Listen to The Mould Show Podcast on ITunes

Fungal Growth in Carpets - Should You Be Worried ?

Uncategorized Mar 22, 2020

Hello and welcome to this week's live stream. My name is Dr. Cameron Jones and I'm an environmental microbiologist. When I was thinking this week about what I wanted to present, obviously we are being inundated with news stories and factual information about COVID-19 and the pandemic spread of this throughout the world and in our own communities. So I'm going to be going back to what we do week in, week out, which is focusing on mould. And towards the end of this show, as part of our breaking news, I am going to be going over some of the breaking news information about COVID-19 and the potential correlation with fungal mediated pneumonias. But that's for later in this show.

So the first ... or the area that I'm going to be focusing on this week is fungal growth in carpets. And I thought that this was an important topic to focus attention on for a couple of reasons. The couple of inspections that I have done, it's very interesting how these inspections tend to cluster together. The last three that I have done personally, the carpet looks like it is the dominant reservoir of mould in these individuals' homes, which is contributing to their adverse health symptomology.

Figure 1. Colonies of Aspergillus fumigatus on a carpet sample tested using a RODAC plate. 

So I thought what better than to focus on this particular area of science and look into the extent of the problem? So some of you are probably thinking with COVID-19 out there, is the carpet a potential contamination with fungi really a problem? Well I think it is, if we get some statistics from 2020 from Suncorp, they state that roughly 17% of all new Suncorp insurance claims are related to water damage. And obviously a lot of these are going to impact on these porous surfaces, carpets.

Before we move into the practicalities of carpets and how they become contaminated, what they're contaminated with, and how we actually move towards solutions to clean them more effectively to reduce the infection or allergen risks inside our homes, I want to sort of review some of the key facts and figures around asthma because asthma is one of those dominant adverse respiratory complaints, which is often associated with exposure to mould and water damaged buildings.

And so if we look some of the statistics we find that the asthma costs in Australia back in 2015 was $28 billion per annum. The comparative costings in the United States was pegged at $80 billion per year. And again, all of these references are going to be provided in the show notes and also at the bottom of this livestream after the livestream. So you'll be able to follow along and look this up and verify these statistics for yourself and also share this to other interested persons.

Now indoor exposure within the home or office is considered to be the number one cause of asthma, since we spend 90% of our time indoors. And some studies suggest that up to 44% of diagnosed asthma is due to exposure to something in the built environment. So it's not always mould. It certainly is something though. And asthma always has more than one trigger, as we certainly know, anyone who watches the show every week.

However, what we do know is that 35% of asthma cases are thought to be caused by exposure to dampness and mould. And that is quite a concerning percentage, 35%. And the total annual costs attributable to mould and dampness, at least in the United States, is considered to be $22.4 billion a year. Of course, we know that that leads to lower quality of life, unexpected and unwanted medical costs, and reduced productivity in the workforce or as part of studying.

So this is the backdrop to today's show. Obviously the topic is on carpet. And I chose this not only because, as I said, the last few inspections I've done have clustered together with carpet being the dominant reservoir of mould and these individuals homes. But also a beautiful paper came out in the Journal of the Built Environment called Morphology And Quantification of Fungal Growth in Residential Dust and Carpets. So this is a recent paper. We're going to be focusing on what this paper says and how we can use this information to effectively decontaminate or clean our own homes more effectively.

So before we jump into what the research says, I want to review the three types of carpets that are available in the marketplace and their comparative costs, because the cost of the carpet is related to the type of quality of carpet. And these different carpets, as you're going to see when we start discussing the research, has a profound impact on their ability support microbial growth, and retain this microbial growth and even resuspend this into the airspace.

So the first type of carpet, the number one type is nylon carpet. And in Australia it ranges anywhere from $125 right up to $300 essentially per meter. And in carpet laying, this is called broadloom meter. Whereas wool carpet ranges from $140 to $500. It's much more expensive. And polypropylene carpet, another type of carpet is really the most inexpensive and ranges from $65 to $180. And polypropylene carpet is also called by the trade name Olefin. And this is the generic name for this type of carpet.

And the research I'm going to be talking to you about, they call the polypropylene carpet Olefin carpet and just bear in mind that this is the least expensive carpet and is certainly are often found in rented premises more often than wool or nylon carpets, mainly due to the fact that it is cheaper and more ... It's cheaper late and purchase. So the source for all this information was from Choice Magazine.

