It is possible for bacteria, fungi, and other microorganisms to flourish in cosmetics, particularly ones that contain a lot of water. To prolong the items' shelf life and assist prevent microbiological contamination, preservatives are required. This is especially crucial for items like makeup, moisturizers, and serums that come into direct touch with our skin. To reduce any possible health hazards, the dosage of preservatives used in cosmetic goods is strictly controlled. The best preservatives should have specific qualities. They should be soluble in water or common cosmetic chemicals, efficient against a variety of germs, non-toxic, non-irritating to the skin, stable and effective across a wide temperature range, and not cause the product to fade, discolor, or smell bad.

Preservatives should not react with other organic compounds in the formulation, should have a neutral pH or at least not significantly affect the product’s pH, and should be economically viable and easily accessible. These compounds could impact a number of microbial cell substructures, such as the cell wall, cell membrane, metabolic enzymes, protein synthesis systems, and genetic material. The precise microorganism being targeted, concentration, and exposure duration are some of the variables that affect a preservative's efficacy.

• Classifications of preservatives: Synthetics chemical preservatives

In the EU, the choice of preservatives in cosmetics must comply with EU (EC) No 1223/2009 Annex V of the cosmetic regulation. Based on their chemical structures, the most used synthetic chemical preservatives can be divided into isothiazolinones, phenolic compounds, parabens, organic acids, polyols, formaldehyde releasers, and halogenated chemicals.
Isothiazolinones Isothiazolinones, specifically methylisothiazolinone (MIT) and methylchloroisothiazolinone/ methylisothiazolinone (CMIT/MIT), are commonly used preservatives in cosmetics due to their strong antimicrobial properties. The mechanism of action of isothiazolinones involves their ability to disrupt the cellular functions of microorganisms. The antimicrobial activity of isothiazolinones is not limited to a specific group of microorganisms. They exhibit broad-spectrum efficacy against bacteria, fungi, and yeasts. This versatility makes isothiazolinones valuable ingredients in cosmetics.
Phenolic compounds
The most used phenol preservatives are phenoxyethanol and benzyl alcohol. Phenoxyethanol is a widely used ingredient in cosmetics and personal care products. It serves multiple functions in these products, including as a preservative and solvent. The European Commission’s Scientific Committee on Consumer Safety (SCCS) and the US Food and Drug Administration (FDA) have deemed phenoxyethanol safe for use in cosmetic products within the recommended concentration range. It can exert a direct inhibitory effect on microbial DNA and RNA synthesis. However, phenoxyethanol alone is not sufficient to preserve cosmetics, it is always combined with other boosters or multifunctionals to provide broad spectrum antimicrobial effects. Furthermore, phenoxyethanol has a relatively low risk of causing skin irritation or sensitization
Parabens
Parabens are the alkyl esters of parahydroxybenzoic acid which are commonly used in cosmetic food, cosmetics and pharmaceutical products for their antimicrobial properties. Methylparaben, ethylparaben, propylparaben and butylparaben are the most used parabens. The antimicrobial effect of the parabens has been shown to increase with increasing chain length of the alcohol component. Parabens are proved to be effective especially against fungi. In addition, more than one paraben is normally used in a single product. The mechanism of action of parabens involves interference with the cell membrane. parabens can penetrate the cell walls of microorganisms and interfere with their metabolic processes.
However, regulatory bodies such as the European Commission and certain state regulations in the U.S. have implemented specific bans on the use of propylparaben and butylparaben in cosmetics, particularly in products designed for children. These restrictions stem from studies indicating that some parabens may accumulate in the body over time and disrupt endocrine function.
Organic acids and their derivatives
Organic acids are naturally occurring compounds and are normally added to foods or cosmetics as preservatives. The commonly used organic acids in cosmetics include benzoic acid, sorbic acid, dehydroacetic acid, caprylhydroxamic acid and their derived salts. Organic acids can be either dissociated or undissociated. The undissociated form is lipid permeable and can therefore diffuse into microbial cytoplasm. The antimicrobial effect of organic acids is stronger under acidic conditions, i.e. the environmental pH is lower than pKa of that acid. After the undissociated form of organic acids enters the cell, it will lead to a decrease of the intracellular pH (pHi) by generating more H+. The lower the pKa value of organic acids (the higher the proportion of dissociated forms), the more obvious the effect of reducing pHi.
Alkanediols
Alkanediols are a group of organic compounds that have multiple hydroxyl groups attached to their carbon atoms. Commonly used alkanediols can be served as effective humectants, helping to attract and retain moisture in the skin, thereby enhancing hydration levels. The most used alkanediols in cosmetics are 1,2-alkanediols, including pentylene glycol, 1,2-hexanediol, caprylyl glycol and ethylhexylglycerin. The primary mechanism through which alkanediols exert antimicrobial effects is membrane disruption. Ethylhexylglycerin (EHG), also known as 3-[2-(Ethylhexyl)oxyl]-1,2-propandiol, is a multifunctional ingredient commonly used in cosmetics.
Formaldehyde releasers
Formaldehyde releasers refer to a class of substances that can be decomposed or degraded to formaldehyde under certain conditions. Good water solubility, wide range of applicable pH values, and excellent antimicrobial efficacy at low dosage have made formaldehyde releasers largely used in cosmetics. Common formaldehyde releasers in cosmetics are DMDM Hydantoin (DMDMH), imidazolidinyl urea and diazolidinyl urea. Based on EU regulation, finished products containing DMDMH must be labeled with the warning “contains formaldehyde” if the (free) formaldehyde concentration exceeds 0.05 %.
Halogenated preservatives
Halogenated preservatives are chemicals and formulations that include chlorine (Cl) and/or bromine (Br) and/or iodine (I). The commonly used halogenated preservatives in cosmetic products include Bronopol (2-Bromo-2-nitropropane-1,3-diol) and IPBC (Iodopropynyl butylcarbamate). Bronopol has a broad-spectrum antibacterial effect and can effectively inhibit most bacteria, especially the excellent antibacterial effect on gram-negative bacteria. The European Union stipulates that the maximum allowable concentration in cosmetics is 0.1 %. The restriction is to avoid the use of amine raw materials, to avoid the formation of nitrosamines. The mechanism of action of bronopol was proved to be action on cell membrane integrity and enzyme systems. IPBC is a fungicide that belongs to the group of carbamates. IPBC has been used for years in water-based paint and wood preservatives, and its role has expanded into the more recent uses in cosmetic products. The usage of IPBC has been reduced during the past few years.
Ethyl lauroyl arginate HCl
Ethyl lauroyl arginate HCl (LAE) is a cationic surfactant that has attracted widespread attention as a new food and cosmetic preservative in recent years. LAE has broad spectrum activity against Gram-positive bacteria, Gram-negative bacteria, yeast and mold. In 2009, the European Commission approved for the first time the use of LAE as a preservative in cosmetics (except lip cosmetics, oral hygiene products, and sprays) at concentrations below 0.4 %, with an additional limit of 0.8 % for use in soaps, anti-dandruff shampoos, and non-spray deodorants. The primary mechanism of LAE is thought to be membrane disruption. LAE can damage cell membranes and cause the leakage of cellular components such as protein, potassium, and nucleic acids.
Cationic surfactants
Cationic surfactants, widely recognized for their antimicrobial properties, are increasingly utilized as preservatives in cosmetic formulations. These surfactants exhibit a positive charge, which allows them to interact effectively with negatively charged bacterial and fungal cell membranes. By disrupting the integrity of these membranes, cationic surfactants can inhibit the growth of microorganisms, thereby extending the shelf life of cosmetic products. Quaternary Ammonium Compounds (Quats) include a variety of ingredients, such as benzalkonium chloride, cetrimonium bromide and benzylkonium chloride which possess both conditioning and antimicrobial properties, making them suitable for use in hair conditioners and creams.

