ASTM G21 – Everything You Need to Know

Standard Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi

The ASTM G21 is a qualitative test that employs a high concentration of spores, from five different fungal species, to determine the resistance of synthetic polymeric materials to fungal growth.

Background on the ASTM G21

Fungi are generally not able to breakdown the main resin component of synthetic polymer to use as a source of carbon (sugar). However, many additives, such as plasticizers, cellulose, lubricants, stabilizers and colorants that are added to impart desirable characterises, are very susceptible to microbial growth.

Once fungi become established by breaking down the easily digested ingredients, the acids they produce as by-products of growth breakdown the resin components into a more useable food source.

A good analogy is how lighter fluid is used to successfully set charcoal on fire. The additives act as a quick burst of energy that unlocks the stored power in the polymer resin.

The ASTM G21 was first released in 1990 as an official method to test the resistance of synthetic polymers to fungal attack.

The current standard (ASTM G21-15) is the sixth version of this test method. Previous versions are listed below:

ASTM G21-13
ASTM G21-09
ASTM G21-96(2002)
ASTM G21-96
ASTM G21-90

Examples of polymers tested using the ASTM G21 include PE, PP, PU, PVC, TPU, TPO, PES, ABS, PC, rubber, etc.  They may be in the form of films, sheets, molded materials, coatings, foam, tubes, pipes, pucks, tiles, plaque, rods, etc.

Other materials, such as paint, dry wall, cardboard, fibers and adhesives may also be tested using the ASTM G21 method.

ASTM G21 Test Method

Test samples are placed on an agar medium that is carbon-free (no readily available energy source) but contains the basic mineral salts required for fungi to grow. Having a carbon-free medium forces the fungi to grow on the test specimen and not on the surrounding agar surface.

After being placed on the mineral salts medium, the test sample is inoculated with a concentrated spore suspension (inoculum) that contains five different fungal species.

ASTM G21 Fungi

  • Aspergillus niger
  • Aureobasidium pullulans
  • Chaetomium globosum
  • Trichoderma virens (formerly known as Gliocladium virens)
  • Penicillium funiculosum (formerly known as Penicillium pinophilum)
Aspergillus niger
Aspergillus niger


Aureobasidium pullulans
Aureobasidium pullulans


Chaetomium globosum
Chaetomium globosum


Penicillium funiculosum
Penicillium funiculosum


Trichoderma virens
Trichoderma virens


These fungi were chosen by the ASTM based on their affinity for using plasticisers, cellulose, lubricants, stabilizers and colorants as food sources.

Test plates are incubated at 28C at 90% relative humidity for 28 days to allow any possible fungal growth to develop. The test samples are examined every seven days to monitor progress.

ASTM G21 Test Assessment

The samples are examined under a dissecting microscope at 40x magnification. A rating score from 0 to 4 is given based on the amount of growth that exists. The description of the rating system is as follows:

0 = Specimen remained free of fungal growth.
1 = Traces of growth on specimen (less than 10%).
2 = Light fungal growth on specimen (10 to 30%).
3 = Medium fungal growth on specimen (30 to 60%).
4 = Heavy fungal growth on specimen (60% to complete coverage)

The ASTM G21 does not outline pass/fail criteria. However, the test method may be terminated earlier for specimens that are experiencing a rating of “2” or higher.

ASTM G21 Untreated vs Treated Test Specimens

The untreated vinyl sample supports heavy amounts of fungal growth (rating of “4”).  The vinyl sample, treated with an antimicrobial agent, remains free of fungal growth (rating of “0”) after 28 days of incubation, as per the ASTM G21 test specifications.

Size of Test Specimens

The desired test specimen size is a 50 mm/2-inch square, 50 mm/2-inch diameter circle or a piece that is at least 76 mm/3-inches in length. Before submitting samples for the first time, contact your chosen test lab for more information and guidance.

Strengths and Weaknesses of the Test Method

ASTM G21 Strengths:

  • Since all the requirements for growth are present (minerals, temperature, humidity, high concentration of spores) except for a readily available energy source, the test specimen’s true capability of resisting fungal attack is evaluated.
  • The ASTM G21 includes several fungi, selected from a broad range of environments.  These fungal organisms have distinct nutrient preferences and utilize different enzymatic pathways. As a result, test samples are exposed to several different modes of attack while using one test method.  Conversely, many other test methods utilize only one test organism.  This reduces the number of possible ways a test sample can be attacked and may mask the sample’s true potential susceptibility to fungal growth.

ASTM G21 Weaknesses:

  • As the scoring guideline is quite subjective, the test results are susceptible to a wide range of interpretation. What may be assessed as 25% coverage (rating of “2”) by one technician, may easily be considered 35% coverage (rating of “3”) by another.
  • The inoculum is prepared by using the five indicated fungal species grown separately on nutrient-rich agar plates. As the spores are harvested, an inexperienced technician may accidentally introduce trace amounts of nutrients into the freshly prepared inoculum, introducing the chance for fungal growth that otherwise may not occur.
  • Some materials are exposed to very high amounts of carbon/sugar during their useful life. As such, the sample’s true ability to withstand fungal attack may not be well reflected using the ASTM G21.
  • The method may be modified by including a nutrient source, if this variation is made clear in the final test report. It is important to discuss the end-use environment with the testing lab to ensure relevant test conditions are employed.

Cost to Test Samples Using the ASTM G21:

The cost for testing specimens using third party laboratories varies widely from lab to lab.

In North America, there are test facilities that charge as low as $175 USD per sample and go up as high as $530 USD for the first sample ($340 USD for each additional sample).


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