c001 — Octinoxate (Ethylhexyl Methoxycinnamate / EHMC / OMC)

Section 1: Identity

Regulatory Status

Typical use concentration: 2–10 % in sunscreen formulations; most common at 5–7.5 %.

Section 2: UV Absorption Spectrum

Octinoxate is a UVB filter (280–320 nm) whose long-wavelength tail extends slightly into UVA II. The molecule shows pronounced solvatochromism (the trans isomer's λmax shifts ~19 nm between cyclohexane and ethanol) and photoisomerism (the cis isomer is photogenerated and has a lower ε).

Reported "headline" value for cosmetic formulators: λmax ≈ 308–311 nm with ε ≈ 23,300–25,000 M⁻¹ cm⁻¹ in alcohol [Wikipedia/Octyl_methoxycinnamate citing Pattanaargson; Cosmetic Ingredient Guide; Couteau et al., Int J Pharm 325 (2006) 28-30] — C1.

E(1%, 1cm) is not commonly reported for octinoxate because formulators normally cite ε; computed from ε = 24,000 / MW × 10 ≈ 826 (at 310 nm in EtOH) — C3.

Section 3: Photodegradation Kinetics

Dilute solution (sub-mM)

• In dilute alcoholic solution, OMC reaches a photostationary state between trans and cis isomers; the cis form has lower ε. After a few seconds of solar simulator exposure, absorptivity drops to a stable plateau:
• Cyclohexane: 79 % ± 1 % of initial absorbance retained at PSS [C1, Hanson 2015].
• Methanol: 68 % ± 2 % retained at PSS [C1, Hanson 2015].
• No further photodegradation between 112 s and 1120 s in dilute solution (<2 % change) — i.e., monomer in solution is essentially photostable once at PSS [C1, Hanson 2015].

Photoisomerization quantum yields [C1, Hanson 2015, broadband solar simulator]: | Solvent | Φ(trans→cis) | Φ(cis→trans) | |---|---|---| | Methanol (solar) | 0.37 ± 0.01 | 0.47 ± 0.06 | | Methanol (330 nm) | 0.41 ± 0.05 | 0.52 ± 0.04 | | Cyclohexane (solar) | 0.28 ± 0.01 | 0.60 ± 0.00 |

Aggregated / neat film / formulated product

This is where the famous photolability appears.

• Quantum yield of irreversible photodegradation of aggregated OMC: Φd = 6.2 (±0.6) × 10⁻⁴ (assuming ε = 24,000 M⁻¹ cm⁻¹) or 1.9 (±0.3) × 10⁻³ (assuming aggregate ε = 8,200) [C1, Hanson et al. 2015].
• Tarras-Wahlberg et al., J Invest Dermatol 113 (1999) 547-553, doi:10.1046/j.1523-1747.1999.00721.x — OMC films lose ≈10 % UV absorption after 35 J/cm² UVA + UVB irradiation; full curve shows 20–30 % drop after a "full day in sun" dose [C1].
• Sayre et al., Photochem Photobiol 81 (2005) 452-456, doi:10.1562/2004-02-12-RA-083.1 — In the presence of avobenzone, OMC undergoes much faster photolysis: a 2+2 cycloadduct between the two filters is formed; OMC + avobenzone irradiated at 1 MED already shows ≥30 % combined degradation [C1].
• Half-life on skin equivalent (4 % OMC sunscreen film): ~95 min under 1.4 MED/h xenon lamp [C2, summarized in Couteau et al., Pharm Res 24 (2007) 1153].

Photoproducts identified

Section 4: Notable Issues

• Endocrine activity: SCCS/1671/24 (June 2025) classified EHMC as an endocrine-active substance with estrogenic and weak antiandrogenic activity in vitro and in vivo, but concluded acceptable Margin of Safety at 10 % [C1]. Schlumpf et al. (Environ Health Perspect 2001) showed uterotrophic effect in rats — among the original triggers for EU re-evaluation [C1].
• Skin penetration: Janjua et al. (2008) measured detectable urinary EHMC and 5–10 % systemic uptake when applied as 2 mg/cm² lotion [C2]. Detected in human breast milk and urine in multiple biomonitoring studies [C2].
• ROS / phototoxicity: Aggregate-form OMC photoproducts photosensitize singlet oxygen at λex 405 nm (Hanson 2015) [C1]. Damiani et al., Free Radic Biol Med 35 (2003) — 4 % OMC sunscreen with avobenzone enhanced UV-induced ROS in skin model [C2].
• Aquatic toxicity: OMC is on the Hawaii/Palau ban list for coral reef bleaching; LC50 (Daphnia magna 48 h) ≈ 0.4–0.6 mg/L [C2, EU SCCS dossier].
• Synergy/antagonism:
• Strong antagonism with avobenzone — the [2+2] cycloaddition destroys both filters [C1, Sayre 2005].
• Modest synergy with octocrylene which can quench OMC's triplet [C1].
• Synergy with bemotrizinol/bisoctrizole (Tinosorb S/M) which are fast triplet quenchers [C2].

Sources

1. Hanson KM, Narayanan S, Nichols VM, Bardeen CJ, "Photochemical degradation of the UV filter octyl methoxycinnamate in solution and in aggregates", Photochem Photobiol Sci 14:1607-1616 (2015), doi:10.1039/C5PP00074B.
2. Tarras-Wahlberg N, Stenhagen G, Larkö O, Rosén A, Wennberg AM, Wennerström O. "Changes in ultraviolet absorption of sunscreens after ultraviolet irradiation". J Invest Dermatol 113 (1999) 547-553, doi:10.1046/j.1523-1747.1999.00721.x.
3. Sayre RM, Dowdy JC, Gerwig AJ, Shields WJ, Lloyd RV. "Unexpected photolysis of the sunscreen octinoxate in the presence of the sunscreen avobenzone". Photochem Photobiol 81 (2005) 452-456.
4. Pattanaargson S, Munhapol T, Hirunsupachot P, Luangthongaram P. "Photoisomerization of octyl methoxycinnamate". J Photochem Photobiol A 161 (2004) 269-274.
5. Pattanaargson S, Limphong-Pand P. Int J Cosmet Sci 23 (2001) 153-160.
6. Couteau C et al. "UV filters, ingredients with a recognized anti-inflammatory effect", Pharm Res 24 (2007) 1153-1160.
7. Schlumpf M et al. "In vitro and in vivo estrogenicity of UV screens". Environ Health Perspect 109 (2001) 239-244.
8. SCCS/1671/24 (final 30 June 2025) — Opinion on Ethylhexyl Methoxycinnamate.
9. Janjua NR et al. "Urinary excretion of phthalates and paraben after repeated whole-body topical application of cosmetics", Int J Androl 31 (2008) 118-130.