Now before I jump into the 2020 research again, what's really good about science is often academics tend to write multiple papers on the same topic. And the 2020 paper was written by a group of authors who wrote an earlier paper that came out in 2016 in the journal Indoor Air. And we're going to review that one first because that sets the stage for what I want to talk to you about today. And so they were looking at the relationship to floor dust. That is in a sense a marker for how dirty the carpet was. And they were investigating a real home which had worn medium pile nylon carpet.

And what they did is they cut out samples of the carpet and they incubated it at different humidity levels called ERH, which is environmental relative humidity. Going to talk to you a little bit about this. And I'd urge you to look up the difference between environmental relative humidity, relative humidity, and water activity. So there's a lot of great Wikipedia links out there that you can understand this area. But essentially it is the amount of water vapor available at a particular temperature, and how this then relates to the growth of microorganisms.

So again, they cut out squares of carpet and they incubated them at 50, 80, 85, 90, 95, and 100% ERH levels. And they then tested these squares of carpet using PCR, which stands for polymerase chain reaction and DNA sequencing of ribosomal DNA for bacteria and fungi. And they were interested in the total concentration of microbes but also any community shifts that were observed in the carpets.

Now after one week, fungi grew only when the ERH was above 80%. And again this is commonsensical. The higher the water activity, the greater the probability that microorganisms are going to be able to use that host material to grow and flourish. But interestingly, bacteria only grew when the ERH was at 100%. And again this is for nylon carpet. So the take home message was that the higher the humidity, the more fungal taxonomic groups were seen. And under PCR analysis they found over a hundred different types of fungi present in the dust within the carpet. And that's quite a wide and large amount of different microbes present.

So the conclusion, and this is the important feature from their 2016 paper, was that 50% of the airborne microbes caused by fungal resuspension from carpet. So it's not just what is in an air space and enters the home, but the resuspension factor when people walk around their essentially dirty carpet and the amount of moisture that fluctuates throughout the year has a huge impact on the micro flora that is found within our homes. And this is the real takeaway from this first paper.

So what did the latest paper, which has just come out in building and environment. These authors now looked at a range of different environmental conditions. They looked at this ERH, equilibrium relative humidity, at now lower percentages starting at 50, 85, 95, and 100. But they looked at a greater range of carpets, which spanned the nylon, the polypropylene or Olefin, as well as wool. And they wanted to see how those three core types of carpet affected fungal colonization. But they also wanted to see if the amount of dust had an impact as well.

And this is a beautiful paper and I've provided the reference down at the bottom. I urge you to download this paper and look at least at the scanning electron micrographs yourself because they put it in very clear the impact that carpet has within your home as a reservoir of mould. And so if we look at a typical scanning electron microscope image of the fibers of this carpet, when it's been incubated at 25 degrees Celsius at a ERH of 95% you can see a whole lot of mould spores at this magnification. And for those that are listening to the podcast version of this later, take my word for it, there's a lot of mould spores present growing in an on the fibers of the carpet.

The next few slides I want to show is almost a visual overview of what the paper is saying. And again for the podcast listeners, this'll be a little bit more difficult to follow for these next few slides. But there is a significant observation under scanning electron microscopy, which is an excellent way of getting a beautiful zoomed in view of exactly what's happening at the micron and even down to the nanoscale. But this is at the micron scale. And we can see that at a 50% humidity level, you can see on the fibers that there are dust particles and microbes like fungi and their hyphae, this is the the way in which fungi reproduce and move. So they reproduce from the spores but they ramify across space seizing hyphae. And you can see that as the humidity increases, you have an increasing amount of microbial growth occurring on the fibers and that at 95% you are then getting a reproductive capacity at the fungal level and you are seeing the production of large volumes of spores down inside your carpets.

And this is quite a amazing phenomena to visualize and this is the key component that makes this 2020 paper so important. And again, they were able to resolve using SEM that the fungal spores were also growing and using the dust, and that these dust particles and flakes that are found within the fibers act to provide additional surfaces for the mould spores to grow and reproduce on. And this is just really quite fascinating to visualize up close. However, let's look at something that you can take home and use now. So there are two types of sub experiments that were done in this paper. And they were looking at the amount of microorganisms or the fungal composition of these three different types of carpet.

