Can Mold Be Safely Left Inside Walls?


By Dr. Harriet Burge

EMLab P&K Chief Aerobiologist and Director of Scientific Advisory Board

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.