ISO 846 vs ASTM G21: Key Differences in Microbial Testing of Plastics

Key summary

• ISO 846 is  used to assess the  deterioration of plastics due to microbial attack
• ASTM G21 assesses the fungal resistance of the plastics and polymeric materials
• ISO 846 allows broader organism selection (bacteria & fungi), while ASTM G21 test product efficacy only again fungal strains
• Both ISO 846 and ASTM G21 include qualitative assessments of microbial growth (typically visual ratings).
• The choice of standard depends on regional regulatory requirements and the intended application of the product.

AEO summary 

The choice between ISO 846 and ASTM G21 depends on regional regulatory requirements and the intended application of the product. ISO 846 goes beyond surface assessment by enabling analysis of material degradation and performance changes.  while ASTM G21 is a qualitative antifungal test that assesses fungal growth using visual ratings.

Why this comparison matters

Different industries and regions expect specific standards, so choosing between ISO 846 and ASTM G21 directly affects regulatory acceptance and market access. Just as important is the intended use of the product whether it needs long-term durability or simple antifungal resistance. The target microorganisms also play a role, since ISO 846 covers a broader range (including bacteria), while ASTM G21 is limited to fungi. Picking the right standard upfront ensures your claims are relevant, compliant, and aligned with real-world performance expectations.

 Ultimately, the choice between these tests affects overall product performance, and using the wrong method can lead to inaccurate claims, reduced lifespan and potential compliance challenges. 

What is ISO 846?

ISO 846 is an international standard developed by the International Organization for Standardization to evaluate the effect of microorganisms such as bacteria and fungi on plastics, including biodeterioration  and changes in material properties. This method has different sub methods, Method A – C, measuring different properties of plastics

ISO 846 measures:

• Microbial growth
• Changes in mass
• Changes in physical and mechanical properties
• Surface damage or discoloration

Methodology:

Method A – Fungal Growth Resistance
Determines whether fungi can grow using the plastic itself as a nutrient source.

Method B – Fungistatic Activity
Evaluates whether the material can inhibit fungal growth in the presence of nutrients.

Method C – Bacterial Resistance
Assesses how the material responds to bacterial exposure and potential colonization.

ISO 846 is applicable to:

• Non-porous Plastics
• Plastics and polymers used in construction
• Polymers used in Agriculture
• Electronic components
• consumer plastics
• Polymers used in High humidity and soil rich environments

What is ASTM G21?

ASTM G21 is a laboratory test method developed by ASTM international to evaluate the resistance of synthetic polymeric materials  to fungal growth under controlled conditions.

ASTM G21 measures

• Fungal growth on material surfaces

Methodology:

Samples are inoculated with a mixture containing several fungal species and are incubated under controlled temperature and humidity. Post incubation, growth is assessed visually over time. 

ASTM G21 is applicable to:

• Plastics and polymers like pipes, rods, tubes
• Coatings of plastic or polymers
• Marine plastics
• Sheets
• Film materials

ISO 846 vs ASTM G21: Key differences

When should you use ISO 846?

Use ISO 846 when your product:

• Needs  biodeterioration assessment
• Claims include bacteria and fungi as target organisms
• Performance depends on resistance to microbial-induced damage over time. 
• Is aligning with European/global regulatory expectations

When should you use ASTM G21?

Use ASTM G21 when:

• You want to understand  antifungal efficacy of plastics and synthetic polymers
• Products intended to be launched in US markets

What do test results mean?

Test results from ASTM G21 and ISO 846 provide valuable insights into product performance, microbial susceptibility and long-term material risks. 

For ASTM G21:

• Results are interpreted on a visual fungal growth rating scale (0–4)
• 0–1 = good resistance (little to no growth)
• 2–4 = poor resistance (increasing fungal coverage)

For ISO 846:

• Results start with visual growth rating (similar concept)
• But go further into material impact, such as mechanical or physical property changes

Why choose MIS

Whether you need rapid fungal resistance screening through ASTM G21 or a deeper, performance-based evaluation using ISO 846, MIS brings proven expertise in both standards. We offer end-to-end microbial testing solutions backed by industry-aligned protocols. 

If you are looking to validate material durability, strengthen product claims and reduce risk, contact our experts now.

FAQs

Is ISO 846 better than ASTM G21?

It is important to understand both standards serve different purposes.ISO 846 is more comprehensive as it evaluates impact of microbial attack  on material performance, while ASTM G21 is ideal for antifungal resistance screening.

Can ASTM G21 be used to detect antibacterial efficacy?

No, ASTM G21 is primarily used to assess fungal resistance of plastics and polymers.

Does ISO 846 measure biodegradation?

No, ISO 846 does not measure biodegradation.

Which test should I choose?

The right standard depends on regional regulatory requirements and intended application. Choose ASTM G21 for a  fungal resistance check and pick up ISO 846 to check product performance against both bacteria and fungi and biodeterioration analysis.