Microbial resistance testing: adding method and meaningful metrics to the madness

This is Part II in a series on Microbial resistance testing. Stay tuned for Part III.

In a previous post, we explored the potential of microbial resistance testing to help manufacturers of building materials and other products better understand their products’ performance in elevated moisture conditions.

For something as organic and variable as mold growth, measurement can be a challenge. Not all microbial resistance testing methods are created equal and it’s important to select testing laboratories and methods that provide the information that best meets your needs.

While many standards for microbial resistance testing exist, UL chooses to conduct testing according to ASTM method D6329. After evaluating several proposed testing methods, UL settled on this method due to its use of metrics to evaluate microbial resistance. Other standards prescribe highly subjective evaluation methods that make it difficult to objectively and quantify the intensity of mold growth.

Other methods call for the use of a control group in the form of a completely susceptible material such as a piece of untreated wood or paper, and then compare mold growth on the item tested with the mold growth on the organic material, to see which grows less. This “pass/fail” approach does little to offer measurable results for comparison purposes such as when a product’s formulation is changed.

ASTM Method D6329 calls for a three-week challenge in which a product is placed in a temperature and humidity controlled chamber and is inoculated with a known amount of mold spores and placed in the chamber at 90% humidity alongside a universally-susceptible control material. The number of mold colonies is measured at the end of three weeks and the percentage increase in mold growth is calculated on the known, susceptible material as well as the material in question. The resulting mold growth is compared using clearly-defined metrics and is calculated for the tested material and the control material.  Note that in this test, the control material is not used as pass/fail determinant, but rather to confirm that the fungal growth conditions were suitable.

The report is valuable in and of itself, but perhaps even more importantly, companies can use results to compare different products’ performance and/or to evaluate the impact of formulation changes.

When testing your products for microbial resistance, be sure to select methods that deliver information that is valuable now and in the future. In the next post, we’ll consider some example of how these results translate to business decisions for many different types of companies. For more information on microbial resistance testing, visit the UL website.