Claim File c001 — Oxybenzone (Benzophenone-3, BP-3)

CAS 131-57-7

Section 1: Identity

• INCI name: Benzophenone-3 [C1]
• USAN / FDA name: Oxybenzone (used in U.S. OTC monograph) [C1]
• IUPAC: (2-Hydroxy-4-methoxyphenyl)(phenyl)methanone [C1]
• Synonyms / trade names: BP-3, MBPH, Eusolex 4360, Escalol 567, Uvinul M40, Uvasorb MET, 2-hydroxy-4-methoxybenzophenone (HMB) [C1]
• CAS: 131-57-7 [C1]
• EC No.: 205-031-5 [C1]
• Molecular formula: C14H12O3 [C1]
• Molecular weight: 228.247 g/mol [C1]
• Physical: pale-yellow crystalline powder; mp 62–65 °C; bp ~226 °C / 14 mmHg [C1]
• Solubility: lipophilic; very poorly soluble in water (~0.0037 g/L at 25 °C); soluble in ethanol, acetone, oils, esters; logKow ≈ 3.79 [C1]

Regulatory status (current as of May 2026)

• Typical formulation use levels: historically 2–6% in sunscreens; in EU products since 2023 typically ≤2.2% in body products [C1]

Section 2: UV Absorption Spectrum

• λmax #1 (UVB): 287–290 nm (commonly reported as 288 nm) [C1]
• λmax #2 (UVA-II): 322–326 nm (commonly reported as 325 nm) [C1]
• Spectral coverage: ~270–350 nm; full-width at half-max envelope crosses UVB and short UVA-II [C1]
• Molar absorption coefficient (ε):
• At ~287–290 nm in ethanol: ε ≈ 1.5–2.1 × 10⁴ M⁻¹·cm⁻¹ (commonly cited 20,381 M⁻¹·cm⁻¹ at 290 nm) [C2]
• At ~325 nm in ethanol: ε ≈ 1.2–1.5 × 10⁴ M⁻¹·cm⁻¹ [C2]
• E(1%, 1 cm) (specific absorbance, 10 g/L): ≈ 880–920 at 287 nm; ≈ 540–660 at 325 nm (computed from MW 228.25 and ε above) [C3]
• Critical wavelength: ~352 nm (boundary of broad-spectrum claim is ≥370 nm — BP-3 alone does not meet this) [C2]
• Solvent dependence (keto-enol / ESIPT effects): BP-3 exists predominantly as the enol tautomer (intramolecular H-bond between 2-OH and adjacent C=O); the ortho-OH/C=O motif drives ultrafast Excited-State Intramolecular Proton Transfer (ESIPT, k ≈ 2 × 10¹² s⁻¹) which is the principal energy-dissipation channel. In nonpolar solvents (hexane, isooctane) the enol is heavily favored and the long-wavelength band sharpens around 325 nm; in polar protic solvents (water, methanol) hydrogen-bond competition can shift the equilibrium, broaden bands, and very slightly red-shift λmax. Adsorption to skin proteins or oil phases of formulations broadens both bands. [C2 — Baker et al. 2015 Photochem Photobiol Sci 14:1814]
• Classification: moderate UVB absorber + weak-to-moderate UVA-II absorber; not a strong UVA-I filter (essentially no absorption beyond 360 nm). Often used historically as a photostabilizer/synergist for avobenzone rather than as the principal UVA active. [C2]

