Claim File c001 — Zinc Oxide (ZnO)

Class: Inorganic / mineral UV filter (wide-bandgap II-VI semiconductor) Research date: 2026-05-07 Author tier scheme: C1 primary verified · C2 well-sourced secondary · C3 inferred (working shown) · C4 graph-digitized · C5 unknown

1. Identity & Physical Chemistry

1.1 Identifiers (C1)

Sources: PubChem CID 14806; Sigma-Aldrich product 255750; ChemicalBook CB3853034; NIST WebBook (C1314132).

1.2 Crystal structure (C1)

ZnO crystallises in three structures: - Wurtzite (hexagonal P6₃mc, a = 0.3296 nm, c = 0.5207 nm) — thermodynamically stable at ambient T/P; the form used in cosmetics. (C1) - Zinc-blende (cubic) — only on cubic substrates. - Rocksalt (B1, NaCl structure) — only at high pressure (>10 GPa).

The Zn²⁺ is tetrahedrally coordinated with O²⁻ in the wurtzite lattice, giving rise to a strong polar c-axis. Source: Özgür et al., J. Appl. Phys. 98, 041301 (2005); SCCS/1489/12 §3.1.

1.3 Bandgap & cutoff wavelength (C1 / C3)

• Bulk wurtzite ZnO direct bandgap, 300 K: E_g = 3.37 eV. (C1; multiple sources — Özgür et al. 2005; Schneider & Lim 2019; Klingshirn 2007 Phys. Stat. Sol. B 244:3027.)
• Some authors cite 3.20–3.30 eV for nanoparticles depending on size / quantum confinement (Smijs & Pavel 2011 cite 3.22 eV at 77 K for wurtzite, 3.32 eV zinc-blende).
• Cutoff wavelength λ_g (C3): Using λ(nm) = 1240 / E_g(eV) → 1240 / 3.37 = 368 nm. Note: the "absorption edge" experimentally is broader; Smijs & Pavel and other reviews quote ~370 nm; nano-ZnO can show absorption out to ≈ 380 nm because of band-tail states / size dispersion. So the useful sunscreen edge is reported variously as 370 nm (theoretical) up to 380 nm (broad-spectrum claim covers UVA1).

1.4 Refractive index (C1)

Standard reference value n ≈ 2.0 in the visible. (Bond, J. Appl. Phys. 36:1674 (1965); refractiveindex.info, Bond-o tabulation.) Lower n than TiO2 → less Mie scattering at any given size → less white-cast for the same particle size. This is the principal cosmetic advantage of ZnO over TiO2 at equal coverage.

1.5 Particle sizes used in sunscreen (C1)

Regulatory threshold (EU): 50 % (number distribution) of particles ≤ 100 nm = "nano". (EU Cosmetic Reg. 1223/2009 Art. 2.1(k); refined definition in Recommendation 2022/C 229/01.) (C1)

SCCS/1489/12 (and 2024 revision) considers safe nano-ZnO only if D50 (volume) > 30 nm and D1 > 20 nm — i.e. very small particles (<20 nm) excluded. (C1)

Wakefield-style commercial dispersions (Croda Z-Cote, BASF, Symrise) commonly specify primary crystallite ~30–50 nm with stabilised aggregates 100–200 nm. (Patent US20060228310 specifies 70–130 nm volume-median.)

1.6 Surface coatings (C1 / C2)

ZnO is chemically reactive (slightly amphoteric, soluble in mild acid, and photocatalytic). Cosmetic-grade ZnO is virtually always coated. SCCS/1518/13 (revision 2014) lists coatings deemed safe at the time:

• Triethoxycaprylylsilane (the most common — C8 silane)
• Dimethicone (PDMS)
• Dimethoxydiphenylsilanetriethoxycaprylylsilane crosspolymer
• Octyl triethoxy silane

Subsequent SCCS opinions (SCCS/1625/20, SCCS/1654/23) extended approvals to silica and additional silanes.

Function of coatings: 1. Suppress photocatalysis (silica, alumina; see §3). 2. Improve dispersion in oil/water phase (silanes, dimethicone, stearate, isopropyl titanate). 3. Reduce ionic Zn²⁺ release (which can otherwise photo-degrade organic filters and cause skin irritation at high load). 4. Improve sensory feel (reduce drag).

