ISO 846

Plastic Resistance to Microbial Growth

ISO 846 method B evaluates whether a treated material can inhibit fungal growth when fungi are supplied with everything they need to grow.

The ISO 846 procedure

ISO 846 method B is performed by exposing samples to fungal spores on an agar medium that contains a full nutrient source, then visually assessing whether fungal growth is inhibited on or around the samples.

1. sample preparation & Incubation

Test pieces (antifungal treated and untreated) are placed onto a complete nutrient medium that supports fungal growth, then inoculated with test fungi. The inoculated plates are then incubated under warm, high-humidity conditions for 4 weeks.

2. Determining ANTIFUNGAL Efficacy

At the end of incubation, the surfaces of the samples and the surrounding agar are examined for fungal growth. Any clear zones in the agar (zones of inhibition) or reduced growth directly on the plastic surface, compared with an untreated control, indicate fungistatic activity or the effect of an incorporated fungicidal treatment.

3. ASSESSMENT of FUNGAL INHIBITION

Results are reported using a growth rating based on the percentage of the sample surfaces covered by fungi. By comparing treated and untreated specimens, ISO 846 method B indicates whether the material resists fungal attack or actively suppresses growth due to added biocides.

Test organisms and applications

ISO 846 specifies an inoculum of mould that is known to colonise and damage polymers, such as Aspergillus. The method is widely used for plastics, coatings, sealants and other materials where resistance to mould growth is important.

Things to note 

Credibility of result

ISO 846 method B is performed under controlled laboratory conditions that maximise the opportunity for fungal growth, which may not fully represent all real-life exposure scenarios. However, by giving fungi an ideal nutrient medium, the test provides a stringent and reproducible way to see whether a material can inhibit growth when conditions strongly favour colonisation.

In real environments, factors such as fluctuating humidity, limited nutrients, cleaning regimes and UV exposure can alter mould growth compared with laboratory tests. Method B offers a comparative tool for product development and claims support, especially when combined with other test methods or field data.

POSSIBLE MODIFICATIONS

The ISO 846 method allows modifications to reflect specific end-use conditions, provided they are clearly documented in the test report. Common variations include changing the fungal inoculum (e.g. adding product-relevant species), adjusting incubation time or temperature or altering the nutrient medium to simulate particular environments. If you are interested in modifying the test to better suit your end goal, please let us know and we will aim to accommodate this.

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