To understand the techniques or even the reasons for remediation of fungal contamination in buildings, we need to answer the question; Why? By now many of us know the answer. Health science has shown a link between exposure to fungal propagules and illness. The linkage goes back hundreds of years but it has not been until recently that we have developed a better understanding of the etiology of fungal diseases. Following, we will discuss the nature of exposure to fungal aerosols and how we mitigate those exposures.
Molds (a.k.a. moulds, fungi) are naturally occurring organisms that utilize organic materials as sources of carbon and energy. It just so happens that modern buildings are prolific sources of carbon including wood framing, paper products, and dust. Given enough moisture, mold can colonize on many of the surfaces and materials in a building. As mold metabolizes its growth substrate, it continues to reproduce, forming colonies and eventually, fuzzy mats of intertwined hyphae called a mycelium. The hyphae are actually microscopic “hair like” strands that form the structures upon which the mold reproduce. A conidiophore is a specialized type of hypha that produces conidia, commonly known as mold spores. Any further discussion of the life cycle of mold I will save for the mycologists other than one important aspect that is of interest to engineers and hygienists.
Mold spores and hyphae (collectively; propagules) are extremely small. The typical sizes of indoor mold spores are in the range of 2 to 15 microns and the diameter of hyphae are in the range of 1-4 microns. A micron (micrometer) is one one-thousandth of a millimeter. The finest human hair is approximately 17 microns. Particles this small aerosolize easily. Even the lightest breeze can dislodge the spores or hyphae from their mycelium. In fact, this is naturally how fungal spores perpetuate their species. Considering that a 1 square centimeter colony may contain well over 100,000 spores, they are quite successful in this endeavor. Under conventional dissemination processes, hyphal fragments are liberated at much higher rates than fungal spores. Studies have shown that concentrations of hyphal fragments are generated at 250 to 500 times the level of spores. Most of these hyphal fragments are smaller than 2.5 microns. What are the consequences of such exposure, knowing that all of these fragments can contain allergens and toxins?
None of the foregoing would be a matter of concern were it not for its implications in the human disease process. Recognizing that fungal growth structures including spores have been idealized by nature to be aerosolized upon even the lightest air currents, inhalation has become the primary path in the etiology of the disease process in humans. Typically, fungal spores are from a few microns to several microns in size. Fungal hyphae may be of various sizes, depending upon how they are dislodged and easily range from a micron in length and far beyond. Settling times for fungal spores in still air will range from several minutes to hours. The smaller the particle, the longer the settling time and the easier it is will re-aerosolize. Particles smaller than 1 micron may not ever settle under anything but laboratory conditions. The simple act of walking is capable of generating significantly amplified fungal aerosols from settled dust in most buildings.
And this is where the indoor environmentalist and physicians become interested because these aerosolized particles are also capable of penetrating deep into the human lung. The aerosolization of fungal spores and hyphae is significant if one considers their route entry into the human body; through the respiratory tract. The inhalable masses of respirable particles (around 10 microns and less) are mostly captured in the human nasal passages. However, particles that are capable of passage deep into the lungs are often less than 4 micrometers. Deep lung penetration increases drastically as particle sizes fall below 1 micron and the nasal capture efficiency approaches zero at that point. Once lodged in the lung, fungal particles can induce the allergenic, toxic or pathogenic effects that have been widely studied and reported upon.
Recent research in mycology has furthered our understanding of the etiology of fungal allergy, toxicology, and infection that has profound impacts upon fungal remediation protocols. So as we endeavor to remove a potentially significant contamination from a building, it is important to not let the cure be worse than the disease. In the course of a fungal remediation, we are likely to be removing trillions upon trillions of spores and miles upon miles of hyphae. Because exposure to fungal aerosols manifests highly variable impacts upon indoor populations, indoor environmentalists have followed, generally, a blanket precautionary approach to resolving the problems of indoor fungal exposures. Accepted remediation protocols are to essentially isolate the contaminant source and remove it, preferably intact with the host substrate. Removal of building structural elements, however, is usually not a practical matter so a variety of means and mechanisms have been developed to address structural contamination. These methods invariably involve some sort of aggressive means to dislodge the fungal mycelium and collect it for disposal. Upon completion, a variety of assessment techniques (aka clearance) are used to validate removal of the mold and byproduct. It may be though, that such “cures” are worse than the disease itself.
Some remediation solutions are so aggressive that the mycelium is literally blasted to pieces as an adjunct to removal. The classic examples are “blasting” technologies, including soda (sodium bicarbonate) blasting and dry-ice blasting. Each of these technologies propels a high-velocity stream of particles and compressed air at the contaminated surface to break up and dislodge the mycelium. The process works very well and the surface appears clean after a pass by one of these machines. But a question remains; what happened to mold? The answer is evident. The mycelium and spores are literally obliterated from the surface and thence disseminated into the surrounding air, eventually to settle out or plate out upon every other surface in the area. Some of the particles will no doubt be so small as to never be resolved with microscopic techniques. If all goes well, many of the particles will be ejected upon the exhaust air stream from the contained area. But there is no way to tell as the hyphae and spores are reduced to fragments so small and obscure that they cannot be identified. Some will remain, plating electrostatically to surfaces, settling in dead air spots or being forced through the minute cracks and crevices of the contained area into adjoining clean areas. The problem is there is no way to know.
These spaces cannot be “cleared”. So what, in essence, have we accomplished with such processes? We have taken a relatively intact and controllable mass of well-defined contaminants, blasted it into innumerable small pieces and blown it all around the contained area and beyond. We have no way of knowing the extent of contamination and have created an exposure scenario that may be far worse than what we had to begin with; many more even smaller particles capable (or virtually guaranteed) of deep impact into lung tissue. Recent studies suggest that these remaining fragments may persist in the affected area after remediation and may be responsible for continued occupant exposure and sickness.
And then there are those that advocate “killing” the mold to mitigate its harmful effects. Utilizing antimicrobials, oxidants (sodium hypochlorite or bleach) or a biocide of choice it is thought that once the mold is dead it will no longer be harmful. Nothing could be further from the truth in most cases. The predominant (though not always the most serious) health effects of mold exposure are due to the allergenic and toxic effects of the mold, which are not mitigated when the mold is no longer viable (i.e. dead). Rarely, some molds are pathogenic and use of a listed biocide has important applications where these fungi have been positively identified. But the vast majority of molds found growing within buildings are not pathogenic to typically healthy persons. Also, it would not be unreasonable to ask about the long-term exposure effects of the liberal application of certain chemicals in an indoor environment (and you should). And if you are really curious, read the labels on some of these chemicals. What you will find is that their use is limited to certain species of fungi or bacteria and their efficacy rates are based on laboratory tests. In other words, they only work on certain species and only so well. A 99.999% efficacy rate, even if it were achievable in the real world on the particular species of mold identified, would leave 1 spore in a 100,000 still viable and capable of continued growth. And that is where a colony starts, with one spore (keep in mind that there may be over 100,000 spores per square centimeter). If all conditions remain ideal, you may have one-inch diameter colonies dotting the entire treated area within a week.
We know fungal contamination in a building must be addressed. Knowing what not to do is a good beginning.