After advising building owners, attorneys and insurance companies on microbial issues for over 10 years, performing hundreds of initial microbial investigations and "clearance" or "post-remediation re-occupancy" studies, like many of you have found myself scratching my head attempting to interpret microbial data in light of the industry's guideline for determination of microbial contamination: Compare inside spore levels with outside spore levels. The ACGIH in there "Bioaerosols Assessment and Control" document explain that:

"The concentration of fungi in indoor air typically is similar to or lower than the concentration seen outdoors. Exceptions are enclosed agricultural and other specialized environments (where indoor fungal concentrations may be much higher). Outdoor concentrations may exceed those measured indoors even where indoor fungal growth is obvious. If outdoor fungal concentrations are very high, indoor/outdoor concentration ratios for total fungi may be low, even in the presence of significant indoor growth. On the other hand, outdoor fungal concentrations my be reduced during times of snow cover or other conditions that suppress the release of fungal spores from outdoor sources, at which times, indoor measurements may be higher than hose outdoors even in the absence of significant indoor sources.1"

On the surface, this guideline, often affectionately referred to as the "Golden Rule", provide an excellent general direction for professionals and scientists in the field of microbial detection and investigation. However, after attempting to follow the Inside vs. Outside guideline, I have found that many awkward and often time problematic scenarios arise that are rarely addressed in the literature and leave the professionals attempting to make critical decisions is the absence of clear scientific understanding.

While the ACGIH is correct in their assertion that outdoor fungal concentrations may be suppressed with snow cover there are a number of other environmental conditions that field investigators need to pay close attention to that have a significant impact on data interpretation. For example, not only does snow cover suppress spore release but remember, environmental fungi proliferate between the temperatures of 4-60°C (40-140°F) with the majority of molds falling into an optimal range of 15-30°C (59-86°F).2 Few molds will be active at temperatures of less than 10°C (50°F). For the sake of argument, we can use EPA's optimal ranges of 4-38°C (40-100°F)3 as an optimal range that the majority of mold genera encountered on water impacted buildings will grow. Mold can also grow in conditions up to 140°F but high heat will kill mold while cooler temperatures tend to place the molds in a dormant state.

What impact does this information have when attempting to interpret ambient environmental fungal data? Well, not only does snow cover suppress mold release, but temperatures below 4°C and greater than 38°C will likely lower outside spore levels. These climatic conditions are commonly encountered in most states in the northern US making comparisons with outside data difficult from anywhere from 3-6 months out of the year. At some point the dependency on exterior total spore levels and genera whether viable or non-viable becomes questionable if not completely meaningless.

For example, in Chicago, total spore levels (using the spore trap method) have been observed in winter months with ambient temperatures of -4°C (25°F), a relative humidity of 17% without snow cover to be as low as 0-10 counts per cubic meter (cts/m3) and with culturable levels at 0 colony forming units per cubic meter (CFU/m3). Meanwhile interior levels where temperatures are maintained at 21°C (70°F) are typically 500-1,000 cts/m3 showing normal mold genera. Some consultants attempting to follow the Golden Rule would fail a contractor or declare there was microbiological reservoir because inside spore counts were higher than outside spore counts which in my opinion is grossly incompetent.

Similar problems occur in the Rocky Mountain states where summer ambient temperatures routinely exceed 38°C (100°F). Exterior spore levels may drop precipitously below normal again due to excessive heat creating this "upside-down" scenario and placing data interpretation in direct conflict with Golden Rule.

Since environmental molds require an abundance of moisture for their growth process, relative humidity also plays a critical role in data interpretation. It is common knowledge that most molds reproduce when RHs meet or exceed 60-70%. A little known fact is that it's not so much the high RH that causes mold growth but it's the building materials that absorb moisture from the atmosphere supplying both the food source and adequate moisture levels simultaneously.4 So even if temperatures are within optimal growing ranges for mold, if there exists inadequate moisture, spore levels may be artificially suppressed as well.

This scenario again may arise in the summer months after prolonged periods of draught. Without rain, humidity levels may plummet along with spore levels even though temperatures are within optimal ranges.

