Tattoo Pigment In Situ as a Determinant of Laser Treatment Response: Composition, Particle Size Distribution, Cellular Localisation, and In Situ Transformations

Introduction: The safety assessment of tattoo removal procedures rests on the assumption that the treatment substrate corresponds to the ink composition declared by the manufacturer. This assumption has rarely been verified. Methods: A structured, comprehensive literature search was conducted across PubMed/MEDLINE, Embase, Scopus, and Web of Science using a four-block strategy encompassing ink composition and impurities, particle characteristics, cellular localisation, and in situ transformations. Analytical chemistry studies, tissue imaging, market composition research, and regulatory documentation were included. Results: The material in the skin may differ from product documentation in four respects. A submicron fraction exists in commercial inks before any procedure. Many products contain undeclared ingredients, and aromatic amines may occur as manufacturing impurities before irradiation. The deposit is probably not inert: photodegradation has been shown outside tissue and remodelling in mice, whereas oxidation remains hypothetical. Finally, Colour Index numbers and INCI declarations do not unambiguously identify the substance. Conclusions: The substrate of the removal procedure remains incompletely characterised and may, depending on the age of the tattoo, predate the harmonisation of ink composition in the European Union. The evidence documents chemical hazards rather than demonstrated clinical risk, with consequences for literature inclusion criteria, the design of degradation studies and informed consent.