# A Survey of Preservatives Used in Cosmetic Products
## 1 Introduction
Some emollients, e.g., milk proteins, hydrolyzates of proteins, and silicon derivatives, may have an antagonistic impact on the preservation system. Undesirable side reactions in the preparations may occur when parabens or phenolic preservatives are combined with non-ionic surfactants. High concentrations of solid minerals, e.g., tal, carbonates, silicates, and solid polymeric organic substances, e.g., cellulose and starch, may act antagonistically by the absorption of preservatives. Some mineral raw materials or even plant extracts may occur to be sources of contamination, especially for spores, mycotoxins, and _Clostridium_[5]. The type of material the packaging of a cosmetic product is made of may affect the effect of preservatives. The use of lipophilic preservatives carries a higher risk of absorption through the containers. Incompatibility between preservatives and containers may appear when using nylon with parabens or polyethylene with some phenolic compounds, mercury, and benzoates [5]. The synergistic effect of preservatives with other recipe ingredients has been quite well described. Ingredients of preparations that have a positive effect on the action of preservatives are called \"preservatives boosters\". Their presence in the cosmetic formulation allow to reduce the concentration of a conventional preservative and minimize the potential adverse effects caused by the preservative. Preservative boosters come from a range of cosmetic ingredients, including surfactants, humectants, emulsifiers, chelating and buffering agents, and antioxidants, and while they do not act as preservatives themselves, they do possess some antimicrobial properties [5; 11; 12]. Some synthetic preservative boosters are non-ionic surfactants and emulsifiers, such as lauric acid, glyceryl caprylate, and sorbitan caprylate. They disrupt cell membranes or increase their porosity, which facilitates penetration of antimicrobial substances. Similarly, ethylhexylglycerin can reduce the surface tension at the cell membrane of microorganisms and allow the preservative to penetrate more effectively. Ethylhexylglycerin can enhance the antimicrobial activity of synthetic preservatives, such as phenoxyethanol, dehydroaroacetic acid, benzyl alcohol, and methylparaben [10; 13]. However, humectants (e.g., glycerol and sorbitol) and gelling substances (e.g., polyacrylic acid, agar, and xanthan gum) can effectively enhance preservative activity by the reduction in the amount of biologically available water. Chelating agents, such as EDTA, may facilitate the activity of many preservatives, e.g., benzalkonium chloride, methylparaben, and propylparaben. EDTA chelates the iron necessary for microbial metabolism and growth [5]. Plant essential oils, extracts and their active constituents, and hydrosols can act as natural preservative boosters by enabling the decrease in or even elimination of preservatives in cosmetic formulations, e.g., _Thymus vulgaris_ (thyme) leaf oil, _Origanum vulgaris_ (oregano) leaf oil, _Salvia officinalis_ leaf extract, and _Schizandra chinensis_ fruit extract [12; 14; 15; 16]. On the other hand, while essential oils and plant extracts are mixtures rich in organic substances, they may be harmful, especially for sensitized individuals and children. Certain essential oils and their constituents used in cosmetic products may cause skin sensitization and eye irritation. Some essential oils are phototoxic, such as lemon, bergamot and bitter orange. Therefore, the use of these substances in cosmetic preparations must comply with Annexes I and II of Regulation (EC) No. 1223/2009, as amended [6]. In addition, IFRA provides guidance on fragrance ingredients, including dermal limits for product categories, and the SCCS's opinion helps identify fragrance allergens, including natural extracts [17; 18].