Section 3: Photodegradation

• Photostability: BP-3 is comparatively photostable as benzophenones go (clearly more stable than avobenzone). Reported retention after standard sunscreen-irradiation protocols (MED 5–25 J/cm² UVA equivalent in vehicle): typically 80–95% intact, depending on matrix. [C2 — Sayre et al. 2005; Gaspar & Maia Campos 2006]
• Photodegradation kinetics: Pseudo-first-order under continuous irradiation. Reported environmental rate constants:
• In ultrapure water (simulated solar irradiation, ~765 W/m²): k ≈ 10⁻⁵–10⁻⁴ s⁻¹, half-life on the order of hours [C2 — Vione et al. 2013]
• In atmospheric aerosol phase: J_eff ≈ 10⁻³–10⁻² s⁻¹, half-life ~1 day [C1 — ACS ES&T Air 2024; Kennedy/Nizkorodov]
• In seawater/freshwater (Sakkas / Vione): k_photo of order 10⁻⁵ s⁻¹, half-life ~1–10 h depending on dissolved organic matter and salt content [C2]
• Quantum yield (Φ_loss): Direct photolysis quantum yield in aqueous solution reported at Φ ≈ 10⁻⁴ to 10⁻³ (i.e. low — ESIPT is dominant non-radiative deactivation; only a small fraction leads to bond breakage). [C2 — published estimates vary by 1 order of magnitude]
• Primary photochemical pathway: ultrafast ESIPT (enol → keto) → vibrational cooling → ground state recovery. Minor pathway (Φ ≈ 10⁻⁴): O–H bond homolysis producing phenoxyl radical → secondary chemistry.
• Photoproducts identified (LC-MS / GC-MS):
• Benzophenone-1 (BP-1, 2,4-dihydroxybenzophenone) — major demethylation product (also a metabolite in vivo) [C1]
• 2,3,4-trihydroxybenzophenone (THB) — hydroxylation product [C1]
• Benzophenone-8 (DHMB, 2,2′-dihydroxy-4-methoxybenzophenone) — minor [C2]
• Benzaldehyde, benzoic acid, benzophenone (parent ketone) — chain-cleavage products, more abundant in aerosol/atmospheric phase [C1 — ACS ES&T Air 2024]
• Hydroxylated and dimeric products in chlorinated water (treatment plants) — including chlorinated BP-3 derivatives with elevated genotoxicity reported. [C1 — Liu et al. 2014, Sci Total Environ]
• Behavior in sunscreen formulation: in oil-based or emulsion vehicles BP-3 is reasonably stable. It can act as a triplet quencher / photostabilizer for avobenzone in some formulations, although it is itself a photosensitizer in others (mixed evidence). In presence of TiO2 or ZnO photocatalysts BP-3 photodegradation is accelerated. [C2]
• ROS generation: BP-3 generates singlet oxygen (¹O₂) and superoxide upon UV excitation at higher quantum yields than next-generation filters such as bemotrizinol or Tinosorb S. Reported ¹O₂ quantum yield Φ_Δ ≈ 0.04–0.06 in organic solvents (compared with Φ_Δ < 0.01 for bemotrizinol). [C2 — Wood et al. 2006; Allen et al. 1996]
• Notable interaction: BP-3 + octinoxate combinations — BP-3 can mildly destabilize octinoxate; with avobenzone, BP-3 can be either stabilizing or destabilizing depending on solvent and presence of triplet quenchers (octocrylene). [C2 — Sayre et al. 2005 Photochem Photobiol 81:452]

Section 4: Notable issues (factual, not alarmist)

4.1 Endocrine activity

• In-vitro: BP-3 is weakly estrogenic via ER-α (E-Screen, MCF-7 proliferation, yeast ER assays); relative estrogenic potency vs 17β-estradiol ≈ 10⁻⁵–10⁻⁶. BP-1, the primary metabolite, is more potent (~10⁻³–10⁻⁴ relative to E2 in some assays). BP-3 also exhibits weak anti-androgenic activity in AR reporter assays. [C1 — Schlumpf et al. 2001 Environ Health Perspect 109:239; Kunz & Fent 2006]
• Animal studies: uterotrophic (uterine weight gain) reported in immature rats at high oral doses (Schlumpf 2001 found ~1500 mg/kg/day to triple uterine weight). At orally-administered or dermally-applied doses in standard 28/90-day studies, NOAELs typically 200–500 mg/kg/day. [C1]
• Human / clinical evidence: Epidemiological associations are inconsistent. Meta-analysis by Ghazipura et al. 2017 of human biomonitoring studies and reproductive outcomes found mixed associations (e.g., shortened anogenital distance in some male infant cohorts; alterations in pubertal timing in some girl cohorts; null associations in others). No single high-quality clinical study has demonstrated a clinically meaningful endocrine effect at typical use exposures, but the SCCS 2021 explicitly stated evidence "warrants further investigation". [C1 — SCCS/1625/20; Mustieles et al. 2023 Environ Int 173:107739 meta-analysis]
• SCCS final opinion (2021): "While there are indications from some studies to suggest that BP-3 may have endocrine effects, the overall evidence is not conclusive enough for the SCCS to ascertain whether or not BP-3 is an endocrine disrupting substance, and this warrants further investigations." Concluded 2.2% in body products is safe with the stated MoS calculation. [C1 — SCCS/1625/20]

4.2 Skin penetration & systemic absorption

• FDA Matta et al. 2019 JAMA (single-product, max-use, single-day): plasma C_max for oxybenzone exceeded 0.5 ng/mL FDA threshold within 2 h and reached up to ~180 ng/mL at end of day for sprays; geometric-mean AUC 4.0–5.0 ng·h/mL/day. [C1 — Matta et al. JAMA 2019;321:2082, doi:10.1001/jama.2019.5586]
• FDA Matta et al. 2020 JAMA (4-day, multiple products): geometric-mean C_max for oxybenzone 209.6 ng/mL (spray 1), 194.9 (spray 2), 169.3 (lotion), 199.9 (pump spray). Plasma >0.5 ng/mL maintained 21 days post-application in some subjects. [C1 — Matta et al. JAMA 2020;323:256, doi:10.1001/jama.2019.20747]
• Population biomonitoring (NHANES): BP-3 detected in ≥96% of urine samples in U.S. NHANES cycles 2003–2014; concentrations higher in women, lighter-skinned individuals, and summer months; 95th percentile ~1000–4000 ng/mL urine. [C1 — Calafat et al. 2008 EHP 116:893; Han et al. 2016]
• Implication: systemic exposure is unequivocal; clinical consequences of measured plasma levels remain unestablished (FDA 2019/2021 explicitly states absorption above threshold does not equate to harm but triggers requirement for additional toxicology data). [C1]

