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How you should interpret your indoor air quality when it says that mould spore levels are greater than 1000 CFU?

Uncategorized Apr 16, 2020

Hello, and my name is Dr. Cameron Jones, and I'm an environmental microbiologist. Thank you for joining me, which could be your second join this morning, because the internet went out. That was entirely unexpected. I apologize for that. In any case, as I said, we're going to be starting today talking about why indoor mould levels above a particular threshold have an impact on your health? The reason is that, in many cases, in order to quantify what the impact of water damage or adverse indoor air quality is, we use these Petri plates to work out what can grow. But not only that, they give you the number of colonies that actually grow, so when these are placed into suspect environments and a pump is used to deliver air onto the top surface of the Petri plate, once these are placed in the incubator, you get to see exactly what colonies are going to grow. But importantly, you count up the numbers, and from the numbers, you can work out the number of colony-forming units per cubic meter of air (CFU per m3), and that is going to be the topic of today's livestream.

I'm going to be going through why these thresholds are important, because in many cases, certainly when people call us out to do indoor air quality and we tell them that we use Petri plates, they want to know, firstly, whether from the results we're going to be able to tell them whether their property is safe for them to continue living in. Or if it's a workplace environment, like this one, you'll want to know that, especially after some sort of adverse environmental threat, like a flood and unexpected water damage, that it's safe to continue working there. Most people ask me, "Do these results allow you to make some sort of determination on the occupancy or the habitability of the building?" I resolutely always answer, "Yes."

However, some people who I speak with, say to me, "Well, we're not very happy about the fact that a single numerical threshold allows me to decide on the habitability or not of a particular building." So today when I was working about what I was going to talk about on this livestream, I wanted to focus on some of the literature which discusses this very topic.

Because it's not just my interpretation of the peer-reviewed literature and the industry documents regarding indoor air quality and the thresholds that you can use, because at the end of the day everyone wants to know whether or not their living environment is in fact safe. So when I was looking at the literature that's been published in the last couple of weeks, I came upon this excellent publication. I'm going to use this as the example on which we're going to build today's livestream topic.

This paper came out on February 21, 2020, and it was talking about the relationship of people in a water damaged building and something called mitochondrial antibodies. I touched on this a couple of weeks ago, because obviously many people are very concerned about the health impacts of working or living in a water damaged building. It turns out that there are these mitochondrial antibodies which actually damage the ability of cells to respire, and that's very strongly linked with exposure to water damage. But in this publication, not only in the introduction but in the introduction to the abstract, they state that airborne levels of mould above 150 to a thousand spores per cubic meter are sufficient to cause a wide range of deleterious human health problems. I'm just putting it up here to say that that is the premise, because obviously people who don't like numerical thresholds often want to deny the validity of these Petri plates being capable of providing a lot of valuable information about exposure to moulds in the built environment.

Now, today's topic, as I said, is whether or not thresholds can be used. An outstanding paper on this topic appeared in the journal, Indoor Air, at the end of 2019. I've got the URL up here, and what I'm going to be talking about is the fact that the premise of their article, because this French research really dives really deeply into this topic about thresholds. In fact, they provide three different ways to interpret indoor air quality using colony-forming unit counts. Later in this live stream, I'm going to compare this French research with the thresholds used in the Australian mould guideline, because you can see and you will discover if you stick around long enough in this livestream, that the Australian mould guideline has done an outstanding job of establishing benchmarks or thresholds at which you should be taking indoor mould very seriously. Very interestingly in a positive way, you'll see that the French research is also coming to the same conclusions in drawing on a lot of other literature.

But what this French research is doing is they've been comparing the mould levels to particular medical illnesses, so this is very important, because in many cases people always ask me, "Is there a correlation then between your Petri plate results and the potential or the probability of having an adverse medical or group of symptoms which could be linked to exposure to mould?" This French research that I've got up here is saying, yes, in fact it can. And again, their premise to their paper is that many different countries have tried to define guidelines to quantify what levels of fungi are considered as appropriate for housing, so this is a really good premise on which to build a paper. Also, they talk about this concept of an unworthy dwelling or an unfit for habitation building, and this is very important wording, because it connects the colony-forming unit levels with housing habitability.

So what do these scientists also state about this whole concept of using colony-forming units from Petri plates, or you can do it from the spore counts using spore counting. Without thresholds and interpretation guidelines, analyzing the domestic environment is a pure waste of time and money, and I've highlighted this because I think that this is fundamental. It is nearly pointless to have someone come into your property to do an indoor air quality and mould inspection without collecting some relevant metrics. And, yes, I'm a scientist and so I show some bias towards the quantitative evaluation of the indoor built environment to see what that data shows, but nevertheless, I think that visual inspections alone cannot possibly quantify the potential adverse health connection with the mould levels. That is also what these French scientists have said as well.