• Classifications of preservatives: Natural preservatives

Natural preservatives derived from plant sources are becoming increasingly popular in the cosmetics industry due to their perceived safety and eco-friendliness. These natural preservatives, such as essential oils, antimicrobial peptides (AMPs) and plant extracts offer antimicrobial properties that help prevent the growth of microorganisms in cosmetic products.
Glyceryl caprylate
Glyceryl caprylate belongs to the class of glyceryl monoesters which comprises esters of glycerin and assorted fatty acids or fatty acid derivatives. It is an ingredient that can be used in cosmetics, and has antibacterial, moisturizing, and fattening effects. The main target of glyceryl caprylate is the cell membrane.
Essential oils
Essential oils are mainly a mixture of complex volatile small molecule compounds. They are concentrated hydrophobic liquids containing volatile aroma compounds from plants. They are typically extracted through methods like steam distillation, cold pressing, or solvent extraction. Essential oil compounds refer to individual chemical constituents found within essential oils. Examples of these compounds include linalool, eugenol, and thymol. These compounds can be isolated for their specific properties, which may include antimicrobial, antifungal, or antioxidant activities. The lipophilic nature of essential oils allows them to penetrate the lipid bilayer of the cell membrane. Once inside, they can cause changes in membrane fluidity, permeability, and integrity, leading to the leakage of cellular contents and ultimately cell death.
Many essential oils contain phenolic compounds, such as thymol and carvacrol, which have been found to inhibit the activity of key enzymes involved in microbial metabolism. These compounds can bind to the active sites of enzymes, preventing their proper functioning and disrupting essential metabolic processes.
Antimicrobial peptides (AMPs)
AMPs are small, naturally occurring peptides that play a crucial role in the innate immune system of various organisms, including humans. In recent years, AMPs have gained significant attention as potential antimicrobial agents in the cosmetics industry. AMPs exhibit a broad spectrum of activity against bacteria, fungi, and viruses, making them an ideal choice for use as preservatives in cosmetics to prevent microbial growth and contamination. The most used AMPs in cosmetics include nisin and defensins. However, the mechanism of action of AMPs has not been fully characterized.
One of the primary mechanisms by which AMPs exert their antimicrobial effects is by disrupting the integrity of microbial cell membranes. Another mechanism by which AMPs exhibit antimicrobial activity is through their ability to inhibit microbial protein synthesis.
Plant extracts
Plant extracts are used as natural preservatives often contain bioactive compounds, such as flavonoids and phenolic compounds, which have antimicrobial properties. These compounds can inhibit the growth of bacteria, fungi, and other microorganisms by disrupting their cellular processes. They may interfere with microbial enzyme activity, disrupt cell membrane integrity, or interfere with DNA replication, leading to microbial cell death. Common plant-derived natural preservatives in cosmetics are tea polyphenols, grape seed extract and lemongrass extract.
One mechanism by which plant extracts exhibit antibacterial activity is by increasing the permeability of bacterial cell membranes. Plant polysaccharides, for example, can disrupt the integrity of the cell membrane, leading to the leakage of cellular contents and eventually bacterial cell death.