Loading: typical surface treatments are 2–8 wt% relative to ZnO. (Croda technical bulletin "Z-Cote HP1"; BASF Z-Cote.)

1.7 Regulatory status (C1)

Key SCCS opinions: SCCS/1489/12 (Sept 2012, base opinion on nano-ZnO as UV filter ≤ 25 % safe except spray); SCCS/1518/13 (April 2014, addendum on coatings); SCCS/1625/20 and revisions through 2024 (S76 revision, expanded coatings).

2. UV Attenuation Spectrum

2.1 Mechanism (C1)

Inorganic UV filters are wide-bandgap semiconductors, not π→π* chromophores. Two physical processes contribute to attenuation:

1. Electronic absorption above the bandgap — photons with E > E_g promote a valence-band electron to the conduction band. Below the gap, the crystal is essentially transparent.
2. Mie scattering — wavelength-comparable particles scatter light. Cross-section depends on size parameter x = πd/λ and refractive-index contrast (n_particle / n_medium).

Cole, Shyr & Ou-Yang (Photodermatol. Photoimmunol. Photomed. 32:5–10, 2016, doi:10.1111/phpp.12214) measured reflection of typical mineral sunscreen films across UV with an integrating sphere: average reflection 4–5 % through the UV — i.e. < SPF 2 from scattering alone. Conclusion: "Metal oxide sunscreens protect skin by absorption, not by reflection or scattering." Reflection rises to ~60 % above the bandgap (visible), which is why thick layers look white. (C1)

2.2 Cutoff (C1 / C3)

• Bandgap-derived sharp cutoff: 1240/3.37 ≈ 368 nm (C3 calculation).
• Practical absorption edge in cosmetics: often quoted 370–380 nm because of (a) particle-size distribution producing band-tail absorption, (b) Mie scattering at the long-UVA end. Smijs & Pavel 2011, Schneider & Lim 2019 both note "broad UVA-UVB absorption" with rolloff ~380 nm. (C2)
• This places ZnO deeper into UVA1 than TiO2 — a key clinical advantage. UVA1 = 340–400 nm.

2.3 Particle size dependence (C1, Mie theory)

Popov, Priezzhev, Lademann, Myllylä (J. Phys. D: Appl. Phys. 38:2564–2570, 2005, doi:10.1088/0022-3727/38/15/006) used Mie theory + Monte Carlo simulation in stratum corneum:

• For 310 nm (UVB) light, optimal TiO2 size = 62 nm (extrapolation to ZnO with lower n shifts optimum slightly larger, ≈ 70–80 nm for UVB).
• For 400 nm (UVA), optimal TiO2 size = 122 nm.

Egerton & Tooley (Int. J. Cosmet. Sci. 34:117–122, 2012, doi:10.1111/j.1468-2494.2011.00689.x) compared 20, 50, 100 nm TiO2 — same physics applies to ZnO with shifted indices: smaller particles (~30 nm) give the highest mass-specific UVB attenuation but minimal UVA attenuation; ~100–150 nm particles give the highest UVA attenuation but visible whitening rises sharply because Mie scattering peak moves into the visible.

Trade-off summary (C2):

2.4 Mass extinction coefficient (C2)

Unlike organic chromophores (where ε is intrinsic), the mass extinction coefficient α (L·g⁻¹·cm⁻¹) of ZnO depends on particle size and dispersion quality. Egerton 2012 reports for typical sunscreen-grade ZnO (~80 nm primary, 150 nm aggregate):

• α(310 nm) ≈ 6–10 L·g⁻¹·cm⁻¹ (UVB)
• α(360 nm) ≈ 4–7 L·g⁻¹·cm⁻¹ (UVA2)
• α(380 nm) ≈ 1–3 L·g⁻¹·cm⁻¹ (UVA1 edge)

(These are roughly half the equivalent figures for nano-TiO2 in the UVB; ZnO holds attenuation longer into UVA.) Caveat C4 — Egerton's figures are graph-derived from Mie computation; actual sunscreen formulas vary widely depending on dispersion.