Now, let's get into threshold limit values, because certainly many people quote the World Health Organization. I've got it up here on screen, and this document came out in 2009. This is an excellent document, but the World Health Organization do discuss threshold limits, but in a sense they have failed to clearly articulate a numerical level, because they too are waiting for the connection between the threshold levels and some medically relevant data. I think that this French paper that came out last year provides that and does definitely provide a solid platform on which to connect colony-forming unit counts and the potential for adverse health. So they haven't, in the World Health Organization, given specific thresholds, but you will note on page 40 to 41 of this, they state that, and I'll read it to you, "For example, a limit valley of 500 colony-forming units per cubic meter for airborne fungal spores in indoor air in urban areas in winter is used in Finland. This value was derived by comparing buildings with and without moisture problems."

And importantly, and I've highlighted it in red, "Concentrations below 500 colony-forming units per cubic meter of air were found to be typical in Finnish buildings without moisture problems." The title of this talk is "1,000 Colony Forming Units." Well, the World Health Organization (WHO) has stated that buildings that don't have moisture problems have colony-forming unit counts of less than 500, and moving on from page 40 and 41 of the World Health Organization, you get into the appendices of that document, and there are quite a lot of references to 500 and 500 to a thousand. So I think that this is a valid jumping off point, and this is why also the French researchers have also quoted from the World Health Organization threshold limit discussion. But remember, it's just a discussion and essentially quite a poorly worded statement by the World Health Organization.

Now, let's move on to other interpretation criteria. Again, I've put the title of the paper and the URL up for you. I suggest that you download this paper, go on to PubMed, read it. Here is the publication. I've had a really good time going through this and highlighting all the relevant facts that I wanted to discuss today, and what they're talking about is essentially three different criteria that they use to assess dwellings that have known occupants who have been exposed to mould and known health symptoms, and then they compare this to a group of individuals that have no allergy. This is really, really great research, because it does provide a very good benchmark from which these thresholds jump out, and the clinicians can use these to make decisions about, for example, asthma and whether that is linked to the building environment.

The first criteria, again, is called the UHB criteria, and this was developed at a particular hospital in France. The second method follows the Portuguese decree, and this is in fact a law that specifies the amount of colony-forming units of mould that dictates whether a building is safe or unsafe. Then we have the French National Agency for Environmental Health, and in a sense they're a little bit like our Department of Health here in Australia, and they have further recommendations for colony-forming unit counts and connecting this with habitability, as these French researchers have done. Now let's jump straight into the data and see what it says.

Well, I'm talking about thresholds and many of you may understand this, and for those who don't, a threshold in a sense provides a numerical level that can be used like a risk consent status matrix. Think of this as like traffic lights. For example, a building either shows low or normal levels of mould. A building after water damage may show elevated or even normal levels, but maybe high or extremely high levels of mould. These are the sort of bands that we want to understand a building or on a room by room basis to fine grain understand exposure history within the built environment. Looking at the first UHB criteria that the French use for the air, they define low as having between zero and 170 colony-forming units per cubic meter.

A little bit later, I'll talk to you a little bit about how you move from the raw count of colonies on a Petri plate to colony-forming unit counts. But it's certainly covered well in the literature that is provided by the manufacturers of the pumps, and it is connected with the number of holes and the volume of air and the time of exposure to the Petri plates to the air, so there's a rich history of how you move between the number of Petri plates on the plate and the number of colony-forming units per cubic meter of air. And as you can see, you get this type of traffic like banding, between low, middle, high, and very high. I've just put up some Petri plates that have grown from air samples over a period of time to just demonstrate the point that, over time, the Petri plates grow, to some extent, what is present in the air. This represents the viable component of the air space fungal micro flora.

Now, the UHB criteria for surfaces is also important, because we are also interested in the built environment, especially after water damage, how surfaces may become contaminated and, importantly, how to measure them. I've put two photographs up here just to demonstrate serious roof leaks through a ceiling. Also, in a new building construction on the right hand side, I have a photograph, and you can actually see the water ponding and coming out from behind the wall. That's not very good for the occupants of that home that was undergoing construction.

