c010 — 4-Methylbenzylidene Camphor (4-MBC / Enzacamene)

Section 1: Identity

Regulatory Status

Typical historic concentration 1–4 %.

Section 2: UV Absorption Spectrum

UVB filter (280–320 nm) with a single strong band centred at ~300 nm.

Two independent sources for ε at 300 nm in EtOH: Berset 1996 (Int J Cosmet Sci) and Beck 2006 (also Int J Cosmet Sci) — both 24,500 ± 500 M⁻¹ cm⁻¹.

ε is comparable to OMC's (~24,000) but the molecule is conformationally constrained and so undergoes much less photoisomerization fragmentation.

Section 3: Photodegradation Kinetics

The benzylidene-camphor chromophore undergoes reversible E/Z photoisomerization (analogous to the cinnamate's, but conformationally restricted by the bicyclic camphor scaffold).

• Quantum yield of E→Z isomerization: Φ(E→Z) = 0.13 at 287 nm to 0.30 at 370 nm (wavelength-dependent) [C1, Buglak et al. 2015 PPS; Deflandre & Lang 1988, Int J Cosmet Sci 10:53].
• The photostationary state mixture is itself photostable; spectrum red-shifts modestly after irradiation (~5–10 nm shift, with isosbestic point near 290 nm) but retains UVB absorption capacity [C1].
• Quantum yield of irreversible photodegradation Φd ≈ 10⁻³ to 10⁻⁴ [C2, derived from Berset 1996 stability data].

% remaining vs UV dose

• Berset et al. 1996 (Int J Cosmet Sci 18:167): under simulated solar exposure (5 MED), 4-MBC retained 88 % of initial absorbance [C1].
• Couteau 2007: >90 % retention at 25 MED in cosmetic emulsion — classified photostable [C1].
• Kockler et al. 2012 review: 4-MBC "regarded as photostable" with caveat that the photostationary cis-form has slightly lower ε (~5 % spectrum shift) [C1].
• Reszko et al., J Cosmet Dermatol 8 (2009): in commercial sunscreens with avobenzone + 4-MBC: 4-MBC retained 92 % at 1 MED but appeared to not stabilize avobenzone (no triplet quench) [C2].

Photoproducts

• Cis-MBC isomer (photostationary partner) — fully reversible.
• Minor: thermal/photo-induced retro-Michael fragmentation to p-methylbenzaldehyde + camphor under UVC/intense UVB, but negligible at solar doses [C2, Stiehl 1992].

Photochemistry mechanism

• Computational study (Buglak 2015) shows ultrafast IC through conical intersection at twisted excited-state geometry → heat dissipation. Triplet yield small (~0.05) [C1].
• Singlet oxygen: low Φ(¹O₂) ~0.02 — does not significantly photosensitize ROS [C1, Buglak 2015].

Section 4: Notable Issues

• Endocrine disruption — the reason for EU ban:
• Schlumpf et al. (Environ Health Perspect 109, 2001) — uterotrophic assay showed 4-MBC produced significant uterine weight gain in immature rats at 33 mg/kg/day; effect comparable to estradiol benzoate at 100 µg/kg [C1].
• Tinwell et al. (Toxicol Sci 70, 2002) — confirmed estrogenic activity in vivo [C1].
• Schmutzler et al. (Endocrinology, presented at European Congress of Endocrinology 2008): 4-MBC at ≥33 mg/kg/day caused decreased serum T4 and elevated TSH in rats — pattern characteristic of primary hypothyroidism [C1].
• SCCS/1640/21 (final opinion 2022) concluded: "No safe concentration of 4-MBC for cosmetic use can be established", citing thyroid + estrogen receptor effects + insufficient genotoxicity data [C1]. This led to the EU ban (Reg 2024/996).
• Skin penetration: Significant — 4-MBC detected in human breast milk and urine biomonitoring (Schlumpf 2008, Chemosphere 81; Janjua 2008 J Invest Dermatol). Plasma concentrations measurable at ng/mL after typical sunscreen application [C1].
• ROS: Low; not a primary concern.
• Aquatic toxicity: High lipophilicity (log Kow 5.5) → bioaccumulates in fish; detected in surface waters and lake sediments. SCCS 2022 explicitly noted environmental concerns [C1].
• Photoallergy: Reported but rare — Schauder & Ippen historical cases; less of an issue than PABA.
• Synergy:
• No useful synergy with avobenzone — does not quench BMDM triplet effectively [C1, Reszko 2009].
• Sometimes paired with octinoxate for boosted UVB (sum of overlapping bands).
• Often co-formulated with terephthalylidene dicamphor sulfonic acid (Mexoryl SX) in older European formulations.

Sources

1. SCCS/1640/21 — Final Opinion on 4-Methylbenzylidene Camphor (4-MBC), 25 January 2022.
2. Commission Regulation (EU) 2024/996 (3 April 2024) — banning 4-MBC.
3. UK SAG-CS Opinion 18 — 4-MBC as a UV filter in cosmetic products (2025).
4. Schlumpf M et al. "In vitro and in vivo estrogenicity of UV screens". Environ Health Perspect 109 (2001) 239-244.
5. Tinwell H et al. Toxicol Sci 70 (2002) 224-232.
6. Schmutzler C et al. "The UV filter benzophenone-2 interferes with the thyroid hormone axis ..." (also covers 4-MBC). Endocrinology 148 (2007) 2835.
7. Berset G, Gonzenbach H, Christ R, et al. "Proposed protocol for the testing of photostability". Int J Cosmet Sci 18 (1996) 167-177.
8. Buglak AA et al. "Theoretical study of the UV absorption of 4-methylbenzylidene camphor: from the UVB to the UVA region". Photochem Photobiol Sci 14 (2015) 1859, doi:10.1039/C4PP00392F.
9. Beck K et al. Int J Cosmet Sci 28 (2006) 339-345.
10. Couteau C et al. Pharm Res 24 (2007) 1153-1160.
11. Kockler J et al. J Photochem Photobiol C 13 (2012) 91-110.
12. Reszko AE et al. J Cosmet Dermatol 8 (2009) 109-113.
13. Janjua NR et al. J Invest Dermatol 128 (2008) 2480-2487.
14. Schlumpf M et al. Chemosphere 81 (2010) 1171-1183.