Environmental Trade-Offs Between Essential Oil and Quaternary Ammonium Biocides in Cultural Heritage Conservation

Quaternary ammonium compounds (QACs) have dominated biocidal practice in cultural heritage conservation for decades, yet growing evidence of environmental persistence, aquatic ecotoxicity, and antimicrobial resistance induction has prompted the search for safer alternatives. Essential oils (EO) have emerged as promising bio-based biocides, though their environmental performance has rarely been quantified through rigorous life cycle approaches. This study presents a comparative Life Cycle Impact Assessment (LCIA) of EObased and QAC-based biocidal formulations across representative conservation scenarios, following ISO 14040/14044 standards and the Environmental Footprint 3.1 methodology with USEtox® 2.1 characterization factors. Three complementary functional units were employed: formulation-based, surface-based, and intervention-based. The results reveal a fundamental trade-off: EO-based systems exhibit 81% higher climate change impacts but 82–89% lower human toxicity and freshwater ecotoxicity impacts compared to QAC-based systems. Surface-normalized comparisons reduce the climate gap to 32%, while toxicity advantages remain robust across all sensitivity scenarios. Monte Carlo analysis confirms the robustness of toxicity findings (p > 99%), whereas climate comparisons remain scenariodependent. These findings support context-dependent adoption of EO-based biocides in conservation practice and demonstrate that EO-related climate impacts are technically mitigable, while QAC toxicity is intrinsic to their molecular structure