But, over time, those surfaces are going to become mould-contaminated, so the UHB criteria states that obviously a surface zero, S0, has no visible mould. S1 has less than 300 centimeters square. Think of that as like a 60 x 50 cm square piece of wall or ceiling or floor that is contaminated with visible mould. Then S2 goes onto between 300 and three square meters, and S3 is greater than three square meters. These are really good ways, and the syntax or the way you describe surface contamination into S0, 1, 2, and 3 makes it very easy to explain to other people about how contaminated something actually is.

Now let's look at what the Portuguese have done, because they have actually made a law about the mould levels. This is a very interesting document because this uses the presence and absence of particular fungal species and then links this with human health, and there are three conditions for determining the adverse indoor living environment from fungi. They are cumulative, and they're looking for particular groups of microorganisms. They say that there are seven concentrations of particular fungi that are going to be present in quite low levels, and they have said under 500 colony-forming units per cubic meter. These are your dominant fungi such as Cladosporium, Penicillium, Aspergillus, Alternaria, Wallemia. They've also then broken down some five more rare moulds such as your Acrimonioum, Curvularia, Nigrospora, and they've said that they should only be present in quite low levels, up to 50 colony-forming units per cubic meter.

Again, then there are eight toxic moulds and they are dangerous and they should, in a sense, not be present in viable culture. These take into consideration Stachybotrys, the infamous black mould, Aspergillus fumigatus, Fusarium, Aspergillus terreus, Trichoderma. These are the moulds that you really don't want to see, and that's how the Portuguese define adverse conditions for habitability. Again, if you ever see any pathogenic microorganisms, which do happen. I often see this, and we immediately autoclave (heat sterilise) these Petri plates. They can grow (from time to time), so it's interesting that the Portuguese have actually defined this into this law.

Now, if we go onto the third way that the French have been defining contamination, again, they call it the ANSES recommendations, and this takes into consideration not just the air space contamination. Again, they say anything over a thousand colony-forming units is considered abnormal and requires professional intervention and remediation, and especially when any of those undesirable species as were present and articulated and identified in the Portuguese decree law. They also go on to talk about surfaces and whether or not they show any visible mould. Again, taking both the air in the surfaces together, the French define this as something called insalubrious building conditions, and I love this word. I've heard it before, but I had to look up its definition, and I've actually put the definition up on this slide, because insalubrious means "unfavorable to health or unwholesome." That's essentially what we're talking about, buildings that are not wholesome or healthy to live in identified using Petri plates. The French define this as substandard home, insalubrity when you have greater than three square meters of visible mould and more than a thousand colony-forming units per cubic meter of mould in your air space.

Now, the French did an experiment, and I'm going to tell you all about it, plus the results and the conclusions and the key takeaways so that you can use this right here in Australia. The experiment that they did was they looked at 1,012 different dwellings. They collected the environmental microbiological results and they compared it to medical data. Now, this is really important, so for all those naysayers who say that there can never be any thresholds that can provide evidence linking exposure to mould with adverse health consequences, this publication does provide that link, and I encourage all of you to take advantage of this information and use this in your own personal circumstances. Now, getting back to the paper in the experiment, 908 patients who suffered from rhinitis, conjunctivitis, asthma, were compared to 104 control patients who didn't have any of these allergies. They applied the Portuguese decree law, and they showed that over half or 55.2% of the dwellings in fact were insalubrious or nonconforming and posed a health risk to the participants.

That's a pretty high number, more than 50%. Looking at the second way of interpreting the indoor air quality and mould data, they found that only 5.2% of the data was nonconforming. Linked back to the medical text, and you can drill into the data and make sense of all of these percentages, but I'm cherry picking the key ones so that you can understand in overview what this publication is saying. Then if you look at what the French hospital standard method was, the UHV method, that showed that 19% of the buildings were nonconforming. So you can see that using different thresholds does give you a different interpretation on how serious the risk is, but importantly, and also not to be left with this dilemma between 5.2% and 55% of dwellings being a potentially dangerous, let's look at how they linked this with the known adverse health in these particular of groups of people.

The results showed that, again, between the 908 and 104 that fungal air concentrations were overwhelmingly higher for allergenic compared to the non-allergenic population of individuals. The medical disease distribution was asthma and rhinitis. The key one here is asthma and rhinitis at 33.7%. Rhinitis at 26.1% and asthma alone at 18.7%, and those with hypersensitivity pneumonitis at 11%. The key takeaway is that the whole number of colonies, that is all of the colonies that are capable of growing during the sampling period in the location, are able to be correlated with asthma and rhinitis, so they're the two key symptoms that are clearly linked with exposure to mould above a thousand colony-forming units.

