Mild Preservatives – Fact or Fiction?

Increasingly, there is a move within the personal care industry to seek out more “mild”, or “natural” ingredients, especially preservatives. This brief paper discusses the problems and myths associated with mild preservation.

By their very nature, preservatives are biologically active, as they are required to exert a killing effect against micro-organisms, which are living organisms. Whilst the activity of some preservatives may be highly specific, and target mostly microorganisms, as a generality, if a compound is biologically active, it has the capacity to harm other living cells. Preservatives, therefore, may cause adverse effects in human cells as a result of their inherent biological activity. Fortunately, as humans are much higher organisms, they are not likely to be killed at the concentrations required for preservation (!), but rather more mild effects such as skin irritation and sensitisation. Whilst these effects are relatively mild, they are certainly undesirable. For this reason, in the EU, all preservatives used in personal case are very carefully assessed for safety before being included on Annex VI of the Cosmetics Directive – the list of permitted preservatives. As part of the approval process, the risk of adverse reactions to the compound is assessed, and a maximum permitted concentration is assigned on the basis of providing an acceptable margin of safety. There are, however, always individuals who are more sensitive and susceptible to skin reactions, and it is not possible to totally remove the risk of an adverse response, although the vast majority of the population is at very little or no risk.

The concept of a “mild” preservative is based on the compound having a good toxicity profile, especially regarding skin irritation/sensitisation. In other words, a higher concentration of the “mild” preservative is required to elicit an adverse reaction. To illustrate this, parabens are considered to be relatively mild preservatives, whereas the isothiazolinones are not. This is reflected in the use concentrations; parabens are required at concentrations varying from 0.2 – 0.4%; isothiazolinones from 5 – 15 parts per million (0.0005 – 0.0015%). Because the parabens are relatively mild, they are not as effective as isothiazolinones and much higher concentrations are required to give the desired preservative effect. In the search for preservatives even more mild than parabens, the successful candidates for preservation will be required at still higher concentrations, as they will not be as effective on a weight for weight basis. Many of the compounds being introduced as new alternatives to traditional (and legal!) preservatives have relatively low antimicrobial activity, and will be required at concentrations in excess of 1% in many, if not most cases.

The difference in the margin of safety between an “aggressive” preservative at 0.0005% and a more “mild” preservative at 1% is very little, as the end result is a concentration high enough to exhibit biological activity – killing micro-organisms! If the search for increasingly “mild” preservatives continues, the successful candidates will be required to be used in increasingly higher concentrations in order to provide sufficient cidal activity to protect the end product. Some of the more recently introduced “natural” preservatives require concentrations a high as 5% to be effective!

So, generally speaking, “mild preservative” is a misnomer, as this simply means that higher concentrations must be used to achieve the desired protection against microbial contamination.

  • Rbuerk

    So, for a small cosmetic manufacturer whose trying to create safe products for a health conscience clientele, are higher concentrations of a “milder” preservative more desireable than lower concentrations of chemical preservatives?

    • Dene62

      Broadly, and simply speaking – no! This will depend upon the precise nature of the preservative systems in question. One reasonably good example is that of organic acids – sorbic, benzoic etc (and their potassium/sodium salts). If the product pH is low (pH 4 – 5) fairly low concentrations may be used. If the pH is fairly high (pH 6 – ish), much higher concentrations will be needed, because these preservatives are massively sensitive to pH, whereas many of the “less mild” preservatives are not, or much less so. Therefore, especially at a pH around 6, it may be much better to use a preservative that is considerably more effective at that pH. Above pH 6, it is almost a complete waste of time using sorbate or benzoate, although dehydroacetic acid is still fairly active up to pH 6.5, although this is the absolute limit for this preservative. This is only one example of a scenario, based solely on pH, but there will be others where pH is not a factor.

      In reality, however, using ANY preservative at concentrations below the maximum permitted in the EU will give you a safe product. Within those restrictions, safety of preservatives is not really an issue, other than for the few unfortunate individuals who may be allergic to one or more specific preservatives – thankfully a rare event! But this is also possible with “mild” preservatives – sorbic acid has one of the (relatively) higher rates of sensitisation, despite its reputation as a mild preservative. If only life were simple!

      • Philippe Papadimitriou

        Just a note on this pH notion (for formulators).

        Everyone willing to work with ingredients having a acid/base equilibrium, but where only ONE form is active (the case of organic acids as preservatives – only the acidic form is active), should inquire about the pKa.

        If pH pKa, the majority is in its basic form.

        But still, this is a majority only.
        I have found this easy to use web site (for its freindly-user calculus method), where one can calculate the active portion of the molecule depending on the pKa and the pH:
        http://www.nurseweek.com/calculators/pka.asp

        Hope this helps.

  • Anonymous

    Another way to think of this is in drug terms. A good preservative should have a high therapeutic index, or therapeutic window. This means that the dose that works is far from the dose that causes harm. Essential oils, although many have antimicrobial properties, have a very low therapeutic window. This means the amount necessary to act as a preservative is very near to the amount that causes skin irritation.

    • Dene62

      Absolutely correct, Cindy. The main thing to remember is that the maximum concentrations permitted in the EU have a significant built-in safety factor applied when the limit is being determined. The SCA could use a positive list of preservatives! (Sorry – couldn’t resist!)