• Factors to consider when selecting preservatives

Selecting the most appropriate preservative system is one of the most important considerations when developing any cosmetic or personal care product formulation. When evaluating preservative options, formulators weigh several key factors including broad-spectrum activity, toxicity profile, pH and temperature stability, interactions with other ingredients, and cost to arrive at an optimal blend for preserving the quality and integrity of the product.
Cosmetic product type
The type of cosmetic product is an important factor to consider when choosing an appropriate preservative system. This is because different product types have different formulation characteristics that affect preservative efficacy and stability. For example.
- Emulsions like creams and lotions typically require a broad-spectrum preservative blend because their water phase provides an optimal environment for fungal, yeast, and bacterial growth
- Anhydrous products like powders and lipsticks have less microbial growth risk and may only require anti-fungal paraben preservatives
- Products with higher pH levels, like shampoos and cleansers, limit the use of pH-dependent preservative options
- Products containing natural botanical ingredients may require extra care to avoid preservative interactions
Microbial risks associated with the product
- The types of microbial contaminants are likely to be risks because a given cosmetic product should guide preservative selection. Formula components like water activity, nutrients, and pH affect the growth of different microorganisms.
- Bacteria such as Pseudomonas aeruginosa and Staphylococcus aureus thrive in moist emulsions and can cause rancidity and odor
- Yeasts like Candida albicans prefer higher pH products. Mold species including Aspergillus niger are risks in anhydrous powders
Consumers introduce bacteria from their fingers and applicators during use. Evaluating product microbiology helps determine which organisms a preservative system must control. High-risk products may require broad-spectrum preservative blends. Low-risk products may need only light preservation. Considering product microbial risks allows matching preservatives that provide sufficient protection while minimizing unnecessary preservative load.
pH and compatibility
The pH of a cosmetic formula plays a key role in preservative effectiveness and should guide selection.
- Many common preservatives like parabens, formaldehyde donors, and phenoxyethanol work best at pH levels below 5.5.
- Higher pH products require preservatives effective in alkaline conditions, like some multifunctional acids or hydantoin derivatives.
- Using a preservative outside its optimal pH range can result in inadequate microbial control and spoilage.
- The pKa of an acidic preservative indicates the pH level where 50% of the molecules are in their active, undissociated form. Lower pKa equals greater acidity and antimicrobial efficacy in higher pH products.

• Trends in cosmetic preservative regulations

There is a growing trend towards stricter cosmetic preservative regulations around the world. This is due to concerns about the safety of certain preservatives, such as parabens and formaldehyde. Here are some of the latest trends in cosmetic preservative regulations:
- Restrictions on parabens: Parabens are a group of preservatives that are widely used in cosmetic products. However, there is growing concern about the safety of parabens, and many consumers are looking for paraben-free products. Some countries, such as Denmark4 and Norway, have banned the use of certain parabens in cosmetics.
- Restrictions on formaldehyde releasers: Formaldehyde releasers are a group of preservatives that release formaldehyde when they are applied to the skin. Formaldehyde is a known carcinogen, and many countries are restricting the use of formaldehyde releasers in cosmetics. For example, the EU has banned the use of certain formaldehyde releasers in cosmetics.
- Rise of natural preservatives: Natural preservatives, such as benzoic acid and sorbic acid, are becoming increasingly popular. Natural preservatives are derived from plants and other natural sources, and they are generally considered to be safe and non-irritating.