Preservatives are natural or synthetic substances that are added to products such as foodstuffs, cosmetics and pharmaceuticals in order to increase their shelf-life and maintain their quality and safety by inhibiting microbial growth and contamination.1
Whilst the term ‘preservatives’ often provokes negative connotations with consumers, without preservatives the moist warm environment of a skin cream would be the perfect breeding ground for microbes.2 A variety of compounds have been used as preservatives in personal care products and, over the years, the safety of these products has been evaluated. We explore the commonly found preservatives used in personal care and explore the evidence for their safety.
How preservative works
Preservatives are essential to prevent products from spoiling.3 Without them, the shelf-life of a product would be much shortened, as after contamination with microbes, there would be no active products present to prevent the microbes from proliferating.3 Even if a cosmetic product was produced in sterile environments and sealed, once opened and exposed to the environment it would be contaminated with microbes and, without any preservatives, would become a breeding ground for microbes.3 Thus, unless our product is single use, all cosmetic products designed for reuse over a period of time need to contain some sort of preservative compound, natural or synthetic. In skincare products, this could lead to skin problems such as infections or dermatitis.2,3 The European Commission maintains a list of scientifically-evaluated safe preservatives for their use in cosmetic products on the EU market.3
The efficacy of preservatives in cosmetic formulations is evaluated in a challenge test as per the European Pharmacopoeia guidelines.4 This test involves the artificial contamination of cosmetics with a pre-determined amount of bacteria and fungi. This includes bacteria such as Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli and fungi including Aspergillus niger and Candida albicans.4 Samples are periodically removed at fixed times and the number of viable microorganisms are counted.4,5
Parabens are the most widely used preservative in cosmetics with previous estimates of 75-90% of cosmetics containing parabens.2,5,7 They have recently come under scrutiny and we will explore the reasons why along with the supporting evidence. With concerns regarding parabens and other artificial preservatives such as benzoates, butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT),1 due to hypersensitivity and carcinogenic effects, more interest has grown in natural preservatives. We will explore the natural preservative options that are being utilised, including organic acids and oils.1
Parabens including methylparaben, butylparaben and propylparaben are common preservatives found in cosmetics. They are alkyl esters of p-hydroxybenzoic acid, which are preserved by reacting the desired alcohol (methyl, ethyl etc) with p-hydroxybenzoic acid in the presence of an acid catalyst such as sulphuric acid.8 They have come under scrutiny in recent years due to health concerns, with many brands now advertising ‘paraben-free’. Parabens
have an oestrogen-like structure, leading to concerns regarding hormonal interference2 and the effect of certain cancers that are oestrogen sensitive, such as breast cancer.9 The concern started after a review by Darbre published in the Journal of Applied Toxicology in 2003 attracted interest into the investigation of the possible effects and implications with regards to breast cancer of parabens being used as preservatives in underarm cosmetics.10
Since this time, there has been many in vitro and animal models which have demonstrated some hormone-like activity of parabens,11 however the activity was much weaker than that of the natural hormones. There is still more work required to elucidate the potential risk, but the current recommendation as per the European Union is that the use of parabens is allowed up to a certain concentration (8g of parabens per kg of cosmetic product), as it is felt to pose a low risk to health.11
BHA and BHT are synthetic antioxidants used as preservatives in skincare products such as moisturisers.4 Both can cause allergic reactions in the skin.12 An incidence of 13-15% for BHA/BHT reactions has been reported.13 BHA is also considered a possible carcinogen and been banned for use in cosmetics by the EU.5 There is a variety of other synthetic preservatives which have raised similar concerns regarding allergic reactions, interference with hormone function or carcinogenic effects, and include coal tar dyes, diethanolamine (DEA)-related ingredients, formaldehyde-releasing preservatives, polyethylene glycols (PEGs), siloxanes and triclosan.6 For many of these, further studies will be needed to elucidate their effect in vivo.
The consumer market is becoming more aware of the potential concerns regarding synthetic preservatives, and this is driving a growing demand for natural preservatives.4
Several essential oils including Lavandula officinalis (lavender), Melaleuca alternifolia (tea tree) and Cinnamomum zeylanicum (cinnamon) have shown comparable or even higher inhibitory activity against tested microorganisms compared to methylparaben.14 Plant extracts including Matricaria chamomilla (chamomile), Aloe barbadensis (aloe vera) and Calendula officinalis (marigold) have also shown to have near comparable antimicrobial action to methylparaben.15
Studies have also looked at combining such natural preservatives with synthetic ones, in order to reduce the required concentrations of synthetic preservatives.15 Whilst there are numerous in vitro antimicrobial studies into the use of pure essential oils and plant extracts as natural preservatives, there has been little research done demonstrating their antimicrobial activity when incorporated directly into cosmetic products.15-18 Further research will be needed in this area in order to demonstrate their effect once incorporated into cosmetic products.
Other natural preservatives include organic acids such as salicylic acid, benzoic acid and sorbic acid.17 It has been demonstrated that these acids meet the required antimicrobial action as required by the European Pharmacopoeia.17 They are all categorised as preservatives in European Regulation No 1223/2009 on cosmetic products,19 and are accepted as natural preservatives.
The main drawbacks limiting the use of natural preservatives include a lack of broad-spectrum activity and poor efficacy at low concentrations, irritation or potential for allergic reaction, poor compatibility with other ingredients, undesirable odour or colour and high cost.17,20,21 These factors will be taken into consideration as limiting factors by researchers and cosmetic manufacturers.
Preservatives are necessary
Preservatives in skincare products are necessary in order to prevent microbial growth within the product and, hence, are found in every formulation. Preservatives can be synthetic or of natural origin. There are concerns regarding the effect on health of synthetic preservatives such as parabens, BHA and BHT.
Natural preservatives, including plant extracts, essential oils and organic acids, have been investigated and show promising in vitro results for their antimicrobial action. Many are limited by factors such as cost or undesirable odour. Further research to evaluate their use within cosmetic preparations and their compatibility with other components found in skincare is required.