2.5 ZnO vs TiO2 spectral comparison (C1)

• TiO2 (rutile, ~20–50 nm): peak attenuation ~280–340 nm; sharply drops above 360 nm because rutile bandgap ≈ 3.0 eV and its higher refractive index means Mie scattering dominates the long-UVA, but mass-specific.
• ZnO: more uniform attenuation across UVB and UVA, extending to ~380 nm. Superior in UVA1 (340–400 nm).
• Hence "broadest spectrum" claim of ZnO. Combination ZnO + TiO2 is common for true broad-spectrum mineral sunscreens. Smijs & Pavel 2011 and Schneider & Lim 2019 both emphasise this complementarity.

3. Photocatalytic activity

3.1 Mechanism (C1)

Critical distinction: ZnO does not photodegrade in the way organic UV filters do. The crystal lattice is stable under UV. However, photogenerated electron–hole pairs (lifetime ~ns–μs) at the particle surface drive redox reactions in adjacent species, catalysing degradation of organic molecules and producing reactive oxygen species (ROS):

• Conduction-band electron + O₂ → O₂•⁻ (superoxide) (E°(O₂/O₂•⁻) = -0.33 V vs NHE; ZnO CB ~ -0.5 V → thermodynamically allowed).
• Valence-band hole + H₂O / OH⁻ → •OH (hydroxyl radical) (E°(•OH/H₂O) = +2.27 V; ZnO VB ~ +2.7 V → allowed).
• Energy transfer to ³O₂ → ¹O₂ (singlet oxygen) (less efficient than charge transfer for ZnO).

Wood, Scaiano et al. (J. Phys. Chem. C 120:2916–2926, 2016; doi:10.1021/acs.jpcc.5b11456) compared ZnO and ZnS — ZnO is more efficient at •OH and ¹O₂ generation; ZnS more efficient at O₂•⁻.

3.2 Quantitative ROS generation (C1 / C2)

• Lewicka, Hsiao, Yu, Colvin et al. (J. Photochem. Photobiol. A 263:24–33, 2013; doi:10.1016/j.jphotochem.2013.04.019) — "Photochemical behavior of nanoscale TiO2 and ZnO sunscreen ingredients". 8 commercial sunscreens compared. ZnO derived from commercial sunscreens produced substantial ROS upon UVA illumination, indicating coatings on ZnO products are less effective than on TiO2.
• Smijs & Pavel (Nanotechnol. Sci. Appl. 4:95–112, 2011): keratinocyte viability fell at ZnO ≥ 15 μg·mL⁻¹ in vitro; Zn²⁺ dissolution is part of the toxicity mechanism (a feature unique to ZnO vs TiO2).
• ZnO is intermediate between anatase TiO2 (most catalytic) and rutile TiO2 (least). Ranking (C2 consensus): anatase TiO2 > ZnO > rutile TiO2.

3.3 Effect of coatings (C1)

• Silica coating is the most effective. Sonochemically synthesised silica-coated ZnO showed >80 % reduction in methylene-blue degradation rate vs uncoated, but exact reduction depends on shell thickness. (Liufu et al., Ultrason. Sonochem. 18:893 (2011).) (C2)
• For TiO2, alumina/silica coatings have been reported to suppress photocatalysis by >90 % (Carlotti et al., J. Photochem. Photobiol. B 96:130 (2009); Egerton review 2012). (C2)
• ZnO coatings are generally less effective than TiO2 coatings because (a) ZnO partially dissolves at slightly acidic pH releasing Zn²⁺ and exposing fresh surface, (b) the coating chemistry is harder to anchor on ZnO than on TiO2.
• SCCS/1518/13 reaffirmed that triethoxycaprylylsilane, dimethicone, dimethoxydiphenylsilanetriethoxycaprylylsilane crosspolymer, and octyl triethoxy silane on nano-ZnO yield acceptable safety. SCCS does not specifically quantify ROS suppression; it considers downstream toxicology.

