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Preservatives in cosmetics: how they work and why they are important

Creams, shampoos, cleansers and make-up products are part of our daily lives, often for months after first opening. During this time, they come into contact with air, hands and the surrounding environment. Yet we expect them to maintain their characteristics and remain safe to use. An important part of this protection is provided by preservatives, ingredients that help control the growth of bacteria, yeasts and moulds. The safety of a cosmetic product, however, also depends on its formulation, manufacturing conditions and packaging. And as attention to natural ingredients grows, it is worth understanding how these substances work and why they are important.

How cosmetic preservatives work

Many cosmetic products contain water: creams, lotions, shampoos and shower gels can create favourable conditions for microbial growth, especially when the water present is available for their development. Bacteria, yeasts and moulds are found in the environment and can contaminate a product during manufacturing or use. Their proliferation can alter its smell, colour and texture, compromise its performance and, in some cases, pose health risks, especially when applied to sensitive areas.
Preservatives limit these phenomena: through different mechanisms, they hinder the growth or survival of microorganisms and help keep the cosmetic product microbiologically safe over time. However, they do not sterilise a formula that is already heavily contaminated and they do not compensate for inadequate manufacturing conditions. Substances used include phenoxyethanol, benzoic acid and sorbic acid, as well as some of their salts, such as sodium benzoate and potassium sorbate. Their effectiveness also depends on the characteristics of the formulation.
An important factor is pH, which measures the degree of acidity or alkalinity. Preservatives such as benzoic acid and sorbic acid are more effective within certain pH ranges. The amount of water available to microorganisms and the interactions between ingredients also matter.
Not all cosmetics therefore require the same level of protection. A water-rich cream presents a different risk from a water-free cosmetic oil. In some formulations, the characteristics of the product itself can hinder microbial growth even without traditional preservatives. The need to use them must be assessed on a case-by-case basis.
Preservatives must also be distinguished from antioxidants. The former counteract microbial growth; the latter slow down oxidation reactions that can alter oils and other ingredients, changing the product’s colour, smell and properties. Both contribute to cosmetic stability, but they perform different functions.
How can we verify that antimicrobial protection is effective? One of the tools used is the challenge test: under controlled conditions, the formula is exposed to specific microorganisms to observe how their presence changes over time. ISO 11930 provides a reference for this assessment. Effectiveness, therefore, does not depend only on the presence of an ingredient in the INCI list, but on the behaviour of the entire formulation. For products with a low microbiological risk, different assessment approaches may be appropriate.

Natural and synthetic preservatives: what really changes

The distinction between natural and synthetic plays an important role in cosmetic communication. Plant origin is often associated with gentleness and safety; substances obtained through synthesis, on the other hand, can arouse distrust. From a scientific point of view, this opposition is too simplistic. Some essential oils and plant extracts, such as rosemary or tea tree oil, contain substances with antimicrobial activity. This does not automatically make them suitable for preserving any cosmetic product: a result observed in the laboratory does not guarantee the same effectiveness within a cream or lotion.
The composition of extracts can vary depending on the raw material and processing. In addition, the concentrations required to achieve antimicrobial action may affect the product’s fragrance, colour, stability and tolerability. Natural does not mean risk-free either: some components of essential oils can cause irritation or allergic reactions.
Synthetic preservatives also have different characteristics and safety profiles. Phenoxyethanol, for example, is used for its preservative action under the specified conditions of use; several parabens, on the other hand, have become the focus of public debate, making them familiar even to non-experts.
The safety of a substance is not determined by its reputation or its origin. It depends on the properties of the ingredient, its concentration, the type of cosmetic product and the conditions of exposure. In the European Union, the reference is Regulation (EC) No 1223/2009 on cosmetic products. Annex V identifies the authorised preservatives and their conditions of use; the regulation also requires a safety assessment of the finished product and the indication of ingredients on the label.
The aim is not to choose between a generically natural formula and a synthetic one, but to obtain a stable and safe cosmetic product. The claim “preservative-free” does not in itself guarantee greater safety either. Some formulations can be protected thanks to their own characteristics, but this condition must be demonstrated through an appropriate assessment. For those developing cosmetics, the challenge is to find a balance between antimicrobial effectiveness, ingredient compatibility, tolerability and consumer expectations.

From production to packaging: protection that continues over time

Preservation begins before the cosmetic product is even opened. Even a well-designed formula can be compromised by contamination during production, transfer or packaging. Hygiene in production areas and on equipment is therefore essential. Surfaces that come into contact with creams, gels and cleansers require appropriate cleaning and control procedures, especially during filling operations, when the product is transferred into the containers intended for sale. Good manufacturing practices, referred to in ISO 22716, concern the organisation and control of production activities. Microbiological safety cannot rely exclusively on preservatives: it requires a process capable of limiting opportunities for contamination.
Packaging also matters. The container helps protect the cosmetic product during transport, storage and use. A jar from which cream is taken with the fingers involves different forms of contact with the environment compared with a bottle fitted with a dispenser. This does not mean that one solution is always preferable to the other: the risk must be assessed in relation to the formula and the way the product is used. Airless systems are designed to dispense the cosmetic product while limiting exchanges with the external environment. Depending on the technology, they can reduce exposure to air and some opportunities for contamination. They are particularly interesting for creams, serums and lotions, but they do not automatically eliminate the need for preservatives.
The consumer also plays a role. Closing packages properly, avoiding introducing water into the product, using it with clean hands and following storage instructions all help reduce risks.
Labelling distinguishes between the date of minimum durability and the period of use after opening, or PAO (Period After Opening). For cosmetic products with a minimum durability of no more than 30 months, the date by which the product, when correctly stored, continues to meet its intended specifications must be indicated. For products with a minimum durability of more than 30 months, the period during which the product can be used safely after opening must be indicated, where relevant. The PAO is represented by the symbol of an open jar, accompanied by a number and the letter M: 12M, for example, indicates twelve months after opening. Following the manufacturer’s instructions remains essential.

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