The scientists did not discover a strong link with conjunctivitis or hypersensitivity pneumonitis, and importantly, out of the different types of fungi that are capable of growing on these Petri plates, consistently they found that exposure to Cladosporium, Aspergillus niger, Aspergillus alternata, and Aspergillus versicolor were linked, again, with the asthma symptoms. And again, these colonies are pretty easy to distinguish in culture and it therefore means that the amount of data that can be extracted from Petri plate assays of the airspace is very high.

The key result, and this is again drawn from the conclusions that the French make, is that dwellings with greater than a thousand colony-forming units per cubic meter of whole mould, either Aspergillus versicolor at 300 colony-forming units or Cladosporium at 495 colony-forming units must be considered as dwellings at risk for allergic patients. I'll just let you reread that, because this is really, really, really important information, and it defines two types of fungi and either a total colony-forming unit count or drilling into the amount or levels of indoor mould of these two fungi can be used to classify an indoor building or living environment or workplace environment, making it at risk for allergic patients.

What were the other conclusions that the scientists made? Well, there were a number of factors that were connected or correlated with exposure to mould, and they include having fewer rooms in the house or building, having a high occupant to surface ratio. Again, the smaller the building and the larger the number of people, the more likely that there'll be exposed to contaminated surfaces if they in fact have become water damaged. Now, the higher the number of inhabitants per cubic meter again is connected, and that's a risk factor for mould. A building that shows a lack of ventilation.

Electric heating again is connected, and I would assume, and my hypothesis in reading this because there's not a lot of detail in the French paper, this is due to installation deficiencies potentially in a property, leading to indoor condensation, often on the interior window glass. That's a subject for another day. Apartments located on the ground floor are overwhelmingly are prone to having problems with water and mould, and apartments or homes that have elevated indoor humidity of course are more likely to have mould. And the no-brainer, a property that has a history of water damage is likely to be correlated with mould exposure.

Now, what are the practical implications of this? Well, they went on to make some suggestions about how indoor air quality and mould inspection should potentially be conducted, and they said that the bathroom measurements of mould alone appeared to reveal the overall contamination level of a dwelling. Now, I have done many thousands of inspections, and we always do the bathroom and en suite, and it has never ever occurred to me to look into whether or not measuring one room only is able to determine the habitability of a building, but I think that that is probably a very fruitful area of research. The French have, in a sense, opened up the possibility of this, but they do make some caveats, and I'm going to talk about them before, because we don't spend all our time in our homes in the bathroom, but bathrooms often have poor ventilation and tend to be damp.

Now, the bedroom, they say is often closed, less ventilated and cleaned less often. We do spend between six, seven or eight hours or more sleeping per day in our bedrooms, and therefore the French stated that it is best to sample at least two indoor rooms, being the bedroom and the bathroom. They do go on a little bit further to state that with the emergence of molecular methods such as PCR and qPCR, the opportunity to get an even more fine-grained understanding of the microflora in the built environment is emerging, and that this is important.

qPCR may in fact be a very useful approach to determine more types of fungi that the person or patient may be exposed to. However, they do definitely state that traditional Petri plate culture using air sampling pumps is the way to go to work out whether or not a building is or is not safe for habitation, or is insalubrious, according to the French terminology. So their conclusion at the end of the paper is pleased take four indoor air samples and four surface samples to build up the amount of information that you use to understand whether or not a particular property is or is not contaminated above normal levels.

So I now just want to spend a few minutes talking about the congruent or the connection or correlation with the Australian mould guideline. The Australian mould guideline came out in 2010. I have published on this, and basically I have taken the data for the number of colony-forming units and the thresholds used in the Australian mould guideline and worked out what the number of colonies translates to in colony-forming units per cubic meter using the as published Australian Mould Guideline (2010) and then using the revisions that I have made and that Dr. Heike Neumeister has also made to her own mould guideline, and essentially there's just some fine tuning with respect to where the absolute colony count on the plate n umber thresholds kick in.

I'm going to put them up here, so it's very clear for everyone. On the left-hand side, we have the French USB method. Again, using that traffic light analogy, we have the low, middle and high and very high cut-offs. We know that the World Health Organization suggests that around about 500 and less, that indicates at least a Finnish building that has no moisture problems, and we know from the French research that I've discussed today that over a thousand colony-forming units is a problem and very high. What does the strain mould guidelines say? Well, the Australian Mould Guideline has got a very close connection with the AMG French work. You can see that when we look at the low band, the first threshold, the USB, say, between zero and 170, the Australian Mould Guideline says between zero and 212.

