A cosmetic formulator often is presented with the challenge of preserving formulations. In Eric Abrutyn's March 2010 column in Cosmetics & Toiletries magazine, titled "Optimizing Formula Preservation," Abrutyn states, “An important consideration in the development of personal care products is the control of microorganism contamination. To ensure that a consumer-ready product is safe, an appropriate preservation strategy must be established for the development, manufacture and packaging of the formula, thus eliminating unacceptable levels of organisms such as pathogenic bacteria, fungi and mold that could grow and pose a health hazard to the consumer.”
Protecting formulations from microorganisms cannot be accomplished when the preservation agent threatens the consumer. For instance, the addition of sodium cyanate to a formulation would clearly prevent microorganism contamination but would have toxic effects on the consumer's health. Formulating to prevent microorganism contamination, yet to produce compositions that are non-toxic to the consumer, requires not just materials that are toxic to the bacterial organism, but materials that are also not harmful to the consumer.
Selective toxicity, a concept developed by German chemist Paul Ehrlich (1854–1915), describes the ability of a chemical to kill a microorganism without harming the host. In line with this concept are certain chemicals that are toxic to some organisms, such as infectious bacteria, but are harmless to other organisms such as humans.
The concept of selective toxicity has been expanded to include not only immediate adverse toxic effects, but more subtle effects of exposure over time, including irritation effects, bioaccumulation effects, estrogen mimic effects, etc. These effects must be considered and are not necessarily reduced by choosing natural preservatives.
In short, it is a balancing act the formulator must handle in choosing a preservative or preservative-free system. The needs to limit microorganism contamination yet protect the consumer in an efficient way must be balanced.