Claim File c003 — Dioxybenzone (Benzophenone-8, BP-8)

CAS 131-53-3

Section 1: Identity

• INCI name: Benzophenone-8 [C1]
• USAN / FDA name: Dioxybenzone [C1]
• IUPAC: (2,2′-Dihydroxy-4-methoxyphenyl)(phenyl)methanone (also written 2,2′-dihydroxy-4-methoxybenzophenone, "DHMB") [C1]
• Synonyms: BP-8, DHMB, Spectra-Sorb UV24, Cyasorb UV24, Eusolex 4360 (in some sources), 2,2′-dihydroxy-4-methoxybenzophenone [C1]
• CAS: 131-53-3 [C1]
• EC No.: 205-026-8 [C1]
• Molecular formula: C14H12O4 [C1]
• Molecular weight: 244.24 g/mol [C1]
• Physical: pale-yellow crystalline powder; mp 68 °C [C1]
• Solubility: lipophilic; very poorly water-soluble; soluble in ethanol, oils, common cosmetic solvents [C1]

Regulatory status (current as of May 2026)

• Typical use levels: rare in modern formulations (sometimes called "the forgotten benzophenone"); occasionally used as a UVA-II booster at 1–3%, often paired with BP-3 or avobenzone. [C2]

Section 2: UV Absorption Spectrum

• λmax #1 (UVB): ~282–285 nm [C2]
• λmax #2 (UVA-II): ~325–352 nm — generally reported with a stronger long-wavelength band than BP-3, extending modestly further into UVA-II owing to the second (2′-hydroxyl) ortho-OH that enables a more extended H-bonded/conjugated chromophore [C2]
• Spectral coverage: ~260–360 nm. Functionally a UVB + UVA-II filter; weak at UVA-I. [C1]
• Molar absorption coefficient (ethanol):
• At λmax UVB (~284 nm): ε ≈ 1.5 × 10⁴ M⁻¹·cm⁻¹ (similar to BP-3) [C3]
• At long-wavelength λmax (~325–352 nm): ε ≈ 1.5–2.0 × 10⁴ M⁻¹·cm⁻¹ — comparable or slightly higher than BP-3 at this wavelength [C3]
• At 254 nm in aqueous matrix: 2.26–5.92 × 10³ M⁻¹·cm⁻¹ depending on pH [C2 — Wang 2023]
• E(1%, 1 cm) at long-wavelength max: ≈ 600–820 (computed) [C3]
• Solvent / pH dependence: Two ortho-hydroxyl groups give two intramolecular H-bonds and two ESIPT-active sites — the molecule has higher photostability potential than BP-3 in principle, although in practice empirical photostability is similar to BP-3. Bands red-shift in alkaline solution as 2-OH and 2′-OH ionize sequentially.
• Functional classification: moderate UVB + moderate UVA-II filter. Provides a slightly longer-wavelength tail than BP-3 alone.

Section 3: Photodegradation

• Photostability: considered photostable; similar profile to BP-3. Empirical data limited compared with BP-3 (commercial use is small). Reported retention >80% after typical sunscreen MED-irradiation protocols. [C3]
• Aqueous photodegradation: under simulated sunlight, pseudo-first-order rate constants in the 10⁻⁵ s⁻¹ range; UV/H₂O₂ and UV/persulfate AOP studies find half-lives of minutes under 254 nm irradiation. [C2 — Wang 2023, Chem Eng J]
• Quantum yield: not separately tabulated in the literature in a way comparable to BP-3; expected Φ_loss ≈ 10⁻⁴–10⁻³ by analogy. [C3 — inferred]
• Photoproducts: demethylation, hydroxylation, and dimerization products reported in AOP studies; specific products less characterized than BP-3.
• Behavior in formulation: stable in lipid-phase; sometimes used as a co-filter in older 1990s–2000s sunscreens.
• ROS generation: limited direct data; expected to be similar to BP-3, possibly slightly lower owing to additional ortho-OH facilitating ESIPT.

Section 4: Notable issues

4.1 Endocrine activity

• In vitro: BP-8 shows weak estrogenic activity in MCF-7 / yeast-ER assays — broadly similar order of magnitude to BP-3, possibly slightly weaker. Has been reported to inhibit human and rat placental 3β-hydroxysteroid dehydrogenase (3β-HSD) at micromolar concentrations (Yan et al. 2023 Toxicol Appl Pharmacol). [C2]
• BP-8 is also a known in-vivo metabolite of BP-3 (along with BP-1 and 2,3,4-trihydroxybenzophenone), so endocrine concerns about BP-3 carry over into BP-8 exposure indirectly. [C1]

4.2 Skin penetration

• Limited published data for dioxybenzone specifically. Owing to similar lipophilicity to BP-3, comparable skin permeation expected. Not evaluated in the FDA Matta absorption studies. The FDA 2019/2021 proposed order specifically lists dioxybenzone among ingredients needing additional safety data including a maximum-use absorption study analogous to those done for BP-3. [C1]

4.3 Aquatic / coral toxicity

• Sparse direct data. As a structural analog of BP-3 (and as a metabolite of BP-3 in coral and human metabolism), it is plausibly active; however, no high-profile coral planula study analogous to Downs 2016 has been published for BP-8. Rare in coastal contamination surveys (low cosmetic use → low environmental loads). [C3]

4.4 Allergenicity

• Reported as a contact and photocontact allergen. Less common than BP-3 in patch-test series, partly reflecting lower commercial usage. Cross-reactivity with BP-3 is documented. [C2]

4.5 Phototoxicity / photo-genotoxicity

• A 2015 dermal cassette-dose study (Sakuratani et al. J Toxicol Sci 40:439) examined phototoxicity of multiple benzophenones; BP-8 was within the same risk range as other benzophenones — modest potential photo-irritation, no strong photo-genotoxic signal in standard 3T3 NRU phototoxicity test at relevant concentrations.

Confidence

• Identity / regulatory → C1 (FDA 3% specifically)
• Spectrum λmax → C2
• Photodegradation kinetics → C3 (sparse)
• Endocrine activity → C2
• Skin penetration / human absorption → C3 (no MaxUse study)
• Coral toxicity → C3 (no direct planula study)

Key citations

1. FDA OTC Monograph M020 — Sunscreen Drug Products (dioxybenzone listed at up to 3%).
2. FDA Proposed Order OTC000008 (24 Sept 2021).
3. CIR Amended Safety Assessment of Benzophenones (2021): https://www.cir-safety.org/sites/default/files/benzop032021FAR.pdf
4. Wang Y et al. Chem Eng J 2023, "Degradation of benzophenone-8 in UV/oxidation processes". doi:10.1016/j.cej.2023.144xxx (search result)
5. Sakuratani Y et al. J Toxicol Sci 2015;40:439-451. doi:10.2131/jts.40.439 — phototoxic risk assessments on benzophenones.
6. DrugBank monograph DB11221 (Dioxybenzone).