Interpreting Low to Mid Levels of Airborne Fungi
Q. How do I interpret moderate to low levels of airborne fungi considering variability in airborne populations?
A. This is a perennial problem, and one that is truly solved only by collecting either high-volume samples (producing a long-term average) or (better) a series of individual samples so that variability over space and time can be estimated. Of course, budgets generally do not allow for this kind of repetitive sampling. Instead, one must understand the magnitude of possible variability, the probability that any one sample represents the whole, and make decisions based on a combination of the air sample data and careful visual inspection.
Variability in fungal spore populations derives from the particulate nature of the spores, the kinds of fungi present (which determines the number of spores produced, and spore release parameters), the size of the space, air and other movement in the space, ventilation and filtration, and other parameters. The fact that spores are particles separates them from volatiles and gases that are sampled using grab techniques. Gases distribute throughout a space by diffusion as well as air mixing, and eventually become more or less uniformly distributed (although there will be a concentration gradient near a source). Spores are relatively large particles, and are removed from the air by settling (at rates dependent on the aerodynamic diameter of the spore), impaction on surfaces, and removal by ventilation. When there is lots of fungal growth, especially with fungi producing small spores as is often the case in indoor environments, spores are usually produced in abundance, released easily over reasonably long periods of time, and tend to stay airborne long enough so that reasonably good mixing can occur. In this case, a single grab sample is likely to be reasonably representative of the space. On the other hand, if only small amounts of growth are present, or if fungi such as Stachybotrys (which has relatively large sticky spores) are dominant, few spores are released over relatively short periods, the larger ones settle quickly, and mixing may be minimal. Here, the probability of capturing the peak aerosol concentration is very small. One could assume that the low counts that often result from these samples indicate minimal exposure, and that may be true. However, a single sample may miss an important peak that could, in fact, have caused symptoms in an occupant.
In spite of these problems, I do recommend air sampling if there is any possibility that you will have to document exposure. The visible presence of fungi is not proof of human exposure, although it is certainly sufficient to indicate that remediation is necessary. If you must document exposure, then sampling protocols will have to be developed that account for the parameters listed above. If there is a great deal of growth of, say, Penicillium and Aspergillus, a short series of samples without then with a fan for disturbance should be sufficient. If, however, the fungal growth is limited, for example, to areas above baseboards as is often the case for Stachybotrys, then documenting exposure (or lack thereof) will involve multiple samples (e.g., 10 for a moderately sized room). In this situation, it is important to remember that you are not trying to prove that exposure is occurring. You want to know the truth: whether or not exposure is occurring, with equal probability for both outcomes. Thus, you need a large number of samples that are essential for proving the negative case. It is rarely appropriate to sample under aggressive conditions that would never normally occur in the space just so you can document that exposure could occur under extreme circumstances. An exception to this is if you want to document exposure to remediators.
So, we are back to what I always say:
1. Sample only to test a carefully constructed hypothesis.
2. Be careful to test the hypothesis, rather than try to prove it.
3. If you can’t sample in a way that will actually test the hypothesis, then don’t sample at all.
Dr. Harriet Burge is director of aerobiology at Environmental Microbiology Laboratory Inc. and associate professor and director of the microbiology laboratory at the Harvard School of Public Health. Widely considered the leading expert in IAQ, Burge pioneered the field more than 30 years ago. She has served as a member of three National Academy of Sciences committees for IAQ, including as vice chair of the Committee on the Health Effects of Indoor Allergens.
To submit a question to Dr. Burge, write to her by e-mail at askdrburge@emlab.com. All questions posed to Burge will receive a reply, although space limitations prevent us from publishing them all. By submitting a question, you agree to have your question and its answer published in a future edition of IE Connections.
H2O911 Mold Remediaiton |