4.3 Coral / aquatic toxicity

• Downs et al. 2016 Arch Environ Contam Toxicol 70:265–288, doi:10.1007/s00244-015-0227-7. In Stylophora pistillata coral planulae:
• LC50 (light, 24 h): ~139 µg/L
• EC20 for deformity: ~6.5 µg/L
• Genotoxicity (DNA-AP lesions) dose-dependent
• Skeletal endocrine disruption: ossification of planulae in their own skeleton
• Photo-toxicity: effects exacerbated under UV
• Environmental concentrations: USVI 75 µg/L–1.4 mg/L at heavily-used snorkel sites; Hawaii 0.8–19.2 µg/L. [C1 — Downs 2016]
• Mechanistic update: Vuckovic et al. 2022 Science 376:644 showed sea anemones and corals glycosylate BP-3 to form 2-O-glucosides that are more phototoxic than parent BP-3 (loss of ESIPT pathway when 2-OH is masked) — supports a mechanistic basis for coral toxicity. [C1 — doi:10.1126/science.abn2600]
• Caveats: Downs 2016 used some artificial-laboratory exposures with DMSO carrier; environmental relevance debated. NOAA 2022 review concluded oxybenzone is "of concern" but is not the sole driver of reef decline; climate change, nutrients, and disease remain dominant stressors. [C2]

4.4 Allergenicity / contact dermatitis

• Most common photoallergen in U.S. and Canadian photopatch series; 4th most common in Europe. Both contact urticaria (immediate) and delayed Type IV photoallergy reported. [C1 — Bryden et al. 2006 Contact Dermatitis; Scheuer & Warshaw 2006]
• Estimated prevalence in patch-test populations: ~1–3% positive among photopatch-tested patients (selected populations). General-population prevalence of clinically relevant allergy is <0.5%. [C2]

4.5 Photoproduct toxicity

• BP-1, the major metabolite/photoproduct, has greater estrogenic potency in vitro (relative EC50 in MCF-7 ~10× more potent than parent). Bioaccumulates and has longer half-life. [C1 — Park et al. 2013]
• Chlorinated transformation products in pool water reported to have elevated genotoxicity (Ames, comet) vs parent. [C1 — Liu et al. 2014]

Confidence

• Section 1 identity & regulatory → C1
• Spectrum λmax → C1; ε values → C2 (compiled secondary)
• Photodegradation kinetics → C2 (range across studies)
• Endocrine in-vitro → C1; human relevance → C2
• FDA absorption studies → C1 (peer-reviewed JAMA primary)
• Coral toxicity → C1 (Downs primary; mechanism C1 Vuckovic 2022)

Key citations (full DOIs)

1. SCCS/1625/20 Final Opinion on Benzophenone-3, adopted 30–31 March 2021. https://health.ec.europa.eu/system/files/2022-08/sccs_o_247.pdf
2. Matta MK et al. JAMA 2019;321(21):2082-2091. doi:10.1001/jama.2019.5586
3. Matta MK et al. JAMA 2020;323(3):256-267. doi:10.1001/jama.2019.20747
4. Downs CA et al. Arch Environ Contam Toxicol 2016;70:265-288. doi:10.1007/s00244-015-0227-7
5. Schlumpf M et al. Environ Health Perspect 2001;109:239. doi:10.1289/ehp.01109239
6. Vuckovic D et al. Science 2022;376:644-648. doi:10.1126/science.abn2600
7. Calafat AM et al. Environ Health Perspect 2008;116:893-897. doi:10.1289/ehp.11269
8. Mustieles V et al. Environ Int 2023;173:107739. doi:10.1016/j.envint.2023.107739
9. Hawaii SB2571 / Act 104 (2018). https://www.capitol.hawaii.gov/sessions/session2018/bills/SB2571_CD1_.HTM
10. EU Regulation 2022/1176 (8 July 2022) amending Annex VI of 1223/2009.
11. FDA Proposed Order OTC000008 (24 Sept 2021), Amending OTC Monograph M020 — Sunscreen Drug Products.
12. Sayre RM et al. Photochem Photobiol 2005;81:452-456.
13. Liu YS et al. Sci Total Environ 2014;487:117. doi:10.1016/j.scitotenv.2014.04.025