And the black bars in the graph represent the polypropylene or olefin type carpet that is the cheapest type of carpet. Then we have the nylon carpet, which is a little bit more expensive. Then we have the wool carpet, which is the most expensive and in a sense is potentially most highly valued due to its thermal properties and the fact that it is wool. And everyone likes to have a wool carpet rather than a less soft in many cases, nylon carpet or olefin carpet. And what they were looking at was the concentration of fungal microorganisms. And you can quickly see in the left hand graph that the nylon carpet actually supported the highest levels of microorganisms when the dust within the carpet was unaltered. Whereas the graph on the right hand side, when it says that the dust was autoclaved, that means that any microorganisms that were found growing on the dust, and as you can see if we go back to the previous micrograph, you can see that the dust supports huge numbers of spores.

So when they actually autoclaved the dust to kill it and then looked at the ability of the spores to grow within the carpets, we quickly see a new distribution here with the wool fibers supporting the highest levels of fungi. The next highest levels were found in nylon fibers. And the lowest levels of fungi were found in the olefin of polypropylene carpet. This is really interesting and important information, which could tip the balance about what type of carpet you elect to either put in or to install or take out of your home, or if you're putting in new carpet, which one you should purchase.

So, the conclusions from the 2020 paper were, number one, the higher the moisture content, the greater the amount of fungal growth. Number two, the higher the level of dust, the greater the amount of fungal growth. Pretty easy, isn't it? The third take home was that the olefin carpet fibers, that is the polypropylene carpet, showed the least amount of fungal growth compared to nylon or wool. And the key conclusion was that allergy sensitive people should consider solid floors over carpet since dust removal is much easier and cleaning is more effective of hard flooring in comparison to carpet.

Which leads me onto the next point, just because we now know that there are differences in the ability of carpets to support fungal growth, how are we going to effectively decontaminate these carpets? And so that means I need to review a paper that we looked at earlier in the year, or last year back in 2019, which reviewed the suggestions for decontaminating carpet. And again, I put the reference up here. And these suggestions and the conclusions from this paper were that three different types of cleaning methods were applied to experimentally contaminated carpets.

And what the authors did is they artificially contaminated the carpets with a fungus called Microsporum camis spores and hairs. And they did this to the carpet, as I said, and then they looked at three different cleaning methods. But they first vacuumed the carpet for either 10, 30, or 60 seconds, then cultured the carpet samples out. And they discovered that they then applied these swatches of carpet to three different ways of cleaning it.

The first one is a beater brush carpet shampooing. The second one is a beater brush carpet shampooing plus the addition of a disinfectant. And the third method is hot water carpet extraction, not to be confused with steam cleaning. But since steam cleaning is very difficult to achieve at scale, most robust applications for domestic and industrial environments use hot water carpet extraction to remove impurities and debris from carpets. So these are the three standard ways of carpet cleaning and the authors also used a particular type of disinfectant that was of their choosing as well. And the carpets were cultured at 24 hours, 78 hours, and seven days after cleaning.

And what do you think they found? They found that after vacuuming, all carpets tested positive for this particular fungal pathogen at over 300 colony forming units. And all of you should be familiar with colony forming units. Colony forming units are the number of discrete fungal colonies that are able to grow on a Petri plate when it's examined. And so simply cleaning your carpets with a vacuum is inefficient at removing fungi. However, it does remove the hairs. But if you still have fungi prison, vacuuming is not very effective.

The use of disinfectants was associated with odor, even when dry, and unfortunately resulted in permanent discoloration of the carpet. So the authors concluded that the use of disinfectants with beater brush shampooing is not a good idea because it does not ... Well, it leaves a very poor result at the end.

Now let's have a look at what the conclusions were from this publication. And I'll jump right to the key conclusion. Hot water carpet extraction is the most effective at removing fungal microorganisms, reducing the population of colony forming units to a very low level after this cleaning method was applied. So, the written conclusion was that hot water carpet extraction cleaning was the best at reducing levels of this particular fungal contaminant from 300 CFU down to 5.5. And this was stable at 24 and 48 hours post cleaning. And hot water carpet extraction cleaning did not discolor the carpet. The next best wash method was two washings with an upright beater brush carpet shampooer. So there you have it, this is the solution to how you move to cleaning your carpet from mould spores. And this is something which I urge you to consider if you want to modify your living environment to minimize your allergen risk.

