Preservatives in Cosmetics-What’s Good and What’s Bad?

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Dr John the Chemist

Close up of a pipette dropping a sample into a test tube on colourful background

What are Preservatives in Cosmetics?

Preservation is an essential part of our everyday life and it allows us to ensure products are safe and effective. We have all seen the effects that the growth of bacteria and mould can have on unpreserved liquid and cream products. Micro-organisms need at least three basic parameters to flourish:

  • Correct temperature
  • water
  • nutrients

If we remove one of those parameters, we can successfully inhibit microbial growth. Products with no water content, for example, do not need preservation.

How Preservatives Work

Preservation for water-based products can take many forms. They can range from typical chemical actives to simple chilling or heating, adjusting the pH or even adding salt. Unfortunately, chemical active preservatives must work by either making life support for micro-organisms difficult or by being cytotoxic. Essentially a good preservative system must be poisonous to bacterial and fungal cells. Fortunately, micro-organisms are much more susceptible to death from chemical attack than us humans.

What Makes A Good Preservative System?

Effective preservation is quite complex because it depends on many parameters. For example, we may have an acne cream which has a pH of about 3, common spoilage organisms very rarely flourish below a pH of about 4 so our product can be “preservative free”. A solution with few bacterial nutrients may have minimal preservation whereas those with large amounts of nutrients will need much more robust systems.

Modern v Historical Preservation

Modern preservation techniques rely on a combination of chemicals as some ingredients are more active against certain organisms than others. Truly broad-spectrum preservatives are rare.

Historically preservatives were selected from highly toxic chemicals that were used in low concentrations. Preservatives including arsenic, mercury, formaldehyde, and chloroform were widely used decades ago. The development of regulatory bodies around the world has ensured preservatives that carried a significant risk to human health were outlawed. To gain perspective on what is safe and what is not safe we need to have some understanding of toxicology.

Natural Preservatives

Nature embodies decay and the prime pathway is by microbial attack, without microorganisms our planet could not survive. Naturally occurring preservatives are therefore rare and often require chemical concentration and/or extraction to be effective. Many products and raw material suppliers tout the use of “natural preservatives” and although well intentioned some are simply not effective enough to be viable. A few of the natural combinations can be useful for systems with low nutrient levels but because of relatively low efficacy compared to their chemical active cousins they frequently fail. Preservative efficacy testing is strongly advised when venturing into natural preservation.

Toxicological Pathway

The toxicological pathway has a lot to do with how safe a product is, for example the intravenous pathway is many orders of magnitude more sensitive to toxins than an oral pathway which, in turn, is many orders of magnitude more sensitive than applying them to the skin. This is simply because the amount of a toxin absorbed trans dermally is infinitely small for most chemicals compared to direct injection. Regulators take this into account when making determinations about the suitability of preservatives for human use.

Therefore, you can look at a Safety Data Sheet for a compound and find all sorts of alarming warnings and precautions but find the same chemical in your skin care products which has no such warnings. This does not mean that your shampoo or moisturiser is unsafe, it merely means that it is safe in the correct concentration for that application.

What’s Good and What’s Bad

Advertising would have you believe that the health effects of preservatives vary widely and are generally undesirable. Social media would have you believe that your life is at risk by using some products. Neither of these positions are technically correct and unfortunately internet based false information is widespread especially for personal care ingredients. There has been a general move away from good preservative systems such as mixed hydroxybenzoates largely because of misinformation.

The modern trend is to utilise preservative systems that are used in food products, remembering the many orders of magnitude of safety that it implies.  Ingredients generally accepted as being “good” include:

  • Ethylhexylglycerin
  • Gluconolactone
  • Sodium or potassium salts of sorbic acid, salicylic acid and benzoic acid.
  • Dehydroacetic acid

These compounds have found recent favour and have excellent safety profiles. 

Those moving out of favour recently and considered “bad” are the formaldehyde donors, these additives are not toxic to bacteria on their own right but gradually break down to form small amounts of formaldehyde which is cytotoxic. Even though the amount of formaldehyde is exceedingly small and considered safe by most regulators for its purpose, its carcinogenic profile would suggest that it is not preferred. There are at least 42 of these compounds and they have almost unpronounceable names such as:

  • Imidazolidinyl urea
  • Diazolidinyl urea
  • DMDM hydantoin or sodium hydroxymethyglycinate

Summary

Formulators need consider many attributes such as formulation type, pH, water content, potential bioburden, packaging type, indications for use, customer preference and many other parameters to ensure that the final product is safe and effective.