Home Stretch | Studying molds: filthy but fascinating

Black spots between the bathroom tiles, a musty smell in the basement, grey window sealants. Almost everyone has such traces of mold around the house, but mold only starts to grow and thrive under favorable temperature and climate conditions, for example in humid spaces. Using a microscope and mathematical formulas, Applied Physics PhD candidate Philip Ruijten investigated indoor mold growth in changing relative humidity.

Even though outsiders often find his field of research rather filthy sometimes – “The study of molds just isn’t that sexy,” he admits with a smile – Flemish TU/e researcher Philip Ruijten would also like to point out the positive sides. After all, the well-known beer produced in his proverbial backyard wouldn’t exist without yeast, which is a single-celled fungus. Nothing beats an Achel Trappist beer, Ruijten says, and the blue mold cheese produced in that same Belgian village is equally hard to resist.

However, mold growth in the abovementioned delicatessen is controlled, which certainly isn’t the case with indoor molds, Ruijten explains. “There are millions of mold spores in the air. That’s not necessarily a problem, but once these molds find their way into an ideal living environment, they start to grow exponentially. Think of humid, poorly ventilated spaces. It’s almost impossible to get the mold off the walls once it starts to appear. Unfortunately, this isn’t just a problem of esthetics, but a health hazard as well. Molds can cause raspatory problems and lead to serious health issues, particularly for people who suffer from asthma and other respiratory diseases.”


Most molds consist of long, white threads that produce large numbers of spores simultaneously. These spores are transported by air, and each spore can subsequently produce a new mold under favorable circumstances. Molds prefer an acidic environment and require suitable nutrients, but the most important factor influencing mold growth is water, Ruijten explains during his mini lecture on mold growth. And that’s where his research differs from the experiments described in the literature. Because whereas relative humidity is constantly kept at a high level in a lab so that molds can grow as quickly as possible, Ruijten simulates an indoor situation. “Cooking or showering rapidly leads to a high concentration of water at specific locations indoors. Because once you’ve finished cooking or showering, the relative humidity level drops back to normal. This means that relative humidity can strongly fluctuate at a microlevel. What effect does that have on molds in plaster, or on wood?”


However, Ruijten, who obtained his master’s degree in theoretical physics, didn’t start his research using plaster or wood, but glass instead. A digital USB microscope – “which enlarges up to 1000x for just a few hundred euros” – allowed him to easily capture the growth process of the three most common indoor molds in the Netherlands. He was also able to make the glass microscope slide water-repellent and extremely clean, so that the only source of water was produced by the local relative humidity. This proved to be a perfect model system to study initial mold growth with. Ruijten used plaster for the further outgrowth, a more natural situation where the molds were able to continue to spread using carbon sources that were present. He arrived at some surprising results. “We saw that a lower relative humidity level leads to the formation of stronger spores. In addition, it turned out that spores aren’t just able to survive under extreme circumstances, but that the fungal threads themselves can also survive longer in a dry environment. We were able to describe in detail how molds ‘pull the water in,’ as it were. Molds turn out to be much more dynamic than we had hitherto imagined.


During the six years Ruijten occupied himself with the study of molds, he combined knowledge from biology, chemistry and materials science – “I learn something new each day” –, however, he also used his own mathematical models. At times, that took some getting used to for the biologists in the field. “I used models to predict mold growth at certain relative humidity conditions, and I was able to test these predictions with biological experiments. Mold experts are a bit surprised at first – even though the formulas appear almost simple to a theoretical physicist – but they eventually become enthusiastic about new possibilities. A crossover that has been gaining ground.”


Even though Ruijten acquired much fundamental knowledge, he unfortunately still hasn’t found the ultimate advice that will help us get rid of that filthy mold in the shower drain. “Proper ventilation and absorbing extreme moisture are good basic rules, but mold is difficult to remove once it has appeared. We’ve certainly laid a good foundation for mold remediation, but we’re not finished with it yet.” Ruijten himself, however, has completed his studies in this field. His PhD ceremony last Friday, fittingly celebrated with a pint of beer, also marked his farewell to the world of academia. Because even though he describes his firm academic base as very valuable, the ‘born chatterbox’ has been functioning perfectly as sales and account manager for the Mathware department at high-tech company Sioux Technologies over the past two years.

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