As a school administrator, I would like to know at what levels of airborne mold is remediation needed ?  For example is there some formula related to outside air samples?- Chris, VA

            There is, of course, no formula that can help a building manager decide whether a .remediation is necessary or not.  I use a variety of indicators.  These include the airborne mold concentration determined from sample collection on culture plates or on greased slides/cassette samplers, the type and amount of mold present compared to outdoor samples, the presence of visible mold growth/water damage/mold odor, etc.

            If visible mold growth is present, water damage is localized or extensive, mold odor is present, remediation is desirable irrespective of what one gets on airborne sampling.  A mold problem exists, it needs remediation.

            Interpreting mold test results is problematic.  Many investigators compare indoor concentrations to the outdoor concentration based on the fact that absent snow cover outdoor concentrations are almost always higher than indoor concentrations.  Indoor concentrations higher than outdoor concentrations thus suggest that a major indoor mold source is present to cause such differences.  In many cases that may be true but probably in as many other cases( if not more) that may not be the case.

            Let’s look at some data recently sent to me by parents.  Six samples were collected on agar medium indoors and one outdoors.  Sample concentrations in 5 rooms ranged from 1166 to 1637 CFU/m³; the sixth was a much lower 495 CFU/m^3.  The outdoor concentration was 848 CFU/m³.

            If one looks at these test results on an overall concentration bases, those indoor numbers are relatively high.  When I conduct similar sampling I rarely get concentrations over 300 CFU/m³ in school classrooms unless there is an obvious infestation problem present.  If one compares the indoor numbers to the outdoor numbers it appears that for the most part indoor concentrations are higher than the outside concentration.  Many investigators would interpret such results as indicating that the school building had an infestation problem that needs remediation.

            In this case, both indoor and the outdoor samples were dominated by Cladosporium and non-sporulating colonies (range 70-90%) and had similar spore types.  Indeed when one looks at the mold types present, it was evident that the major source of observed indoor mold levels was the outdoor environment.  As such, absent any visible mold growth, the school building did not have a mold problem that needed remediating.

            In another school building investigations collected airborne onto cassette samplers.  Concentrations determined from counts at 600X magnification varied from 71-947 S/m³ with an outdoor value of 410 S/m³.  Clearly in this case the outside concentration was significantly higher than indoor concentrations.  Indoor airborne mold included many mold types that are much more common outdoors than indoors.  Despite this and the absence of any visible mold growth, the mold consultants in this case recommended a full scale remediation of the building including carpeting and air handling systems. Such advice is unfortunate and of course expensive. This kind of circumstance argues strongly for a need of a second opinion.

December 10, 2004