Fortunately, investigators do have other criteria available to aid in the interpretation of mold data. An experienced investigator will look at numerous other factors including:

• General Dissimilarities - When hydrophilic signature genera such as Chaetomium, Stachybotrys, Fusarium and Memnoniella are present in indoor samples and not outdoors, this is a clear indication of microbial contamination. This is a common occurrence during post-remediation studies since total spore levels may have fallen dramatically because of remediation (even below exterior levels) but remnants of these molds remain due to incomplete decontamination.
• General Composition - When hydrophiles such as Penicillium, Aspergillus and Cladosporium show up in both indoor and outdoor samples, a relatively common occurrence, the percent composition of these genera within the sample may provide clues as to whether or not microbial contamination persists. For example, I frequently encounter interior total spore levels that are below exterior levels but the percent Penicillium/Aspergillus (P/A) is upside-down; that is, interior percentages are much greater than exterior percentages. As a rule of thumb, if interior percentages are great than 15-25% of exterior percentages, even though total spore levels are lower inside, I would be inclined to fail the clearance or look for additional mold reservoirs.
• Toxigenic/Pathogenic Molds - Should Stachybotrys, Fusarium, Memnoniella, Trichoderma, A. fumigatus, A. flavus or other known toxigenic or pathogenic molds be present, even though total spore levels inside are lower than outside levels, its probable that microbial contamination persists. In most instances I have taken a  zero tolerance approach, that is, occupants are advised to find other accommodations and no amount of these organisms are permitted to remain in the contaminated area.

One other note, the ACGIH authors recommend:

"Fungi whose presence may indicate excessive moisture or a specific health hazard have been termed indicator organisms. Interpreting the presence or absence of an indicator species requires the ability to identify fungi to the species level(emphasis added) and a knowledge of the prevalence of the indicator species in both the indoor and outdoor environments.1"

In other words, speciation, not just genera determination of bioaerosol and surface samples is recommended by the ACGIH. This suggests that simply identifying P/A with a spore trap method falls short of their recommended approach. The impact this has on microbial investigations is significant. Only the more expensive and time consuming agar and polymerase Chain Reaction (PCR) methods allow for fungal speciation. Should an expert use the spore trap or surface tape methods for microbial detection, they may find themselves conflicted with an informed attorney. While admittedly I do not use agar and PCR methods exclusively, I do use them in conjunction with optical methods anytime medical or legal issues are at stake. On lower profile projects where the neither of these concerns exist, I find optical methods adequate for investigative purposes, particularly since they are less expensive and time consuming.

And you thought asbestos detection and investigation was difficult! In my opinion, asbestos in its rather planar state is much simpler to evaluate then the far more dynamic environmental fungal issues.

So what should a field investigator do? First of all, don't legalistic with the ACGIH Golden Rule. While it is a helpful guideline, it is only one of several factors taken into consideration when interpreting fungal results. Stay away from strict interpretations of data based solely on indoor versus outdoor comparisons. Integrate several of the other factors (genera dissimilarities, genera composition and toxigenic/pathogenic molds) into your understanding of the microbial problems. Document temperature and RH conditions anytime bioaerosols are collected. If indoor and outdoor temperatures and relative humidities show large discrepancies between them, it is probable that your data will be influenced as well. An awareness of climatic seasonal trends may also help understanding hard to explain data. Or when in doubt, you can always say "insufficient data" and obtain more samples to bring clarity to your findings.

Table 1

Typical Mold Temperature and Moisture Relationships⁵

Species	°F	Optimum °C	Aw	°F	Limits °C	Aw
A. amstelodami	91	33	0.93	50-108	10-42	0.71
A. niger	91	33	>0.98	54-109	12-43	0.78
A. gumigatius	104	40	>0.97	54-127	12-53	0.82
P. martensii	73	23	>0.98	<41-90	<5-32	0.79
P. islandicum	88	31	>0.97	50-100	10-38	0.83
Stachybotrys atra	73	23	>0.98	45-99	7-37	0.94

Aw - Availability of water.

Bibliography

1) "Bioaerosols Assessment and Control", American Conference of Governmental Industrial Hygienists, 1999; p. 7.4.2.1-2.

2) "Bioaerosols", Harriet Burge, Center for Indoor Air Research, CRC Press, 1995; p.12.

3) "Indoor Air Quality in Large Buildings", US EPA, Appendix C: Moisture, Mold and Mildew, p.1.

4) Block, S.S.; Applied Microbiology, 1:287-293, 1953.

5) Flannigan, Brian; "Approaches to Assessment of the Microbial Flora of Buildings"; presented at IAQ-92 - Environments for People; 1993.