Cosmetic challenge test
The degree of microbiological risk for the products depends on the ability of a product to support the growth of microorganisms and the probability that those microorganisms can cause harm to the user.
The likelihood of microbiological contamination for some cosmetic products is extremely low (or non-existent) due to product characteristics (which include the composition of the product, the production conditions, packaging and a combination of these factors) that create a hostile environment for survival/growth of microorganisms. Those products identified as “hostile”, but produced in compliance with GMP, pose a very low risk to the user, and therefore don’t require a Cosmetic challenge test.
The physico-chemical characteristics in a product that can create a hostile environment (and any combination of them):
- Water activity, aw, of formulation: water is one of the most important factors controlling the rate of growth of an organism. However, it is the available water in the formulation, not the total moisture content that determines the potential for growth. The most useful measurement of water availability in a product formulation is water activity, defined as the ratio of the water vapour pressure of the product to that of pure water at the same temperature. Even though water activity values are important in assisting in the risk analysis for microbial contamination, water activity should not be used as a sole indicator in determining whether product testing is necessary for a particular product formulation. Other factors such as manufacturing and filling temperature should be taken into consideration to determine if a product requires further microbiological testing.
- pH of formulation: acidic pH (below pH 3) creates hostile environment for the growth of yeast. On the other hand, also alkaline pH (above pH 10) prevents the growth of microorganisms that would be likely to contaminate cosmetic products.
- Alcohol content: Products containing alcohol levels above 20% by volume mass do not require microbiological testing (challenge test and end product testing). At levels below 20%, other physico-chemical factors need to be evaluated to determine potential risk.
- Raw materials that can create a hostile environment: the use of certain raw materials in cosmetic formulations helps to create an environment that is hostile to microbial growth, however, data to support the conclusion that microbial growth has been inhibited may need to be generated, either through literature reference, experimental design or review of product history:
- Strong oxidizing agents (e.g. hydrogen peroxide), or strong reducing agents (e.g. thiol compounds)
- Polar organic solvents (e.g. ethyl acetate)
- Oxidising dyes (e.g. hair dyes)
- Aluminum chlorohydrate and related salts (above 25%)
- Propellant gases: where propellant gas is used to help deliver the product, microbial growth is hindered by the fall in the partial pressure of oxygen, and in certain cases by intrinsic inhibiting effect of the propellant gas
- Other substances: if supported by data
- Production conditions: Cosmetic challenge test on product formulations that are filled at a temperature above 65 degrees Celsius is not required.
- Certain types of packaging (e.g. pressurized delivery) or unit dose, provide full protection of the cosmetic formulation from contamination during use. If the product is microbiologically acceptable when marketed, it will remain so throughout its use.
- Pump dispensers, single dose units provide physical protection against contamination from the consumer use.
- Combined factors: Combinations of the factors mentioned in this International Standard can create an environment that is hostile to microbial growth or survival. These combined factors should be taken into account when determining if a product is subject to the appropriate microbiological standards regarding testing and/or product stability.
The exemption from cosmetic challenge test should be based on appropriate justification. This determination is the responsibility of the manufacturer. Data to support the conclusion that the microbiological risk is low may need to be generated, either through literature reference, experimental design, review of product history etc.
Table 1: Example of low-risk products
|pH||u 3,0||Skin peels (glycolic acid)|
|pH||W 10,0||Hair relaxers|
|Ethanol or other alcohol||W 20 %||Hair sprays, tonics, perfumes|
|Filling temperature||W 65,0 °C||Lip balms, lipsticks, cream blushes|
|Water activity (aw)||u 0,75a|
|Solvent-based products||Nail enamels|
|Oxidizing products||Hair dyes|
|Aluminium chlorohydrate||W 25 %||Anti-perspirants|
Source: ISO 29621, 2010