3.4 Bleaching of co-formulated organic filters (C1)

A direct, formulation-relevant consequence: uncoated/poorly-coated nano-ZnO and nano-TiO2 can accelerate degradation of avobenzone, octinoxate and other organic filters. Documented examples:

• Lewicka et al. 2013 measured ROS generation in commercial sunscreens — ZnO-containing products generated detectable ROS that correlated with degradation of dye markers.
• Tampucci, Burgalassi, Chetoni et al. (Photochem. Photobiol. Sci. 20:1593, 2021; doi:10.1007/s43630-021-00101-2) — "Zinc oxide-induced changes to sunscreen ingredient efficacy and toxicity under UV irradiation" — quantified loss of avobenzone and OMC in O/W emulsions co-formulated with ZnO.
• Avobenzone loss in TiO2/ZnO formulations can exceed 50 % after 2 MED of UV unless stabilisers (octocrylene, DEHN, polycrylene) are present, and the inorganics are well coated.

3.5 Implications for skin (C1 / C2 consensus)

• In vitro keratinocyte / fibroblast studies show ROS-mediated cytotoxicity at ZnO ≥ 10–30 μg·mL⁻¹ (uncoated, nano).
• In vivo coated nano-ZnO does not reach viable keratinocytes (see §4) — therefore intracellular ROS damage is unlikely under intact-skin use.
• SCCS, TGA, and FDA agree that coated nano-ZnO at ≤ 25 % is safe on intact skin; uncertainty remains for damaged skin and chronic / spray exposure.

4. Safety / Regulatory

4.1 Skin penetration (C1)

• Filipe, Silva, Steiner, Cardoso et al. Skin Pharmacol. Physiol. 22:266–275, 2009 (doi:10.1159/000235554) — nuclear microscopy of human skin biopsies after 2 h sunscreen application: ZnO and TiO2 detectable only on surface and outermost stratum corneum, even after 48 h occluded exposure. (C1)
• Cross, Innes, Roberts, Tsuzuki et al. Skin Pharmacol. Physiol. 20:148, 2007 — coated nano-ZnO (26–30 nm) penetration through human epidermis < 0.03 % over 24 h. (C1)
• Gulson, McCall, Korsch et al. Toxicol. Sci. 118:140–149, 2010 (doi:10.1093/toxsci/kfq243) — used isotope-labelled ⁶⁸ZnO sunscreen (19 nm) on humans for 5 days; detected trace ⁶⁸Zn in blood/urine — but at ~1/1000 of total Zn body load. The authors interpret this as partial dissolution to ionic Zn²⁺, not particulate penetration. (C1)
• Mavon, Miquel, Lejeune et al. Skin Pharmacol. Physiol. 20:10–20, 2007 — 20 nm TiO2 in decyl-glucoside vehicle: 83 % in stratum corneum (15 strippings), 5 % in viable epidermis, ~0.1 % in dermis (after exhaustive analytical methods). Largely consistent with "no significant penetration." (C1)

Consensus: intact stratum corneum is an effective barrier; both SCCS and TGA accept this for coated material at typical particle sizes. Damaged or flexed skin is an uncertain area.

4.2 Inhalation hazard (C1)

• IARC Monograph 93 (2010): TiO2 = Group 2B (possibly carcinogenic by inhalation, based on chronic rat studies). ZnO is not classified by IARC as a carcinogen.
• However, both ZnO and TiO2 nanoparticles can cause pulmonary inflammation by inhalation. SCCS/1489/12 explicitly states nano-ZnO must not be used in spray products — same restriction for nano-TiO2 (SCCS/1583/17).
• EU Annex VI entries 30/30a and 27/27a for non-spray formats only. (C1)

4.3 Coral / aquatic safety (C1 / C2)

• Hawaii Act 104 (effective 2021) bans organic filters oxybenzone & octinoxate (not ZnO/TiO2). Subsequent Act 15 (2021) bans avobenzone & octocrylene from 2023. ZnO is sometimes marketed as the "reef-safe" alternative — but this is partial:
• Corinaldesi, Marcellini, Nepote et al. Sci. Total Environ. 637:1279, 2018 (doi:10.1016/j.scitotenv.2018.05.108) — uncoated nano-ZnO induced rapid coral bleaching at 300 μg·L⁻¹; coated ZnO substantially less toxic.
• Tang, Liu, Wang et al. (J. Hazard. Mater. 2024) — transcriptomic effects on Galaxea fascicularis at sub-mg/L nZnO and nTiO2.
• Surface-coated, larger-particle ZnO is the preferred "reef safer" option but not zero-impact. The label "reef safe" has no FDA/EU regulatory definition.

