We just had a mold test done in some older single-family homes, which will be torn down in about 12 months to make room for new apartments, so we are not interested in expensive remediation.  There is no mold visible in any of the houses, and the tape tests did not discover any mold, but the air tests showed elevated levels of Cladosporium and Penicillium.  The highest reading for Cladosporium was 1440.  There were also high readings for Smuts, Periconia, etc., the highest being 13000.  We are preparing to rent these units.  None of the previous tenants was ill.  All areas of moisture intrusion have been repaired, and the units cleaned and painted.  Do you think there would be a health hazard to new tenants?  Thank you for your consideration.-Arlee, California 

            The test results you report indicate a dominating influence of outdoor mold on indoor mold test results.  Cladosporium occurs both indoors and outdoors with higher concentrations found outdoors.  The presence of very large Cladosporium herbarum would indicate that outdoor mold was significantly affecting indoor concentrations.

            Smuts and Periconia are both members of a group of fungi called Basidiomycetes. They are most often wood decay fungi  but can also be plant pathogens(i.e. smut on corn).  They are outdoor fungal genera that may be found indoors as a result of the intrusion of outdoor air or in some cases very much decayed wood timbers.

            Many mold consultants compare indoor and outdoor mold levels to interpret airborne sampling results.  Such comparisons can be helpful if the consultant can distinguish between indoor and outdoor mold types.  Species of the genera Penicillium and Aspergillus are more likely to be found in higher numbers indoors when a mold problem exists indoors.  However, lab counts of spores from species from these two genera are not very specific.  They may include yeasts that are often very abundant in outdoor samples.

            I usually recommend that windows in a house be closed for a minimum of 24 hours to minimize the effect of outdoor mold on indoor levels at the time of sampling.  This is effective to some degree as larger spores that have an outdoor source will settle out in a matter of hours.  However, activities that result in the re-suspension of settled dust will cause such spores to be re-suspended and thus affect airborne concentrations at the time of sampling.

            In houses that have windows open a lot during the warmer months of the year, re-suspended outdoor mold spores can affect indoor concentrations for months into the heating season.

            Studies on culturable mold at various seasons of the year in house dust have revealed that outdoor mold species often dominate such samples.  The impact of outdoor mold on indoor airborne mold concentrations are significant and need to be seriously considered in interpreting sampling results.

September 30, 2005