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November 2005

Volume 3 | Issue 11

Hello hello,

I hope you're enjoying the beginning of the holidays and will find the following discussion about mold growth in walls by Dr. Harriet Burge, and the article about ascospores by Murali Putty, interesting and informative.

With best wishes,
Dave Gallup

Can Mold Be Safely Left Inside Walls?
By: Dr. Harriet Burge

This ends up being a question of relative risk, and a complicated one at that. First, in the ideal situation, all the mold growth would be removed, either by removal of the growth itself, or of the material supporting the growth. So - what is an ideal situation? Here is where the relative risk comes in. The ideal situation for complete removal is when the risk of leaving the mold far outweighs the risk of removal. I know some of you will say - "there is no risk associated with removal". I will say the opposite: there is little risk associated with dried mold in walls, and significant financial and emotional risk associated with its removal. Here are a few examples.

1. Penicillium chrysogenum is known to have grown extensively inside and on the occupied surfaces of walls in a school room. All the occupied space mold has been removed, the water problem repaired, a sequence of air samples has documented the absence of culturable P. chrysogenum, and concentrations of Pen/Asp spores are low. Thus, there appears to be little if any health risk, and any risk to the building would require a water event which would precipitate new (possibly different) mold regardless of whether or not the existing mold is removed. The parents and teachers don't believe or understand this, and want the mold removed. On the other hand, the school board has facts and figures that indicate that undertaking removal of the mold means that the school will have to be closed for the remainder of the year, causing disruption of the children in this and in whatever school they have to move to. It impacts the teachers - no school, no job. The school board, contrary to popular belief, does not have the funds at hand to do the removal job and support all of the other essential school expenses (salaries, supplies, services, etc.). So, who gets laid off? To me, these few statements justify leaving the mold, making sure no new water events occur, and monitoring routinely for several months, looking only for P. chrysogenum or for sharp increases in Pen/Asp spores.

2. Here's an example from a hospital. Contrary to popular belief, hospitals do have mold, especially behind baseboards, and near sinks and other water sources. They are there in most hospitals, and present no apparent risk (e.g., no increases in infection rates). In fact, the fungi that grow in these areas are generally not those that cause infections. So, remember that we are not dealing with an initially pristine environment. Now, you are called in to evaluate a hospital that has had a flood on the lowest level. The flood water has been removed, all the carpeting dried and cleaned, and the walls thoroughly washed. Air samples reveal very little mold of any kind. However, because of the heightened awareness of mold, hospital staff have discovered some of the mold in other parts of the hospital and are clamoring for its removal. Because there are small amounts of mold at nearly every nurses station (e.g., in cabinets under the sink) and every baseboard that has been pulled back reveals some signs of water damage and mold, removal becomes a significant problem. Hospital administration has to make the decision whether or not there is funding for such a project, which would entail removal of rooms from service, potential release of mold that at the present poses virtually no danger, and a great deal of expense. If the hospital happens to be wealthy (few are these days), then the risk lies primarily in the potential for mold release during remediation. If money is short, the hospital must make the same decisions as for the school. Can they afford to have rooms out of service? Does the financial risk, and the risks associated with remediation outweigh the health risks of leaving the mold in place? Since the mold probably developed within weeks of opening, and it is unlikely that remediation will prevent further development (unless they have all the sinks inspected monthly at least, and stop wet-mopping patient rooms and steam cleaning hallway carpets).

The bottom line is, making decisions about whether or not to remove hidden mold requires an analysis of the risks associated with leaving it there balanced against the risks of removal. Obviously, if Aspergillus fumigatus has colonized the inner walls of a hospital, then it must be removed, because the risk is high, even if only a little escapes into a transplant patient's room or a surgical suite. If the Penicillium chrysogenum in the school example is routinely recovered from the room air, then of course it will have to be removed because of the potential effects on asthmatic children. However, removal based on the mere fact of its presence, or based on nonspecific symptoms that are not related to mold exposure, is often not appropriate.

Fungus of the month: Ascospores
By: Murali Putty

Ascospores are sexual spores of fungi in the class Ascomycete. Ascomycetes are ubiquitous, and include more than 3,000 genera, including common genera like Chaetomium, Peziza, and Eurotium. The cellulolytic ascomycetes, Chaetomium and Ascotricha, are frequently found growing indoors on damp substrates. They are among the organisms responsible for the destruction of cellulosic fabrics.