4.4 Other regulatory observations

• California Prop 65 lists "Titanium dioxide (airborne, unbound particles of respirable size)"; ZnO not listed.
• OSHA / NIOSH have inhalation REL/PEL for ZnO fume (0.5–10 mg·m⁻³), distinct from cosmetic use.

5. Confidence summary

6. Key references (full list in /sources.md)

1. Schneider SL, Lim HW. A review of inorganic UV filters zinc oxide and titanium dioxide. Photodermatol. Photoimmunol. Photomed. 35:442–446 (2019). doi:10.1111/phpp.12439
2. Smijs TG, Pavel S. Titanium dioxide and zinc oxide nanoparticles in sunscreens: focus on their safety and effectiveness. Nanotechnol. Sci. Appl. 4:95–112 (2011). doi:10.2147/NSA.S19419
3. Cole C, Shyr T, Ou-Yang H. Metal oxide sunscreens protect skin by absorption, not by reflection or scattering. Photodermatol. Photoimmunol. Photomed. 32:5–10 (2016). doi:10.1111/phpp.12214
4. Popov AP, Priezzhev AV, Lademann J, Myllylä R. TiO2 nanoparticles as an effective UV-B radiation skin-protective compound in sunscreens. J. Phys. D: Appl. Phys. 38:2564–2570 (2005). doi:10.1088/0022-3727/38/15/006
5. Lewicka ZA, Yu WW, Oliva BL, Contreras EQ, Colvin VL. Photochemical behavior of nanoscale TiO2 and ZnO sunscreen ingredients. J. Photochem. Photobiol. A 263:24–33 (2013). doi:10.1016/j.jphotochem.2013.04.019
6. Egerton TA, Tooley IR. UV absorption and scattering properties of inorganic-based sunscreens. Int. J. Cosmet. Sci. 34:117–122 (2012). doi:10.1111/j.1468-2494.2011.00689.x
7. Filipe P, Silva JN, Silva R, et al. Stratum corneum is an effective barrier to TiO2 and ZnO nanoparticle percutaneous absorption. Skin Pharmacol. Physiol. 22:266–275 (2009). doi:10.1159/000235554
8. Mavon A, Miquel C, Lejeune O, et al. In vitro percutaneous absorption and in vivo stratum corneum distribution of an organic and a mineral sunscreen. Skin Pharmacol. Physiol. 20:10–20 (2007). doi:10.1159/000096167
9. Gulson B, McCall M, Korsch M, et al. Small amounts of zinc from zinc oxide particles in sunscreens applied outdoors are absorbed through human skin. Toxicol. Sci. 118:140–149 (2010). doi:10.1093/toxsci/kfq243
10. Tampucci S, Burgalassi S, Chetoni P, et al. Zinc oxide-induced changes to sunscreen ingredient efficacy and toxicity under UV irradiation. Photochem. Photobiol. Sci. 20:1593–1606 (2021). doi:10.1007/s43630-021-00101-2
11. Corinaldesi C, Marcellini F, Nepote E, Damiani E, Danovaro R. Impact of inorganic UV filters contained in sunscreen products on tropical stony corals. Sci. Total Environ. 637–638:1279–1285 (2018). doi:10.1016/j.scitotenv.2018.05.108
12. SCCS/1489/12 (2012) — Opinion on Zinc Oxide (nano form) as UV-filter.
13. SCCS/1518/13 (2014) — Addendum on coatings for nano-ZnO.
14. EU Reg. 2016/1143 — Annex VI amendment, ZnO (nano) entry 30a.
15. US FDA — 21 CFR 352.10; OTC Monograph M020 final administrative order (2021).
16. TGA Australia — Literature review on safety of TiO2 and ZnO nanoparticles in sunscreens (2017 update).
17. Özgür Ü, Alivov YI, Liu C, et al. A comprehensive review of ZnO materials and devices. J. Appl. Phys. 98:041301 (2005). doi:10.1063/1.1992666