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What are “Normal” Levels of Mold Exposure?
An interview with David MacIntosh, Sc.D.
by Linda Goodspeed

As more and more attention is focused on indoor air quality and its effect on the health and well-being of building occupants, mold has emerged as a major issue and potential liability for building owners and managers. Several scientific studies have associated fungal material and dampness in indoor environments with respiratory complaints and non-specific illness in adults and children. The legal system has elevated this association even further with several high-profile settlements and jury awards in cases brought by tenants claiming harmful and excessive exposure to mold.

“It is true that mold has properties that can adversely affect people’s health,” said David MacIntosh, Principal Scientist at Environmental Health & Engineering (EH&E) in Newton, MA. “Some molds produce toxins and irritants that can cause respiratory symptoms and other non-specific illness. Mold also has allergens which can cause allergic reactions in some people.” But while it seems clear that mold can indeed cause health problems, the actual level of exposure required to cause symptoms is not. “Different levels of mold become toxic at different points to different people,” MacIntosh said. “Science is inconclusive on the extent to which mold can affect the general public, especially at the levels we see in almost all buildings. And the simple fact is that outdoor levels of mold dwarf indoor levels.”

So what is a building owner or manager to do if mold is suspected – or even not suspected – within a building? The typical approach in testing for mold is to take indoor samples of air and then compare those measurements with outdoor air samples. Higher mold spore counts indoors could be a sign of fungi growth indoors. Similarly, specific types of fungi found indoors but not present in the outdoor air could also be a sign of active mold growth in the building.  While helpful, the problem with this approach, MacIntosh said, is that we still do not know what “normal” exposure is to mold. Just because there are more mold spores inside a building than out does not necessarily mean occupants are being exposed to an abnormal or harmful level of fungal material. “Mold is everywhere,” MacIntosh said. “If you look for it you’re going to find it.”

In a 1998 study for the Environmental Protection Agency, EH&E surveyed 100 buildings across the country in an effort to determine normal levels of mold exposure. “What made this study different is that buildings are not usually measured until there is some kind of complaint,” MacIntosh said. “We take samples because we think there’s a reason to take samples. That’s part of the reason why we have a limited understanding of what typical mold levels are. In this study we did just the opposite. We wanted to characterize indoor air qualities of what were thought to be normal, healthy buildings.”

The survey measured indoor-outdoor ratios of both total and specific fungal material at different times of the day and in different locations within buildings. “One of the fundamental problems, in my opinion, in the way we sample for mold in the air is that samples are collected over just a few minutes. As a result they are really nothing more than a snapshot in time,” MacIntosh said. “If mold levels are changing substantially hour by hour, mold samples may not be a valid representation of what mold levels really are over time. In this study we took standard short-term samples, but at three locations at two different times of the day. This enabled us to look at both within-building variations by looking across the three locations, and also at time variations between morning and afternoon samples.”

The study has given EH&E a unique understanding of the “normal” exposure levels to mold within buildings. This can be very important in ruling out mold as a possible cause of building complaints or as a potential health hazard. “It helps us to better interpret results because we can now compare them against a more accurate picture of typical exposure levels,” MacIntosh said. While the study results are available publicly, few other companies currently have the level of expertise and experience that EH&E, as the study authors, brings to mold testing and analysis. “Data is one thing,” MacIntosh noted. “But useful information comes from analyzing the data, and no one’s done that except us. We’re working hands-on with this study every day. As a result, we are in a unique position to provide the best interpretation of fungal levels in a building with respect to its potential consequences for health.”

For more information on this study contact MacIntosh at:


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