In any case, we are now up to the breaking news segment, which we started in 2020, and it is becoming increasingly important, especially as we have these enormous health threats such as COVID-19. In any case, I'm going to be reviewing a paper which is available online. It can certainly be purchased and the abstract is freely available. It has just been published and it's been published in a journal called Cell Host and Microbe. It's coming out in April 2020 and it focuses on how mould damages the lung. It is a complex read. There is a lot of specific vocabulary and I'm going to try to summarize the key points that have been made in this paper. But why this is an important publication is that really with a great degree of elegance, the paper demonstrates that mould in fact damages the lung. And this is how it damages the lung. And this is why mould and water damage exposure needs to be taken extremely seriously.

And so what the authors discovered was that an allergan called an inhaled fungal protease actually damages what's called the junctions of the bronchiolar club cells. And I have a schematic in a few minutes, which I'm going to put up on the screen. But just bear with me now as I go through this. Now these particular junctions, think of those as the branching pathways in the bronchioles, which lead to finer and finer airways within the lung. Now, injury to the junction triggers something called allergic inflammation. And this is detected in the lungs by something called the mechanosensor, which is called TRPV4. And this is able to sense that there is a change in the ability of these bronchiales to actually inflate with air. And so this allergic inflammation is the key problem with the mould challenge. And so when TRPV4, the mechanosensor, this is sufficient to cause inflammation in mice. And this paper shows that it is the same mechanosensor which is acting in human lung cells.

And so, this is a fundamental and very important paper which inextricably links exposure to mould with a very serious health outcomes, adverse health outcomes in humans. And so there is something to do with calcium signaling. Again, it's too complicated in an area to get into how calcium regulates cell metabolism and the ability of the cell to exchange information and nutrients with the cytoplasm and the wider cell matrix it finds itself in. But there is a mediation pathway with calcium which also initiates further allergic lung inflammation.

And this is the key publication which I urge you to download. And reference and cite when you are attempting to explain to other individuals why exposure to water damage and mould is so significant and is such a trigger for potential adverse health in your own body or family or work or living environment. And the beautiful schematic that the oldest have prepared in this publication looks at how these bronchial junction cells are mediated with very different proteases and how this leads to an allergic inflammation reaction. And again I urge you to download this publication, as I said, and use this to help others become aware at the very real threat posed by mould in our environment, especially when it's present at concentrations which are much higher than normal or expected.

Now, the next publication I want to talk about is focusing on COVID-19 and this came out in Travel Medicine and Infectious Disease. This paper is actually a couple of weeks old. But why I've highlighted this is I considered that this is definitely worth downloading and reading carefully. You can get this on Pub Med. There are various different servers which provide access to this journal. And I really want to ... I've highlighted in red one of the key take-homes of this particular paper and why I'm going to link this to something which also just come out in the literature in the last couple of days. And we all know that COVID-19 is a very real and present a threat to all of us. And that the unfortunate reality is that many individuals, especially with preexisting immune compromise are possibly likely to need intensive care hospitalization.

And the statistics are very concerning because this has a case fatality rate of approximately 13% for people with COVID-19 who underwent a hospitalization when there was the presence of a preexisting factor which made them more susceptible to a respiratory problem. But the next thing I want to focus on is that being admitted to intensive care is a risk factor in itself because before COVID-19, just entering the ICU results in average mortality rates of between 8 and 19%. And why this is important is this new paper which just came out. Well actually, it came out at the end of last year.

But why I wanted to link this to the COVID-19 results because there is this respiratory illness called ARDS, which is Acute Respiratory Distress Syndrome and it is increasingly being seen in COVID-19 patients and it is a severe pulmonary infection. And there has been a lot of work by epidemiologists and microbiologists and the medical community focusing on this ARDS or this Acute Respiratory Distress Syndrome and what other infections appear to complicate the issue, especially with regard to admission to ICU and complications that could be encountered that need to be looked at and carefully screened for at the hospital.

And the number one complication is infection with aspergillosis. So when individuals have this ARDS, or Acute Respiratory Distress Syndrome, the potential connection with aspergillosis is very high. And the publications as recently as 2020, but also this particular one that I've put up from 2019 and has come out, states that because there are rising reports of influenza associated pulmonary aspergillosis being ... patients being admitted to ICU, these are in individuals that were not considered at risk of fungal infection. So anyone with a pneumonia even caused by the virus COVID-19, there needs to be an increased clinical awareness of the potential infection with other microorganisms, especially aspergillosis challenged microorganisms. Anyway, there's going to be a lot more appearing in the literature over the coming months on this association I'm sure. And that's really the take-home that I wanted to leave you with.

