 1%, and a pre-dispersed active listed as 'Zinc Oxide, Caprylic/Capric Triglyceride, Polyhydroxystearic Acid' at 29.5%. Note this last line: the compound INCI name DOES disclose that this particular ZnO is pre-coated/pre-dispersed with polyhydroxystearic acid in a triglyceride carrier - coating disclosure is possible but inconsistent and depends on the raw-material supplier choosing to list the dispersion's full ingredient string rather than 'Zinc Oxide' alone. 3) WHAT A LABEL CAN AND CANNOT TELL YOU (regulatory, OBSERVED). - US: of the 16 legacy monograph actives, FDA's 2019 proposed rule (84 FR 6204) placed only zinc oxide and titanium dioxide (up to 25% each) in Category I / proposed GRASE (generally recognized as safe and effective); 12 organic filters (avobenzone, homosalate, octisalate, octocrylene, oxybenzone, octinoxate, cinoxate, dioxybenzone, ensulizole, meradimate, padimate O, sulisobenzone) were placed in Category III ('insufficient data'), and PABA/trolamine salicylate in Category II (not GRASE). Two qualifications matter and are stated rather than smoothed over. First, that 2019 proposed rule was never finalized, and the Proposed Order that would import its categories into the operative monograph (OTC000008, issued Sept. 24, 2021 / 86 FR 53322) is still pending as of Aug. 2026 - so the Category III organics continue to be lawfully marketed under the CARES Act Deemed Final Order while their final GRASE status is undetermined. Second, the list is no longer 16: FDA Final Administrative Order OTC000039 (announced June 9, 2026; Federal Register document 2026-11578, published June 10, 2026; effective Aug. 9, 2026) added bemotrizinol at up to 6% as GRASE - the first new US sunscreen active in more than 20 years and the first ORGANIC filter to reach GRASE status here. That matters for this record: it means 'GRASE in the US' no longer maps onto 'inorganic/particulate', and therefore no longer maps onto the cast-producing filter class. Source: FDA OTC Monograph M020 (2021), FDA's GRASE Q&A page, and Final Administrative Order OTC000039 (2026). - EU: Regulation (EC) No 1223/2009, Annex VI, lists titanium dioxide (entry 27, non-nano; entry 27a, nano, capped 25%) and zinc oxide (entry 30, non-nano; entry 30a, nano, capped 25%) alongside roughly 25-30 organic filters (e.g., bemotrizinol, bisoctrizole, ecamsule, drometrizole trisiloxane) not all permitted in the US. The EU REQUIRES the '(nano)' qualifier on the ingredient list when the nano form is used; the US does not require this qualifier - it is voluntary. - In NEITHER jurisdiction does the ingredient label disclose primary particle size, particle-size distribution, or the specific surface coating chemistry (e.g., silica, alumina, triethoxycaprylylsilane, dimethicone) of zinc oxide or titanium dioxide, UNLESS a supplier chooses to spell out the coating as part of a compound/trade INCI string (as in the Koster Keunen example above) rather than declaring 'Zinc Oxide'/'Titanium Dioxide' alone. This is why cast CANNOT be predicted from a standard INCI list: two products both listing 'Zinc Oxide' can differ by an order of magnitude in primary particle size and have entirely different measured cast, and the label gives no way to distinguish them. This limitation is OBSERVED (a direct reading of what the regulations require to be disclosed) even though the underlying particle/coating values themselves are usually UNDISCLOSED and UNTESTED by any source available to the public. SUMMARY OF EVIDENCE TIERS: - OBSERVED (published, sourced, quantitative): particle-size specs and whiteness (ΔL) data in US7390355; ZnO%-vs-L* correlation in the 2025 PLOS One clinical study; FDA/EU regulatory filter lists and nano-labeling rules; the Koster Keunen formula's actual INCI percentages. - HYPOTHESIS (mechanism proposed by a source, not independently clinically confirmed in this record): light-scattering-coefficient reasoning in US2017/0065505A1; masking-agent optical claims attributed to patent literature on wax/ester/silicone bases. - UNTESTED / NOT PUBLISHED: coating chemistry and particle size for the great majority of commercial zinc oxide/titanium dioxide raw materials used in marketed sunscreen sticks; any head-to-head measured cast ranking of named commercial stick products (no source reviewed measured this, so none is reported here); any generalized claim that a given base type (wax vs ester vs silicone) reduces cast more than another across products - this record found no controlled comparative study of base chemistry alone (holding filter constant) and treats that comparison as unpublished/untested rather than inferring an answer.

System name

Sunscreen-stick white-cast system: filter class × base (inorganic filter particle size/coating + wax-ester-silicone carrier)

Member ingredients

• INORGANIC (PARTICULATE) FILTERS: Zinc Oxide (CAS 1314-13-2); Titanium Dioxide (CAS 13463-67-7) - both GRASE up to 25% under US FDA OTC Monograph M020; both listed non-nano and nano (capped 25%, nano forms barred from sprayable formats) under EU Reg. 1223/2009 Annex VI entries 27/27a (TiO2) and 30/30a (ZnO).
• ORGANIC/CHEMICAL FILTERS - US monograph (GRASE status pending, 'insufficient data'): Avobenzone (Butyl Methoxydibenzoylmethane); Homosalate; Octisalate (Ethylhexyl Salicylate); Octocrylene; Oxybenzone (Benzophenone-3); Octinoxate (Ethylhexyl Methoxycinnamate); Cinoxate; Dioxybenzone; Ensulizole (Phenylbenzimidazole Sulfonic Acid); Meradimate; Padimate O; Sulisobenzone. Not GRASE: Aminobenzoic Acid (PABA); Trolamine Salicylate.
• ORGANIC/CHEMICAL FILTERS - additional entries approved in EU Annex VI but not in the US monograph list above (non-exhaustive; ~25-30 total EU-approved organic filters exist): Bemotrizinol (Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine); Bisoctrizole (Methylene Bis-Benzotriazolyl Tetramethylbutylphenol); Ecamsule (Terephthalylidene Dicamphor Sulfonic Acid); Drometrizole Trisiloxane.
• BASE / CARRIER - waxes: Beeswax (Cera Alba); Candelilla Wax (Euphorbia Cerifera Cera); Carnauba Wax (Copernicia Cerifera Cera); Microcrystalline Wax; Paraffin; Oryza Sativa (Rice) Bran Wax; Theobroma Cacao (Cocoa) Seed Butter; Behenyl Alcohol; Cetyl Alcohol; Cetostearyl Alcohol.
• BASE / CARRIER - esters and oils: Lauryl Laurate; Polyhydroxystearic Acid; Caprylic/Capric Triglyceride; Cocos Nucifera (Coconut) Oil; Squalane; Isohexadecane; Mineral Oil (Paraffinum Liquidum).
• BASE / CARRIER - silicones and cast-masking agents named in patent literature (mechanism = HYPOTHESIS, not independently clinically confirmed here): Dimethicone; Dimethicone Copolyol; PPG-10 Butanediol; Cyclopentasiloxane; C30-45 Alkyl Dimethicone.
• SURFACE-COATING AGENTS applied to ZnO/TiO2, named in patent/technical literature but rarely disclosed on retail labels: Triethoxycaprylylsilane; Silica; Alumina; Dimethicone (as a particle coating, distinct from its use as a base ingredient); Polyhydroxystearic Acid (as a particle coating, as seen in the Koster Keunen compound-INCI example).

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