As pathogens of crop plants, timber, and ornamental trees, the ascomycetes are among our worst fungal enemies, causing such diseases as apple scab, brown rot of stone fruits, powdery mildews, foot rot of cereals, ear rot of corn, blue stain, etc. Some members of Eurotiales infect wild mammal populations; related species cause infections known as ringworm and athlete's foot in humans. Some are pathogens of fish. Ascomycetes are closely associated with insects. Some, such as Ophiostoma, Ambrosiella, Raffaelea, and Symbiotaphrina, are found in insect structures and provide or detoxify the food on insects. Species of Beuveria, Metarhizium, and Fusarium kill insects and are being studied for use as biological insecticides. Ascomycetes also provide us with a huge array of metabolic processes and products, such as antibiotics, and serve as important model systems for scientific discovery.

While many of the ascospores are grouped together, certain ascospores belonging to certain genera such as Chaetomium, Microascsus, and Petriella can be easily identified on spore traps. Spores identified simply as "ascospores" on spore traps are commonly recovered in the outside air with recovery rates between a low of 69% in December and a high of 86% in October. The levels recovered are moderate, with the 50th percentile value, when recovered, ranging from 107 spores per cubic meter in December and January, to 160 in September and October (see figure 1). The recovery rate is notably influenced by the weather with rates varying from a low of 45% in light and moderate snow to a high of 95% in moderate rain. When recovered, the spore density ranges from a low of 53 spores/m3 in light and moderate snow to a high of 960 in moderate rain. Note the correlation between the spore dispersal method and the higher recovery rate and spore density during periods of moderate rain (see figure 2).

Figure 1: Frequency of detection and spore density of ascospores by month.
The gray bars represent the frequency of detection, from 0 to 1 (1=100%), graphed against the left axis. The red, green, and purple lines represent the 2.5, 50, and 97.5 percentile airborne spore densities, when recovered, graphed against the right hand axis. (Source: EMLab MoldRange data. Total sample size used for this graph: 39,878.)


Figure 2: Frequency of detection and spore density ascospores by weather.
The gray bars represent the frequency of detection, from 0 to 1 (1=100%), graphed against the left axis. The red, green, and purple lines represent the 2.5, 50, and 97.5 percentile airborne spore densities, when recovered, graphed against the right hand axis. (Source: EMLab MoldRange data. Total sample size used for this graph: 20,191.)

Allergens in ascospores are poorly studied; they are highly variable and dependent on genus and species. Many ascomycetous fungi can produce toxic cyclopeptides, coumarins, phenolics, terponiods, polysaccharides, and glycoproteins, some of which are known as mycotoxins that make food unpalatable and dangerous.

Ascomycetes may have two distinct reproductive phases, one sexual, involving the formation of asci and ascospores, and the other asexual, with spore production occuring at different times on the same vegetative bodies (i.e. the hyphae). Some of the common asexual fungi such as Penicillium and Aspergillus produce sexual forms under certain conditions; these are classified in the ascomycete group and given distinct names. For example, the most common sexual forms of Penicillium are Talaromyces and Eupenicillium; the most common sexual forms of Aspergillus are Eurotium and Emericella.

Ascospores are formed within a sac-like structure called an ascus, in which nuclear fusion and meiosis occur. Ascus shape varies and asci may be spherical to elongated with cylindrical, ovoid, or globose forms (see picture 1).

Picture 1: Ascospores in an ascus.
The most common number of ascopores produced per ascus in most species is eight. Ascospores come in a variety of sizes and shapes ranging from long, thin, and threadlike to globose and even hat shaped in appearance. Ascospores may be one celled or septate (see picture 2)

When ascospores are mature, they are released and subsequently dispersed. In many ascomycetes, ascopores are released by forcible ejection. Spores are predominantly forcibly discharged during periods of high humidity or rain. Consequently, outdoor levels of ascospores are often high after a rain. Ascospores may also be dispersed by wind, animals or other agents. There can sometimes be a delay in how indoor levels of spores correspond to changes in outdoor levels. This may result in situations where the indoor counts are higher than the outside and not indicative of fungal growth indoors, but rather of earlier high outside counts.

Picture 2: Ascospores on a spore trap.
In spore traps, ascospores are recognizable by the fact that they have no attachment points, and are sometimes enclosed in gelatinous sheaths or within a sac. On tape samples, many ascomycetes are distinctive, especially if fruiting bodies are present. While some ascomycetes sporulate in culture (Chaetomium, Pleospora), many are parasitic plant pathogens, and sporulate only on living host plants.


1) Alexopoulos, C.J., Mims, C.W. & Blackwell, M. Introductory Mycology. 1996. 4th ed., John Wiley & Sons, Inc., USA.

The data and other information contained in this newsletter are provided for informational purposes only and should not be relied upon for any other purpose. Environmental Microbiology Laboratory, Inc. hereby disclaims any liability for any and all direct, indirect, punitive, incidental, special or consequential damages arising out of the use or interpretation of the data or other information contained in, or any actions taken or omitted in reliance upon, this newsletter.

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