Essential Oils and Cultural Heritage Conservation: Are They Safe, Environmentally Friendly, Sustainable, and Negligibly Toxic?

Microbial colonization of heritage materials is a well-known conservation issue. When necessary, it is removed using mechanical, physical, or chemical methods, with biocide formulations being a common choice. The need to reduce dependence on conventional biocides has led to the exploration of innovative alternative methods and new formulations with biocidal properties for the conservation of heritage objects. Alternative approaches include natural compounds such as plants' essential oils. While these natural options show promise, they present challenges-such as inconsistent effectiveness, possible toxicity, and the need for thorough compatibility testing with historic materials. Therefore, although some concerns are legitimate, the "run" to alternative substances is a growing concern as well. A comprehensive selection and examination of international research articles from the past two decades on this subject has been conducted. The detailed and critical analysis of existing data on essential oils, hydrolates, and other plant-derived extracts studied to prevent and/or eradicate the colonization of microbial communities on heritage objects focused on the effect on microorganisms in controlled environments, in situ applications on microorganisms, encapsulation in hydrogels and emulsions, toxicity and ecological impact, and alterations of heritage materials. The review also discusses the advantages, limitations, and practical implications of these strategies.