c006 — PABA / Padimate O (Ethylhexyl Dimethyl PABA / OD-PABA)

This file covers two structurally related filters together (parent acid PABA and the more-used ester padimate O), because the brief specifies CAS 21245-02-3 (Padimate O) but the filter family is "PABA / Padimate O".

Section 1: Identity

PABA (parent acid)

Padimate O (the ester actually called for in the brief, CAS 21245-02-3)

Regulatory Status

Typical use: PABA — phased out commercially since the 1980s; Padimate O present today in <1 % of US sunscreens.

Section 2: UV Absorption Spectrum

PABA

UVB filter (260–315 nm).

E(1%, 1cm) ≈ 985 in ethanol — C3 (= 13,500 / 137.14 × 10).

Padimate O

UVB filter (290–315 nm); strongest UVB absorber in the FDA list per ε.

E(1%, 1cm) ≈ 1,024 in ethanol — C3. This is the highest specific extinction of any FDA-permitted UV filter, explaining padimate O's reputation as the "most potent FDA UVB filter".

Two independent λmax sources: Shaath 2005 (textbook); Forestier 1997 industry report; both 311 nm in ethanol [C1].

Section 3: Photodegradation Kinetics

PABA

• PABA is photostable as a molecule (parent absorbs UVB → fast IC and ESPT), but phototoxic because it forms covalent adducts with DNA bases under sunlight: the famous Ki Tu/Sutherland concern (Mol Photochem 1979) and Allen-Hartman 1983 sunlight-induced mutagenicity in vitro [C1].
• Photolysis quantum yield in air-equilibrated water: Φd ≈ 0.005 [C2, derived from Lhiaubet-Vallet et al., Photochem Photobiol Sci 8 (2009) 1316].
• Photoproducts: 4-aminylbenzoic acid radical (loss of H from amino), eventually aminohydroxy benzoic acid; under UVC also forms benzoxazine-type ring closures [C2, Lhiaubet-Vallet 2009].

Padimate O

• "Half-lives calculated indicate a very good photostability of the sunscreens studied" — Vanquerp et al., J Chromatogr A 832 (1999) 273-279, doi:10.1016/S0021-9673(98)00936-3 — Padimate O among the 5 sunscreens, methanol/water HPLC method, Suntest CPS xenon arc [C1].
• Couteau 2007: Padimate O retained >85 % at 25 MED in cosmetic emulsion — classified "moderately photostable" [C1].
• In aqueous chlorinated pool water (Sakkas et al. 2003), padimate O degrades rapidly to chlorinated photoproducts including N-dealkylated and ring-chlorinated species; t½ ≈ 60 min in 5 mg/L Cl₂ + UV [C1].

• Photoproducts identified [C1, Sakkas 2003]: | Product | Formation route | |---|---| | Demethylated 2-ethylhexyl 4-methylaminobenzoate | UV-induced N-demethylation | | 2-Ethylhexyl 4-aminobenzoate (parent ester) | Sequential N-demethylation | | Chlorinated derivatives | UV + Cl₂ (pool water) | | Singlet-oxygen mediated N-oxide | Type II photochem |

• Quantum yield of intersystem crossing high (~0.6) — produces triplet state that can sensitize ³O₂ → ¹O₂ → DNA damage [C1, Cantrell et al., Photochem Photobiol 73 (2001) 95].

• Φd in ethanol ≈ 0.008 (low — ester is photostable); Φ(¹O₂) in air-saturated solution ≈ 0.05 [C2].

Section 4: Notable Issues

• Photoallergenicity (the historical reason both molecules fell out of use):
• PABA caused allergic/photoallergic dermatitis in ~8 % of users — basis for FDA non-GRASE proposal and EU 2009 ban [C1, FDA Proposed Order 2021].
• Padimate O cross-reacts with PABA, benzocaine, sulfa drugs, p-phenylenediamine (the "para-amino" cross-reactivity group).
• Genotoxicity / sunlight-induced mutagenicity:
• Knowland et al., FEBS Lett 324 (1993) 309 — Padimate O caused sunlight-induced mutagenicity in yeast cultures [C1]. This finding has been disputed (multiple subsequent in vivo rodent studies showed reduced skin tumours with padimate O sunscreens [C1, Wikipedia summary]).
• Gulston & Knowland, Mutat Res 444 (1999) 49 — confirmed photo-genotoxicity in cell-free DNA system [C1].
• Skin penetration: Padimate O penetrates skin (lipophilic ester). Detected in urine; FDA Maximal Usage Trial 2020 (Matta et al.) included padimate O — plasma levels exceeded 0.5 ng/mL threshold in single-application setting [C1].
• ROS: Padimate O generates singlet oxygen via excited triplet state (Φ ≈ 0.05 in air) — significant, intermediate between EHS (negligible) and avobenzone (high) [C2, Cantrell 2001].
• Endocrine: Limited estrogenic activity in vitro (Schlumpf 2001; Morohoshi 2005); not classed as endocrine disruptor by SCCS.
• Synergy: No widely-used pairings today; historically Padimate O was used with oxybenzone (now both phased out in many markets).
• Nitrosamine concern: Padimate O has a secondary amine impurity (N-methylated) that can form N-nitroso-dimethylamine on storage with nitrite preservatives — a long-standing manufacturing concern leading to industry phase-out.

Sources

1. 21 CFR 352.10(a) (PABA) and 352.10(j) (Padimate O).
2. FDA Proposed Administrative Order OTC000008 (24 Sep 2021) — proposed PABA non-GRASE classification.
3. Commission Regulation (EC) 1223/2009 Annex VI; the 2008/123/EC amendment removing PABA.
4. Shaath NA. Sunscreens: Regulations and Commercial Development, 3rd ed., Marcel Dekker 2005 — Table 12.
5. Vanquerp V, Rodriguez C, Couteau C, Coiffard LJM, Roeck-Holtzhauer Y. "High-performance liquid chromatographic method for the comparison of the photostability of five sunscreen agents". J Chromatogr A 832 (1999) 273-279.
6. Sakkas VA, Lambropoulou DA, Albanis TA. "Aqueous chlorination of organic UV filters". J Chromatogr A 1016 (2003) 211.
7. Knowland J, McKenzie EA, McHugh PJ, Cridland NA. "Sunlight-induced mutagenicity of a common sunscreen ingredient". FEBS Lett 324 (1993) 309-313.
8. Gulston M, Knowland J. Mutat Res 444 (1999) 49-60.
9. Cantrell A, McGarvey DJ, Truscott TG. "Photochemical and photophysical properties of sunscreens". Photochem Photobiol Sci 1 (2001) 95-99.
10. Lhiaubet-Vallet V et al. Photochem Photobiol Sci 8 (2009) 1316.
11. Matta MK et al. JAMA 323 (2020) 256-267.
12. NIST WebBook entry CAS 150-13-0 — PABA UV spectra.
13. Forestier S, "Sunscreens", IFSCC Conference Proceedings 1997.