When you look at the number of colonies that you would actually count on the Petri plate, the French size zero to 10 is low. The Australians say zero to 12 is low. For the middle band, the French say between 10 and 32. The Australians say between 13 and 28. But now we move into the changing traffic lights as it goes from normal to potentially problematic, and we can see that High for the French is between 32 and 57 colonies, and Elevated in Australia is between 29 and 57. In France, >1000, over a thousand or greater than 57 is a problem. In Australia, 58 and above means that that building shows a high level of viable mould. That's what I wanted to present today as the dominant piece of research that has come to my attention in the last couple of weeks that I thought was very salient to bring to your attention, because it definitely demonstrates unequivocally that mould thresholds are important and are linked with adverse health.

Now, I'm going to do the Breaking News as we do every week, and I will ... Well, what am I going to be talking about with news this week? Well, I think I can't go past talking about SARS-CoV-2 and about one aspect of this particular viral pandemic. A very interesting piece of research came out a couple of days ago on one of the pre-print servers, and I'm just going to get that up for you now. This was talking again about exposure to this particular virus in the workplace, but they were of course linking not just the exposure to the RNA virus, but whether or not individuals became sickened with the illness, COVID-19. So they were looking at whether or not some jobs are more or less likely to be connected with the experience of illness from this virus, and they looked at this in this study by looking at all of the confirmed cases in Hong Kong, Japan, Singapore, Taiwan, Thailand and Vietnam.

What they did is in each country, they followed for 40 days the first locally transmitted cases, but excluded the imported cases. They looked at the work-related cases, which they defined as close contact with another confirmed case due to work or an unknown contact history, but likely to have become infected at work. Again, you can get this paper for free on the pre-print server. I encourage you to read it. It leads into another topic about surface contamination with virus, which is not too dissimilar to surface contamination with mould in that they are both events that we don't want to become exposed to. But I think that this is a very important paper to be aware of, because it does list those occupations which are most connected with contracting the virus and this becoming an illness.

What they discovered from 690 local transmissions, the authors identified 103 which were workplace-related, and the five occupation groups with the most cases where healthcare workers. Well, that's a bit of a no-brainer. We know that, but the paper goes into a lot more detail. I'm going to cover that in a few minutes. Second, drivers and transport workers at 18%. The third most risky occupation, service and sales workers at 18%, same as drivers and transport workers. And at 9%, cleaning and domestic workers, and the number five rounding it out, public safety workers.

But there is more in this publication, and what they stated is that there is a connection between early and late outbreak, and they sit that possible work-related transmission played a substantial role in early outbreak cases, up to nearly 50% (or exactly 47.7%), and the dominant at risk occupations where service and sales drivers, construction workers and religious professionals. In the later outbreak stage, the at-risk occupations were healthcare workers, drivers, cleaning personnel and domestic workers, police officers and religious personnel. I think we need to be very mindful in here to note that cleaners can potentially be exposed to this virus in the workplace, as can other people who are transporting our goods and services. Even think of Uber Eats and all the potential contact roles and responsibilities that a lot of people who are still working are exposed to within the community. In any case, this is it for today's livestream. The takeaway from this particular paper was that elevated risk of infection at work is definitely not limited to healthcare professionals.

Now, I'm an environmental microbiologist. My name's Dr. Cameron Jones. I do live streams every week focusing on the built environment and pathogens, be they standard microbiological ones, and increasingly frequently, I'm commenting on viral transmission, viral surface contamination, and what we should be aware of in the built environment today. I thought it very important to review thresholds in rounding out your understanding of mould-contaminated buildings, and I'll be back next week with more information that could help all of us as we stay at home and work from home. I hope everyone who is watching this remains free of contact with this virus, and all I can say is what a serious time we are all in, and I wish the best to everyone. In any case, I'll see you next week. Bye for now.

YouTube Link:


Reboux G, Rocchi S, Laboissière A et al. Survey of 1012 moldy dwellings by culture fungal analysis: Threshold proposal for asthmatic patient management. Indoor Air. 2018;29(1):5-16. doi:10.1111/ina.12516.

L L. Mold Exposure and Mitochondrial Antibodies. Altern Ther Health Med. 2020 Feb 21. pii: AT5799. [Epub ahead of print] - PubMed - NCBI. Published 2020. Accessed April 15, 2020.

Jones, C.L. (2019). Guidelines For The Assessment of Viable Fungal Hygiene On Indoor Surfaces Using RODAC Petri Plates. J Bacteriol Mycol Open Access. 2019;7(5):116‒126.

Work-related Covid-19 transmission
Fan-Yun Lan, Chih-Fu Wei, Yu-Tien Hsu, David C Christiani, Stefanos N Kales
medRxiv 2020.04.08.20058297; doi:




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