The issue of aspergillosis, whenever there is a concern about ARDS, clinicians and microbiologists pathologists should certainly be on the lookout for aspergillosis as a coinfection. Now, obviously you're probably watching this on Facebook Live. As I said, we always put all of these shows in video format onto YouTube and we also save these out as an audio file only and put them up onto iTunes and Spotify. We have a dedicated website for all of the live streams that we've done right back to last year at

So, for those of you who have not subscribed, I'd certainly ask you to jump onto iTunes or to Spotify and to The Mould Show or at We put a lot of work into these live streams each week and it's part of my role and responsibility as a public health advocate to bring you this type of information. And I enjoy doing it, but it also is good for us and good feedback for us if you actually subscribe and do more than just maybe leaving a like on our Facebook page.

So I wanted to put this up here. I rarely put a final screens on any of our live streams, but I'm doing it today because I do want to focus on the fact that there is a podcast which is associated with this live stream. In any case, my name is Doctor Cameron Jones. I'm an environmental microbiologist. I wish everyone good health in the coming days and weeks. And be extremely mindful of all the public health warnings and suggestions which are out there on our televisions, radio, and across the internet. In any case, bye for now, and I'll see you next week. Bye.

YouTube Link:


Dannemiller K, Weschler C, Peccia J. Fungal and bacterial growth in floor dust at elevated relative humidity levels. Indoor Air. 2016;27(2):354-363. doi:10.1111/ina.12313.

Nastasi N, Haines S, Xu L et al. Morphology and quantification of fungal growth in residential dust and carpets. Build Environ. 2020;174:106774. doi:10.1016/j.buildenv.2020.106774

Wiesner D, Merkhofer R, Ober C et al. Club Cell TRPV4 Serves as a Damage Sensor Driving Lung Allergic Inflammation. Cell Host Microbe. 2020. doi:10.1016/j.chom.2020.02.006

Moriello K. Decontamination of carpet exposed to Microsporum canis hairs and spores. J Feline Med Surg. 2016;19(4):435-439. doi:10.1177/1098612x16634390

Tursynbek Nurmagambetov, Robin Kuwahara, Paul Garbe. The Economic Burden of Asthma in the United States, 2008 - 2013. Annals of the American Thoracic Society, 2018; DOI: 10.1513/AnnalsATS.201703-259OC

The National Asthma Council Australia. Published 2020. Accessed March 18, 2020.

Klepeis, N., Nelson, W., Ott, W., Robinson, J., Tsang, A., Switzer, P., et al. (2001). The National Human Activity Pattern Survey (NHAPS): A resource for assessing exposure to environmental pollutants. Journal of Exposure Analysis and Environmental Epidemiology, 11(3). LBNL Report #: LBNL-47713. Retrieved from

Bruce P. Lanphear, Robert S. Kahn, Omer Berger, Peggy Auinger, Steven M. Bortnick and Ramzi W. Nahhas. Contribution of Residential Exposures to Asthma in US Children and Adolescents. Pediatrics June 2001, 107 (6) e98; DOI:

Mudarri D. Valuing the Economic Costs of Allergic Rhinitis, Acute Bronchitis, and Asthma from Exposure to Indoor Dampness and Mold in the US. J Environ Public Health. 2016;2016:1-12. doi:10.1155/2016/2386596

Rodriguez-Morales AJ, Cardona-Ospina JA, Gutiérrez-Ocampo Estefaní, Villamizar-Peña R, Holguin-Rivera Y, Escalera-Antezana JP, Alvarado-Arnez LE, Bonilla-Aldana DK, Franco-Paredes C, Henao-Martinez AndréF, Paniz-Mondolfi A, Lagos-Grisales GJ, Ramírez-Vallejo E, Suárez JA, Zambrano LI, Villamil-Gómez WE, Balbin-Ramon GJ, Rabaan AA, Harapan H, Dhama K, Nishiura H, Kataoka H, Ahmad T, Sah R, On behalf of the Latin American Network of Coronavirus Disease 2019-COVID-19 Research (LANCOVID-19), Clinical, laboratory and imaging features of COVID-19: A systematic review and meta-analysis, Travel Medicine and Infectious Disease (2020), doi:


50% Complete

Get Your FREE Housing Mould Hazard Check

In less than 5 minutes you'll know if you need to take action about your water damage or mould concerns.